# What is the largest possible size for a brown dwarf?

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What is the largest possible size for a brown dwarf before it has enough mass, capable of sustained fusion, thus becoming a star?

I think what you are asking is what is the radius of an object that has the maximum mass to be classed as a brown dwarf?

The mass threshold for this is about $0.075 M_{odot}$. The boundary is a bit fuzzy - there is still some hydrogen fusion, but not enough to produce the luminosity of the object), it also depends slightly on the chemical composition of the object.

However, the fact that you can still see (with a telescope!) brown dwarfs raises a more fundamental problem with your question. Their luminosity comes from gravitational potential energy; they are contracting all the time.

So you can only ask what is the radius of the most massive brown dwarf at a given age?

The oldest (about 10 billion years, though there is no good way to estimate the age of a brown dwarf in the field, or measure its mass), most massive brown dwarf's should be about the size of Jupiter. A similar brown dwarf in a star forming region, born a few million years ago, should be several times bigger.

The plot below (from Nelson, Rappaport & Joss 1986 - newer models are available, but show the same behaviour) shows the radius of substellar objects as a function of time. The plateau at early times is caused by Deuterium fusion in object more massive than about $0.012M_{odot}$.

## In a Curious Case for Astronomers, a Brown Dwarf Goes Missing

Scientists trained one of the world's largest telescopes on the star and found nothing.

Astronomers are trying to crack a strange new case: the mystery of the missing dwarf star.

Using one of the biggest telescopes in the world, the ESO's Very Large Telescope in Chile, researchers were on the hunt for what they had confidently predicted to be a brown dwarf, only to discover that the object was nowhere to be found.

Brown dwarfs are cool, dim objects that actually resemble planets more than stars. (Related: "Dimmest Stars in Universe Spotted?")

While they do give off heat and have chemical properties similar to those of ordinary stars like our sun, these weird objects are often referred to as "failed stars" because they don't have enough mass to ignite thermonuclear reactions in their cores. (See "Coldest Star Found—No Hotter Than Fresh Coffee.")

In this case, the dwarf in question was thought to be orbiting a double-star system called V471 Tauri in the constellation Taurus, the Bull, about 163 light-years from Earth. The two stars orbit each other every 12 hours, causing dips in the pair's brightness every six hours as one star passes in front of the other.

When astronomers precisely timed the dips in light created by the stars' orbits, they found that the timing wasn't always quite on schedule. The only explanation seemed to be that an orbiting brown dwarf was gravitationally tugging on the stars, slightly throwing off the timing.

To look for the dwarf, the scientists trained a giant telescope on the binary star system using a powerful new camera system called SPHERE, designed to directly image planets around distant stars.

To their surprise, they saw nothing where the brown dwarf was predicted to be.

That sends astronomers back to the drawing board.

"This is how science works," said Adam Hardy, lead author on the new study published this week in the Astrophysical Journal Letters. "Observations with new technology can either confirm or, as in this case, disprove earlier ideas."

Though the brown dwarf at the heart of this mystery is missing, its home cluster happens to be one of the brightest and biggest deep-sky objects visible in the night sky.

This scattering of stars is known collectively as the Hyades cluster, named after the mythical Greek sisters who bring the rain.

## In a first, NASA measures wind speed on a brown dwarf

For the first time, scientists have directly measured wind speed on a brown dwarf, an object larger than Jupiter (the largest planet in our solar system) but not quite massive enough to become a star. To achieve the finding, they used a new method that could also be applied to learn about the atmospheres of gas-dominated planets outside our solar system.

Described in a paper in the journal Science, the work combines observations by a group of radio telescopes with data from NASA's recently retired infrared observatory, the Spitzer Space Telescope, managed by the agency's Jet Propulsion Laboratory in Southern California.

Officially named 2MASS J10475385+2124234, the target of the new study was a brown dwarf located 32 light-years from Earth -- a stone's throw away, cosmically speaking. The researchers detected winds moving around the planet at 1,425 mph (2,293 kph). For comparison, Neptune's atmosphere features the fastest winds in the solar system, which whip through at more than 1,200 mph (about 2,000 kph).

