The Hubble
Space Telescope has spied one of the smallest objects ever detected around a
normal star. The object further blurs the line between stars and planets and raises new questions about
how planets should be defined outside our solar system.
Announcement
of the discovery comes two weeks after the International Astronomical Union
(IAU) approved the first official definition of "planet" for our
solar system and downgraded
Pluto to dwarf
planet status.
The newly
spotted object is a companion to CHXR
73, a low-mass red
dwarf star located 500 light-years from Earth, and is itself called CHXR
73 B.
With 12
times the mass of Jupiter, CHXR 73
B straddles the line between the largest planets and the smallest stars. The
latter, called brown
dwarfs or "failed
stars," don't have enough mass to sustain the types of thermonuclear
reactions that keep larger stars alight for billions of years.
The finding
will be detailed in the Sept. 20 issue of Astrophysical
Journal.
Formation history
CHXR 73 B
is located about 19.5 billion miles (31.3 billion km) from its parent star, or
roughly 200 times farther than Earth is from our Sun. This distance is so great that even
though CHXR 73 B has about the right mass to be a planet, it likely didn't form
in the same way that planets in our solar system did, scientists say.
According
to standard planet
formation theories, planets are created from the disks of gas and dust
surrounding newborn
stars. But the circumstellar disks of red dwarf
stars are typically no more than 10 billion miles (16 billion km) in diameter.
Furthermore, theory predicts that gas-giant planets like Jupiter should form no
more than about 3 billion miles (4.8 billion km) from their stars. CHXR 73 B is
located far beyond both these limits.
"This
object is too far from the star to have formed within a disk of gas and
dust," said Kevin Luhman of Pennsylvania State University, leader of the international team
that discovered CHXR 73 B.
More
likely, scientists say, CHXR 73 B formed in the manner of stars: from the
gravitational collapse of large, diffuse clouds of hydrogen gas.
For this
reason, Luhman believes CHXR 73 B should be regarded
as a brown dwarf. Luhman believes an extrasolar
object's formation history is more important than mass when determining whether
it is a planet or not.
Planet or Star?
In a twist
that surprised many astronomers, the definition of planet recently adopted by
the IAU is not meant to apply to objects around other stars. Also, it does not
take formation history into account. According to the new
definition, an object is a planet if it is round, orbits a star but does
not orbit a planet, and clears a path around its star.
CHXR 73 B
"certainly fulfills all three of those criteria, but the IAU definition
was never meant to be applied to other solar systems," Luhman
told SPACE.com. "It was just for
our solar system."
The new
discovery is a reminder that objects in nature do not always fit into the neat
categories created by scientists. Recently, astronomers spied even stranger
planetary-mass objects that drift freely through space, far away from any star.
Some scientists are calling the bizarre objects "planemos,"
but Luhman says the name is unnecessary and that the
objects are really just brown dwarfs.
Next step
Scientists
say the question of whether or not CHXR 73 B is a planet could be settled when
the James
Webb Space Telescope launches, sometime around 2013. The high resolution
Webb telescope would be sharp enough to determine if, like some other brown
dwarfs, CHXR 73 B has a circumstellar disk of its
own.
"If it
does, it's clearly an object that did not form in a disk around the star,
because it wouldn't have a disk by itself then," Luhman
said.
He added
that if CHXR 73 B does have a disk, it would be some 5 AU in diameter, much
wider than the debris rings found around gas giant planets like Saturn and Jupiter. One AU is equal to
the distance between Earth and the Sun.
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