Not one failed brown dwarf but two planets Trent Dupuy, William Best, Michael Liu/Keck Observatory

Finding massive planets is nothing new these days. But finding them orbiting each other instead of orbiting a star is unprecedented. An object initially thought to be a single brown dwarf is actually a pair of giant worlds. It’s not yet clear how this binary system formed, but the discovery may help redefine the line between planets and brown dwarfs – failed stars with tens of times the mass of Jupiter.

This pair of planets is made up of two balls of gas the size of Jupiter but almost four times more massive, separated by some 600 million kilometres, and slowly circling each other once per century or so. The young couple only emits light at infrared wavelengths, with residual heat from their formation, just 10 million years ago.

Observations with the 10-metre Keck II telescope, by a team led by William Best of the University of Hawaii, uncovered the binary system, with the help of adaptive optics that correct for the blurring effects of Earth’s atmosphere.


“This is a careful piece of work and a very nice discovery,” says David Latham of the Harvard-Smithsonian Center for Astrophysics.

Two planets, no sun

No one really understands the formation of rogue worlds that don’t orbit a star. So a binary system is even harder to understand, according to Gibor Basri of the University of California at Berkeley.

Gravitational interactions may slingshot single planets out of their solar systems, but the newly found pair of planets most likely formed from the fragmentation of a condensing protostar.

According to Alex de Koter of the University of Amsterdam, the discovery shows that various scenarios to produce free-floating planetary-mass objects are at work in the universe. Because they’re small and faint, they can only be discovered in our cosmic neighbourhood. This new find – called 2MASS J1119−1137 – is only 85 light years away, and the team thinks there may be many more similar planetary-mass binaries out there.

But are they really planets? Maybe not. In the past, the dividing line between planets and brown dwarfs was generally placed at 14 Jupiter masses, when nuclear fusion of deuterium in the object’s core sets in.

But Latham argues that the best way to distinguish between the two is not by their mass but by how they form: brown dwarfs result from collapsing clouds of gas and dust, while planets form out of a stellar disk.

‘The highest mass planets can be more massive than the lowest mass brown dwarfs,’ he says. ‘This binary is the nicest example that I know of for the overlap of planetary and brown dwarf masses.’

And if other brown dwarfs are similar – that is, if they’re not brown dwarfs at all but sneaky double bodies – we may have underestimated how many free-floating planets there are in our universe.

Journal Reference:

Journal reference: arxiv.org, DOI: 1706.01883