Astronomers Explain the Varying Glow of Brown Dwarfs
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Astronomers Explain the Varying Glow of Brown Dwarfs
AUGUST 18, 2017
This artist’s concept shows a brown dwarf with bands of clouds, thought to resemble those seen at Neptune and the other outer planets. Credits: NASA/JPL-Caltech
Using data from NASA’s Spitzer Space Telescope, astronomers reveal that the varying glow of brown dwarfs over time can be explained by bands of patchy clouds rotating at different speeds.
Dim objects called brown dwarfs, less massive than the Sun but more massive than Jupiter, have powerful winds and clouds — specifically, hot patchy clouds made of iron droplets and silicate dust. Scientists recently realized these giant clouds can move and thicken or thin surprisingly rapidly, in less than an Earth day, but did not understand why.
Now, researchers have a new model for explaining how clouds move and change shape in brown dwarfs, using insights from NASA’s Spitzer Space Telescope. Giant waves cause large-scale movement of particles in brown dwarfs’ atmospheres, changing the thickness of the silicate clouds, researchers report in the journal Science. The study also suggests these clouds are organized in bands confined to different latitudes, traveling with different speeds in different bands.
“This is the first time we have seen atmospheric bands and waves in brown dwarfs,” said lead author Daniel Apai, associate professor of astronomy and planetary sciences at the University of Arizona in Tucson.
Just as in Earth’s ocean, different types of waves can form in planetary atmospheres. For example, in Earth’s atmosphere, very long waves mix cold air from the polar regions to mid-latitudes, which often lead clouds to form or dissipate.
The distribution and motions of the clouds on brown dwarfs in this study are more similar to those seen on Jupiter, Saturn, Uranus and Neptune. Neptune has cloud structures that follow banded paths too, but its clouds are made of ice. Observations of Neptune from NASA’s Kepler spacecraft, operating in its K2 mission, were important in this comparison between the planet and brown dwarfs.
“The atmospheric winds of brown dwarfs seem to be more like Jupiter’s familiar regular pattern of belts and zones than the chaotic atmospheric boiling seen on the Sun and many other stars,” said study co-author Mark Marley at NASA’s Ames Research Center in California’s Silicon Valley.
Brown dwarfs can be thought of as failed stars because they are too small to fuse chemical elements in their cores. They can also be thought of as “super planets” because they are more massive than Jupiter, yet have roughly the same diameter. Like gas giant planets, brown dwarfs are mostly made of hydrogen and helium, but they are often found apart from any planetary systems. In a 2014 study using Spitzer, scientists found that brown dwarfs commonly have atmospheric storms.
Due to their similarity to giant exoplanets, brown dwarfs are windows into planetary systems beyond our own. It is easier to study brown dwarfs than planets because they often do not have a bright host star that obscures them.
“It is likely the banded structure and large atmospheric waves we found in brown dwarfs will also be common in giant exoplanets,” Apai said.
Using Spitzer, scientists monitored brightness changes in six brown dwarfs over more than a year, observing each of them rotate 32 times. As a brown dwarf rotates, its clouds move in and out of the hemisphere seen by the telescope, causing changes in the brightness of the brown dwarf. Scientists then analyzed these brightness variations to explore how silicate clouds are distributed in the brown dwarfs.
Researchers had been expecting these brown dwarfs to have elliptical storms resembling Jupiter’s Great Red Spot, caused by high-pressure zones. The Great Red Spot has been present in Jupiter for hundreds of years and changes very slowly: Such “spots” could not explain the rapid changes in brightness that scientists saw while observing these brown dwarfs. The brightness levels of the brown dwarfs varied markedly just over the course of an Earth day.
To make sense of the ups and downs of brightness, scientists had to rethink their assumptions about what was going on in the brown dwarf atmospheres. The best model to explain the variations involves large waves, propagating through the atmosphere with different periods. These waves would make the cloud structures rotate with different speeds in different bands.
University of Arizona researcher Theodora Karalidi used a supercomputer and a new computer algorithm to create maps of how clouds travel on these brown dwarfs.
“When the peaks of the two waves are offset, over the course of the day there are two points of maximum brightness,” Karalidi said. “When the waves are in sync, you get one large peak, making the brown dwarf twice as bright as with a single wave.”
The results explain the puzzling behavior and brightness changes that researchers previously saw. The next step is to try to better understand what causes the waves that drive cloud behavior.
