Astronomy

How close to a brown dwarf would a planet need to be to receive as much light as Earth?

How close to a brown dwarf would a planet need to be to receive as much light as Earth?


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How close to a brown dwarf would an orbiting planet need to be in order to receive as much sunlight as Earth receives from the Sun?


Brown dwarfs are born moderately hot and luminous and then they contract and cool. Thanks to electron degeneracy in their cores, they never become hot enough to ignite hydrogen (though there is a brief deuterium burning phase) and as a result their fate is to cool and fade.

The plot below (from Burrows et al. 1997) shows how the luminosity behaves as a function of time since birth (age) for objects of various mass. The brown dwarfs are the green and red tracks (some would call the red tracks giant planets). The curves are labelled in Jupiter masses. The highest mass brown dwarf is 73 Jupiter masses, the next one down is 70$M_J$ and then the green curves count down in steps of 5$M_J$ to 15$M_J$, then the first red curve is at 13$M_J$ and the red curves count down in steps of 1$M_J$.

The axes are logarithmic (to base 10). Thus when the y-axis says -2, it means the brown dwarf is one hundredth the luminosity of the Sun, -3 = one thousandth etc. The x-axis is logarithmic in units of billions of years. Most brown dwarfs observed in the Galaxy will be in the range 1-10 billion years (i.e. between 0 and 1 on this x-axis).

The flux received by a planet in orbit around a star/brown dwarf will be proportional to the its luminosity divided by orbital radius squared. From this we can write that the orbital radius where a similar flux to that of the Earth around the Sun will be received is $$ R = left( frac{L_{BD}}{L_{odot}} ight)^{1/2} { m au} $$ $$ logleft(frac{R}{1 { m au}} ight) = 0.5 log left(frac{L}{L_{odot}} ight)$$

This you can get you answer immediately from the graph below. Choose the mass of your brown dwarf and its age. Find the log luminosity on the y-axis and halve this value. That is the the log of the orbital radius in astronomical units where a planet would receive the same amount of flux as the Earth from the Sun.

e.g. Consider a 50$M_J$ brown dwarf that is a billion years old. Find the appropriate green curve and see that the y-axis value for x=0 (1 billion years) is -4.2. The log of the orbital radius (in au) at which a planet would receive the same amount of flux from the brown dwarf as does the Earth from the Sun would be -2.1. In linear units, this is $10^{-2.1} = 0.0079$ au, or 1.19 million km.

A further issue to consider is that the brown dwarf surface will be far cooler than the Sun and also changes (cools) with time. This means that the radiation received from the planet will push further and further into the infrared as the brown dwarfs gets older and hardly any of the received radiation will be in the visible part of the spectrum.


Brown dwarves don't have fusion in their core, thus they don't have their own light. They may have a little heat production from other processes (for example, contraction, or radioactive decay in their core). These aren't enough strong to heat even the dwarf significantly.

Many brown dwarve were found orbiting a star closely, they may have surface temperature even in the order of some thousands K. But they get their temperature from their stars.

For example, the Jupiter's temperature is only 40K more warm as it would be reasoned by the Sun.

In the case of red dwarves, the situation is better. The distance depends on the star. You can very easily calculate it: check its absolute luminosity, so you can see its total power output, compared to the Sun.

The radiation what a planet receives (per surface area) depends quadratically from its distance.


T, L, Y Type Stars - Brown Dwarf Star

Brown Dwarf Stars are giant balls of gas, too big to be classed as a gas planet and too small to start nuclear fusion. If Jupiter, the biggest planet in our solar system had bigger mass then it would not be a gas giant but a brown dwarf. Brown Dwarfs will have the Spectral type of T or L. These types of stars are sometimes referred to as failed stars as they have not successfully started nuclear fusion There are no Giant, Supergiant or Hypergiant equivalents.

Brown Dwarves need to gain more mass before they can start nuclear fusion. Quite possibly if the brown dwarf were to gain mass then possibly then they could. Brown Dwarves colliding with one another could possibly create the right conditions where the new brown dwarf has sufficient mass. There's no citation, just thinking out aloud. the Milky Way and Andromeda Galaxy will billions of years time will merge. Some stellar collisions will happen and a brown dwarf may collide and gain the necessary mass required.

The composition/make-up of a Brown Dwarf is like that of a main sequence star such as our own, the Sun. The dwarf would be made up of hydrogen and possibly other elements but would not start burning because there's not enough gravity to cause the fusion. Space

Unlike other stars, Brown Dwarfs never die. The smaller a star is, the longer it lives for. Small stars are very energy efficient and Brown Dwarves are the most efficient of them all. There is no fusion so no burning takes place. C Seligman

They are larger than gas giants which have also been referred to as failed stars. A Brown Dwarf star is may double digits larger than a gas giant such as Jupiter.

