A parental star has been found for a planet previously thought to be wandering unaccompanied through space. Dr. Niall Deacon and Dr. Simon Murphy have found that the object known as 2MASS J2126−8140 is not, in fact, an isolated body, but is instead accompanied by, and probably orbits, a small red dwarf. Nevertheless, they are still puzzled by the enormous distance between planet and star.

Eight years ago, 2MASS J2126−8140 (full name 2MASS J21265040−8140293) was discovered by an infrared survey seeking brown dwarfs – objects with masses between planets and stars. Shortly beforehand, a separate study noticed that the red dwarf star TYC 9486-927-1 is only 100 light-years from Earth – close by galactic standards.

The two objects, however, were so far apart that no one realized they might be connected. Now, in the Monthly Notices of the Royal Astronomical Society, Murphy and Deacon demonstrate the two objects are the same distance from us and are moving across the sky in the same direction and at a very similar pace.

^{TYC 9486-927-1 and 2MASS J2126. The arrows show their projected movement over 1,000 years. Light from the star takes about a month to travel to the planet. 2MASS/S. Murphy/ANU}
Murphy, of the Australian National University, told IFLScience that archival data on the locations of the two objects is not precise enough to confirm that they are definitely orbiting each other. Nevertheless, it appears this is the most distant spacing of a planet and star ever detected. The pair are 6,900 times further apart than the Earth and Sun, which is 0.1 light-years or 140 times the distance to Pluto. “The planet is not quite as lonely as we first thought, but it’s certainly in a very long distance relationship,” said University of Hertfordshire's Deacon in a statement.

The nearest star to the Sun, Proxima Centauri, orbits more than twice as far from Alpha Centauri A and B, at more than 0.2 light-years distance. Other binary stars are further still. However, Murphy told IFLScience that “Proxima Centauri, while a small star, is a lot larger than this planet, and is bound around two much larger stars. All the wide binaries we know have far more mass than this system.” If 2MASS J2126−8140 is orbiting TYC 9486-927-1, it is three times as distant as the next widest planet-star pair.

Nevertheless, Murphy says we have found five examples of planets hugely distant from stars, and we need to work out how this occurs. “There is no way it formed in the same way as our solar system did, from a large disc of dust and gas,” he said in a statement, adding to IFLScience that even the largest protoplanetary disks are a small fraction of this extent from their star.

It is possible that the planet was once in a more normal orbit before being disrupted, but Murphy thinks it more likely the two formed in the same filament of gas.

Lithium abundance, which declines with age, suggests TYC 9486-927-1 is 10 to 45 million years old. If 2MASS J2126−8140 is similar, our knowledge of planetary evolution indicates it must have 12 to 15 times the mass of Jupiter, right at the point where planets become brown dwarfs.