White dwarf's impact on orbiting bodies

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Recently the Kepler telescope in its study of white dwarfs detected the first planetary object transiting a white dwarf in the data from the K2 mission. It was consistent with earlier theories' prediction that a planetary object orbiting a white dwarf would slowly disintegrate. Why would a planetary object that is orbiting a white dwarf disintegrate? I read it here.

The original paper published in Nature (preprint): A Disintegrating Minor Planet Transiting a White Dwarf

I think Aabaakawad's link gives a complete answer, but to give an astronomy for dummies answer, there's nothing about a white dwarf that causes a planet's orbit to decay at least, not directly. Your article (I've pulled quote below the caption):

Slowly the object will disintegrate, leaving a dusting of metals on the surface of the star.

That's only talking about this particular situation and there's a difference between disintegrate and decay. This planetoid is enormously close to the white dwarf. So close, that what we think of as normal white dwarf/planet dynamics (very cold) is no longer true. This planetoid is slowly being vaporized.

Looking at the orbital period of 4.5 hours (about 1948 orbital periods in 365.25 days). The orbital distance to orbital ratio relation is exponential to the power of 2/3 (this varies a bit due to eccentricity, but it's generally correct), so an orbital period 1948 times faster means about 156 times closer, and giving the white dwarf equal mass to our sun, that puts the planetoid at a bit under 1 million KM. If this white dwarf is lighter than the sun, the planetoid would need to be even closer. That's close to the Roche limit and would be inside it if the planetoid wasn't dense and rocky/metallic.

If we estimate the white dwarf to be about the size of the Earth, which is a common size given for white dwarfs, An Earth sized object from 1 million KM would be larger in the sky than the sun appears from Earth, and presumably quite a bit hotter than the surface of our sun too, so this isn't a tiny white dwarf in the sky from the perspective of the planetoid. It's a blazing furnace of a sun, so hot, it vaporizes metallic gas and dust off the surface of the planet.

The article mentions this (end of page 3), that Poynting-Robertson drag see here and here, and that may be a factor in any orbital decay in this scenario. The article is clear that there's a good deal of uncertainty on with that effect, and that only affects tiny particles, but enough tiny particles could create a drag over time… (maybe). The general scenario with this orbit is a planet scorched and as a result, is losing material. It's likely the very high heat that's driving any orbital decay, not gravity.

Gravitational decay / orbital decay does happen, usually much more slowly. That's probably not what's happening here.

There are some interesting orbital effects that can happen when a main sequence star goes red dwarf and later when it creates a planetary nebula, significant increases in tidal forces due to the star's greater size in the first case and increased drag in the 2nd, but at the white dwarf stage, there's no significant orbital decay effects.

Update:

Why not Poynting Robertson drag and Orbital decay effect the planetoid when white dwarf was a star or even red giant? Is there any "interesting orbital effects" when a star undergoes red giant?.Can you update your answer to summarize the forces and their effect on the planetoid in each phase of the star. and also what do you mean by orbital decay? Does it have something to deal with the Roche limit.

OK, I think, having read more about it, Poynting-Robertson effect only matters when the orbiting objects are very small. I've linked it twice above, but the simple explanation is that objects in orbit move and so any light or debris from the sun hits the moving object at an angle, not direct on. If the object is small enough, this over time drives the dust and maybe grain of sand sized particles into the sun. This doesn't affect larger objects, so it's not really relevant to any planets or planetoids.

As far as "interesting red dwarf" effects. That really has to do with tides. Using the Moon/Earth example, the Moon creates tides on the Earth, a tidal bulge towards the Moon, but because the Earth Rotates faster than the Moon orbits, this tidal bulge is always ahead of the Moon and this creates a gravitational tug on the moon that pulls it away from the Earth - very slowly.

The same thing happens with planets around stars, but even more slowly, lets pretend it's just the Earth and the Sun - a 2 body system (in reality, with several planets it's much more complicated), but just Earth and sun, teh Earth creates a tidal bulge on the sun, the sun rotates ahead of the earth, this causes the earth to very slowly spiral away from the sun - so slowly that it might take a trillion years for the Earth to spiral away.

Now when the sun goes Red Giant, the sun is essentially the same mass but much more spread out and parts of it, much closer to the Earth and less gravitationally bound to the sun. This creates a far larger tidal tug. Also, as the sun expands it's orbital velocity drops, because orbital momentum is concerved, so when the Sun is Red Giant, the tidal bulge will be behind the Earth which drags it in towards the Sun. Due to the size and proximity of the Red Giant star, this draws the remaining near-by planets towards the sun fairly quickly, at least compared to main sequence stages which, provided the sun rotates faster than the planets orbit, has a much smaller outward tidal pressure on the planets.

And when the sun goes planetary nebula, any debris in the planet's path can also cause the planets to slow down slightly - the precise process there I'm less clear on, but in general, any orbital debris creates drag and can slow down a planet's orbit. This may be a key factor in the formation of hot jupiters, cause they can't form close to their suns but enough orbital debris can drive them in closer to their suns. (or planet to planet gravitational interactions can too).

That's the gist of the Sun-Planet orbital relation. When the sun is young, planets are mostly driven outwards, and young suns can have far greater solar flares and stronger solar wind. How much that effects the planets, I'm not sure.

During the Main sequence stage, stars tend to push planets outwards (unless they rotate very slowly, in which case the tidal effect is reversed), but this tidal effect is very small and very gradual.

During the Red Giant stage, stars tend to drag planets in wards, and I assume, during the planetary nebula stage as well. This effect is larger for closer planets.

You also asked about Orbital Decay - if you click on the link, there are examples of that. That probably gives a better explanation than I could. In general, Orbital decay happens very slowly unless you're talking Neutron Star or Black hole in which case the relativistic effects can cause orbital decay to happen quite fast. There's nothing about a white dwarf star that would cause faster than normal orbital decay but a white dwarf star would lose any tidal bulge tugging that a main sequence star has, so there would be essentially no tidal outwards pressure either which could in theory speed up decay cause you've lost a small outwards pressure but you would still have any debris or space dust clouds causing a small inwards pressure. (if that makes sense?)

That's my layman's explanation anyway.

Let's assume the white dwarf has a mass of $0.6 M_{odot}$ (there's probably a more accurate value, but most white dwarfs are close to this… ). With a period of 4.5 hours we can use Kepler's third law, assuming the planetary mass is negligible compared to the white dwarf, to infer an orbital radius of 0.0054 au ($8.1 imes 10^{8}$ m).

The tidal forces this close to a white dwarf are very large. The Roche limit for the total tidal disintegration of a satellite, in synchronous rotation, held together only by its own gravity is roughly $$d = 1.44 R_{WD} left( frac{ ho_{WD}}{ ho_p} ight)^{1/3},$$ where $R_{WD}$ is the radius of the white dwarf (similar to the radius of the Earth), $ho_{WD}$ is the average density of the white dwarf (a few times $10^{9}$ kg/m$^3$) and $ho_p$ the density of the planet (let's assume 5000 kg/m$^3$).

Thus $d simeq 6 imes 10^{8}$ m and is very similar to the actual orbital radius of the planet. i.e. It will be tidally disintegrating.

I guess it will be an observational selection effect that such objects will be detected at the tidal breakup radius, since if they were further way they would not be disintegrating and would not be detected, and if they were closer they would have already disintegrated and wouldn't be seen!

EDIT: On reading the paper - the authors claim that these objects are not tidally disintegrating. In fact they argue that this must be debris from a rocky planet precisely because the density must be large enough to avoid tidal disintegration according to the formula above. However I find the whole discussion rather incoherent. They specifically talk about "disintegrating planetesimals" (note the tense) which are being evaporated in a Parker-type wind due to heating by the radiation from the white dwarf. I cannot see where they explain then how the planetesimals disintegrate.

Astronomers See Space Twist Around A White Dwarf 12,000 Light Years Away

The theory of general relativity is packed with strange predictions about how space and time are affected by massive bodies. Everything from gravitational waves to the lensing of light by dark matter. But one of the oddest predictions is an effect known as frame-dragging. The effect is so subtle it was first measured just a decade ago. Now astronomers have measured the effect around a white dwarf, and it tells us how some supernovae occur.