Exoplanets and LIfe

An exoplanet has been discovered in orbit round a Brown Dwarf star. It is unlikely that the planet would have alien life. For a Brown Dwarf, the planet would have to be nearer to its star than Earth is to the Sun. The reason for having to be close is light and heat. The further from the star, the colder and darker it is. The star can be located in the constellation of Sagittarius


4 Answers 4

My answer takes its detail from this paper regarding habitable moons. This is my first attempt at answering one of these questions so be nice!

The following aspects affect the climate of the moon:

  • Insolation from the main star
  • Reflected main starlight from the Brown Dwarf. This may be low considering the likely low albedo of the dwarf.
  • Radiated heat from the brown dwarf
  • periodic eclipses of the main star when the brown dwarf moves between the moon and the sun.
  • the tidal locking of the moon
  • the thickness of the atmosphere which affects heat transport from the sub-stellar point of the moon to the other side. This has not been specified in the OP.
  • the ocean heat transfer from water moving from the substellar side to the other side and then back again. You have not specified in the OP the depth of the ocean or what continents exist which block this cycling in the N-S or E-W directions. These are very very important parameters to determine to what extent open seas exist and where.
  • the inclination of the dwarf/moon system which you have specified at 45 degrees.
  • the eccentricity of the moon's orbit around the dwarf. This has not been specified. It may well be zero considering the moon is tidally locked but this is not essential (earths moons orbit is not circular). However if there is any significant degree of tidal heating it will alter the moons climate radically.
  • whether the orbit of the moon/dwarf pair round the main star is circular or not. If not then this will also induce additional effects, although they will be mitigated by the 1:1 ratio of dwarf/star flux.
  • how long it takes the moon to orbit the dwarf

According to the linked paper, the system's inclination of 45 degrees will ensure pretty even reception of heat across the moon's surface (see Figure 7, bottom right panel), averaged over one orbit of the dwarf/moon pair round the main star. And note that the details in the paper have the ratio to flux from the star and planet/dwarf at much more than 1:1, so in your case, the flux smoothing effect is more even than in the paper.

However, in the same panel, you will note a 'northern summer' and 'southern summer' is depicted which to my understanding results from the inclination of the orbit of the pair round the main star. Again, yours should be less pronounced all else equal thanks to that 1:1 ratio.

Despite smooth average heat flux, there will, of course, be interesting weather patterns depending on your atmospheric and oceanic and geological parameters due to the interplay of all the factors. Although the moon is locked, it will not have a "cold" side because of the contribution from both the star and the dwarf. However, the flux from the dwarf is presumably mostly infra-red which may be beyond usefulness for photosynthesis. So when the dwarf eclipses the main star, the moon will be dark but likely not much colder than the other side.

I feel that all these factors are going to create a weather system which is not the classic 'eyeball world' scenario for a tidally locked planet directly orbiting an M-Dwarf close in, and might actually be quite chaotic.

Also, see the paper for important constraints regarding how close the moon can orbit the planet before it suffers runaway greenhouse effect due to a combination of tidal heating and illumination. You'll have to work out how to modify these parameters based on how you judge the effect of brown dwarf radiative heating versus the radiative heating described in the paper.

Like most tidally locked planets, there will be a "hot pole" facing the heat source (in this case the Brown Dwarf) and a "cold pole" facing space. These two poles will be the drivers of much of the planetary climate, as the atmosphere and water vapor is heated and expands away from the hot pole and moves in large convective cells towards the cold pole, where it sinks and heads back towards the hot pole. So there will be a constant wind at ground level blowing towards the hot pole and a "jet stream" like wind system moving from the hot pole towards the cold pole. There will be a more or less solid ring of clouds around the hot pole as water vapour condenses out at higher altitudes, and possibly another ring of clouds somewhere on the night side as remaining vapour condenses out in the cold temperatures, so there will be two planetary "belts" with high levels of precipitation rain around the hot pole and snow around the cold pole.

In terms of ecospheres, you could think of the planet as a beach ball with a series of concentric rings radiating away from the hot pole to the cold pole, and the ecology is determined by the varying amounts of energy and moisture being received in each "ring". The distant sun will provide a small amount of energy and illumination, although if it is as far as you say, this could be minimal (maybe even as little as a full moon on Earth). Because the planet would be orbiting the Brown Dwarf fairly closely, the illumination would also be rather sporadic, with the distant sun rising and setting quite quickly. Most native life would have to be adapted to "seeing" and harvesting energy in the infrared band, so plants would probably be black in colour and the eyes of native creatures would be very large to gather enough light, to begin with, and have the proper adaptations to receive and image infrared wavelengths, so the eyes would look different from ours.