Measuring wind speed on Earth means clocking the motion of our gaseous atmosphere relative to the planet's solid surface. But brown dwarfs are composed almost entirely of gas, so "wind" refers to something slightly different. The upper layers of a brown dwarf are where portions of the gas can move independently. At a certain depth, the pressure becomes so intense that the gas behaves like a single, solid ball that is considered the object's interior. As the interior rotates, it pulls the upper layers -- the atmosphere -along so that the two are almost in synch.

In their study, the researchers measured the slight difference in speed of the brown dwarf's atmosphere relative to its interior. With an atmospheric temperature of over 1,100 degrees Fahrenheit (600 degrees Celsius), this particular brown dwarf radiates a substantial amount of infrared light. Coupled with its close proximity to Earth, this characteristic made it possible for Spitzer to detect features in the brown dwarf's atmosphere as they rotate in and out of view. The team used those features to clock the atmospheric rotation speed.

To determine the speed of the interior, they focused on the brown dwarf's magnetic field. A relatively recent discovery found that the interiors of brown dwarfs generate strong magnetic fields. As the brown dwarf rotates, the magnetic field accelerates charged particles that in turn produce radio waves, which the researchers detected with the radio telescopes in the Karl G. Jansky Very Large Array in New Mexico.

Planetary Atmospheres

The new study is the first to demonstrate this comparative method for measuring wind speed on a brown dwarf. To gauge its accuracy, the group tested the technique using infrared and radio observations of Jupiter, which is also composed mostly of gas and has a physical structure similar to a small brown dwarf. The team compared the rotation rates of Jupiter's atmosphere and interior using data that was similar to what they were able to collect for the much more distant brown dwarf. They then confirmed their calculation for Jupiter's wind speed using more detailed data collected by probes that have studied Jupiter up close, thus demonstrating that their approach for the brown dwarf worked.

Scientists have previously used Spitzer to infer the presence of winds on exoplanets and brown dwarfs based on variations in the brightness of their atmospheres in infrared light. And data from the High Accuracy Radial velocity Planet Searcher (HARPS) -- an instrument on the European Southern Observatory's La Silla telescope in Chile -- has been used to make a direct measurement of wind speeds on a distant planet.

But the new paper represents the first time scientists have directly compared the atmospheric speed with the speed of a brown dwarf's interior. The method employed could be applied to other brown dwarfs or to large planets if the conditions are right, according to the authors.

"We think this technique could be really valuable to providing insight into the dynamics of exoplanet atmospheres," said lead author Katelyn Allers, an associate professor of physics and astronomy at Bucknell University in Lewisburg, Pennsylvania. "What's really exciting is being able to learn about how the chemistry, the atmospheric dynamics and the environment around an object are interconnected, and the prospect of getting a really comprehensive view into these worlds."

The Spitzer Space Telescope was decomissioned on Jan. 30, 2020, after more than 16 years in space. JPL managed Spitzer mission operations for NASA's Science Mission Directorate in Washington. Spitzer science data continue to be analyzed by the science community via the Spitzer data archive located at the Infrared Science Archive housed at IPAC at Caltech. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech in Pasadena. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. Caltech manages JPL for NASA.

## Astronomers Capture a Direct Image of a Brown Dwarf

The field of exoplanet photography is just getting underway, with astronomers around the world striving to capture clear images of the more than 4000 exoplanets discovered to date. Some of these exoplanets are more interesting to image and research than others. That is certainly the case for a type of exoplanet called a brown dwarf. And now scientists have captured the first ever image of exactly that type of exoplanet.

Brown dwarfs are “substellar objects” – they do not have enough mass to spark nuclear fusion in their core, and therefore were not able to become an actual star, but are much more massive than any traditional planet. The one imaged by a team of astronomers at the Subaru Telescope and the W. M. Keck observatory in Manuakea has a mass 46 times that of Jupiter.

This particular brown dwarf is interesting for reasons other than its size though. Of primary interest is its orbital path and the stellar system it resides in. The planetary system it resides in is known as HD 33632. The star in HD 33632 is a main sequence star, in many ways similar to our sun. The brown dwarf, now named very creatively as HD 33632Ab, orbits around the star at a distance of about 20 AUs (approximately the distance from Mercury to Pluto).

