Little star or big planet? Dim brown dwarfs illuminate the difference

Is it time for a stellar identity crisis? Astronomers studying cold brown dwarfs have found that the boundary between star and planet might be much blurrier than once thought.

These strange ‘failed’ stars, described online in the journal Science, could help shed light on the atmospheres of distant exoplanets.

Scientists have managed to find out a lot about stars and their atmospheres by studying their light, including their temperatures and their distances. But that’s easier said than done for exoplanets: The operating ground rules are very different between a massive, hot, bright star and a very small, cool, dark planet. So these brown dwarfs, which sit on the scale between normal stars and gas-giant planets, can bridge the gap in scientists’ knowledge.

There could be a whole lot of them in the sky, too -- so many that some scientists suggested they could help partly explain the mysterious Universe-wide effects attributed to dark matter.

These brown dwarfs are essentially the D-listers of celestial Hollywood – their career goals to become stars never quite took off. That’s because these relative lightweights never became dense enough in their cores to start their hydrogen-fueled nuclear fusion engines.

Ultra-cool brown dwarfs, described in the last two years using NASA’s Wide-field Infrared Survey Explorer telescope, are the coldest of the bunch, and very dim and small. They generally sit right around the borderline between a tiny star and a really big gas giant – 13 Jupiter masses.

But brown dwarfs are easier to pick out in the sky than exoplanets, said study coauthor Adam Kraus, an astronomer at the University of Texas at Austin.

“Those planets are normally quite hard to study - not only are planets faint, but they’re right next to something very bright!” Kraus wrote in an email.

And given the number of properties the two share, studying these brown dwarfs and their atmospheres could help astronomers understand exoplanet atmospheres too.

But because brown dwarfs don’t shine the way that successful stars do, it’s hard to judge a lot of their characteristics, including one of the most basic – how far they are. So the researchers decided to directly measure the distances of 16 brown dwarfs with a different method, using NASA’s Spitzer Space Telescope as it trailed behind the Earth.

As Spitzer moves, it watches how the brown dwarfs ‘move’ in front of the background stars. It’s the same reason your finger seems to ‘jump’ against the background if you hold it up and look at it with one eye closed and then quickly switch to the other eye. Each eye has a slightly different perspective because it’s in a slightly different position on your face.

And just as your two eyes’ slightly different perspectives allow your brain to determine depth, the scientists used Spitzer’s snapshots of the same brown dwarf from different positions to calculate the failed star’s distance. It’s a painstaking but time-tested method known as the parallax method.

The researchers found that these failed stars were between 20 and 50 light years away. They also ended up being hotter than expected. Many of the ultra-cool brown dwarfs are thought to be room temperature, but the brown dwarfs surveyed here were around 260 to 350 degrees Fahrenheit.

“The coldest brown dwarfs (and by extension, the extrasolar planets that closely resemble them) have their appearance shaped by many properties, and not just by their temperature,” said Kraus, who co-wrote the report with Trent Dupuy of the Harvard-Smithsonian Center for Astrophysics. “We need to understand the influence of these other properties (such as chemical composition, cloud cover, and surface gravity) in order to interpret the many exoplanet discoveries that we expect to see from ongoing and future discovery programs.”

Perhaps the planet-star boundary needs to be redrawn – but not in terms of mass, Kraus added. Size should not matter as much. What should matter is how they formed.

“If an object formed in orbit around a star, out of the material left over from that star’s formation process, then it should be called a planet,” he said. “Objects that formed freely floating out in space (including these objects) should be regarded as the most extremely low-mass cases of the same process that forms stars, and hence should be called brown dwarfs.”

[For the record, Sept. 7, 10:26 a.m.: An earlier version of this story incorrectly indicated that NASA’s WISE satellite had discovered brown dwarfs in the last two years. It has discovered ultra-cool brown dwarfs, the coldest failed stars.]

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