Although you have not mentioned other moons of the Brown Dwarf, it is quite likely that they would exist due to the intense gravitational field. Multiple moons would probably settle into resonant orbits (moons in nonresonant orbits get "pumped up" with energy and change orbits, either being expelled, absorbed by the Brown Dwarf or settling in stable resonant orbits. This should provide an extra source of energy for the moon's various tectonic, hydrological and atmospheric cycles, as the core is "kneaded" by the interaction with the other moons and heated more than it otherwise might be. This internal heat is probably what is keeping the oceans liquid (much like the internal oceans on Europa and other Jovian moons).

The last thing which might affect the moon is the presence of a powerful magnetic field around the Brown Dwarf. Jupiter's magnetosphere provides a great deal of energy to the environment, and if the moon in your system is at the right distance, it might be interacting with the magnetosphere much like the Jovian moon Io, which is working like an armature in a dynamo and creates a multimillion amp "flux tube" between itself and Jupiter. Something like that would certainly cause a great deal of disruption to any planetary atmospheres, as well as make scientific discovery interesting (you might discover electricity early, but the roar of noise on radio frequencies will prevent the development of radio broadcast technology and radio astronomy, for example).


How close could a brown dwarf be to us?

I've heard that it is possible that there are brown dwarves closer than proxima centauri, but that they are hard to detect because they give off so little light.

How close could one of these be without having been detected until now via direct observation or its gravitational effects?

The WISE survey sought to answer your question and it completed it's surveying in 2014. The WISE survey relied on a highly sensitive infrared detector and was designed partly to look for large planets orbiting our sun or even brown dwarfs. A Neptune sized object would have been detected out to 700 AU and a Jupiter sized planet would have been detected as far out as 1 light year. A brown dwarf, which is several times the mass of Jupiter, puts out enough radiant energy to be spotted if it was as close as the outer Oort cloud (1.5 lys).

Brown dwarfs are actually incredibly hot compared to empty space and it was easy for WISE to spot them. By 2011 WISE had already detected some 100 brown dwarfs nearby, but none of them were closer than proxima centauri. In fact this nasa press release from 2012 actually goes into the fact that we found far fewer brown dwarfs nearby then we expected. The WISE survey has concluded, very very convincingly, that there are no brown dwarfs closer to us then proxima centauri is and there are no Neptune or jupiter sized objects hiding in the Kuiper belt.

The people that still claim there are large objects lurking in the kuiper belt ready to cause armagedon are, at this point, crank conspiracy theorists and their theories have been proven wrong.


Newfound Star System Is Third-Closest to Sun

Scientists have discovered the closest star system to the sun found in nearly a century.

With a dim duo of "failed stars" known as brown dwarfs at its center, the new neighbor is the third-nearest to our solar system overall, and it could be a good place to look for exoplanets, researchers say.

"The distance to this brown dwarf pair is 6.5 light-years — so close that Earth's television transmissions from 2006 are now arriving there," Kevin Luhman, a researcher at Penn State's Center for Exoplanets and Habitable Worlds, said in a statement. "It will be an excellent hunting ground for planets because it is very close to Earth, which makes it a lot easier to see any planets orbiting either of the brown dwarfs." [The Strangest Alien Planets]

Brown dwarfs are strange objects that are bigger than planets but too small to trigger the internal nuclear fusion reactions required to become full-fledged stars.

This pair is slightly farther away than Barnard's star, a red dwarf discovered in 1916 that lies 6.0 light-years from the sun. The closest system to Earth is Alpha Centauri, whose two main stars, Alpha Centauri A and Alpha Centauri B, form a binary pair that are about 4.4 light-years from the sun.

Last year an Earth-size planet was discovered in the Alpha Centauri system, suggesting it may host other alien worlds as well.

Officially named WISE J104915.57-531906, the newly discovered system was spotted in a map obtained by NASA's Wide-field Infrared Survey Explorer (WISE) spacecraft, which spent its 13-month mission scanning the entire sky 1 1/2 times, taking about 1.8 million images of asteroids, stars and galaxies.

"One major goal when proposing WISE was to find the closest stars to the sun," said the mission's principal investigator, Ned Wright of UCLA. "WISE 1049-5319 is by far the closest star found to date using the WISE data, and the close-up views of this binary system we can get with big telescopes like Gemini and the future James Webb Space Telescope will tell us a lot about the low-mass stars known as brown dwarfs."