That solar distance combined with the similarities between HD 33632’s star and our sun make the existence of a brown dwarf in that system highly informative to models predicting how those systems might be formed. The image the scientists captured also provides valuable data points for the analysis of other directly imaged exoplanets. There is a chance the atmosphere of the HD 33632Ab may contain carbon monoxide and water, making it a useful barometer for comparing other exoplanets atmospheres.

Our ability to see any exoplanet’s atmosphere, even one as big as HD 33632Ab, are thanks to advances in adaptive optics and near-infrared imaging systems. Those systems on the Subaru and Keck observatories joined together to snap this unique image. Subaru leveraged it’s exoplanet hunting system SCExAO/CHARIS while Keck contributed images from a near-infrared camera called NIRC-2. These combined instruments resulted in a much more clearly defined picture than would have been possible with only one of the observatories.

Graphic depicting the sizes of brown dwarfs compared to planets and stars.
Credit: NASA

This finding certainly won’t be the last application of that combination of powerful exoplanet imaging technologies. Nor will it be the last exoplanet, or brown dwarf, that we as a species will directly image. But as these images start to trickle in, what we will begin to find will hopefully become more and more fascinating as we begin to take a peek at these newly discovered worlds.

Lead Image: Direct image of the stellar system HD 33632, including the brown dwarf on the right of the screen.
Credit: W. M. Keck Observatory

## Contents

### 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.

## References

Nakajima, T. et al. Nature 378 378–465 (1995).

Berger, E. et al. Nature 410, 338–340 (2001).

Rutledge, R. E., Basri, G., Martín, E. L. & Bildsten, L. Astrophys. J. 538, L141–L144 (2000).

Guedel, M. & Benz, A. O. Astrophys. J. 405, L63–L66 (1993).

Benz, A. O. & Guedel, M. Astron. Astrophys. 285, 621–630 (1994).

Smith, K. W., Guedel, M. & Benz, A. O. Astron. Astrophys. 349, 475–484 (1999).

Krucker, S. & Benz, A. O. Solar Phys. 191, 341–358 (2000).

## Astronomers weigh the coldest brown dwarfs with astronomy's sharpest eyes

Astronomers have used ultrasharp images obtained with the Keck Telescope and Hubble Space Telescope to determine for the first time the masses of the coldest class of "failed stars," a.k.a. brown dwarfs. With masses as light as 3 percent the mass of the sun, these are the lowest mass free-floating objects ever weighed outside the solar system. The observations are a major step in testing the theoretical predictions of objects that cannot generate their own internal energy, both brown dwarfs and gas-giant planets. The new findings, which are being presented in a press conference today at the American Astronomical Society meeting in St. Louis, show that the predictions may have some problems.

"Mass is the fundamental parameter that governs the life-history of a free-floating object, and thus after many years of patient measurements, we are delighted to report the first masses of the very faintest, coldest brown dwarfs," said Dr. Michael Liu of the Institute for Astronomy at the University of Hawaii (IfA/UH). "After weighing these tiny, dim, cold objects, we have confirmed that the theoretical predictions are mostly correct, but not entirely so." The team announcing the results is composed of Dr. Liu, Mr. Trent J. Dupuy (IfA/UH), and Dr. Michael J. Ireland (University of Sydney).

Brown dwarfs are a class of objects that represent the missing link between the lowest-mass stars and the gas-giant planets, such as Jupiter and Saturn. Brown dwarfs are the faintest and coolest objects that can be directly observed outside the solar system. They emit as little as about 1/300,000 the energy of the sun and have surface temperatures comparable to the inside of a pizza oven (800° F), more than 9,000° F cooler than the surface of the sun.

"Astronomers have measured the energy output and temperatures for a myriad of brown dwarfs. However, the most important property of all is the hardest one to measure--the mass," said Dr. Ireland.

To determine the masses, the team has spent the last several years studying brown dwarfs that occur in binaries, that is two brown dwarfs that are mutually bound together by gravity and orbit each other, in a fashion similar to how Earth orbits the sun. As first shown by Johannes Kepler in the 17th century, the total mass of any binary system can be determined by precisely measuring the orbit's size and how long it takes for the two objects to complete one orbital cycle.

