Is it star? No. Is it a planet? No. It’s a planemo-and it comes with a twin!
Astronomers have discovered a pair of planemos (short for planetary mass objects) that circle not around a star but around each other. The existence of such a twin system is surprising the astronomy world and challenging the current understanding of how stars and planets are formed.
“The same kind of process that forms stars like the sun seems to be somehow able to form objects that are [a] hundred times lower [in] mass, and form them as a binary,” explained professor Ray Jayawardhana of the department of astronomy and astrophysics at U of T, who reported this new discovery with Dr. Valentin D. Ivanov of the European Southern Observatory (ESO) in the journal Science.
Not quite a star but not quite a planet, planemos are strange members in the family of celestial objects, sitting between low mass stars and giant planets like Jupiter. Planemos are big in size, roughly a third the radius of the sun when planemos are young, although they have very low masses-only one per cent the mass of the sun. Like the sun, however, planemos do shine.
“[Planemos] are very faint, but when they are young they are still hot, because they are still contracting. They shine because they convert gravitational energy into heat,” explained Jayawardhana.
Half of all stars come in pairs, dubbed a binary star, and one sixth of brown dwarfs-“failed stars” whose mass is too low to sustain hydrogen fusion found in stars but can still fuse deuterium, a heavier form of hydrogen-also come in pairs.
Where do planemos fit in? A few dozen single planemos, which are too low in mass to fuse deuterium, have been discovered by scientists since 2000. Oph1622 is the first planemo found to have a companion.
What is fascinating about the discovery of this pair is that they were found with no stars nearby, very much unlike traditional planets which orbit around a star. Furthermore, they are found in binary form-much like many stars.
Stars form from clouds of gas and dust that collapse as gravity pulls them together. Over millions of years, this spinning cloud reaches a high enough density and temperature in the core to initiate hydrogen fusion, or nuclear fusion, which makes a star shine. The gas and dust that is left in a disc around newborn stars may collide and accumulate to form large chunks and eventually give rise to planets, or other smaller objects such as asteroids and comets.
Jayawardhana speculates the newly discovered planemo twin may have originated much like stars, forming “out of a contracting gas cloud that fragmented, like a miniature stellar binary.”
Jayawardhana and Ivanov estimated the planemo companion to be about seven times the mass of Jupiter whereas Oph1622 is about 14 times the mass of Jupiter. Separated by 35 billion kilometers, or approximately six times the distance between Pluto and the Sun, the binary pair is located in the Ophiuchus star-forming region about 400 light years away.
The scientists were able to determine the pair is still young-barely a million years old, and too cold to be stars.
“Recent discoveries have revealed an amazing diversity of worlds out there,” said Jayawardhana. “Still, the Oph1622 pair stands out as one of the most intriguing, if not peculiar.”