Luhman noticed that this particular system seemed to be racing across the sky in the WISE images.

"In these time-lapse images, I was able to tell that this system was moving very quickly across the sky, which was a big clue that it was probably very close to our solar system," Luhman said.

Turning to older surveys of the sky, he spotted the system in images obtained between 1978 and 1999 from the Digitized Sky Survey, the Two Micron All-Sky Survey and the Deep Near Infrared Survey of the Southern Sky.

"Based on how this star system was moving in the images from the WISE survey, I was able to extrapolate back in time to predict where it should have been located in the older surveys and, sure enough, it was there," Luhman explained in a statement.

The star system's distance was measured by trigonometric parallax, which can only be done if an object is close enough to show an apparent shift in position relative to much farther background stars, due to the Earth's orbit around the sun.

After obtaining a spectrum of the system with the Gemini South telescope on Cerro Pachón in Chile, Luhman also determined that the system has a very cool temperature and is actually made up of two objects.

"It was a lot of detective work," Luhman said. "There are billions of infrared points of light across the sky, and the mystery is which one — if any of them — could be a star that is very close to our solar system."


Brown Dwarfs can Spin so Fast They Almost Tear Themselves Apart

We tend to image planets as spheres. Held together by gravity, the material of a planet compresses and shifts until gravity and pressure reach a balance point known as hydrostatic equilibrium. Hydrostatic equilibrium is one of the defining characteristics of a planet. If a planet were stationary and of uniform density, then at equilibrium, it would be a perfect sphere. But planets rotate, and so even the largest planets aren’t a perfect sphere.

When a planet rotates, the region around the equator moves more quickly than the regions near the poles. Gravity has a harder time holding on to the equatorial matter, and so the equator bulges out slightly. Careful measurements of Earth, for example, show that it bulges slightly. The diameter of Earth along the equator is about 40 kilometers greater than the diameter from pole to pole. But this is tiny compared to the overall size of Earth, which is why it looks like a perfect sphere when viewed from space.

The dwarf planet Haumea has a rapid rotation that flattens it. Credit: Stephanie Hoover

Some planets rotate so quickly we can see their flattening with the naked eye. Saturn is perhaps the best example of this. It has an average density less than water, and its “day” is only 10 hours long. The most extreme case in our solar system is likely the dwarf planet Haumea, which rotates every 4 hours. We don’t have high-resolution images of Haumea, but observations of its varying brightness indicate it has an equatorial diameter more than twice that of its polar diameter.

Things get interesting as a planet is more massive. Jupiter, for example, rotates a bit faster than Saturn but is significantly less flattened. That’s because Jupiter has three times the mass of Saturn, so Jupiter’s gravity can better hold things together. With greater mass comes stronger gravity, so you would think that planets larger than Jupiter would be spherical. But as a recent study shows, that might not always be so.

The study focuses on brown dwarfs, which lie on the mass scale between stars and planets. A brown dwarf has a mass between about 13 and 78 Jupiters. Below 78 Jupiter masses, a body isn’t large enough to fuse hydrogen like a proper star. Above 13 Jupiter masses, a body can fuse a bit of deuterium, so they aren’t really a planet. Up close, most massive brown dwarfs would look like small reddish stars, while the smallest brown dwarfs (known as y-dwarfs) would appear very Jupiter-like. Even though they are more massive than Jupiter, y-dwarfs would be about the same size, just more dense because of their stronger gravity.

How the light curve of a planet tells us its rotation. Credit: Robert Hurt (IPAC/Caltech)

Astronomers have found more than 2,800 brown dwarfs. For most of them, we only know their basic properties, but for 78 of them, we know their rotation periods. We can measure this by observing the dwarf’s variation in brightness over time. Since their cloud layer has weather features like Jupiter, the periodicity of their brightness tells us their rate of rotation. When the team analyzed the rotational periods of these worlds, they found that the shortest rotational periods were just over an hour. Three of the brown dwarfs studied had periods of about an hour, which would indicate an upper limit.

This is extraordinarily fast. It means near the equator some of these brown dwarfs are rotating at more than 100 kilometers per second. If we assume brown dwarfs have a composition similar to Jupiter, then these fast-rotating bodies would have a flattened shape similar to Saturn, even with a much higher surface gravity.

It isn’t clear why the maximum rotation is about an hour, but one idea is that the brown dwarf would rip itself apart if it spun any faster. Since brown dwarfs don’t produce heat through fusion, they gradually cool as they age. With less heat and pressure, gravity squeezes them more tightly, causing them to shrink. With more mass closer to the axis of rotation, the brown dwarf would spin faster. So brown dwarfs could reach this upper limit as they age, and then start to break apart as they try to spin faster.

