ASTRONOMERS are getting their first look at the atmosphere of a planet beyond
the Solar System. They have detected sodium atoms in the atmosphere of a planet
orbiting a Sun-like star 150 light years away.
The composition of a planet鈥檚 atmosphere is one clue to whether it might
harbour life: finding oxygen, carbon dioxide and water would be a clear sign
that it did. To date, astronomers have found 76 planets orbiting other suns, but
they are too faint to see directly with today鈥檚 telescopes and it hasn鈥檛 been
possible to detect their atmospheres.
The new discovery is the first step towards being able to measure the
composition and physical properties of the atmospheres of these planets. 鈥淚t鈥檚
an idea people would have laughed at seven years ago,鈥� says Tim Brown of the
National Center for Atmospheric Research in Colorado.
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Brown has been studying one planet with an orbit that takes it directly
between us and its star, HD 209458. The giant planet is a 鈥渉ot Jupiter鈥� or
鈥渞oaster鈥�, only 7.5 million kilometres from its sun. It passes in front of the
star every 3.5 days, and when it does it blocks around 1.6 per cent of the
star鈥檚 total light, meaning its diameter must be about 30 per cent larger than
闯耻辫颈迟别谤鈥檚.
Different gases absorb light at different wavelengths. So by looking at what
happens to these particular wavelengths as the planet passes in front of the
star, Brown and his colleague Dave Charbonneau, now at the California Institute
of Technology, thought they might be able to glean information about the gases
in the planet鈥檚 atmosphere.
Using data from the Hubble Space Telescope, the astronomers measured the
yellow-green light absorbed by sodium. They report in a future issue of
Astrophysical Journal that the planet absorbs slightly more light at this
wavelength than at others鈥攃lear evidence of sodium and an atmosphere.
鈥淭his opens up new possibilities, and turns the page on studies of giant planets
in extremis,鈥� says astronomer Adam Burrows of the University of Arizona in
Tucson. Brown and Charbonneau now plan to look at light absorbed by other
substances, such as water.
This particular planet is very close to its star, and would be far too hot to
support life as we know it. But the same technique could be used to investigate
other planets, so long as they pass directly between us and their parent
star.
To look for signs of life on planets that aren鈥檛 aligned in this way, we鈥檒l
have to wait for ESA鈥檚 Darwin project, currently planned for launch in 2014.
Darwin will use an array of six telescopes to detect the light from distant
planets directly.