Hubble Reveals a New Type of Planet
February 21, 2012
GJ1214b, shown in this artist's conception, is a super-Earth
orbiting a red dwarf star 40 light-years from Earth. New
observations from NASA's Hubble Space Telescope show that it is
a waterworld enshrouded by a thick, steamy atmosphere. GJ1214b
therefore represents a new type of world, like nothing seen in
our solar system or any other planetary system currently known.
Credit: David A. Aguilar (CfA)
High Resolution Image (jpg)
Low Resolution Image (jpg)
Cambridge, MA - Our
solar system contains three types of planets: rocky, terrestrial
worlds (Mercury, Venus, Earth, and Mars), gas giants (Jupiter
and Saturn), and ice giants (Uranus and Neptune). Planets
orbiting distant stars come in an even wider variety, including
lava worlds and "hot Jupiters."
Observations by NASA's Hubble Space Telescope have added a new
type of planet to the mix. By analyzing the previously
discovered world GJ1214b, astronomer Zachory Berta
(Harvard-Smithsonian Center for Astrophysics) and colleagues
proved that it is a waterworld enshrouded by a thick, steamy
atmosphere.
"GJ1214b is like no planet we know of," said Berta. "A huge
fraction of its mass is made up of water."
GJ1214b was
discovered in
2009 by the ground-based MEarth (pronounced "mirth") Project,
which is led by CfA's David Charbonneau. This super-Earth is
about 2.7 times Earth's diameter and weighs almost 7 times as
much. It orbits a red-dwarf star every 38 hours at a distance of
1.3 million miles, giving it an estimated temperature of 450 °
Fahrenheit.
In 2010, CfA scientist Jacob Bean and colleagues reported that
they had measured the atmosphere of GJ1214b, finding it likely
that the atmosphere was composed mainly of water. However, their
observations could also be explained by the presence of a
world-wide haze in GJ1214b's atmosphere.
Berta and his co-authors used Hubble's WFC3 instrument to study
GJ1214b when it crossed in front of its host star. During such a
transit, the star's light is filtered through the planet's
atmosphere, giving clues to the mix of gases.
"We're using Hubble to measure the infrared color of sunset on
this world," explained Berta.
Hazes are more transparent to infrared light than to visible
light, so the Hubble observations help tell the difference
between a steamy and a hazy atmosphere.
They found the spectrum of GJ1214b to be featureless over a wide
range of wavelengths, or colors. The atmospheric model most
consistent with the Hubble data is a dense atmosphere of water
vapor.
"The Hubble measurements really tip the balance in favor of a
steamy atmosphere," said Berta.
Since the planet's mass and size are known, astronomers can
calculate the density, which works out to about 2 grams per
cubic centimeter. Water has a density of 1 g/cm3, while Earth's
average density is 5.5 g/cm3. This suggests that GJ1214b has
much more water than Earth, and much less rock.
As a result, the internal structure of GJ1214b would be very
different than our world.
"The high temperatures and high pressures would form exotic
materials like 'hot ice' or 'superfluid water' - substances that
are completely alien to our everyday experience," said Berta.
Theorists expect that GJ1214b formed farther out from its star,
where water ice was plentiful, and migrated inward early in the
system's history. In the process, it would have passed through
the star's habitable zone. How long it lingered there is
unknown.
GJ1214b is located in the direction of the constellation
Ophiuchus, and just 40 light-years from Earth. Therefore, it's a
prime candidate for study by the next-generation James Webb
Space Telescope.
A paper reporting these results has been accepted for
publication in The
Astrophysical Journal and
is available online.
This release is being issued jointly with NASA.
Headquartered in Cambridge, Mass., the
Harvard-Smithsonian Center for Astrophysics (CfA) is a joint
collaboration between the Smithsonian Astrophysical Observatory
and the Harvard College Observatory. CfA scientists, organized
into six research divisions, study the origin, evolution and
ultimate fate of the universe.
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
Source: cfa.harvard
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