A closer look at the motions of a small star and a misfit planet shows the two are linked by gravity - and can teach us about planets and stars alike

An artist’s impression of 2MASS J2126 University of Hertfordshire / Neil Cook

It was almost a missed connection. A red dwarf star and a loner giant planet share the same patch of sky, but no one had linked the two. Now a closer look at their motions suggests the planet isn’t so lonely – it just orbits its star at a larger orbital distance than we’ve ever seen before.

The small star, called TYC 9486, was first discovered in 2006, while its small neighbor 2MASS J2126 was spotted in 2008. In the years since their discovery, both objects have drifted in the same direction. That seemed suspicious to Niall Deacon of the University of Hertfordshire in Hatfield.

After taking new measurements of both objects, Deacon and his colleagues think they are travelling through space together. That would mean the two are tethered by gravity, with the planet orbiting a trillion kilometres away – 6900 times farther out from its small star than the Earth is from the sun . For perspective, the potential new “Planet Nine” proposed last week is at most 1200 times as far from the sun as Earth is.

The arrows show the projected movement of the star and planet on the sky over 1000 years. 2MASS/S. Murphy/ANU

The team estimates that 2MASS J2126 is roughly a dozen times heavier than Jupiter. That puts it on the edge between brown dwarfs – failed stars, too small to burn hydrogen – and the very biggest planets. “It’s not me saying yes, this a hundred percent definitely a planet,” Deacon says. “These things are a little fuzzy sometimes.”

But as a planet-brown dwarf hybrid, J2126 and its host star may have a lot to teach us, says Jonathan Gagné at the Carnegie Institute in Washington, DC. In previous work , Gagné had argued that J2126 belonged with a different group of stars, but he says that the new observations have convinced him that TYC 9486 is its closest kin.

The pair may reveal things about planets and brown dwarfs that neither could alone, Gagné says. J2126 is far enough from its star that its own brightness isn’t overwhelmed by starlight. But since they are linked, we know the overgrown planet probably formed at the same time as the star.

“This gravitational linking gives us the liberty to translate what we measure from one object to the other object,” he says. “I think it’s going to be one of these important benchmark systems in the future.”

Journal reference: Monthly Notices of the Royal Astronomical Society, in press