It’s a fascinating idea, but we’ll need more observational data to test it. For now, we can say pretty certainly that even some of the largest planet-like bodies are flattening the spherical curve.


An Earth-sized planet may be igniting an aurora around a nearby red dwarf star

Over 4,100 exoplanets have been found, worlds orbiting other stars. A variety of methods has been used to discover them, including transits, radial velocity measurements, and even direct imaging (see the planets themselves in images).

A team of astronomers has just announced they may have found a planet orbiting the red dwarf star GJ 1151, but the technique is entirely novel: The planet is interacting with the star magnetically, creating an aurora not on the planet but on the star itself!

More Bad Astronomy

An aurora on Earth is caused when subatomic particles streaming away from the Sun slam into our atmosphere, guided by the Earth's magnetic field. These particles slam into molecules in our atmosphere, knocking electrons off them. When the electrons recombine, the molecule gives off a characteristic glow, and we call that the aurora.

This happens on other planets too, but in different ways as well. For example, Jupiter has an aurora caused by its moon Io. The moon is volcanic, sending sulfur atoms up off the surface. As Jupiter spins, its strong magnetic field strips those atoms away, and they get trapped in the planet's magnetic field and fall down to Jupiter's poles. As they do, they spiral around the magnetic field lines, emitting radio waves (called synchrotron emission). The radio emission we see from Jupiter strengthens when Jupiter and Io align in a certain way with respect to us observing from Earth, so we see radio emission going up and down, with its strength tied to Io's orbit.

Not only that, but the magnetic field of the planet affects the way the light is emitted, polarizing it. This means the waves are to degree aligned, and that can be measured. In fact, this type of polarization is an excellent way to confirm that the light you see from a planet is actually from an aurora.

This light is very faint, though, so what the astronomers did was clever: Using the Gaia database, they cross-checked a radio wave all-sky survey recently done by the Low Frequency Array (or LOFAR) against the locations of stars known to be within 65 light years of the Sun. This radio aurora glow is relatively weak, so they constrained their efforts to nearby stars to maximize their chance of seeing it.

They found quite a few matches, most of which were red dwarf stars, dim bulbs that are faint and cool. The first viable target they got was the red dwarf GJ 1151, which is a mere 26 light years from Earth. It has a mass 1/6th the Sun's, and a diameter 1/5th of the Sun, so it's indeed pretty dinky.

Then things got interesting. The star was observed by LOFAR four times, and only once was it seen emitting radio waves out of those observations. Many red dwarfs are very magnetically active, blasting out stellar storms (like flares and such), but as it happens GJ 1151 is quiescent. In other words, it doesn't do that. Not only that, but the polarization of the radio waves is exactly what you'd expect from an aurora.

LOFAR observations of the star GJ 1151 show that sometimes it emits low-frequency radio waves (left) and sometimes it doesn’t (right). This may indicate auroral activity generated by an orbiting planet. Credit: Vedantham et al.

If this is the case, then the best fit to their data is a planet orbiting the star, interacting with the star's magnetic field. Perhaps the planet is volcanic, or has an atmosphere that's getting stripped by the star. But one way or another, atoms are "leaking" into space, following the star's magnetic field lines, and emitting polarized radio waves as they slam into the gas above the star's surface, creating an aurora there.

The reason it appears in one observation but not the other three makes sense if that's the case, too, because the planet and star have to be aligned just the right way with respect to Earth for us to see these waves. Again, if this is true, the planet would have to have an orbit of 1–5 days to match the observations.

As it happens, GJ 1511 is a target of planet hunters already, and (very) recently announced observations show that no planet is detected there of more than 5.6 times the mass of the Earth. All that means is that no gas giants orbit the star, but there's lots of wiggle room for smaller ones. And in fact we know that red dwarfs tend to make smaller planets more readily than big ones… and many such systems have their planets all huddled close to the star, so a 1–5 day orbit is no problem.

Hopefully, even more follow-up observations of this star will reveal the existence of this planet (or these planets). If confirmed, this would be a brand new way to find planets around nearby stars: by their aurorae.

Artwork depicting a solitary brown dwarf with an aurora. Credit: Chuck Carter, Caltech, NRAO/AUI/NSF

Incidentally, this method might work for brown dwarfs, too objects intermediate in mass between planets and stars. In 2018 one was found that appeared to have an aurora, and while it's hard to know for sure it may be low mass enough to be considered a rogue planet, a planet without a star. A massive one for sure, more massive than Jupiter! But still, the auroral emission may indicate it has a moon orbiting it, and it's creating an aurora like Io does with Jupiter. So not only is this a good way to find exoplanets, it might even help us find exomoons.

