Exoplanet caught masquerading as brown dwarf

What was previously thought to be a brown dwarf has been discovered to be an exoplanet lying on the cusp between the two. The exoplanet could help in the study of exoplanet atmospheres.


Brown dwarfs share properties of both planets and stars, and so act as a bridge between the two in our knowledge.
Credit: NASA/JPL, slightly modified by Jonathan Gagné

The closest brown dwarf to Earth has been unmasked as a free-floating exoplanet. It’s thought that the object bridges the gap between brown dwarf and planet, and could be useful to the study of atmospheres around gas giants.

Brown dwarfs are larger than planets, but still too small to sustain the hydrogen fusion which fuels stars. Instead, after formation brown dwarfs slowly cool and contract over time.

"This means that the temperatures of brown dwarfs can range from as hot as stars to as cool as planets, depending on how old they are," says Jackie Faherty, from the American Museum of Natural History, who helped make the discovery.

The team used the temperature of an object previously thought to be a brown dwarf, known as SIMP J013656.5+093347, in a group of stars called Carina-Near to help determine the planet’s mass. As all stars in the group are 200 million years-old, the researchers knew the object’s age and could use this along with the temperature to find its mass.

They discovered SIMP0136 was around 13 times the mass of Jupiter, putting it right on the boundary that separates brown dwarf from planet.

Read more about brown dwarfs in BBC Sky at Night Magazine:

The find could be a boon to another area of exoplanet research – planetary atmospheres.

"The implication that the well-known SIMP0136 is actually more planet-like than we previously thought will help us to better understand the atmospheres of giant planets and how they evolve," says Johnathan Gagné from the Insitute of Research on Exoplanets at Université de Montréal and who led the discovery.

Studying atmospheres is challenging as usually the infrared radiation emitted by a planet is overwhelmed by the brightness of the host star.

Free-floating planets, such as SIMP0136, do not have a nearby star, and so do not blind instruments. Finding such objects is challenging however, as they could be anywhere on the sky and are often confused for other objects, as happened here.

The discovery also sheds a new light on previous measurements taken of SIMP0136.

"This newest addition to the very select club of free-floating planetary like objects is particularly remarkable, because we had already detected fast-evolving weather patterns on the surface of SIMP0136, back when we thought it was a brown dwarf,” says Étienne Artigau, who led the original discovery the object in 2006.


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