In general relativity, gravity is not a force. The presence of a mass bends space around it, and this means that objects moving near the mass are deflected from a straight path. This deflection looks as if the object is being pulled toward the mass as if by a force we call gravity. When a large mass is rotating, space also twists slightly in the direction of rotation. It is this effect that is known as frame-dragging.

An illustration of frame dragging. Credit: Simon Tyran, via Wikipedia

You can see an illustration of frame-dragging in the figure above. The central object is a massive rotating body, such as a black hole. The red dots represent points that are “at rest,” which means they aren’t moving through space. Instead, they move because space around the body is twisting due to the rotation. This frame-dragging effect is in addition to any orbital motion an object might have, and it is part of the reason why the accretion disk around a black hole can get so extremely hot.

Near Earth, the frame-dragging effect is very small. So small that it took a special satellite to measure it. Known as Gravity Probe B, the spacecraft contained one of the most spherical objects ever made. Once in space, the sphere was set spinning and watched over time.

The precession effect of Gravity Probe B. Credit: Gravity Probe B Team, Stanford, NASA

Without frame-dragging, a spinning sphere orbiting the Earth should always keep the same orientation, like a gyroscope. Earth’s gravity can’t cause it to twist on its own. But frame-dragging can. Because of Earth’s rotation, the region of space closer to the Earth twists just slightly faster than the region of space farther away. This means the part of the sphere that’s closer to Earth gets a little push, and as a result, it’s orientation changes over time. We call this Lense–Thirring precession. In 2015 the team measured this precession, and it agreed perfectly with general relativity.

While the frame-dragging effect is larger around massive bodies like white dwarfs and neutron stars, it isn’t easy to measure. To measure the frame-dragging of a body you need to have something orbiting it. Luckily for us, many white dwarfs and neutron stars are part of a binary system. So recently a team used a binary system to study frame dragging.

In 1999, the Australian Parkes Radio Telescope discovered the pulsar PSR J1141-6545. It is a neutron star that’s in a binary orbit with a white dwarf star. The distance between these two stars is only about the width of the Sun, and they orbit each other every five hours.

Because pulsars emit a sharp radio pulse at regular intervals, astronomers can use them to make extremely accurate measurements of the pulsar’s motion and orbit. The measurements are so precise that we can use them to measure the effects of general relativity, including frame dragging. Because the white dwarf is rotating, the orbit of the pulsar precesses slightly over time. The amount of precession depends on the mass and rotational speed of the white dwarf.

Parkes radio telescope viewed from the visitor’s area. Credit: Stephen West

After observing the pulsar for twenty years, the team not only observed frame-dragging, they used it to measure the rotational speed of the white dwarf. They found that it rotates once every 100 seconds, which is quite fast for a white dwarf.

The results agree with a popular model about how close binary systems evolve. Pulsars form when large stars die and become supernovae. This means the binary system was once a binary system where a large star orbited the white dwarf. As the star reached the end of its life, material from its outer layer would have been captured by the white dwarf, causing it to spin faster. The observations show that the white dwarf formed before the pulsar.

All this from an amazing work of astronomy, measuring relativistic frame-dragging in a star 12,000 light-years away.

Astronomers find the first intact planet orbiting a white dwarf… and it's far bigger than its star!

Astronomers have just announced they have found the first intact planet orbiting a white dwarf * , the dead core of a star that was once much like the Sun.

There are lots of cool things about this discovery, but one of the weirdest is that the planet is bigger than the star!

But then, it almost has to be. Here's how this works.

WD 1856+354 (let's just call it WD 1856) is a white dwarf about 80 light years away in the constellation of Draco. It may be part of a triple system, orbiting a pair of stars called G 229-20, although it's not clear if it actually orbits them or is coincidentally close to them in the sky.

It was observed by TESS, the Transiting Exoplanet Survey Satellite, which is scanning the whole sky looking for planets orbiting other stars. If we happen to see a planet's orbit edge-on, then once per orbit it'll pass directly in front of the star, dimming it a bit. TESS has already detected quite a few planets since it started work in 2018.

The transit seen by the Gran Telescopio Canarias (left) shows the white dwarf’s light dropping by over half, which was also seen in the infrared by Spitzer (right). The transit lasts 8 minutes. Credit: Vanderburg et al.

The planet found using TESS, WD 1856b, is about 10 times bigger than Earth (so slightly smaller than Jupiter) and orbits the star at a distance of just 3 million kilometers, which is close — Mercury's orbit around the Sun is 15 times wider — and circles the white dwarf once every 34 hours.

I love this next part. When a star like the Sun dies, the core becomes extremely dense, and the outer layers of the star blow away. Eventually the core is exposed to space, a tiny and exceptionally hot little ball: a white dwarf. WD 1856 is no exception. It has about half the mass of the Sun squeezed down into a sphere about 18,000 kilometers wide — only about 1.5 times the diameter of Earth. It really is quite small.

But the planet is ten times bigger than Earth. which means the planet itself is about seven times wider than the star!

That's a little unsettling. Normal stars (meaning ones that fuse hydrogen into helium in their cores) are much bigger than planets. Jupiter is about as large a planet as you can get, and it's still only 1/10th the diameter of the Sun. So it's a little odd to think of a planet being bigger than a star, but we're not talking about a normal star here.

Artwork depicting the white dwarf WD 1856 and its massive planet, far larger than the star itself. Credit: NASA/JPL-Caltech/NASA's Goddard Space Flight Center

Because the planet is bigger than the star, it could in principle block out all the light from a star as it passes in front. However, we see its orbit at a very slight angle, so it's a grazing or partial transit with a depth (how much light was blocked) of about 56%. That's still enormous most exoplanetary transits are more like 1% dips at most. So this one is truly extraordinary.

Ironically, it's so weird that the automatic software used to scan TESS data rejected the transit as being too deep! It was found when astronomers looked at the data for various white dwarfs by eye. When they saw it they knew they had something interesting, so they followed up with telescopes on the ground as well as with Spitzer Space Telescope (shortly before it was decommissioned).

This actually helped them get the mass of the planet. White dwarfs don't generate their own heat once they form they just cool slowly over time. This allows their age to be estimated. WD 1856 probably became a white dwarf about 6 billion years ago, and was a normal star before that for about 4 billion years. Planets don't generate their own heat, either, but cool after they form for billions of years the rate at which they cool depends on their mass.

If WD 1856b were above about 14 times Jupiter's mass, its warm glow in infrared would've been seen by Spitzer. It wasn't, so it must be less massive than that. Given the age of WD 1856 it's likely to be less than 12 times Jupiter's mass, firmly in the range of planetary masses. If it were much more massive it would be a brown dwarf, similar to a planet but with various different properties.

It likely formed farther out from the star all those eons ago. When the star died, the planetary system became unstable, with planets moving around. An encounter with another massive planet could've dropped WD 1856b closer in to the star, putting it on the tight orbit it's on now.

What's odd is that we haven't found any planets around white dwarfs until this one. Given how deep the transits are, it should be relatively easy to find them. Since we don't, they may be rare. We know that a lot of white dwarfs have asteroids orbiting them. These may be the remains of planets that got too close to their host stars and got torn apart by the fiercely intense gravity of the white dwarfs — in fact, the very first evidence for planets orbiting other star was from observations of a white dwarf made in 1917, and it was from the remains of a planet ripped asunder and eaten by the star.

One reason we may not find planets is that they commonly get too close in and are destroyed. Finding one far enough out to survive may be rare.

The only way to know is to keep observing them and look for transits. There are thousands of white dwarfs seen by TESS, so it may only be a matter of time before more are found. Hopefully many more.

* Technically, this is an exoplanet candidate, since it hasn't been confirmed using other methods. However, given the nature of the observations I'm pretty confidant it's real even if the paper author's have to hedge their bet a bit.

A White Dwarf’s Surprise Planetary Companion

For the first time, an intact, giant exoplanet has been discovered orbiting close to a white dwarf star. This discovery shows that it is possible for Jupiter-sized planets to survive their star’s demise and settle into close orbits around the remaining stellar ember, near the habitable zone. This foretells one possible future for our own Solar System when the Sun ages into a white dwarf.