"These are very challenging measurements, because brown dwarf binaries have tiny separations on the sky and orbit each other very slowly. We needed to obtain the sharpest measurements that are possible with current telescopes to precisely monitor their motion," said Mr. Dupuy.

The astronomers obtained images using the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii. Keck II is equipped with a powerful adaptive optics system that corrects for the blurring of astronomical images caused by turbulence in Earth's atmosphere. The Keck system can also employ a low-power laser to create an "artificial" star to enable such correction for almost anywhere in the sky.

The resulting images have an angular resolution as good as 1/20 of an arc second, about 1/40,000 the diameter of the full moon. A person with vision as sharp as the Keck adaptive optics system would be able to read a magazine that was about a mile away. In fact, the positional accuracy achieved with such sharp images is equivalent to hitting a bull's-eye on a dartboard that is 8,000 miles away.

By regularly monitoring binaries with Keck adaptive optics and analyzing previous data obtained by the Hubble Space Telescope, the team was able to precisely measure the size and duration of the binaries' orbits, and thereby determine the masses.

The team measured the masses of two brown dwarf binaries. One, known as 2MASS 1534-2952AB, is composed of two "methane" brown dwarfs, the coolest type of brown dwarf, which is characterized by the presence of methane gas in their atmospheres. This is the first mass measurement for this type of brown dwarf. The team found that the total mass of 2MASS 1534-2952AB is only 6 percent of the sun's mass, and each brown dwarf in it has a mass of about 3 percent of the sun's (about 30 times the mass of Jupiter). The other binary system, HD 130948BC, is a pair of slightly warmer "dusty" brown dwarfs with a total mass of only 11 percent of the sun's mass and individual masses of about 5.5 percent of the sun's.

Theoretical models predict the masses of brown dwarfs based on their energy output and temperature. But when the team compared their mass measurements to the theoretical predictions, they did not agree. For example, the surface temperature of 2MASS 1534-2952AB was much cooler than expected given its current level of energy output, while HD 130948BC was much warmer.

"While there is general agreement between our data and the predictions, something is not quite right with the theoretical studies of brown dwarfs, either in determining their temperatures or in predicting their energy output. Or perhaps both," said Dr. Liu. "These findings will be a challenge for the theorists, and we are inspired to measure the masses of more brown dwarfs in the coming years to better understand the problem."

The two binaries, located in the constellations of Libra (the Scales) and Bootes (the Herdsman), are about 45-60 light-years from Earth. The two components of each binary have a typical separation of about 2 astronomical units (AU), where 1 AU is the distance from Earth to the sun (93 million miles). This is somewhat larger than the 1.5 AU distance between Mars and the sun. Their orbital periods are about 10-15 years, compared with 2 years for Mars around the sun.

The team's results are described in two upcoming papers submitted to the Astrophysical Journal. This research has been supported by the National Science Foundation and the Alfred P. Sloan Foundation.

First discovered in 1995, brown dwarfs represent a class of objects with masses less than 7 percent the mass of the sun (about 70 times Jupiter's mass). While ordinary stars become hot and dense enough in their interiors to generate their own energy via nuclear fusion, brown dwarfs have insufficient mass to do this, so instead they steadily fade and cool over their lifetime. In many ways, brown dwarfs are very similar to gas-giant planets like Jupiter and Saturn, since both types of objects are unable to steadily generate their own energy and have very low surface temperatures.

Scientists have discovered hundreds of brown dwarfs within 100 light-years of Earth. About 15 percent of them are binary systems. Dr. Adam Burgasser (then at the University of California, Los Angeles, now at MIT) and Dr. Daniel Potter (then at IfA/UH) used the Hubble Space Telescope and the Gemini-North Observatory, respectively, to identify 2MASS 1534-2952AB and HD 130948BC as binaries around 2001.

Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.

