Newly discovered celestial object defies categories

This is an image of the ROXs 42B system obtained with the Keck telescope. 

The star is located in the center of the masked region. 

ROXs 42Bb orbits at about 150 astronomical units (AU). 

(1 AU=the distance from Earth to the Sun.) 

The other object ("c") is a likely unrelated background star. 

Credit: Thayne Currie

An object discovered by astrophysicists at the University of Toronto (U of T) nearly 500 light years away from the Sun may challenge traditional understandings about how planets and stars form.

The object is located near and likely orbiting a very young star about 440 light years away from the Sun, and is leading astrophysicists to believe that there is not an easy-to-define line between what is and is not a planet.

Thayne Currie

"We have very detailed measurements of this object spanning seven years, even a spectrum revealing its gravity, temperature, and molecular composition. Still, we can't yet determine whether it is a planet or a failed star – what we call a 'brown dwarf'. Depending on what measurement you consider, the answer could be either," said Thayne Currie, a post-doctoral fellow in U of T's Department of Astronomy & Astrophysics and lead author of a report on the discovery published this week in Astrophysical Journal Letters.

Named ROXs 42Bb for it's proximity to the star ROXs 42B, the object is approximately nine times the mass of Jupiter, below the limit most astronomers use to separate planets from brown dwarfs, which are more massive.

However, it is located 30 times further away from the star than Jupiter is from the Sun.

"This situation is a little bit different than deciding if Pluto is a planet. For Pluto, it is whether an object of such low mass amongst a group of similar objects is a planet," said Currie.

"Here, it is whether an object so massive yet so far from its host star is a planet. If so, how did it form?"

Most astronomers believe that gas giant planets like Jupiter and Saturn formed by core accretion, whereby the planets form from a solid core that then accretes a massive gaseous envelope.

Core accretion operates most efficiently closer to the parent star due to the length of time required to first form the core.

An alternate theory proposed for forming gas giant planets is disk instability – a process by which a fragment of a disk gas surrounding a young star directly collapses under its own gravity into a planet. This mechanism works best farther away from the parent star.

More information: The discovery is reported in a study titled "Direct imaging and spectroscopy of a candidate companion below/near the deuterium-burning limit in the young binary star system, ROXs 42B" which can also be viewed on arXiv.org at arxiv.org/abs/1310.4825.

Currie will present these and other findings at the annual meeting of the American Astronomical Society in Washington, DC this week.