Astronomers have used the international Gemini Observatory, a Program of NSF’s NOIRLab, and other telescopes around the globe and in space to find and characterize a giant planet, less than 13.8 times as massive as Jupiter [1], orbiting a white dwarf star [2][3]. The research is published in the journal Nature.

This is the first example of an intact giant planet orbiting close to a white dwarf star — in this case a particularly cool and dim stellar ember known as WD 1856+534. “The discovery came as something of a surprise,” according to lead author Andrew Vanderburg, assistant professor at the University of Wisconsin-Madison. “A previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid.” [4]

After detecting the planet with the TESS satellite, which observed it transiting its white dwarf star, the team took advantage of the tremendous light-collecting power of Gemini North’s 8.1-meter mirror and used the sensitive Gemini Near-Infrared Spectrograph (GNIRS) to make detailed measurements of the white dwarf star in infrared light from Maunakea, Hawai‘i. The spectroscopic observations captured the unique fingerprint of the star, but not that of the planet or any debris surrounding this system [5][6]. “Because no debris from the planet was detected floating on the star’s surface or surrounding it in a disk we could infer that the planet is intact,” said Siyi Xu, an assistant astronomer at Gemini Observatory and one of the researchers behind the discovery.

“We were using the TESS satellite to search for transiting debris around white dwarfs, and to try to understand how the process of planetary destruction happens,” explains Vanderburg. “We were not necessarily expecting to find a planet that appeared to be intact.”

“Additionally, because we didn’t detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we’ve ever found.” [7]. Xu adds that the precise upper limit of the planet’s temperature was measured by NASA’s Spitzer Space Telescope to be 17 °C (63 °F), which is similar to the average temperature of Earth.

“We’ve had indirect evidence that planets exist around white dwarfs and it's amazing to finally find a planet like this,” said Xu [8]. White dwarfs are extremely dense and very small, so the exoplanet is much larger than its tiny parent star, making the system extremely unusual.

The surprising discovery of this planet, known as WD 1856b, raises interesting questions about the fate of planets orbiting stars destined to become white dwarfs (like our Sun). Of the thousands of planets outside the Solar System that astronomers have discovered, most orbit stars that will eventually evolve into red giants and then into white dwarfs. During this process, any planets in close orbits will be engulfed by the star, a fate that WD 1856b somehow managed to avoid.

“Our discovery suggests that WD 1856b must have originally orbited far away from the star, and then somehow journeyed inwards after the star became a white dwarf,” said Vanderburg. “Now that we know that planets can survive the journey without being broken up by the white dwarf's gravity, we can look for other, smaller planets.”

“The study of planets in extreme locations is giving us new perspectives on the history and fate of the billions of worlds around other stars,” said Martin Still, NSF Program Director for the international Gemini Observatory partnership. “Gemini’s sensitivity was critical in following up the TESS space-based detection of this planet, revealing a more complete story of the exoplanetary system.”

This new discovery suggests that planets can end up in or near the white dwarf's habitable zone, and potentially be hospitable to life even after their star has died. “We’re planning future work to study this planet's atmosphere with Gemini North,” concludes Xu. “The more we can learn about planets like WD 1856b, the more we can find out about the likely fate of our own Solar System in about 5 billion years when the Sun becomes a white dwarf.” [9]

Notes

[1] The upper limit of the object’s mass is 13.8 Jupiter masses. This mass is close to the dividing line astronomers use to distinguish between a planet and a brown dwarf.

[2] White dwarfs are common stellar remnants left behind by the deaths of low-mass stars like the Sun. Though they have a mass comparable to the Sun’s, they are roughly the size of Earth, making them incredibly dense. White dwarfs generate no energy of their own and glow faintly with leftover thermal energy, slowly fading over billions of years.

[3] The discovery of WD 1856b relied on observations from facilities including Gemini North, NASA’s Transiting Exoplanet Survey Satellite (TESS), NASA’s Spitzer Space Telescope, various professional telescopes around the world, and a handful of privately operated telescopes.

[5] The light from a star is spread over many wavelengths, and not all these wavelengths radiate equally. The distribution of emission at different wavelengths makes up the emission spectrum of a star, and features of this spectrum act as very recognizable “fingerprints.” When an orbiting planet gravitationally tugs at a star, it causes a star to wobble and these spectral fingerprints shift slightly. This technique is often used to gather information about exoplanets, but in the case of WD 1856, the stellar spectrum obtained by Gemini North showed no identifying features — no “fingerprints” — showing that the orbiting planet is intact.

[6] The first “polluted white dwarf” — a white dwarf with planet debris in its outer layer — was discovered in 1917 by Adriaan van Maanen using Mount Wilson observatory’s 60-inch telescope. The star is known as van Maanen’s Star and has an interesting backstory.

[7] The team was searching at a wavelength of 4.5 microns.

[8] In a result widely reported last year, a team using ESO facilities detected gas disk orbiting, and accreting onto, a white dwarf. The gas seems to have a composition similar to that of Neptune and Uranus, so it is hypothesized that the gas must have come from such a planet. The planet itself was not detected, only the gas debris.

[9] This could be the final fate of Earth and the other rocky planets in the Solar System. When the Sun expands into a red giant it will swell and become vastly more luminous, charring and then engulfing Mercury, Venus, and possibly Earth. However, there’s nothing to worry about yet — our Sun is only halfway through its 10-billion-year lifetime.

This research was presented in the paper A Giant Planet Candidate Transiting a White Dwarf to appear in the journal Nature.

The team was composed of Andrew Vanderburg (University of Wisconsin-Madison and University of Texas at Austin), Saul A. Rappaport (Massachusetts Institute of Technology), Siyi Xu (NSF’s NOIRLab/Gemini Observatory), Ian Crossfield (University of Kansas), Juliette C. Becker (California Institute of Technology), Bruce Gary (Hereford Arizona Observatory), Felipe Murgas (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Simon Blouin (Los Alamos National Laboratory), Thomas G. Kaye (Raemor Vista Observatory and The University of Hong Kong), Enric Palle (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Carl Melis (University of California, San Diego), Brett Morris (University of Bern), Laura Kreidberg (Max Planck Institute for Astronomy and Center for Astrophysics | Harvard & Smithsonian), Varoujan Gorjian (NASA Jet Propulsion Laboratory), Caroline V. Morley (University of Texas at Austin), Andrew W. Mann (University of North Carolina at Chapel Hill), Hannu Parviainen (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Logan A. Pearce (University of Arizona), Elisabeth R. Newton (Dartmouth College), Andreia Carrillo (University of Texas at Austin), Ben Zuckerman (University of California, Los Angeles), Lorne Nelson (Bishop’s University), Greg Zeimann (University of Texas at Austin), Warren R. Brown (Center for Astrophysics | Harvard & Smithsonian), René Tronsgaard (Technical University of Denmark), Beth Klein (University of California, Los Angeles), George R. Ricker (Massachusetts Institute of Technology), Roland K. Vanderspek (Massachusetts Institute of Technology), David W. Latham (Center for Astrophysics | Harvard & Smithsonian), Sara Seager (Massachusetts Institute of Technology), Joshua N. Winn (Princeton University), Jon M. Jenkins (NASA Ames Research Center), Fred C. Adams (University of Michigan), Björn Benneke (Université de Montréal), David Berardo (Massachusetts Institute of Technology), Lars A. Buchhave (Technical University of Denmark), Douglas A. Caldwell (NASA Ames Research Center and SETI Institute), Jessie L. Christiansen (Caltech/IPAC-NASA Exoplanet Science Institute), Karen A. Collins (Center for Astrophysics | Harvard & Smithsonian), Knicole D. Colón (NASA Goddard Space Flight Center), Tansu Daylan (Massachusetts Institute of Technology), John Doty (Noqsi Aerospace, Ltd.), Alexandra E. Doyle (University of California, Los Angeles), Diana Dragomir (University of New Mexico, Albuquerque), Courtney Dressing (University of California, Berkeley), Patrick Dufour (Université de Montréal), Akihiko Fukui (Instituto de Astrofísica de Canarias and The University of Tokyo), Ana Glidden (Massachusetts Institute of Technology), Natalia M. Guerrero (Massachusetts Institute of Technology), Xueying Guo (Massachusetts Institute of Technology), Kevin Heng (University of Bern), Andreea I. Henriksen (Technical University of Denmark), Chelsea X. Huang (Massachusetts Institute of Technology), Lisa Kaltenegger (Cornell University), Stephen R. Kane (University of California, Riverside), John A. Lewis (Center for Astrophysics | Harvard & Smithsonian), Jack J. Lissauer (NASA Ames Research Center), Farisa Morales (NASA Jet Propulsion Laboratory and Moorpark College), Norio Narita (National Astronomical Observatory of Japan, Instituto de Astrofísica de Canarias and The University of Tokyo), Joshua Pepper (Lehigh University), Mark E. Rose (NASA Ames Research Center), Jeffrey C. Smith (SETI Institute and NASA Ames Research Center) Keivan G. Stassun (Vanderbilt University and Fisk University), Liang Yu (Massachusetts Institute of Technology and ExxonMobil Upstream Integrated Solutions).

