We know all too little about planetary-mass objects — planemos, as Centauri Dreams is learning to call them — that are not associated with a star. But we’ve learned a bit more with the discovery of an unusual binary system. Discovered in optical imagery taken by the European Southern Observatory’s 3.5 meter instrument in La Silla (Chile), Oph162225-240515 (Oph1622 for short) is a 14-Jupiter mass planemo apparently orbited by a companion of about half that mass.
“This is a truly remarkable pair of twins – each weighing some hundred times less than our sun,” says Ray Jayawardhana, an associate professor of astronomy and astrophysics at the University of Toronto. “Their mere existence is a surprise, and their origin and fate a bit of a mystery.”
Following up the find with optical spectra and infrared work, Jayawardhana and ESO’s Valentin Ivanov established that both members of the pair are at the same distance from the Sun and far too cool to be stars. They’re also young, perhaps a million years old, and separated by about six times the Pluto/Sun distance. Their place of birth is the Ophiuchus star-forming region some 400 light years away.
Centauri Dreams‘ take: This discovery, announced in the August 3 Science Express (abstract here), again puts the investigation of binaries into sharp focus. Evidently half of all Sun-like stars are binary, and current estimates are that about one sixth of all brown dwarfs come in pairs. These planemos are far below the brown dwarf ‘cut-off,’ which at 75 Jupiter masses is the level below which an object cannot sustain nuclear fusion. They are, in fact, the same mass as many of the extrasolar planets detected thus far, but they circle no nearby star.
Do free-floating planetary mass objects commonly occur as binaries? The difficulty in detecting them means we have no real idea, but the answer could tell us much about how such objects form. The current thinking is that this pair emerged from a contracting gas cloud that fragmented, creating a tiny version of a stellar binary. The weak gravitational interaction between the two members of Oph1622 puts constraints on other possibilities, including ejection from a developing multi-star system.