Publication: D. Apai, et al., “Zones, spots, and planetary-scale waves beating in brown dwarf atmospheres,” Science 18 Aug 2017: Vol. 357, Issue 6352, pp. 683-687; DOI: 10.1126/science.aam9848
Source: Elizabeth Landau, Jet Propulsion Laboratory
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Hubble & Spitzer See Weather Patterns in a Brown Dwarf
JANUARY 9, 2013
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. Image credit: NASA/JPL-Caltech
By using the Hubble Space Telescope and Spitzer Space Telescope to simultaneously view the brown dwarf planet 2MASSJ22282889-431026, a team of astronomers has created a detailed “weather map” and a technique to better understand the atmospheres of planets beyond our solar system.
Pasadena, California — Astronomers using NASA’s Spitzer and Hubble space telescopes have probed the stormy atmosphere of a brown dwarf, creating the most detailed “weather map” yet for this class of cool, star-like orbs. The forecast shows wind-driven, planet-sized clouds enshrouding these strange worlds.
This artist’s illustration shows the atmosphere of a brown dwarf called 2MASSJ22282889-431026, which was observed simultaneously by NASA’s Spitzer and Hubble space telescopes. Image credit: NASA/JPL-Caltech
This graph shows the brightness variations of the brown dwarf named 2MASSJ22282889-431026 measured simultaneously by both NASA’s Hubble and Spitzer space telescopes. As the object rotates every 1.4 hours, its emitted light periodically brightens and dims. Surprisingly, the timing, or phase, of the variations in brightness changes when measured at different wavelengths of infrared light. Spitzer and Hubble’s wavelengths probe different layers in the atmosphere of the brown dwarf. The phase shifts indicate complex clouds or weather patterns that change with altitude. Image credit: NASA/JPL-Caltech/University of Arizona
Brown dwarfs form out of condensing gas, as stars do, but lack the mass to fuse hydrogen atoms and produce energy. Instead, these objects, which some call failed stars, are more similar to gas planets with their complex, varied atmospheres. The new research is a stepping-stone toward a better understanding not only of brown dwarfs, but also of the atmospheres of planets beyond our solar system.
“With Hubble and Spitzer, we were able to look at different atmospheric layers of a brown dwarf, similar to the way doctors use medical imaging techniques to study the different tissues in your body,” said Daniel Apai, the principal investigator of the research at the University of Arizona in Tucson, who presented the results at the American Astronomical Society meeting Tuesday in Long Beach, California.
A study describing the results, led by Esther Buenzli, also of the University of Arizona, is published in the Astrophysical Journal Letters.
The researchers turned Hubble and Spitzer simultaneously toward a brown dwarf with the long name of 2MASSJ22282889-431026. They found that its light varied in time, brightening and dimming about every 90 minutes as the body rotated. But more surprising, the team also found the timing of this change in brightness depended on whether they looked using different wavelengths of infrared light.
These variations are the result of different layers or patches of material swirling around the brown dwarf in windy storms as large as Earth itself. Spitzer and Hubble see different atmospheric layers because certain infrared wavelengths are blocked by vapors of water and methane high up, while other infrared wavelengths emerge from much deeper layers.
“Unlike the water clouds of Earth or the ammonia clouds of Jupiter, clouds on brown dwarfs are composed of hot grains of sand, liquid drops of iron, and other exotic compounds,” said Mark Marley, research scientist at NASA’s Ames Research Center in Moffett Field, California, and co-author of the paper. “So this large atmospheric disturbance found by Spitzer and Hubble gives a new meaning to the concept of extreme weather.”
Buenzli says this is the first time researchers can probe variability at several different altitudes at the same time in the atmosphere of a brown dwarf. “Although brown dwarfs are cool relative to other stars, they are actually hot by earthly standards. This particular object is about 1,100 to 1,300 degrees Fahrenheit (600 to 700 degrees Celsius),” Buenzli said.
“What we see here is evidence for massive, organized cloud systems, perhaps akin to giant versions of the Great Red Spot on Jupiter,” said Adam Showman, a theorist at the University of Arizona involved in the research. “These out-of-sync light variations provide a fingerprint of how the brown dwarf’s weather systems stack up vertically. The data suggest regions on the brown dwarf where the weather is cloudy and rich in silicate vapor deep in the atmosphere coincide with balmier, drier conditions at higher altitudes — and vice versa.”
Researchers plan to look at the atmospheres of dozens of additional nearby brown dwarfs using Spitzer and Hubble.
“From studies such as this we will learn much about this important class of objects, whose mass falls between that of stars and Jupiter-sized planets,” said Glenn Wahlgren, Spitzer program scientist at NASA Headquarters in Washington. “This technique will see extensive use when we are able to image individual exoplanets.”
