HONOLULU—A team
of amateur and professional astronomers has discovered a mammoth orb more than
13 times the mass of Jupiter that whips around its parent star in fewer than four
days and is considered an "oddball" planet among its exoplanet relatives.
The new
exoplanet, dubbed XO-3b, was described here at a meeting of the American
Astronomical Society. The discovery came out of the XO Project, a collaboration
between amateur and professional astronomers.
"Of
the 200-plus exoplanets found so far, XO-3b is an oddity in several
respects," said XO Project director Peter McCullough, an astronomer at the
Space Telescope Science Institute in Baltimore.
For one,
it's the largest and most massive planet found in such a tight orbit around its
parent star. Secondly, with such a close proximity between the planet and star, astronomers would expect a
circular orbit as the parent star's gravity tugs and shapes the orbit. But
XO-3b treks along an elliptical orbit.
And during
every orbit, XO-3b passes in front of its star, making it a "transiting
planet," of which just a few dozen have been identified.
Planet
or brown dwarf?
The finding
also stirs up an already frothy debate concerning the boundary between brown
dwarfs and massive planets.
"We
are intrigued that its mass is on the boundary between planets and 'brown
dwarfs,'" said Christopher Johns-Krull, an astronomer at Rice University.
"There's still a lively debate among astronomers about how to classify
brown dwarfs."
Brown
dwarfs are too massive to be considered planets,
yet they don't meet the "Sumo-weight" requirements for hydrogen
fusion (about 80 Jupiters) so they fall short of being stars. The new object's
mass is right on the boundary where brown-dwarf status begins, a mass
requirement for the burning of deuterium, or heavy hydrogen.
"The
controversy lies at the lower end of the scale," Johns-Krull said.
"Some people believe anything capable of fusing deuterium, which in theory
happens around 13 Jupiter masses, is a brown dwarf. Others say it's not the
mass that matters, but whether the body forms on its own or as part of a
planetary system."
Brown
dwarf desert
Even if
scientists determine XO-3b to be a brown dwarf, the object would still have
"celebrity" status. Due to their mass, brown dwarfs should be easy for
astronomers to spot. That's because when hunting for exoplanets,
astronomers usually look for them indirectly via the wobbles
in stars caused by the gravitational tug of the orbiting planet or planets.
The heftier the orbiting planet, the more it should shake its parent star.
However, sky scanners haven't spotted a treasure trove of brown dwarfs and this
scarcity has become known as the "brown dwarf desert."
"There
are many astrophysical systems out there that mimic transiting planets,"
McCullough said. "The only way to sort out the real planets from the rest
is to observe the stars more carefully," and come up with accurate mass and
other measurements.
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