Brown dwarfs reveal exoplanets' secrets

Brown dwarfs reveal exoplanets' secrets

Brown dwarfs are smaller than stars, but more massive than giant planets. As such, they provide a natural link between astronomy and planetary science. However, they also show incredible variation when it comes to size, temperature, chemistry, and more, which makes them difficult to understand, too.

New work led by Carnegie's Jacqueline Faherty surveyed various properties of 152 suspected young in order to categorize their diversity and found that atmospheric properties may be behind much of their differences, a discovery that may apply to planets outside the solar system as well. The work is published by The Astrophysical Journal Supplement Series.

Scientists are very interested in brown dwarfs, which hold promise for explaining not just planetary evolution, but also stellar formation. These objects are tougher to spot than more-massive and brighter stars, but they vastly outnumber stars like our Sun. They represent the smallest and lightest objects that can form like stars do in the Galaxy so they are an important "book end" in Astronomy.

For the moment, data on brown dwarfs can be used as a stand-in for contemplating extrasolar worlds we hope to study with future instruments like the James Webb Space Telescope.

"Brown dwarfs are far easier to study than planets, because they aren't overwhelmed by the brightness of a host star," Faherty explained.

Brown dwarfs are smaller than stars, but more massive than giant planets. As such, they provide a natural link between astronomy and planetary science. However, they also show incredible variation when it comes to size, temperature, chemistry, and more, which makes them difficult to understand, too. New work surveyed various properties of 152 suspected young brown dwarfs in order to categorize their diversity and found that atmospheric properties may be behind much of their differences. Credit: Jacqueline Faherty and David Rodriguez.

But the tremendous diversity we see in the properties of the brown dwarf population means that there is still so much about them that remains unknown or poorly understood.

Brown dwarfs are too small to sustain the hydrogen fusion process that fuels stars, so after formation they slowly cool and contract over time and their surface gravity increases. This means that their temperatures can range from nearly as hot as a star to as cool as a planet, which is thought to influence their atmospheric conditions, too. What's more, their masses also range between star-like and giant planet-like and they demonstrate great diversity in age and .

By quantifying the observable properties of so many young brown dwarf candidates, Faherty and her team—including Carnegie's Jonathan Gagné and Alycia Weinberger—were able to show that these objects have vast diversity of color, spectral features, and more. Identifying the cause of this range was at the heart of Faherty's work. By locating the birth homes of many of the brown dwarfs, Faherty was able to eliminate age and chemical composition differences as the underlying reason for this great variation. This left atmospheric conditions—meaning weather phenomena or differences in cloud composition and structure—as the primary suspect for what drives the extreme differences between objects of similar origin.

All of the brown dwarf birthplaces identified in this work are regions also host exoplanets, so these same findings hold for orbiting nearby stars.

"I consider these young brown dwarfs to be siblings of giant exoplanets. As close family members, we can use them to investigate how the planetary aging process works," Faherty said.


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When it comes to brown dwarfs, 'how far?' is a key question

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Aug 15, 2016
"WISE data has revealed hundreds of new brown dwarfs. Of these, fourteen are classified as cool Ys .. Its temperature has since been revised and newer estimates put it in the range of 250 to 400 C (-23-127 ° C - 10-260 ° F).
In April 2014, WISE 0855-0714 was announced with a temperature profile estimated around 225 to 260 K .. "
https://en.wikipe..._class_Y

Earth 184 to 330 ° K, Venus 737 ° K and both of have a molten interior without radiation.

Aug 15, 2016
Well the upper mass limit on a brown dwarf is well established, what is the lower limit? Is it one of those arbitrary limits like for dwarf planets?

Aug 16, 2016
Brown color is related to body with very slow rotations and small bodies that are located in transition between "planet" and "classic" star.
If are an independent or away from the central body the word is only rotation and somewhat less mass. With increasing rotation or mass color changes to the red.
Brown dwarf mass may be greater than 50% of the stars of our galaxy.
The lower limit is less than Venus at faster, the central body and closer orbits (the effects of the binary system).
On the color body affected: weight, speed of rotation, the effects of a binary system and the mass and speed of rotation of the central body.

Aug 16, 2016
The lower mass limit for a brown dwarf is ~13 Jupiter masses because that is where deuterium fusion begins. The upper mass limit is ~75-80 Jupiter masses because that is where hydrogen fusion begins. https://en.wikipe...wn_dwarf

There are a couple other related properties of brown dwarfs that really sets them apart, i.e., density and gravity. For example, COROT-3b has a surface gravity about twice that of the sun, about 54 g, and a density greater than osmium. I recall reading somewhere that the most dense brown dwarfs can have surface gravity of over 100 g.

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