In the early 1990s we didn't know if other planets even existed. Now we know of thousands, and there are more than a dozen methods used to find them. It's funny what you can do once you know it's actually possible.


Citizen scientists spot closest young brown dwarf disk yet

This artist's conception illustrates the brown dwarf named 2MASSJ22282889-431026. NASA's Hubble and Spitzer space telescopes observed the object to learn more about its turbulent atmosphere. Brown dwarfs are more massive and hotter than planets but lack the mass required to become sizzling stars. Their atmospheres can be similar to the giant planet Jupiter's. Spitzer and Hubble simultaneously observed the object as it rotated every 1.4 hours. The results suggest wind-driven, planet-size clouds. Credit: NASA

Brown dwarfs are the middle child of astronomy, too big to be a planet yet not big enough to be a star. Like their stellar siblings, these objects form from the gravitational collapse of gas and dust. But rather than condensing into a star's fiery hot nuclear core, brown dwarfs find a more zen-like equilibrium, somehow reaching a stable, milder state compared to fusion-powered stars.

Brown dwarfs are considered to be the missing link between the most massive gas giant planets and the smallest stars, and because they glow relatively dimly they have been difficult to spot in the night sky. Like stars, some brown dwarfs can retain the disk of swirling gas and dust left over from their initial formation. This material can collide and accumulate to form planets, though it's unclear exactly what kind of planets brown dwarfs can generate.

Now researchers at MIT, the University of Oklahoma, and elsewhere, with the help of citizen scientists, have identified the closest young brown dwarf with the kind of disk that could potentially form planets. The brown dwarf, named W1200-7845, is a mere 3.7 million years old and sits at a nearby 102 parsecs, or about 332 light years from Earth.

At this proximity, scientists may be able to zoom in on the young system with future high-powered telescopes, to examine the earliest conditions of a brown dwarf's disk and perhaps learn more about the kind of planets brown dwarfs might support.

The new system was discovered through Disk Detective, a crowdsourced project funded by NASA and hosted by Zooniverse that provides images of objects in space for the public to classify, with the aim of picking out objects that are likely stars with disks that could potentially host planets.

The researchers are presenting their findings, as well as announcing a new version of the Disk Detective website, this week at the all-virtual meeting of the American Astronomical Society.

"Within our solar neighborhood"

Users of Diskdetective.org, which first launched in 2014, can look through "flipbooks"—images of the same object in space, taken by NASA's Wide-field Infrared Survey Explorer, or WISE, which detects infrared emissions such as thermal radiation given off by the gas and dust debris in stellar disks. A user could classify an object based on certain criteria such as whether the object appears oval—a shape that more resembles a galaxy—or round —a sign that the object is more likely a disk-hosting star.

"We have multiple citizen scientists look at each object and give their own independent opinion, and trust the wisdom of the crowd to decide what things are probably galaxies and what things are probably stars with disks around them," says study co-author Steven Silverberg, a postdoc in MIT's Kavli Institute for Astrophysics and Space Research.

From there, a science team including Silverberg follows up on crowd-classified disks, using more sophisticated methods and telescopes to determine if indeed they are disks, and what characteristics the disks may have.

In the case of the newly discovered W1200-7845, citizen scientists first classified the object as a disk in 2016. The science team, including Silverberg and Maria Schutte, a graduate student at the University of Oklahoma, then looked more closely at the source with an infrared instrument on the Magellan 6.5-meter telescopes at Las Campanas Observatory in Chile.

With these new observations, they determined that the source was indeed a disk around a brown dwarf that lived within a "moving group"—a cluster of stars that tend to move as one across the night sky. In astronomy, it's far easier to determine the age of a group of objects rather than one alone. Because the brown dwarf was part of a moving group of about 30 stars, previous researchers were able to estimate an average age for the group, about 3.7 million years old, that was likely also the age of the brown dwarf.

The brown dwarf is also very close to the Earth, at about 102 parsecs away, making it the closest, young brown dwarf detected yet. For comparison, our nearest star, Alpha Centauri, is 1 parsec from Earth.

"When it's this close, we consider it to be within the solar neighborhood," Schutte says. "That proximity is really important, because brown dwarfs are lower in mass and inherently less bright than other objects like stars. So the closer these objects are to us, the more detail we'll be able to see."