The W. M. Keck Observatory operates twin 10-meter telescopes located on the summit of Mauna Kea on the island of Hawaii and is managed by the California Association for Research in Astronomy, a non-profit corporation whose board of directors includes representatives from Caltech, the University of California and NASA. For more information, please visit: http://www.keckobservatory.org.

The Hubble Space Telescope is operated by the Space Telescope Science Institute with funding from NASA.

## Trio of Fast-Spinning Brown Dwarfs May Reveal a Rotational Speed Limit

The faster a brown dwarf spins, the narrower the different-colored atmospheric bands on it likely become, as shown in this illustration. Some brown dwarfs glow in visible light, but they are typically brightest in infrared wavelengths, which are longer than what human eyes can see.

Brown dwarfs, sometimes known as “failed stars,” can spin at upwards of 200,000 mph, but there may be a limit to how fast they can go.

Using data from NASA’s Spitzer Space Telescope, scientists have identified the three fastest-spinning brown dwarfs ever found. More massive than most planets but not quite heavy enough to ignite like stars, brown dwarfs are cosmic in-betweeners. And though they aren’t as well known as stars and planets to most people, they are thought to number in the billions in our galaxy.

In a study appearing in the Astronomical Journal, the team that made the new speed measurements argue that these three rapid rotators could be approaching a spin speed limit for all brown dwarfs, beyond which they would break apart. The rapidly rotating brown dwarfs are all about the same diameter as Jupiter but between 40 and 70 times more massive. They each rotate about once per hour, while the next-fastest known brown dwarfs rotate about once every 1.4 hours and Jupiter spins once every 10 hours. Based on their size, that means the largest of the three brown dwarfs whips around at more than 60 miles per second (100 kilometers per second), or about 220,000 miles per hour (360,000 kilometers per hour).

The speed measurements were made using data from Spitzer, which NASA retired in January 2020. (The brown dwarfs were discovered by the ground-based Two Micron All Sky Survey, or 2MASS, which ran until 2001.) The team then corroborated their unusual findings through observations with the ground-based Gemini North and Magellan telescopes.

NASA Spitzer Space Telescope has identified the fastest-spinning brown dwarf known. Brown dwarfs are generally more massive than planets but not massive enough to become stars.

Brown dwarfs, like stars or planets, are already spinning when they form. As they cool down and contract, they spin faster, just like when a spinning ice skater draws her arms into her body. Scientists have measured the spin rates of about 80 brown dwarfs, and they vary from less than two hours (including the three new entries) to tens of hours.

With so much variety among the brown dwarf speeds already measured, it surprised the authors of the new study that the three fastest brown dwarfs ever found have almost the exact same spin rate (about one full rotation per hour) as each other. This cannot be attributed to the brown dwarfs having formed together or being at the same stage in their development, because they are physically different: One is a warm brown dwarf, one is cold, and the other falls between them. Since brown dwarfs cool as they age, the temperature differences suggest these brown dwarfs are different ages.

The authors aren’t chalking this up to coincidence. They think the members of the speedy trio have all reached a spin speed limit, beyond which a brown dwarf could break apart.

All rotating objects generate centripetal force, which increases the faster the object spins. On a carnival ride, this force can threaten to throw riders from their seats in stars and planets, it can tear the object apart. Before a spinning object breaks apart, it will often start bulging around its midsection as it deforms under the pressure. Scientists call this oblation. Saturn, which rotates once every 10 hours like Jupiter, has a perceptible oblation. Based on the known characteristics of the brown dwarfs, they likely have similar degrees of oblation, according to the paper authors.

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All spinning objects, from carousels to planets, generate centripetal force. If a planet rotates too fast, that force can pull it apart. Before that happens, the planet will experience “flattening,” or bulging around its midsection, as seen in this animated illustration of a brown dwarf, Jupiter, and Saturn.

Reaching the Speed Limit

Considering that brown dwarfs tend to speed up as they age, are these objects regularly exceeding their spin speed limit and being torn apart? In other rotating cosmic objects, like stars, there are there natural braking mechanisms that stop them from destroying themselves. It’s not clear yet if similar mechanisms exist in brown dwarfs.

