Astronomers have found the
heavyweight champion of extrasolar planets in the form of an odd alien world
slightly bigger than Jupiter, but more than eight times as massive.
Dubbed HAT-P-2b, the super-dense
planet is the most massive known to transit across its parent star, but the
weirdness doesn't stop there. Its oval orbit is so extreme that it first bakes
the planet, and then cools it off during an annual trip that takes just more
than five days.
"This planet is so unusual that at
first we thought it was a false alarm--something that appeared to be a planet
but wasn't," said astronomer Gaspar Bakos, who led the team at the Harvard-Smithsonian Center
for Astrophysics. "But we eliminated every other possibility, so we knew we had
a really weird planet."
The planet is a gas giant in orbit
around the star HD 147506, which is about twice the size of our own Sun and
burns a bit hotter in a system 440 light-years from Earth in the constellation
Hercules. Bakos and his colleagues used a series of
small automated telescopes known as the HATNet to
discover the planet. Their findings are detailed in a paper submitted Tuesday
to the Astrophysical Journal.
One weird world
Astronomers have found about 230
extrasolar planets beyond our own solar system, and last week announced the
discovery
of an Earth-like planet could support liquid water.
Every five days and 15 hours--the
time it takes the planet to complete a full trip around its star--HAT-P-2b
crosses in front of its stellar parent, as seen from Earth, in what astronomers
call a transit. During such transits, researchers can
determine the physical size of extrasolar planets by measuring how much they
dim the light of their central star.
Bakos and his team found that the newly
discovered planet is about 1.18 times brighter than Jupiter and 8.2 times as massive. A
150-pound (60-kilogram) person on Earth would weigh 2,100 pounds
(952 kilograms), or just over one ton, and experience about 14 times Earth's
gravity at the visible cloud top surface of HAT-P-2b, researchers said.
The planet's extremely elliptical
orbit brings it within about 3.1 million miles (4.9 million kilometers) of its
parent star on the inside, and swings out to a distance of about 9.6 million
miles (15.4 million kilometers). For comparison, Earth orbits the Sun at a
distance of about 93 million miles (150 million kilometers), but would range
between the orbits of Mercury and Mars if its orbital path mimicked the
extremes of HAT-P-2b.
Eccentricity explained
Astronomers believe that the odd
eccentricity of the planet's orbit--all previous extrasolar worlds found via the
transit method have circular orbits--may be due to another, outer world whose
gravitational pull disturbs the path of HAT-P-2b.
If the planet contained about 50
percent more mass, it could have fired up nuclear fusion and burn as a star for
a short while, researchers added.
"HAT-P-2b is hot, but it's not a
Jupiter," CfA astronomer Robert Noyes, a co-author on the study, said, adding
that previous planets found via the transit method have been billed as 'hot
Jupiters.' "It's much denser than a Jupiter-like planet; in fact, it is as
dense as Earth even though it's mostly made of hydrogen."
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