Looking for Peter Pan

The team plans to zoom further in on W1200-7845 with other telescopes, such as ALMA, the Atacama Large Millimeter Array in Chile, comprising 66 huge radio dishes that work together as one powerful telescope to observe the universe between the radio and infrared bands. At this range and precision, the researchers hope to see the brown dwarf's disk itself, to measure its mass and radius.

"A disk's mass just tells you how much stuff is in the disk, which would tell us if planet formation happens around these systems, and what sorts of planets you'd be able to produce," Silverberg says. "You could also use that data to determine what kinds of gas are in the system which would tell you about the disk's composition."

In the meantime, the researchers are launching a new version of Disk Detective. In April 2019, the website went on hiatus, as its hosting platform, the popular citizen scientist portal Zooniverse, briefly retired its previous software platform in favor of an updated version. The updated platform has prompted Silverberg and his colleagues to revamp Disk Detective. The new version, launching this week, will include images from a full-sky survey, PanSTARRS, that observes most of the sky in high-resolution optical bands.

"We're getting more current images with different telescopes with better spatial resolution this time around," says Silverberg, who will be managing the new site at MIT.

Where the site's previous version was aimed at finding any disks around stars and other objects, the new site is designed to pick out "Peter Pan" disks—disks of gas and dust that should be old enough to have formed planets, but for some reason haven't quite yet.

"We call them Peter Pan disks because they seem to never grow up," Silverberg says.

The team identified its first Peter Pan disk with Disk Detective in 2016. Since then, seven others have been found, each at least 20 million years old. With the new site, they hope to identify and study more of these disks, which could help to nail down conditions under which planets, and possibly life, may form.

"The disks we find will be excellent places to look for exoplanets," Silverberg says.

"If planets take longer to form than we previously thought, the star they orbit will have fewer gigantic flares when the planets finally form. If the planet receives fewer flares than it would around a younger star, that could significantly impact our expectations for discovering life there."


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Early theorizing Edit

The objects now called "brown dwarfs" were theorized by Shiv S. Kumar in the 1960s to exist and were originally called black dwarfs, [9] a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However: (a) the term black dwarf was already in use to refer to a cold white dwarf (b) red dwarfs fuse hydrogen and (c) these objects may be luminous at visible wavelengths early in their lives. Because of this, alternative names for these objects were proposed, including planetar [ check spelling ] and substar. In 1975, Jill Tarter suggested the term "brown dwarf", using "brown" as an approximate color. [6] [10] [11]

The term "black dwarf" still refers to a white dwarf that has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarf to cool to this temperature is calculated to be longer than the current age of the universe hence such objects are expected to not yet exist.

Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a population I object with a mass less than 0.07 solar masses ( M ) or a population II object less than 0.09 M would never go through normal stellar evolution and would become a completely degenerate star. [12] The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07 and 0.08 solar masses for population I objects. [13] [14]

Deuterium fusion Edit

The discovery of deuterium burning down to 0.013 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.

Since then, numerous searches by various methods have sought these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions of main-sequence dwarfs and white dwarfs, surveys of young star clusters, and radial velocity monitoring for close companions.

GD 165B and class "L" Edit

For many years, efforts to discover brown dwarfs were fruitless. In 1988, however, a faint companion to a star known as GD 165 was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All-Sky Survey (2MASS) which discovered many objects with similar colors and spectral features.

Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". [15] [16]

Although the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very-low-mass star, because observationally it is very difficult to distinguish between the two. [ citation needed ]

Soon after the discovery of GD 165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True stars burn their lithium within a little over 100 Myr, whereas brown dwarfs (which can, confusingly, have temperatures and luminosities similar to true stars) will not. Hence, the detection of lithium in the atmosphere of an object older than 100 Myr ensures that it is a brown dwarf.

Gliese 229B and class "T" – the methane dwarfs Edit

The first class "T" Brown Dwarf was discovered in 1994 by Caltech astronomers Shrinivas Kulkarni, Tadashi Nakajima, Keith Matthews, and Rebecca Oppenheimer, [17] and Johns Hopkins scientists Sam Durrance and David Golimowski. It was confirmed in 1995 as a substellar companion to Gliese 229. Gliese 229b is one of the first two instances of clear evidence for a brown dwarf, along with Teide 1. Confirmed in 1995, both were identified by the presence of the 670.8 nm lithium line. The latter was found to have a temperature and luminosity well below the stellar range.

Its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in the atmospheres of giant planets and that of Saturn's moon Titan. Methane absorption is not expected at any temperature of a main-sequence star. This discovery helped to establish yet another spectral class even cooler than L dwarfs, known as "T dwarfs", for which Gliese 229B is the prototype.