“It would be pretty spectacular to find a brown dwarf rotating so fast it is tossing its atmosphere out into space,” said Megan Tannock, a Ph.D. candidate at Western University in London, Ontario, and lead author on the new study. “But so far, we haven’t found such a thing. I think that must mean that either something is slowing the brown dwarfs down before they hit that extreme or that they can’t get that fast in the first place. The result of our paper supports some sort of limit on the rotation rate, but we’re not sure of the reason yet.”

Brown dwarfs are more massive than most planets but not quite as massive as stars. Generally speaking, they have between 13 and 80 times the mass of Jupiter. A brown dwarf becomes a star if its core pressure gets high enough to start nuclear fusion.

The maximum spin rate of any object is determined not only by its total mass but by how that mass is distributed. That’s why, when very rapid spin rates are involved, understanding a brown dwarf’s interior structure becomes increasingly important: The material inside likely shifts and deforms in ways that could change how fast the object can spin. Similar to gas planets such as Jupiter and Saturn, brown dwarfs are composed mostly of hydrogen and helium.

But they are also significantly denser than most giant planets. Scientists think the hydrogen in the core of a brown dwarf is under such tremendous pressures that it starts behaving like a metal rather than an inert gas: It has free-floating conducting electrons, much like a copper conductor. That changes how heat is conducted through the interior and with very fast spin rates, may also affect how the mass inside an astronomical object is distributed.

“This state of hydrogen, or any gas under such extreme pressure, is still very enigmatic,” said Stanimir Metchev, co-author on the paper and the Canada Research Chair in Extrasolar Planets at the Institute for Earth and Space Exploration at Western University. “It is extremely challenging to reproduce this state of matter even in the most advanced high-pressure physics laboratories.”

Physicists use observations, laboratory data, and mathematics to create models of what brown dwarf interiors should look like and how they should behave, even under extreme conditions. But current models show that the maximum brown dwarf spin speed should be about 50% to 80% faster than the one-hour rotation period described in the new study.

“It is possible that these theories don’t have the full picture yet,” said Metchev. “Some unappreciated factor may be coming into play that doesn’t let the brown dwarf spin faster.” Additional observations and theoretical work may yet reveal whether there’s some braking mechanism that stops brown dwarfs from self-destruction and whether there are brown dwarfs spinning even faster in the darkness.

NASA's Jet Propulsion Laboratory, a division of Caltech, managed Spitzer mission operations for NASA's Science Mission Directorate in Washington. Science operations were conducted at the Spitzer Science Center at IPAC at Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. The Spitzer data archive is housed at the Infrared Science Archive at IPAC at Caltech in Pasadena, California. The international Gemini Observatory is a Program of the National Science Foundation’s NOIRLab.

## Long-Period Giant Planets Form Differently than Brown Dwarfs, Say Astronomers

Using the 10-m Keck II telescope at the W.M. Keck Observatory and Subaru Telescope, a team of astronomers has studied the orbits of 27 long-period giant planets and brown dwarfs in their planetary systems. Combined with modeling of the orbits, the data allowed the researchers to determine that the brown dwarfs in these systems formed like stars, but the gas giants formed like planets.

An artist’s impression of a brown dwarf and its parent star. Image credit: Sci-News.com.

Brown dwarfs are cool, dim objects that have a size between that of a gas giant, such as Jupiter or Saturn, and that of a Sun-like star.

Sometimes called failed stars, they are too small to sustain hydrogen fusion reactions at their cores, yet they have star-like attributes.

Typically, they have masses between 11-16 Jupiters (the approximate mass at which deuterium fusion can be sustained) and 75-80 Jupiters (the approximate mass to sustain hydrogen fusion).

Dr. Brendan Bowler from the University of Texas at Austin and colleagues wanted to settle the question: are gas giant planets on the outer fringes of planetary systems the tip of the planetary iceberg, or the low-mass end of brown dwarfs?

Using the Near-Infrared Camera, second generation (NIRC2) instrument on the Keck II telescope, as well as the Subaru Telescope, they took images of giant planets and brown dwarfs as they orbit their parent stars.

They combined their new observations of 27 systems with all of the previous observations published by other astronomers or available in telescope archives.