Source: Whitney Clavin, Jet Propulsion Laboratory; NASA
Images: NASA/JPL-Caltech; NASA/JPL-Caltech/University of Arizona
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Spitzer Discovers Turbulent Weather on Brown Dwarfs
JANUARY 8, 2014
This artist’s concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. Image Credit: NASA/JPL-Caltech/University of Western Ontario/Stony Brook U
Using NASA’s Spitzer Space Telescope, astronomers have discovered that many brown dwarfs have turbulent weather similar to Jupiter’s “Great Red Spot.”
Swirling, stormy clouds may be ever-present on cool celestial orbs called brown dwarfs. New observations from NASA’s Spitzer Space Telescope suggest that most brown dwarfs are roiling with one or more planet-size storms akin to Jupiter’s “Great Red Spot.”
“As the brown dwarfs spin on their axis, the alternation of what we think are cloud-free and cloudy regions produces a periodic brightness variation that we can observe,” said Stanimir Metchev of the University of Western Ontario, Canada. “These are signs of patchiness in the cloud cover.”
Metchev is principal investigator of the brown dwarf research. The results were presented at a news conference today at the 223rd annual meeting of the American Astronomical Society in Washington by Metchev’s colleague, Aren Heinze, of Stony Brook University, New York.
Brown dwarfs form as stars do, but lack the mass to fuse atoms continually and blossom into full-fledged stars. They are, in some ways, the massive kin to Jupiter.
Scientists think that the cloudy regions on brown dwarfs take the form of torrential storms, accompanied by winds and, possibly, lightning more violent than that at Jupiter or any other planet in our solar system. However, the brown dwarfs studied so far are too hot for water rain; instead, astronomers believe the rain in these storms, like the clouds themselves, is made of hot sand, molten iron or salts.
In a Spitzer program named “Weather on Other Worlds,” astronomers used the infrared space telescope to watch 44 brown dwarfs as they rotated on their axis for up to 20 hours. Previous results had suggested that some brown dwarfs have turbulent weather, so the scientists had expected to see a small fraction vary in brightness over time. However, to their surprise, half of the brown dwarfs showed the variations. When you take into account that half of the objects would be oriented in such a way that their storms would be either hidden or always in view and unchanging, the results indicate that most, if not all, brown dwarfs are racked by storms.
“We needed Spitzer to do this,” said Metchev. “Spitzer is in space, above the thermal glow of the Earth’s atmosphere, and it has the sensitivity required to see variations in the brown dwarfs’ brightness.”
The results led to another surprise as well. Some of the brown dwarfs rotated much more slowly than any previously measured, a finding that could not have been possible without Spitzer’s long, uninterrupted observations from space. Astronomers had thought that brown dwarfs sped up to very fast rotations when they formed and contracted, and that this rotation didn’t wind down with age.
“We don’t yet know why these particular brown dwarfs spin so slowly, but several interesting possibilities exist,” said Heinze. “A brown dwarf that rotates slowly may have formed in an unusual way — or it may even have been slowed down by the gravity of a yet-undiscovered planet in a close orbit around it.”
The work may lead to a better understanding of not just brown dwarfs but their “little brothers”: the gas-giant planets. Researchers say that studying the weather on brown dwarfs will open new windows onto weather on planets outside our solar system, which are harder to study under the glare of their stars. Brown dwarfs are weather laboratories for planets, and, according to the new results, those laboratories are everywhere.
Other researchers on the team include: Daniel Apai and Davin Flateau of the University of Arizona, Tucson; Mark Marley of NASA Ames Research Center, Moffett Field; Jacqueline Radigan of the Space Telescope Science Institute, Baltimore, Maryland; Etienne Artigau of Universite de Montreal, Canada; Adam Burgasser of University of California San Diego; Peter Plavchan of NASA’s Exoplanet Science Institute at the California Institute of Technology, Pasadena; and Bertrand Goldman of Max-Planck Institute for Astronomy, Germany.
NASA’s Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech.
Source: Whitney Clavin, Jet Propulsion Laboratory; NASA
Image: NASA/JPL-Caltech/University of Western Ontario/Stony Brook U
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ESO’s VLT Charts First Map of Weather on the Surface of a Brown Dwarf
JANUARY 30, 2014
ESO’s Very Large Telescope has been used to create the first ever map of the weather on the surface of the nearest brown dwarf to Earth. An international team has made a chart of the dark and light features on WISE J104915.57-531906.1B, which is informally known as Luhman 16B and is one of two recently discovered brown dwarfs forming a pair only six light-years from the Sun. The figure shows the object at six equally spaced times as it rotates once on its axis. Credit: ESO/I. Crossfield
ESO’s Very Large Telescope has charted the first ever map of the weather on the surface of a Brown Dwarf.