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (in cooperation with DOE’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawaiʻi, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O'odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

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For the first time, an intact, giant exoplanet has been discovered orbiting close to a white dwarf star. This discovery shows that it is possible for Jupiter-sized planets to survive their star&rsquos demise and settle into close orbits around the remaining stellar ember, near the habitable zone. This foretells one possible future for our own Solar System when the Sun ages into a white dwarf.

Astronomers have used the international Gemini Observatory, a Program of NSF&rsquos NOIRLab, and other telescopes around the globe and in space to find and characterize a giant planet, less than 13.8 times as massive as Jupiter [1], orbiting a white dwarf star [2][3]. The research is published in the journal Nature.

This is the first example of an intact giant planet orbiting close to a white dwarf star &mdash in this case a particularly cool and dim stellar ember known as WD 1856+534. &ldquoThe discovery came as something of a surprise,&rdquo according to lead author Andrew Vanderburg, assistant professor at the University of Wisconsin-Madison. &ldquoA previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid.&rdquo [4]

After detecting the planet with the TESS satellite, which observed it transiting its white dwarf star, the team took advantage of the tremendous light-collecting power of Gemini North&rsquos 8.1-meter mirror and used the sensitive Gemini Near-Infrared Spectrograph (GNIRS) to make detailed measurements of the white dwarf star in infrared light from Maunakea, Hawai&lsquoi. The spectroscopic observations captured the unique fingerprint of the star, but not that of the planet or any debris surrounding this system [5][6]. &ldquoBecause no debris from the planet was detected floating on the star&rsquos surface or surrounding it in a disk we could infer that the planet is intact,&rdquo said Siyi Xu, an assistant astronomer at Gemini Observatory and one of the researchers behind the discovery.

&ldquoWe were using the TESS satellite to search for transiting debris around white dwarfs, and to try to understand how the process of planetary destruction happens,&rdquo explains Vanderburg. &ldquoWe were not necessarily expecting to find a planet that appeared to be intact.&rdquo

&ldquoAdditionally, because we didn&rsquot detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we&rsquove ever found.&rdquo [7]. Xu adds that the precise upper limit of the planet&rsquos temperature was measured by NASA&rsquos Spitzer Space Telescope to be 17 °C (63 °F), which is similar to the average temperature of Earth.

&ldquoWe&rsquove had indirect evidence that planets exist around white dwarfs and it's amazing to finally find a planet like this,&rdquo said Xu [8]. White dwarfs are extremely dense and very small, so the exoplanet is much larger than its tiny parent star, making the system extremely unusual.

The surprising discovery of this planet, known as WD 1856b, raises interesting questions about the fate of planets orbiting stars destined to become white dwarfs (like our Sun). Of the thousands of planets outside the Solar System that astronomers have discovered, most orbit stars that will eventually evolve into red giants and then into white dwarfs. During this process, any planets in close orbits will be engulfed by the star, a fate that WD 1856b somehow managed to avoid.

&ldquoOur discovery suggests that WD 1856b must have originally orbited far away from the star, and then somehow journeyed inwards after the star became a white dwarf,&rdquo said Vanderburg. &ldquoNow that we know that planets can survive the journey without being broken up by the white dwarf's gravity, we can look for other, smaller planets.&rdquo

&ldquoThe study of planets in extreme locations is giving us new perspectives on the history and fate of the billions of worlds around other stars,&rdquo said Martin Still, NSF Program Director for the international Gemini Observatory partnership. &ldquoGemini&rsquos sensitivity was critical in following up the TESS space-based detection of this planet, revealing a more complete story of the exoplanetary system.&rdquo

This new discovery suggests that planets can end up in or near the white dwarf's habitable zone, and potentially be hospitable to life even after their star has died. &ldquoWe&rsquore planning future work to study this planet's atmosphere with Gemini North,&rdquo concludes Xu. &ldquoThe more we can learn about planets like WD 1856b, the more we can find out about the likely fate of our own Solar System in about 5 billion years when the Sun becomes a white dwarf.&rdquo [9]

[1] The upper limit of the object&rsquos mass is 13.8 Jupiter masses. This mass is close to the dividing line astronomers use to distinguish between a planet and a brown dwarf.

[2] White dwarfs are common stellar remnants left behind by the deaths of low-mass stars like the Sun. Though they have a mass comparable to the Sun&rsquos, they are roughly the size of Earth, making them incredibly dense. White dwarfs generate no energy of their own and glow faintly with leftover thermal energy, slowly fading over billions of years.

[3] The discovery of WD 1856b relied on observations from facilities including Gemini North, NASA&rsquos Transiting Exoplanet Survey Satellite (TESS), NASA&rsquos Spitzer Space Telescope, various professional telescopes around the world, and a handful of privately operated telescopes.

[4] Result reported by NASA.

[5] The light from a star is spread over many wavelengths, and not all these wavelengths radiate equally. The distribution of emission at different wavelengths makes up the emission spectrum of a star, and features of this spectrum act as very recognizable &ldquofingerprints.&rdquo When an orbiting planet gravitationally tugs at a star, it causes a star to wobble and these spectral fingerprints shift slightly. This technique is often used to gather information about exoplanets, but in the case of WD 1856, the stellar spectrum obtained by Gemini North showed no identifying features &mdash no &ldquofingerprints&rdquo &mdash showing that the orbiting planet is intact.

[6] The first &ldquopolluted white dwarf&rdquo &mdash a white dwarf with planet debris in its outer layer &mdash was discovered in 1917 by Adriaan van Maanen using Mount Wilson observatory&rsquos 60-inch telescope. The star is known as van Maanen&rsquos Star and has an interesting backstory.

[7] The team was searching at a wavelength of 4.5 microns.

[8] In a result widely reported last year, a team using ESO facilities detected gas disk orbiting, and accreting onto, a white dwarf. The gas seems to have a composition similar to that of Neptune and Uranus, so it is hypothesized that the gas must have come from such a planet. The planet itself was not detected, only the gas debris.

[9] This could be the final fate of Earth and the other rocky planets in the Solar System. When the Sun expands into a red giant it will swell and become vastly more luminous, charring and then engulfing Mercury, Venus, and possibly Earth. However, there&rsquos nothing to worry about yet &mdash our Sun is only halfway through its 10-billion-year lifetime.

This research was presented in the paper A Giant Planet Candidate Transiting a White Dwarf to appear in the journal Nature.

