Poised between stars and planets

Two bodies difficult to categorize have been discovered: solitary giant planets or brown dwarfs extremely low mass? Both formed very recent – 12 million years ago, the other just 2 million years ago – and they are respectively 80 and 500 light-years from us. Studies out of ApJ and A & A.


One is wandering the constellation of Capricorn, the other wanders, just as introverted, for that of the Chameleon. They are the nightmare of taxonomists, the platypus astronomy. Young celestial orphan bodies of a star mother who roam the galaxy without identity. Too big to be planets, too small to be stars, their home is the frontier that strips of uncertainty that aspires to distinguish the gas giants from brown dwarfs. Baptized respectively PSO J318 -22 and OTS 44 – if they had friends, PSO and OTS – have just earned the pages of the Astrophysical Journal Letters the first and the second Astronomy & Astrophysics Letters.

Before drawing the ambiguous identikit of the two, it is worth summarizing what should be the specific traits of a star and of such a planet. A star is an object with sufficient mass to allow the trigger nuclear fusion of hydrogen atoms in its core. The brown dwarf – a class of objects not coincidentally nicknamed “failed stars ” – the mass is too small for this to happen . But it’s still big enough to kick off, although for a limited time and only in the surface layers of the star, in another kind of nuclear fusion: that of deuterium, or heavy hydrogen. In short, the key to everything is the mass. And what is the minimum mass so that primed the fusion of deuterium? About 13 times the mass of Jupiter, astronomers say. Below they remain planet. But as often happens when one moves along a continuum, the boundary lines tend to be arbitrary and a bit ‘ragged’, as he showed us the recent downgrading of Pluto.

Now that we have traced some coordinates, we return to our two young champions. PSO, which, with its 12 million years old is the teenager of the couple, has a mass of approximately 6 times that of Jupiter: well below the threshold, then. But its spectral signature – weak and extremely red-shifted – that is being discovered just looking brown dwarfs. “Never before have had we come across a similar object that was floating free in space. It is equal in all respects to the young planets that we see orbiting other stars, but that it is drifting on its own, “says Michael Liu of the University of Hawaii that led the team which studied PSO.”I have long wondered if similar could exist. We now know that the answer is yes. ”

If PSO is a teenager, OTS with its 2 million year-old is little more than an infant. But the stronger: its mass is about 12 times that of Jupiter. Enough to place it dangerously close to the boundary line between planet and a brown dwarf. Not only subjected to scrutiny by ESA’s Herschel space telescope and analyzed by the SINFONI spectrograph on ESO’s Very Large Telescope in Chile, OTS has proved to be wrapped in a disk of gas, just like a young star; gas, which still continues to increase in its mass, exactly as it would do for a star . “For anyone involved in star formation , to discover that the same processes are at work even for objects of planetary mass is crucial information ,” says Viki Joergens , a researcher at the Max Planck Institute for Astronomy and first author of the article on OTS .

So OTS can become bigger, and who knows how big will become. It is certain that the existing categories are beginning to show their limitations. To the point that the bodies of the border like these – lonely planets or brown mini dwarfs – there are others who, to be sure, coined the label free-floating planetary -mass objects: objects of planetary mass on leave.

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