An international team has made a chart of the dark and light features on WISE J104915.57-531906.1B, which is informally known as Luhman 16B and is one of two recently discovered brown dwarfs forming a pair only six light-years from the Sun. The new results are published in the 30 January 2014 issue of the journal Nature.
Brown dwarfs fill the gap between giant gas planets, such as Jupiter and Saturn, and faint cool stars. They do not contain enough mass to initiate nuclear fusion in their cores and can only glow feebly at infrared wavelengths of light. The first confirmed brown dwarf was only found twenty years ago and only a few hundred of these elusive objects are known.
This video shows the first ever map of the weather on the surface of the nearest brown dwarf to Earth. An international team has used ESO’s Very Large Telescope to create a chart of the dark and light features on WISE J104915.57-531906.1B, which is informally known as Luhman 16B and is one of two recently discovered brown dwarfs forming a pair only six light-years from the Sun. This video shows the object as it rotates on its axis. Credit: ESO/I. Crossfield
The closest brown dwarfs to the Solar System form a pair called Luhman 16AB [1] that lies just six light-years from Earth in the southern constellation of Vela (The Sail). This pair is the third closest system to the Earth, after Alpha Centauri and Barnard’s Star, but it was only discovered in early 2013. The fainter component, Luhman 16B, had already been found to be changing slightly in brightness every few hours as it rotated — a clue that it might have marked surface features.
Now astronomers have used the power of ESO’s Very Large Telescope (VLT) not just to image these brown dwarfs, but to map out dark and light features on the surface of Luhman 16B.
Ian Crossfield (Max Planck Institute for Astronomy, Heidelberg, Germany), the lead author of the new paper, sums up the results: “Previous observations suggested that brown dwarfs might have mottled surfaces, but now we can actually map them. Soon, we will be able to watch cloud patterns form, evolve, and dissipate on this brown dwarf — eventually, exometeorologists may be able to predict whether a visitor to Luhman 16B could expect clear or cloudy skies.”
This video sequence is an artist’s impression based on the first ever map of the weather on the surface of the nearest brown dwarf to Earth. An international team used ESO’s Very Large Telescope to make a chart of the dark and light features on WISE J104915.57-531906.1B, which is informally known as Luhman 16B and is one of two recently discovered brown dwarfs forming a pair only six light-years from the Sun. Luhman 16A appears in the background in this sequence. Note that the faint fine detail on the surface of Luhman 16B has been added for artistic effect. Credit: ESO/I. Crossfield/N. Risinge
To map the surface the astronomers used a clever technique. They observed the brown dwarfs using the CRIRES instrument on the VLT. This allowed them not just to see the changing brightness as Luhman 16B rotated, but also to see whether dark and light features were moving away from, or towards the observer. By combining all this information they could recreate a map of the dark and light patches of the surface.
The atmospheres of brown dwarfs are very similar to those of hot gas giant exoplanets, so by studying comparatively easy-to-observe brown dwarfs [2] astronomers can also learn more about the atmospheres of young, giant planets — many of which will be found in the near future with the new SPHERE instrument that will be installed on the VLT in 2014.
Crossfield ends on a personal note: “Our brown dwarf map helps bring us one step closer to the goal of understanding weather patterns in other solar systems. From an early age I was brought up to appreciate the beauty and utility of maps. It’s exciting that we’re starting to map objects out beyond the Solar System!”
Notes
[1] This pair was discovered by the American astronomer Kevin Luhman on images from the WISE infrared survey satellite. It is formally known as WISE J104915.57-531906.1, but a shorter form was suggested as being much more convenient. As Luhman had already discovered fifteen double stars the name Luhman 16 was adopted. Following the usual conventions for naming double stars, Luhman 16A is the brighter of the two components, the secondary is named Luhman 16B and the pair is referred to as Luhman 16AB.
[2] Hot Jupiter exoplanets lie very close to their parent stars, which are much brighter. This makes it almost impossible to observe the faint glow from the planet, which is swamped by starlight. But in the case of brown dwarfs there is nothing to overwhelm the dim glow from the object itself, so it is much easier to make sensitive measurements.
Publication: I. J. M. Crossfield, et al., “A global cloud map of the nearest known brown dwarf,” Nature 505, 654–656 (30 January 2014); doi:10.1038/nature12955
Source: European Southern Observatory
Image: ESO/I. Crossfield