Largest
Known Exoplanet Discovered by
Ker Than, Space.com, Aug 07, 2007
A
computer-generated simulation of TrES-4, with its host star on the
right. The planet's home star is bigger and hotter than the sun, and
is about ten times larger than the planet.
he largest
planet ever discovered is also one of the strangest and
theoretically should not even exist, scientists say.
Dubbed TrES-4, the planet is about
1.7 times the size of Jupiter and belongs to a small subclass of
"puffy" planets that have extremely low densities. The finding
will be detailed in an upcoming issue of Astrophysical
Journal.
"Its mean density is only about
0.2 grams per cubic centimeter, or about the density of balsa
wood," said study leader Georgi Mandushev of the Lowell
Observatory in Arizona. "And because of the planet's relatively
weak pull on its upper atmosphere, some of the atmosphere
probably escapes in a comet-like tail."
The planet's large mass-to-density
ratio makes it an anomaly among known exoplanets, and its
existence cannot be explained by current models.
A planet Anomoly
"TrES-4 is way bigger than it's
supposed to be," Mandushev told SPACE.com. "For its mass, it
should be much smaller. It basically should be about the size of
Jupiter and instead it's almost twice as big."
"TrES-4 appears to be something of
a theoretical problem," said study team member Edward Dunham,
also of the Lowell Observatory. "Problems are good, though,
since we learn new things by solving them."
The planet is located about 1,400
light years away from Earth and zips around its parent star in
only three and a half days. An international team of astronomers
discovered it using a network of automated telescopes called the
Trans-Atlantic Exoplanet Survey. TrES-4 was detected as it
passed in front of, or "transited," its parent star, called GSC
02620-00648. The transit technique is the only planet-finding
method that allows scientists to calculate the size of a planet.
TrES-4 "is the largest planet
found so far for which we actually know the size," Mandushev
said in a telephone interview. "There could be larger planets,
but we have no way of measuring their sizes because they don't
transit."
Speeded up star
The parent star of TrES-4 is also
unusual in that it is about the same age as our sun but much
farther along in its evolutionary history. "Because it is more
massive, it has evolved much faster," Mandushev explained. "It
has become what astronomers call a subgiant, or a star that has
exhausted all of its hydrogen fuel in the core and is on its way
to becoming a red giant."
Our sun is not expected to become
a red giant for another 5 to 7 billion years. When it does, the
sun will expand to engulf the inner planets and possibly Earth.
Because TrES-4 orbits so close to its star, it is certain to be
consumed once the star becomes a red giant in about a billion
years, Mandushev said.
GSC 02620-00648 is much more
luminous than our sun and emits three to four times more energy
per second. As a general rule, gas planets that orbit larger,
more luminous stars tend to be puffier, but this does not
explain TrES-4's anomalously large size.
"We actually looked at the energy
which the planet gets from the star, and there's no way this can
be the only explanation for how big the planet is," Mandushev
said.
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