09-Dec-2008: The NASA/ESA Hubble Space Telescope has
discovered carbon dioxide in the atmosphere of a planet orbiting
another star. This is an important step along the trail of finding the
chemical biotracers of extraterrestrial life, as we know it.
The
Jupiter-sized planet, called HD 189733b, is too hot for life. But new
Hubble observations are a proof-of-concept demonstration that the basic
chemistry for life can be measured on planets orbiting other stars.
Organic compounds can also be a by-product of life processes and their
detection on an Earth-like planet may someday provide the first
evidence of life beyond Earth.
Previous observations of HD
189733b by Hubble and the Spitzer Space Telescope found water vapour.
Earlier this year Hubble found methane in the planet’s atmosphere.
"This
is exciting because Hubble is allowing us to see molecules that probe
the conditions, chemistry, and composition of atmospheres on other
planets," says first author Mark Swain of The Jet Propulsion Laboratory in Pasadena, USA. "Thanks
to Hubble we’re entering an era where we are rapidly going to expand
the number of molecules we know about on other planets."
The
international team of astronomers used Hubble’s Near Infrared Camera
and Multi-Object Spectrometer (NICMOS) to study infrared light emitted
from the planet, which lies 63 light-years away. Gases in the planet’s
atmosphere absorb certain wavelengths of light from the planet’s hot
glowing interior. The team identified not only carbon dioxide, but also
carbon monoxide. The molecules leave their own unique spectral
fingerprint on the radiation from the planet that reaches Earth. This
is the first time a near-infrared emission spectrum has been obtained
for an extrasolar planet."The carbon dioxide is kind of the main
focus of the excitement, because that is a molecule that under the
right circumstances could have a connection to biological activity as
it does on Earth," Swain says. "The very fact that we’re able to
detect it, and estimate its abundance, is significant for the long-term
effort of characterizing planets both to find out what they’re made of
and to find out if they could be a possible host for life."
This
type of observation is best done for planets with orbits tilted edge-on
to Earth. They routinely pass in front of and then behind their parent
stars, phenomena known as eclipses. The planet HD 189733b passes behind
its companion star once every 2.2 days. This allows an opportunity to
subtract the light of the star alone (when the planet is blocked) from
that of the star and planet together prior to eclipse), thus isolating
the emission of the planet alone and making possible a chemical
analysis of its "day-side" atmosphere.
In this way, Swain
explains that he’s using the eclipse of the planet behind the star to
probe the planet’s day side, which contains the hottest portions of its
atmosphere. "We’re starting to find the molecules and to figure out
how many of them there are to see the changes between the day side and
the night side,”"Swain says.
This successful demonstration of
looking at near-infrared light emitted from a planet is very
encouraging for astronomers planning to use the NASA/ESA/CSA James Webb
Space Telescope when it is launched in 2013. These biomarkers are best
seen at near-infrared wavelengths.
Astronomers look forward to
using JWST to spectroscopically look for biomarkers on a terrestrial
planet the size of Earth, or a "super-Earth" several times our planet’s
mass.
Swain and colleagues next plans to search for molecules in
the atmospheres of other extrasolar planets, as well as trying to
increase the number of molecules detected in extrasolar planet
atmospheres. He also plans to use molecules to study changes that may
be present in extrasolar planet atmospheres to learn something about
the weather on these distant worlds.
Co-author Giovanna Tinetti from University College London adds: "In
the terrestrial planets of our solar system, carbon dioxide plays a
crucial role for the stability of climate. On Earth, carbon dioxide is
one of the ingredients of the photosynthesis and a key element for the
carbon cycle. Our observations represent a great opportunity to
understand the role of carbon dioxide in the atmospheres of hot Jupiter
type planets".
Notes for editors:
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
Image credit: ESA, NASA, M. Kornmesser (ESA/Hubble) and STScI
The
science team members are M.R. Swain (JPL, USA), G. Vasisht (JPL, USA),
G. Tinetti (University College London, UK), J. Bouwman (Max-Planck
Institute for Astronomy, Germany), Pin Chen (JPL, USA), Y. Yung
(Caltech, USA) & D. Deming (Goddard Space Flight Center USA)