Nearby Brown Dwarf Appears to be a Free-Floating ‘Planet’

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An international team of astronomers that included astrophysicists at UC San Diego has discovered that one of the closest brown dwarfs to our Sun has the same mass as a giant planet.

However, because the object isn’t orbiting a star, the discovery challenges the very definition of a planet.

The analysis by the team—which was led by the Carnegie Institution for Science’s Jonathan Gagné and included researchers from the Institute for Research on Exoplanets (iREx) at Université de Montréal and the American Museum of Natural History (AMNH) —appears in a paper that will be published in The Astrophysical Journal Letters.

“It used to be obvious that planets were distinct from stars,” said Adam Burgasser, a professor of physics at UC San Diego and a co-author of the paper. “But this object and others are ‘breaking’ these definitions.”

The object in question is SIMP J013656.5+093347, or SIMP0136 for short, a well-studied brown dwarf only 21 light-years from the Sun in the constellation Pisces.

Smaller than stars, but bigger than giant planets, brown dwarfs are too small and low-mass to sustain the hydrogen fusion process that fuels stars like the Sun and allows them to remain hot and bright for a long time. Instead, after a period of early contraction, brown dwarfs simply cool off over their long lifetimes.

“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,” said the AMNH’s Jacqueline Faherty, a co-author of the discovery.

The team determined that SIMP0136 is a planetary-mass—and planetary-like—member of a 200-million-year-old group of stars called Carina-Near. Groups of similarly aged stars moving together through space are prime targets in searches for free-floating planets, because they provide one of the only means of age-dating these cold and isolated worlds. Knowing the age and temperature of a brown dwarf enables astronomers to determine its mass.

Gagné and the research team were able to demonstrate that SIMP0136 has a mass of about 13 times that of Jupiter, right at the boundary between brown dwarfs and giant planets. This boundary is set by the short-lived fusion of deuterium in the cores of brown dwarfs.

Free-floating planetary-mass objects are valuable because they are similar to the traditional gas giant exoplanets that orbit stars, like our own Solar System’s Jupiter or Saturn. However, free-floating planets are easier to study because their dim light isn’t overwhelmed by the brightness of their host stars, which blinds the instruments that astronomers use to characterize an exoplanet’s atmosphere.

“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,” Gagné said.

Free-floating worlds are hard to find, because they can be located anywhere in the sky and are very hard to tell apart from brown dwarfs or very small stars. For this reason, researchers have confirmed only a handful of free-floating planetary-mass objects so far.

Étienne Artigau, a co-author on the study who lead the original discovery of SIMP0136, added: “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.”

In a field where analyzing exoplanet atmospheres is of the utmost interest, evidence of weather patterns on an easier-to-observe free-floating object is an exciting realization.

“This discovery highlights how much there remains to be learned about the stars, brown dwarfs and exoplanets in the immediate vicinity of the Sun,” said Burgasser, whose previous work led to the discovery of the “T spectral class” of brown dwarfs, of which SIMP0136 is a member. “To find that one of our nearest neighbors is at this fuzzy boundary between stars and planets suggests many more of these remarkable sources may be out there.”

Other members of the research team were Daniella Bardalez Gagliuffi of UC San Diego, and Sandie Bouchard, Loïc Albert, David LaFrenière and René Doyon of iREx.