A new
technique has uncovered an extrasolar planet hidden in Hubble Space Telescope images
taken 11 years ago
The new
strategy may allow researchers to uncover other distant alien worlds potentially
lurking in over a decade's worth of Hubble archival data.
The method
was used to find an exoplanet that went undetected in Hubble images taken
in 1998 with its Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Astronomers
knew of the planet's existence from images taken with the Keck and Gemini North
telescopes in 2007 and 2008, long after Hubble snapped its first picture of the
system.
The planet
is estimated to be at least seven times the mass of Jupiter. It is the
outermost of three massive planets known to orbit the dusty young star HR 8799,
which is 130 light-years away from Earth. NICMOS could not see the other two
planets because its coronagraphic spot — a device that blots out the glare of
the star —blocked its viewof the two inner planets.
"We've
shown that NICMOS is more powerful than previously thought for imaging
planets," said the scientist who found the planet, David Lafreniere of the
University of Toronto in Canada. "Our new image-processing technique
efficiently subtracts the glare from a star that spills over the coronagraph's
edge, allowing us to see planets that are one-tenth the brightness of what
could be detected before with Hubble."
Taking the
image of an exoplanet is not an easy task. Planets can be billions of times
fainter than the star around which they orbit and are typically located at
separations smaller than 1/2,000th the apparent size of the full moon, as seen
from Earth, from their star. The planet recovered in the NICMOS data is about
100,000 times fainter than the star when viewed in the near-infrared spectrum.
Over the
last two decades, scientists have spotted more than 300 extrasolar
planets circling other stars in our Milky Way galaxy.
Lafreniere
adapted an image reconstruction technique that was first developed for
ground-based observatories.
Using the
new technique, he recovered the planet in NICMOS observations taken 10 years
before the Keck/Gemini discovery. The Hubble picture not only provides
important confirmation of the planet's existence, it provides a longer baseline
for demonstrating that the object is in an orbit about the star.
"To
get a good determination of the orbit we have to wait a very long time because
the planet is moving so slowly (it has a 400-year period)," Lafreniere
said. "The 10-year-old Hubble data take us that much closer to having a
precise measure of the orbit."
Hubble is
due to be serviced by a NASA shuttle crew in May for the fifth and final time.
The shuttle
Atlantis was rolled out for the mission on Tuesday and is due to launch May
12.
NICMOS's
view provided new insights into the physical characteristics of the planet,
too. This was possible because NICMOS works at near-infrared wavelengths that
are severely blocked by Earth's atmosphere due to absorption by water vapor.
"The
planet seems to be only partially cloud covered and we could be detecting the
absorption of water vapor in the atmosphere," said team member Travis
Barman of Lowell Observatory in Flagstaff, Ariz. "Measuring the water
absorption properties will tell us a great deal about the temperatures and
pressures in the atmospheres, in addition to the cloud coverage."
With the
success of this planet hunt, scientists hope they can find more extrasolar
planets lurking in the enormous catalogue of images that Hubble has taken in
its lifetime.
"During
the past 10 years Hubble has been used to look at over 200 stars with
coronagraphy, looking for planets and disks. We plan to go back and look at all
of those archived images and see if anything can be detected that has gone
undetected until now," said Christian Marois of the Herzberg Institute of
Astrophysics, Victoria, Canada.
If the team
sees a companion object to a star in more than one NICMOS picture, and it
appears to have moved along an orbit, follow-up observations will be made with
ground-based telescopes. If researchers see something once but its brightness
and separation from the star would be reasonable for a planet, they will also
do follow-up observations with ground-based telescopes.
NASA's recently-launched
Kepler mission will also be hunting for extrasolar planets in our home
galaxy, though it will be looking for ones that are Earth-sized.
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