The team was composed of Andrew Vanderburg (University of Wisconsin-Madison and University of Texas at Austin), Saul A. Rappaport (Massachusetts Institute of Technology), Siyi Xu (NSF&rsquos NOIRLab/Gemini Observatory), Ian Crossfield (University of Kansas), Juliette C. Becker (California Institute of Technology), Bruce Gary (Hereford Arizona Observatory), Felipe Murgas (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Simon Blouin (Los Alamos National Laboratory), Thomas G. Kaye (Raemor Vista Observatory and The University of Hong Kong), Enric Palle (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Carl Melis (University of California, San Diego), Brett Morris (University of Bern), Laura Kreidberg (Max Planck Institute for Astronomy and Center for Astrophysics | Harvard & Smithsonian), Varoujan Gorjian (NASA Jet Propulsion Laboratory), Caroline V. Morley (University of Texas at Austin), Andrew W. Mann (University of North Carolina at Chapel Hill), Hannu Parviainen (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Logan A. Pearce (University of Arizona), Elisabeth R. Newton (Dartmouth College), Andreia Carrillo (University of Texas at Austin), Ben Zuckerman (University of California, Los Angeles), Lorne Nelson (Bishop&rsquos University), Greg Zeimann (University of Texas at Austin), Warren R. Brown (Center for Astrophysics | Harvard & Smithsonian), René Tronsgaard (Technical University of Denmark), Beth Klein (University of California, Los Angeles), George R. Ricker (Massachusetts Institute of Technology), Roland K. Vanderspek (Massachusetts Institute of Technology), David W. Latham (Center for Astrophysics | Harvard & Smithsonian), Sara Seager (Massachusetts Institute of Technology), Joshua N. Winn (Princeton University), Jon M. Jenkins (NASA Ames Research Center), Fred C. Adams (University of Michigan), Björn Benneke (Université de Montréal), David Berardo (Massachusetts Institute of Technology), Lars A. Buchhave (Technical University of Denmark), Douglas A. Caldwell (NASA Ames Research Center and SETI Institute), Jessie L. Christiansen (Caltech/IPAC-NASA Exoplanet Science Institute), Karen A. Collins (Center for Astrophysics | Harvard & Smithsonian), Knicole D. Colón (NASA Goddard Space Flight Center), Tansu Daylan (Massachusetts Institute of Technology), John Doty (Noqsi Aerospace, Ltd.), Alexandra E. Doyle (University of California, Los Angeles), Diana Dragomir (University of New Mexico, Albuquerque), Courtney Dressing (University of California, Berkeley), Patrick Dufour (Université de Montréal), Akihiko Fukui (Instituto de Astrofísica de Canarias and The University of Tokyo), Ana Glidden (Massachusetts Institute of Technology), Natalia M. Guerrero (Massachusetts Institute of Technology), Xueying Guo (Massachusetts Institute of Technology), Kevin Heng (University of Bern), Andreea I. Henriksen (Technical University of Denmark), Chelsea X. Huang (Massachusetts Institute of Technology), Lisa Kaltenegger (Cornell University), Stephen R. Kane (University of California, Riverside), John A. Lewis (Center for Astrophysics | Harvard & Smithsonian), Jack J. Lissauer (NASA Ames Research Center), Farisa Morales (NASA Jet Propulsion Laboratory and Moorpark College), Norio Narita (National Astronomical Observatory of Japan, Instituto de Astrofísica de Canarias and The University of Tokyo), Joshua Pepper (Lehigh University), Mark E. Rose (NASA Ames Research Center), Jeffrey C. Smith (SETI Institute and NASA Ames Research Center) Keivan G. Stassun (Vanderbilt University and Fisk University), Liang Yu (Massachusetts Institute of Technology and ExxonMobil Upstream Integrated Solutions).

(Nature) (Astrophysical Journal Letters)
• IPAC release
• Bishop&rsquos University release
• Vanderbilt University release
• Heising-Simons Foundation release
• University of Kansas release
• University of California, Riverside release
• Instituto de Astrofísica de Canarias release
• University of Hong Kong release
• University of Tokyo/Japan Science and Technology Agency release

Science Contacts:

Andrew Vanderburg
Cell: +1 512-484-8392
Email [email protected]

Siyi Xu
NSF&rsquos NOIRLab
Cell: +1 808 765 9596
Email: [email protected]

PIO Contacts:

Amanda Kocz
Press and Internal Communications Officer
NSF&rsquos NOIRLab
Cell: +1 626 524 5884
Email: [email protected]

Artist&rsquos impression of WD 1856b. In this illustration, WD 1856b, a giant planet, orbits its dim white dwarf star every day and a half. Credit: NASA&rsquos Goddard Space Flight Center

Jupiter-sized planet found orbiting white dwarf. For the first time, an intact, Jupiter-sized, exoplanet has been discovered orbiting close to a white dwarf star. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. Pollard

A Vietnamese delicacy

China is not the only one to blame for the not-so-secret pangolin trade. In both China and Vietnam, pangolins are considered a sign of affluence and status — not when kept as a pet but when cooked and eaten. A single pangolin dish will cost more than the annual income of most Vietnamese adults.

Vietnam also shares with China a perception that pangolins are useful in traditional medicine. They are perceived to cure severe illnesses, to bring on good health, and to help make other medicines more effective. Pangolin scales and blood are supposed to clear up rashes, detox the body, increase milk production in new mothers, and even cure cancer. It goes without saying that there is not a shred of scientific evidence for this.

The perception that pangolins are both delicious and medicinal means Vietnam is now the second leading black market for pangolins. Vietnam criminalized the trade in 2018, with a sentence of up to 15 years, but it's too little, too late. It's estimated that 80 to 90 percent of all Vietnamese pangolins have been hunted to near extinction in the last few decades.

Giant ‘survivor’ planet found orbiting dead star

Artist’s concept of the Jupiter-sized planet WD 1856 b orbiting its white dwarf. Image via NASA/ Goddard Space Flight Center.

Exoplanets have been found orbiting various types of stars, including some similar to our own sun and including red dwarf stars, the most common stars in our galaxy. But now, astronomers using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and the now-retired Spitzer Space Telescope have announced something brand new: a Jupiter-sized planet whirling around a white dwarf star. White dwarfs are the small, very dense leftover cores of once sun-like stars that are now dead. If confirmed, this will be the first still-intact planet found orbiting a white dwarf: a survivor of a star’s death throes.

The peer-reviewed results were published in Nature on September 16, 2020. From the abstract:

Here we report the observation of a giant planet candidate transiting the white dwarf WD 1856+534 (TIC 267574918) every 1.4 days. We observed and modeled the periodic dimming of the white dwarf caused by the planet candidate passing in front of the star in its orbit. The planet candidate is roughly the same size as Jupiter and is no more than 14 times as massive (with 95% confidence).

Finding a planet still orbiting a white dwarf is exciting because most planets orbiting a sun-like star would be destroyed when that star first expands into a red giant star before shrinking down to a hot, white remnant core. This planet, if further confirmed, survived that process.

The planet, called WD 1856 b, is about the size of Jupiter, and orbits the white dwarf once every 34 hours. That’s 60 times faster than Mercury orbits the sun. This system is 80 light-years away in the northern constellation Draco. The white dwarf is about 11,000 miles (18,000 kilometers) across, may be up to 10 billion years old, and is a distant member of a triple star system.

Normally, of course, we think of planets as being much smaller than their stars. But in this case, the planet is orbiting a white dwarf, which is only 40% larger than Earth, so this planet is much larger than the white dwarf it orbits.

Andrew Vanderburg at the University of Wisconsin-Madison, who led the study, said in a statement:

WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece. The white dwarf creation process destroys nearby planets, and anything that later gets too close is usually torn apart by the star’s immense gravity. We still have many questions about how WD 1856 b arrived at its current location without meeting one of those fates.

Co-author Siyi Xu, at the international Gemini Observatory in Hilo, Hawaii, said:

We’ve known for a long time that after white dwarfs are born, distant small objects such as asteroids and comets can scatter inward towards these stars. They’re usually pulled apart by a white dwarf’s strong gravity and turn into a debris disk. That’s why I was so excited when Andrew told me about this system. We’ve seen hints that planets could scatter inward, too, but this appears to be the first time we’ve seen a planet that made the whole journey intact.

View larger. | Infographic from NOIRLab detailing the discovery of WD 1856 b. Image via International Gemini Observatory/ NOIRLab/ NSF/ AURA/ J. Pollard.