Teide 1 – the first class "M" brown dwarf Edit

The first confirmed class "M" brown dwarf was discovered by Spanish astrophysicists Rafael Rebolo (head of team), María Rosa Zapatero Osorio, and Eduardo Martín in 1994. [18] This object, found in the Pleiades open cluster, received the name Teide 1. The discovery article was submitted to Nature in May 1995, and published on 14 September 1995. [19] [20] Nature highlighted "Brown dwarfs discovered, official" in the front page of that issue.

Teide 1 was discovered in images collected by the IAC team on 6 January 1994 using the 80 cm telescope (IAC 80) at Teide Observatory and its spectrum was first recorded in December 1994 using the 4.2 m William Herschel Telescope at Roque de los Muchachos Observatory (La Palma). The distance, chemical composition, and age of Teide 1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and substellar evolution models at that moment, the team estimated for Teide 1 a mass of 55 ± 15 M J, [21] which is below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works.

In theory, a brown dwarf below 65 M J is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles used to judge the substellar nature of low-luminosity and low-surface-temperature astronomical bodies.

High-quality spectral data acquired by the Keck 1 telescope in November 1995 showed that Teide 1 still had the initial lithium abundance of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations confirmed that Teide 1 is a brown dwarf, as well as the efficiency of the spectroscopic lithium test.

For some time, Teide 1 was the smallest known object outside the Solar System that had been identified by direct observation. Since then, over 1,800 brown dwarfs have been identified, [22] even some very close to Earth like Epsilon Indi Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12 light-years from the Sun, and Luhman 16, a binary system of brown dwarfs at 6.5 light-years from the Sun.


New Brown Dwarf discovered

Artist’s conception of the brown dwarf WISE J085510.83-071442.5. The Sun is the bright star directly to the right of the brown dwarf.
Credit: Robert Hurt/JPL, Janella Williams/Penn State University
[Click to enlarge image]

“It is very exciting to discover a new neighbor of our solar system that is so close,” said Kevin Luhman, an associate professor of astronomy and astrophysics at Penn State and a researcher in the Penn State Center for Exoplanets and Habitable Worlds. “In addition, its extreme temperature should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.”

Brown dwarfs start their lives like stars, as collapsing balls of gas, but they lack the mass to burn nuclear fuel and radiate starlight. The newfound coldest brown dwarf, named WISE J085510.83-071442.5, has a chilly temperature between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius). Previous record holders for coldest brown dwarfs, also found by WISE and Spitzer, were about room temperature.

Although it is very close to our solar system, WISE J085510.83-071442.5 is not an appealing destination for human space travel in the distant future. “Any planets that might orbit it would be much too cold to support life as we know it” Luhman said.

“This object appeared to move really fast in the WISE data,” said Luhman. “That told us it was something special.” The closer a body, the more it appears to move in images taken months apart. Airplanes are a good example of this effect: a closer, low-flying plane will appear to fly overhead more rapidly than a high-flying one.

WISE was able to spot the rare object because it surveyed the entire sky twice in infrared light, observing some areas up to three times. Cool objects like brown dwarfs can be invisible when viewed by visible-light telescopes, but their thermal glow — even if feeble — stands out in infrared light.

After noticing the fast motion of WISE J085510.83-071442.5 in March, 2013, Luhman spent time analyzing additional images taken with Spitzer and the Gemini South telescope on Cerro Pachon in Chile. Spitzer’s infrared observations helped to determine the frosty temperature of the brown dwarf.

WISE J085510.83-071442.5 is estimated to be 3 to 10 times the mass of Jupiter. With such a low mass, it could be a gas giant similar to Jupiter that was ejected from its star system. But scientists estimate it is probably a brown dwarf rather than a planet since brown dwarfs are known to be fairly common. If so, it is one of the least massive brown dwarfs known.

Combined detections from WISE and Spitzer, taken from different positions around the Sun, enabled the measurement of its distance through the parallax effect. This is the same principle that explains why your finger, when held out right in front of you, appears to jump from side to side when you alternate left-eye and right-eye views.

In March of 2013, Luhman’s analysis of the images from WISE uncovered a pair of much warmer brown dwarfs at a distance of 6.5 light years, making that system the third closest to the Sun. His search for rapidly moving bodies also demonstrated that the outer solar system probably does not contain a large, undiscovered planet, which has been referred to as “Planet X” or “Nemesis.”

“It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the Sun’s nearest neighbors,” said Michael Werner, the project scientist for Spitzer at NASA’s Jet Propulsion Laboratory (JPL), which manages and operates Spitzer. “This exciting new result demonstrates the power of exploring the universe using new tools, such as the infrared eyes of WISE and Spitzer.”