Hi,
As we keep discovering extra-solar planets, it seems like
they're in some sort of contest to be stranger and less-expected
than the last strange world system discovered. Now it's two planets
that orbit around each other -- no star, just two planets, all on their
own, and not orbiting closely either, but more than 200 AU apart.
OK, a multi-planet system without a star... What's next?
Sterling K. Webb
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<http://www.sciencedaily.com/releases/2006/08/060804084105.htm>
Baby 'Planemos' Can Be Born As Twins
A team led by a University of Toronto astronomy professor is challenging
an existing theoretical model and thrilling the astronomy community with its
discovery of a seven-Jupiter-mass companion next to a planemo, or planetary
mass object, only twice as heavy. Both objects have masses similar to those
of extra-solar giant planets, usually found in orbit around a star.
Unexpectedly, these bodies appear to circle each other.
"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."
Jayawardhana and Valentin D. Ivanov of the European Southern Observatory
(Chile) report the discovery in the August 3 issue of Science Express, the
rapid online publication service of the journal Science.
The researchers discovered the companion candidate in an optical image
taken with the European Southern Observatory's 3.5-meter New Technology
Telescope on La Silla, Chile, and investigated it further with optical
spectra and infrared images obtained with ESO's 8.2-meter Very Large
Telescope on Paranal, Chile.
These followup observations confirmed that both objects are young, at
the same distance, and much too cool to be stars. By comparing the companion
to widely used theoretical models, Jayawardhana and Ivanov estimate that it
weighs about seven times as much as Jupiter, while the primary planemo is an
estimated 14 times Jupiter's mass. The newborn pair, barely a million years
old, are separated by about six times the distance between the sun and
Pluto, and are located in the Ophiuchus star-forming region approximately
400 light years away.
"Roughly half of all sun-like stars, and about a sixth of brown dwarfs,
come in pairs," says Jayawardhana. Brown dwarfs are 'failed stars' that
weigh less than 75 Jupiter masses and are unable to sustain nuclear fusion.
"Oph 162225-240515, or Oph1622 for short, is the first planemo to be
resolved into a double."
The existence of this wide pair poses a challenge to a popular theory
which suggests that brown dwarfs and planemos are embryos ejected from
multiple proto-star systems. Since the two objects in Oph1622 are so far
apart, and only weakly bound to each other by gravity, they would not have
survived such a chaotic birth.
Planets are thought to form out of disks of gas and dust that surround
stars, brown dwarfs and even some planemos. The researchers think that these
planemo twins formed together out of a contracting gas cloud that
fragmented, like a miniature stellar binary.
"We are resisting the temptation to call it a 'double planet' because
this pair probably didn't form the way that planets in our solar system
did," says Ivanov. "Now we're curious to find out whether such pairs are
common or rare. The answer could shed light on how free-floating
planetary-mass objects form."
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