The researchers aren’t sure how the planet managed to survive intact, but it may have been in a long, elliptical orbit. According to co-author Juliette Becker at Caltech:

The most likely case involves several other Jupiter-size bodies close to WD 1856 b’s original orbit. The gravitational influence of objects that big could easily allow for the instability you’d need to knock a planet inward. But at this point, we still have more theories than data points.

In a press release for NOIRLab, Vanderburg commented on how the discovery was surprising, since since previously, only rocky debris had been seen around a white dwarf:

The discovery came as something of a surprise. A previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid.

It’s also possible that the other two stars in the system, red dwarfs G 229-20A and G 229-20B, exerted a gravitational tug on the planet over billions of years, with a subsequent flyby of a rogue star perturbing the system.

The researchers used Spitzer to observe the system in infrared, just a few months before the space telescope was decommissioned and the mission ended. They concluded the object was a planet and not a brown dwarf or low-mass star since it did not emit any light on its own. When the researchers compared the Spitzer data to visible light transit observations taken with the Gran Telescopio Canarias in the Canary Islands, they saw no discernible difference. The age of the system and other data also told the researchers that the object was most likely a planet.

White dwarf stars are very small, typically about the size of the Earth. Image via European Space Agency.

Vanderburg also commented:

Additionally, because we didn’t detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we’ve ever found.

We’ve had indirect evidence that planets exist around white dwarfs and it’s amazing to finally find a planet like this.

As measured by Spitzer, the upper limit of the temperature on the planet is 63 degrees Fahrenheit (17 degrees Celsius), which is very Earth-like. However, the planet is a gas giant, not a rocky world, so unlikely to be habitable in this case.

Follow-up observations were also conducted using the 10-meter Hobby-Eberly Telescope at the McDonald Observatory at the University of Texas. Other observations included ones from Gemini Observatory, part of NOIRLab.

So what’s next? Could there even be habitable planets around white dwarfs?

That answer may come with NASA’s upcoming James Webb Space Telescope (JWST). Vanderburg, along with co-author Lisa Kaltenegger and others, found that, using simulated observations, water and carbon dioxide could be detected on hypothetical rocky worlds orbiting white dwarfs by observing just five transits. Those results are available in another paper published in Astrophysical Journal Letters on September 16, 2020.

NASA’s upcoming James Webb Space Telescope will be able to detect possible biosignatures in the atmospheres of planets orbiting white dwarfs. Image via Jack Madden/ Carl Sagan Institute/ Cornell Chronicle.

The near-term search for life beyond the solar system currently focuses on transiting planets orbiting small M dwarfs, and the challenges of detecting signs of life in their atmospheres. However, planets orbiting white dwarfs (WDs) would provide a unique opportunity to characterize rocky worlds … rocky planets in the WD habitable zone therefore represent a promising opportunity to characterize terrestrial planet atmospheres and explore the possibility of a second genesis on these worlds.

Even more impressively, Webb could detect gas combinations potentially indicating biological activity on such a world in as few as 25 transits. WD 1856 b suggests planets may survive white dwarfs’ chaotic histories. In the right conditions, those worlds could maintain conditions favorable for life longer than the time scale predicted for Earth. Now we can explore many new intriguing possibilities for worlds orbiting these dead stellar cores.

In the Cornell Chronicle, Kaltenegger also said:

If rocky planets exist around white dwarfs, we could spot signs of life on them in the next few years.

Co-lead author Ryan MacDonald expanded a bit on this, saying:

When observing Earth-like planets orbiting white dwarfs, the James Webb Space Telescope can detect water and carbon dioxide within a matter of hours. Two days of observing time with this powerful telescope would allow the discovery of biosignature gases, such as ozone and methane.

We know now that giant planets can exist around white dwarfs, and evidence stretches back over 100 years showing rocky material polluting light from white dwarfs. There are certainly small rocks in white dwarf systems. It’s a logical leap to imagine a rocky planet like the Earth orbiting a white dwarf.

Andrew Vanderburg at the University of Wisconsin-Madison, who led the new study. Image via McDonald Observatory.

That’s quite an amazing thought, that life could still exist on a planet orbiting a star that is long-dead. As Kaltenegger mused:

What if the death of the star is not the end for life? Could life go on, even once our sun has died? Signs of life on planets orbiting white dwarfs would not only show the incredible tenacity of life, but perhaps also a glimpse into our future.

Bottom line: For the first time, astronomers have detected a planet orbiting a white dwarf star.

Planet Hugging a White Dwarf May Be a Survivor of Star's Death Throes

AUSTIN, Texas — An international team of astronomers has used NASA’s Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope to discover what may be the first intact planet found closely orbiting a white dwarf, the dense leftover of a sun-like star only 40% larger than Earth. The work, led by Andrew Vanderburg of The University of Texas at Austin, included follow-up observations with the 10-meter Hobby-Eberly Telescope at the university’s McDonald Observatory.

The Jupiter-size object, called WD 1856 b, is about seven times as large as the white dwarf named WD 1856+534. It circles this stellar cinder every 34 hours, more than 60 times as fast as Mercury orbits our sun.

“WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece,” said Vanderburg, who was a NASA Sagan Fellow at UT Austin while completing this work and is now an assistant professor at the University of Wisconsin-Madison. “The white dwarf creation process destroys nearby planets, and anything that later gets too close is usually torn apart by the star’s immense gravity. We still have many questions about how it arrived at its current location without meeting one of those fates.”

A paper about the system, consisting of several co-authors including UT Austin’s Caroline Morley and Andreia Carillo, appears in Nature and is available online.

TESS monitors large swaths of the sky, called sectors, for nearly a month at a time. This long gaze allows the satellite to find exoplanets, or worlds beyond our solar system, by capturing changes in stellar brightness caused when a planet crosses in front of, or transits, its star.

The satellite spotted WD 1856 b about 80 light-years away in the northern constellation Draco. It orbits a cool, quiet white dwarf that is about 11,000 miles (18,000 km) across, may be up to 10 billion years old, and is a distant member of a triple star system.

When a sun-like star runs out of fuel, it swells up to hundreds to thousands of times its original size, forming a cooler red giant star. Eventually, it ejects its outer layers of gas, losing up to 80% of its mass. The remaining hot core becomes a white dwarf. Any nearby objects are typically engulfed and incinerated during this process, which in this system would have included WD 1856 b in its current orbit. The research team estimates the possible planet must have originated at least 50 times farther away from its present location.

“We’ve known for a long time that after white dwarfs are born, distant small objects such as asteroids and comets can scatter inward towards these stars. They’re usually pulled apart by a white dwarf's strong gravity and turn into a debris disk,” said co-author Siyi Xu, an assistant astronomer at the international Gemini Observatory, which is a program of the National Science Foundation’s NOIRLab. “That’s why I was so excited when Andrew told me about this system. We’ve seen that planets could scatter inward, too, but this appears to be the first time we’ve seen a planet that made the whole journey intact.”

The team suggests several scenarios that could have nudged WD 1856 b onto an elliptical path around the white dwarf. This trajectory would have become more circular over time as the star’s gravity stretched the object, creating enormous tides that dissipated its orbital energy.

“The most likely case involves several other Jupiter-size bodies close to WD 1856 b’s original orbit,” said co-author Juliette Becker, a 51 Pegasi b fellow in planetary science at Caltech. “The gravitational influence of objects that big could easily allow for the instability you’d need to knock a planet inward. But at this point, we still have more theories than data points.”

Other possible scenarios involve the gradual gravitational tug of the two other stars in the system, red dwarfs G229-20 A and B, over billions of years and a flyby from a rogue star perturbing the system. Vanderburg’s team thinks these and other explanations are less likely because they require finely tuned conditions to achieve the same effects as the potential giant companion planets.

Jupiter-size objects can occupy a huge range of masses, however, from planets only a few times the mass of Earth to low-mass stars thousands of times Earth’s mass. Others are brown dwarfs, which straddle the line between planet and star. Usually scientists turn to observations to measure an object’s mass, which can hint at its composition and nature. This method works by studying how an orbiting object tugs on its star and alters the color of its light. But in this case, the white dwarf is so old that its light has become both too faint and too featureless for scientists to detect noticeable changes.