At this point, computer modeling comes in. They created an orbit-fitting code called Orbitize!’ which uses Kepler’s laws of planetary motion to identify which types of orbits are consistent with the measured positions, and which are not.

The code generates a set of possible orbits for each companion. The slight motion of each giant planet or brown dwarf forms a ‘cloud’ of possible orbits. The smaller the cloud, the more the researchers are closing in on the companion’s true orbit. And more data points — that is, more direct images of each object as it orbits — will refine the shape of the orbit.

“Rather than wait decades or centuries for a planet to complete one orbit, we can make up for the shorter time baseline of our data with very accurate position measurements,” said Dr. Eric Nielsen, an astronomer at Stanford University.

“A part of Orbitize! that we developed specifically to fit partial orbits, OFTI (Orbits For The Impatient), allowed us to find orbits even for the longest period companions.”

Finding the shape of the orbit is key: objects that have more circular orbits probably formed like planets. That is, when a cloud of gas and dust collapsed to form a star, the distant companion (and any other planets) formed out of a flattened disk of gas and dust rotating around that star.

On the other hand, the ones that have more elongated orbits probably formed like stars.

In this scenario, a clump of gas and dust was collapsing to form a star, but it fractured into two clumps.

Each clump then collapsed, one forming a star, and the other a brown dwarf orbiting around that star.

This is essentially a binary star system, albeit containing one real star and one brown dwarf.

“Even though these companions are millions of years old, the memory of how they formed is still encoded in their present-day eccentricity,” Dr. Nielsen said.

“Eccentricity is a measure of how circular or elongated an object’s orbit is.”

“The punchline is, we found that when you divide these objects at this canonical boundary of more than about 15 Jupiter masses, the things that we’ve been calling planets do indeed have more circular orbits, as a population, compared to the rest. And the rest look like binary stars,” Dr. Bowler said.

The findings were published in the Astronomical Journal.

Brendan P. Bowler et al. 2020. Population-level Eccentricity Distributions of Imaged Exoplanets and Brown Dwarf Companions: Dynamical Evidence for Distinct Formation Channels. AJ 159, 63 doi: 10.3847/1538-3881/ab5b11

## TESS Discovers Its First Brown Dwarf

Astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) have discovered an intermediate-mass brown dwarf orbiting a young star about 841 light-years from Earth. Named TOI-503b, the object is the first brown dwarf discovered by TESS.

An artist’s impression of TOI-503b and its star. Image credit: Sci-News.com.

Brown dwarfs are cool, dim objects that have a size between that of a gas giant, such as Jupiter or Saturn, and that of a Sun-like star.

Sometimes called failed stars, they are too small to sustain hydrogen fusion reactions at their cores, yet they have star-like attributes.

Typically, they have masses between 11-16 Jupiters (the approximate mass at which deuterium fusion can be sustained) and 75-80 Jupiters (the approximate mass to sustain hydrogen fusion).

The newly-discovered brown dwarf, TOI-503b, is just 1.34 times bigger than Jupiter but 53.7 times more massive.

It orbits its host star, TOI-503, once every 3.7 days at a distance of only 0.06 AU in a circular orbit.

Also known as BD+13 1880, TIC 186812530 and TYC 802-751-1, TOI-503 is an A-type star with 1.8 times the Sun’s mass and a radius that’s 1.7 times larger than the Sun.

The star has a surface temperature of 13,311 degrees Fahrenheit (7,377 degrees Celsius) and is just 180 million years old.

“We argue that this brown dwarf formed in-situ, based on the young age of the system and the long circularization timescale for this brown dwarf around its host star,” said lead author Dr. Jan Subjak and colleagues.

“TOI-503b joins a growing number of known short-period, intermediate-mass brown dwarfs orbiting main sequence stars, and is the second such brown dwarf known to transit an A star, after HATS-70b.”

“With the growth in the population in this regime, the driest region in the brown dwarf desert (35-55 Jupiter masses) is reforesting and its mass range shrinking.”

The team’s paper will be published in the Astronomical Journal.

Ján Šubjak et al. 2019. TOI-503: The first known brown dwarf-Am star binary from the TESS mission. AJ, in press arXiv: 1909.07984