A newly spotted pair of planet-sized objects floating far away from any star
has astronomers puzzling over how such a bizarre system could have formed.
Using the
European Southern Observatory's telescopes in Chile, astronomers have spotted a seven-Jupiter
mass object paired to another 14-Jupiter-mass companion. Instead of orbiting
around a star, however, the two planetary mass objects, or "planemos," are circling each other.
Planemos
are objects similar to brown dwarfs, failed stars too small to sustain the
nuclear reactions required for stellar ignition. But at only a few times more
massive than Jupiter, they resemble
planets more than stars.
The
finding, detailed in the August 4 issue of the journal Science, places
strong constraints on planemo formation theories and
could shed light on how the strange objects form.
A first
About half
of all Sun-like stars have stellar companions. About a sixth of all brown
dwarfs are also paired. But the new system, called Oph
162225-240515, or Oph1622 for short, is the only known planemo
binary.
"Recent
discoveries have revealed an amazing diversity of worlds out there. Still, the
Oph1622 pair stands out as one of the most intriguing, if not peculiar,"
said study team member Ray Jayawardhana of the
University of Toronto in Canada.
The pair is
barely a million years old and located in the Ophiuchus
star-forming region, about 400 light years away. The objects are separated by
about six times the distance between the Sun
and Pluto. Infrared observations by
NASA's Spitzer Space Telescope suggest that at least one, and possibly both, of
the planemos are ringed by a disk of gas and dust.
Light and
color analysis of the
planemos strongly suggests they were born at the
same time but scientists aren't sure how they formed. The answer to this latter
question could determine whether or not the objects are classified as planets.
Are they
planets?
If size
were the only criteria for being called a planet, then most of the few dozen planemos discovered thus far would certainly qualify.
Astronomers sometimes draw the cut off line for whether or not an object is a
planet at 13-Jupiter-masses. Objects more massive than this can burn a form of
hydrogen called deuterium for a short time early in their lives while those
below this limit cannot.
But some
scientists argue that planethood shouldn't be
bestowed based solely on mass, and that how an object was created should also
be considered.
Most known planets, including those in our solar system, form out of discs of
gas and dust surrounding
stars or brown dwarfs. As far as scientists can tell, the new planemo pair did not form this way.
Instead,
scientists think the twins formed the same way that many binary
star systems do, when a contracting gas cloud splits in two before
condensing into a stellar core.
"We
are resisting the temptation to call it a 'double planet' because this pair
probably didn't form in the way that planets in our solar system did,"
said ESO astronomer and study team member Valentin Ivanov.
A weak
link
According
to another theory, planemos are embryos ejected from
nascent stellar
nurseries. However, the two objects in Oph1622 are so far apart and so
weakly bound to each other by gravity that they would not have survived such an
ejection, the researchers say. Their connection is so tenuous, in fact, that a
passing star or brown dwarf could permanently separate the two objects.
The
researchers say the next step is to determine if binary planemos
are the rule or whether the new discovery is a rare exception.
"The
answer could shed light on how free-floating planetary mass objects form,"
Ivanov said.
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