Instead, the team observed the system in the infrared using Spitzer, just a few months before the telescope was decommissioned. If WD 1856 b were a brown dwarf or low-mass star, it would emit its own infrared glow. This means Spitzer would record a brighter transit than it would if the object were a planet, which would block rather than emit light. When the researchers compared the Spitzer data with visible light transit observations taken with the Gran Telescopio Canarias in Spain’s Canary Islands, they saw no discernable difference. That, combined with the age of the star and other information about the system, led them to conclude that WD 1856 b is most likely a planet no more than 14 times Jupiter’s size. Future research and observations may be able to confirm this conclusion.

Media Contacts:
Rebecca Johnson, Communications Mgr.
McDonald Observatory
The University of Texas at Austin
512-475-6763

Characterizing the Planet

After its initial discovery, the authors knew the rough size of the object (called WD 1856 b) because of how much light it blocked out, but did not know its mass. Jupiter-sized objects can be anything from a giant planet (which has a mass of 1/10th of Jupiter’s mass) to a low-mass star (which can weigh as much as 10 Jupiters). Astronomers usually determine the mass of an orbiting object by looking at the spectrum of the host star and using Doppler monitoring (which relies on the fact that an orbiting body will Doppler shift the light from its host star as the object moves toward and away from us). Because this WD’s spectrum does not have any strong emission or absorption lines, they had to use other techniques, namely thermal monitoring. The team observed the object in the infrared using NASA’s Spitzer Space Telescope. They measured the amount of thermal emission (heat) from the object compared to that of the WD and found that the transiting body emitted no more than 6.1% of the flux (energy) of the star, confirming that it was either a planet or a very low-mass brown dwarf. The motion of the star indicated that it is part of the thin disk of the galaxy, meaning it is less than 10 million years old. Using this along with the mass (

14 Jupiter masses), they were able to confirm (using information from mass models) that it is a planetary body and not a brown dwarf.

A White Dwarf’s Surprise Planetary Companion: First-of-Its-Kind Exoplanet Detected Around Dead Star

For the first time, an intact, giant exoplanet has been discovered orbiting close to a white dwarf star. This discovery shows that it is possible for Jupiter-sized planets to survive their star’s demise and settle into close orbits around the remaining stellar ember, near the habitable zone. This foretells one possible future for our own Solar System when the Sun ages into a white dwarf.

Astronomers have used the international Gemini Observatory, a Program of NSF’s NOIRLab, and other telescopes around the globe and in space to find and characterize a giant planet, less than 13.8 times as massive as Jupiter [1] , orbiting a white dwarf star. [2][3] The research is published in the journal Nature.

This is the first example of an intact giant planet orbiting close to a white dwarf star — in this case a particularly cool and dim stellar ember known as WD 1856+534. “The discovery came as something of a surprise,” according to lead author Andrew Vanderburg, assistant professor at the University of Wisconsin-Madison. “A previous example of a similar system, where an object was seen to pass in front of a white dwarf, showed only a debris field from a disintegrating asteroid.” [4]

After detecting the planet with the TESS satellite, which observed it transiting its white dwarf star, the team took advantage of the tremendous light-collecting power of Gemini North’s 8.1-meter mirror and used the sensitive Gemini Near-Infrared Spectrograph (GNIRS) to make detailed measurements of the white dwarf star in infrared light from Maunakea, Hawai’i. The spectroscopic observations captured the unique fingerprint of the star, but not that of the planet or any debris surrounding this system. [5][6] “Because no debris from the planet was detected floating on the star’s surface or surrounding it in a disk we could infer that the planet is intact,” said Siyi Xu, an assistant astronomer at Gemini Observatory and one of the researchers behind the discovery.

“We were using the TESS satellite to search for transiting debris around white dwarfs, and to try to understand how the process of planetary destruction happens,” explains Vanderburg. “We were not necessarily expecting to find a planet that appeared to be intact.”

“Additionally, because we didn’t detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we’ve ever found.” [7] Xu adds that the precise upper limit of the planet’s temperature was measured by NASA’s Spitzer Space Telescope to be 17 °C (63 °F), which is similar to the average temperature of Earth.

“We’ve had indirect evidence that planets exist around white dwarfs and it’s amazing to finally find a planet like this,” said Xu. [8] White dwarfs are extremely dense and very small, so the exoplanet is much larger than its tiny parent star, making the system extremely unusual.

The surprising discovery of this planet, known as WD 1856b, raises interesting questions about the fate of planets orbiting stars destined to become white dwarfs (like our Sun). Of the thousands of planets outside the Solar System that astronomers have discovered, most orbit stars that will eventually evolve into red giants and then into white dwarfs. During this process, any planets in close orbits will be engulfed by the star, a fate that WD 1856b somehow managed to avoid.

“Our discovery suggests that WD 1856b must have originally orbited far away from the star, and then somehow journeyed inwards after the star became a white dwarf,” said Vanderburg. “Now that we know that planets can survive the journey without being broken up by the white dwarf’s gravity, we can look for other, smaller planets.”

“The study of planets in extreme locations is giving us new perspectives on the history and fate of the billions of worlds around other stars,” said Martin Still, NSF Program Director for the international Gemini Observatory partnership. “Gemini’s sensitivity was critical in following up the TESS space-based detection of this planet, revealing a more complete story of the exoplanetary system.”

This new discovery suggests that planets can end up in or near the white dwarf’s habitable zone, and potentially be hospitable to life even after their star has died. “We’re planning future work to study this planet’s atmosphere with Gemini North,” concludes Xu. “The more we can learn about planets like WD 1856b, the more we can find out about the likely fate of our own Solar System in about 5 billion years when the Sun becomes a white dwarf.” [9]

[1] The upper limit of the object’s mass is 13.8 Jupiter masses. This mass is close to the dividing line astronomers use to distinguish between a planet and a brown dwarf.

[2] White dwarfs are common stellar remnants left behind by the deaths of low-mass stars like the Sun. Though they have a mass comparable to the Sun’s, they are roughly the size of Earth, making them incredibly dense. White dwarfs generate no energy of their own and glow faintly with leftover thermal energy, slowly fading over billions of years.

[3] The discovery of WD 1856b relied on observations from facilities including Gemini North, NASA’s Transiting Exoplanet Survey Satellite (TESS), NASA’s Spitzer Space Telescope, various professional telescopes around the world, and a handful of privately operated telescopes.

[4] Result reported by NASA.

[5] The light from a star is spread over many wavelengths, and not all these wavelengths radiate equally. The distribution of emission at different wavelengths makes up the emission spectrum of a star, and features of this spectrum act as very recognizable “fingerprints.” When an orbiting planet gravitationally tugs at a star, it causes a star to wobble and these spectral fingerprints shift slightly. This technique is often used to gather information about exoplanets, but in the case of WD 1856, the stellar spectrum obtained by Gemini North showed no identifying features — no “fingerprints” — showing that the orbiting planet is intact.

[6] The first “polluted white dwarf” — a white dwarf with planet debris in its outer layer — was discovered in 1917 by Adriaan van Maanen using Mount Wilson observatory’s 60-inch telescope. The star is known as van Maanen’s Star and has an interesting backstory.

[7] The team was searching at a wavelength of 4.5 microns.

[8] In a result widely reported last year, a team using ESO facilities detected gas disk orbiting, and accreting onto, a white dwarf. The gas seems to have a composition similar to that of Neptune and Uranus, so it is hypothesized that the gas must have come from such a planet. The planet itself was not detected, only the gas debris.

[9] This could be the final fate of Earth and the other rocky planets in the Solar System. When the Sun expands into a red giant it will swell and become vastly more luminous, charring and then engulfing Mercury, Venus, and possibly Earth. However, there’s nothing to worry about yet — our Sun is only halfway through its 10-billion-year lifetime.

Reference: “A Giant Planet Candidate Transiting a White Dwarf” 16 September 2020, Nature.
DOI: 10.1038/s41586-020-2713-y

The team was composed of Andrew Vanderburg (University of Wisconsin-Madison and University of Texas at Austin), Saul A. Rappaport (Massachusetts Institute of Technology), Siyi Xu (NSF’s NOIRLab/Gemini Observatory), Ian Crossfield (University of Kansas), Juliette C. Becker (California Institute of Technology), Bruce Gary (Hereford Arizona Observatory), Felipe Murgas (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Simon Blouin (Los Alamos National Laboratory), Thomas G. Kaye (Raemor Vista Observatory and The University of Hong Kong), Enric Palle (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Carl Melis (University of California, San Diego), Brett Morris (University of Bern), Laura Kreidberg (Max Planck Institute for Astronomy and Center for Astrophysics | Harvard & Smithsonian), Varoujan Gorjian (NASA Jet Propulsion Laboratory), Caroline V. Morley (University of Texas at Austin), Andrew W. Mann (University of North Carolina at Chapel Hill), Hannu Parviainen (Instituto de Astrofísica de Canarias and Universidad de La Laguna), Logan A. Pearce (University of Arizona), Elisabeth R. Newton (Dartmouth College), Andreia Carrillo (University of Texas at Austin), Ben Zuckerman (University of California, Los Angeles), Lorne Nelson (Bishop’s University), Greg Zeimann (University of Texas at Austin), Warren R. Brown (Center for Astrophysics | Harvard & Smithsonian), René Tronsgaard (Technical University of Denmark), Beth Klein (University of California, Los Angeles), George R. Ricker (Massachusetts Institute of Technology), Roland K. Vanderspek (Massachusetts Institute of Technology), David W. Latham (Center for Astrophysics | Harvard & Smithsonian), Sara Seager (Massachusetts Institute of Technology), Joshua N. Winn (Princeton University), Jon M. Jenkins (NASA Ames Research Center), Fred C. Adams (University of Michigan), Björn Benneke (Université de Montréal), David Berardo (Massachusetts Institute of Technology), Lars A. Buchhave (Technical University of Denmark), Douglas A. Caldwell (NASA Ames Research Center and SETI Institute), Jessie L. Christiansen (Caltech/IPAC-NASA Exoplanet Science Institute), Karen A. Collins (Center for Astrophysics | Harvard & Smithsonian), Knicole D. Colón (NASA Goddard Space Flight Center), Tansu Daylan (Massachusetts Institute of Technology), John Doty (Noqsi Aerospace, Ltd.), Alexandra E. Doyle (University of California, Los Angeles), Diana Dragomir (University of New Mexico, Albuquerque), Courtney Dressing (University of California, Berkeley), Patrick Dufour (Université de Montréal), Akihiko Fukui (Instituto de Astrofísica de Canarias and The University of Tokyo), Ana Glidden (Massachusetts Institute of Technology), Natalia M. Guerrero (Massachusetts Institute of Technology), Xueying Guo (Massachusetts Institute of Technology), Kevin Heng (University of Bern), Andreea I. Henriksen (Technical University of Denmark), Chelsea X. Huang (Massachusetts Institute of Technology), Lisa Kaltenegger (Cornell University), Stephen R. Kane (University of California, Riverside), John A. Lewis (Center for Astrophysics | Harvard & Smithsonian), Jack J. Lissauer (NASA Ames Research Center), Farisa Morales (NASA Jet Propulsion Laboratory and Moorpark College), Norio Narita (National Astronomical Observatory of Japan, Instituto de Astrofísica de Canarias and The University of Tokyo), Joshua Pepper (Lehigh University), Mark E. Rose (NASA Ames Research Center), Jeffrey C. Smith (SETI Institute and NASA Ames Research Center) Keivan G. Stassun (Vanderbilt University and Fisk University), Liang Yu (Massachusetts Institute of Technology and ExxonMobil Upstream Integrated Solutions).

NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC-Canada, ANID-Chile, MCTIC-Brazil, MINCyT-Argentina, and KASI-Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and the Vera C. Rubin Observatory. It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai?i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

Planet Hugging a White Dwarf May Be a Survivor of Star’s Death Throes

An international team of astronomers has used NASA's Transiting Exoplanet Survey Satellite (TESS) and retired Spitzer Space Telescope to discover what may be the first intact planet found closely orbiting a white dwarf, the dense leftover of a sun-like star only 40% larger than Earth. The work, led by Andrew Vanderburg of The University of Texas at Austin, included follow-up observations with the 10-meter Hobby-Eberly Telescope at the university's McDonald Observatory.

The Jupiter-size object, called WD 1856 b, is about seven times as large as the white dwarf, named WD 1856+534. It circles this stellar cinder every 34 hours, more than 60 times as fast as Mercury orbits our sun.

"WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece," said Vanderburg, who was a NASA Sagan Fellow at UT Austin while completing this work and is now an assistant professor at the University of Wisconsin-Madison. "The white dwarf creation process destroys nearby planets, and anything that later gets too close is usually torn apart by the star's immense gravity. We still have many questions about how it arrived at its current location without meeting one of those fates."

A paper about the system, consisting of several co-authors including UT Austin's Caroline Morley and Andreia Carillo, appears in Nature and is available online.

TESS monitors large swaths of the sky, called sectors, for nearly a month at a time. This long gaze allows the satellite to find exoplanets, or worlds beyond our solar system, by capturing changes in stellar brightness caused when a planet crosses in front of, or transits, its star.

The satellite spotted WD 1856 b about 80 light-years away in the northern constellation Draco. It orbits a cool, quiet white dwarf that is about 11,000 miles (18,000 km) across, may be up to 10 billion years old, and is a distant member of a triple star system.

When a sun-like star runs out of fuel, it swells up to hundreds to thousands of times its original size, forming a cooler red giant star. Eventually, it ejects its outer layers of gas, losing up to 80% of its mass. The remaining hot core becomes a white dwarf. Any nearby objects are typically engulfed and incinerated during this process, which in this system would have included WD 1856 b in its current orbit. The research team estimates the possible planet must have originated at least 50 times farther away from its present location.

"We've known for a long time that after white dwarfs are born, distant small objects such as asteroids and comets can scatter inward towards these stars. They're usually pulled apart by a white dwarf's strong gravity and turn into a debris disk," said co-author Siyi Xu, an assistant astronomer at the international Gemini Observatory, which is a program of the National Science Foundation's NOIRLab. "That's why I was so excited when Andrew told me about this system. We've seen hints that planets could scatter inward, too, but this appears to be the first time we've seen a planet that made the whole journey intact."

The team suggests several scenarios that could have nudged WD 1856 b onto an elliptical path around the white dwarf. This trajectory would have become more circular over time as the star's gravity stretched the object, creating enormous tides that dissipated its orbital energy.

"The most likely case involves several other Jupiter-size bodies close to WD 1856 b's original orbit," said co-author Juliette Becker, a 51 Pegasi b fellow in planetary science at Caltech. "The gravitational influence of objects that big could easily allow for the instability you'd need to knock a planet inward. But at this point, we still have more theories than data points."

Other possible scenarios involve the gradual gravitational tug of the two other stars in the system, red dwarfs G229-20 A and B, over billions of years and a flyby from a rogue star perturbing the system. Vanderburg's team thinks these and other explanations are less likely because they require finely tuned conditions to achieve the same effects as the potential giant companion planets.

Jupiter-size objects can occupy a huge range of masses, however, from planets only a few times the mass of Earth to low-mass stars thousands of times Earth's mass. Others are brown dwarfs, which straddle the line between planet and star. Usually scientists turn to radial velocity observations to measure an object's mass, which can hint at its composition and nature. This method works by studying how an orbiting object tugs on its star and alters the color of its light. But in this case, the white dwarf is so old that its light has become both too faint and too featureless for scientists to detect noticeable changes.

Instead, the team observed the system in the infrared using Spitzer, just a few months before the telescope was decommissioned. If WD 1856 b were a brown dwarf or low-mass star, it would emit its own infrared glow. This means Spitzer would record a brighter transit than it would if the object were a planet, which would block rather than emit light. When the researchers compared the Spitzer data with visible light transit observations taken with the Gran Telescopio Canarias in Spain's Canary Islands, they saw no discernable difference. That, combined with the age of the star and other information about the system, led them to conclude that WD 1856 b is most likely a planet no more than 14 times Jupiter's size. Future research and observations may be able to confirm this conclusion.