Hubble Finds First
Organic Molecule On Extrasolar Planet
(19 March 2008) The NASA/ESA Hubble
Space Telescope has made the first detection ever of an organic molecule in the
atmosphere of a planet orbiting another star.
This breakthrough
is an important step in eventually identifying signs of life on a planet
outside our Solar System.
Hubble found the tell-tale signature of
methane in the atmosphere of the Jupiter-sized extrasolar planet HD 189733b.
Under the right circumstances, methane can play a key role in prebiotic
chemistry - the chemical reactions considered necessary to form life as we know
it. Although methane has been detected on most of the planets in our Solar
System, this is the first time any organic molecule has been detected on a
world orbiting another star.
This discovery proves that Hubble and
upcoming space missions, such as the NASA/ESA/CSA James Webb Space Telescope,
can detect organic molecules on planets around other stars using spectroscopy,
which splits light into its components to reveal the fingerprints of various
chemicals.
Artist’s impression of the extrasolar planet HD 189733b, now known to have methane and water. Astronomers used the Hubble Space Telescope to detect methane — the first organic molecule found on an extrasolar planet. Hubble also confirmed the presence of water vapour in the Jupiter-size planet’s atmosphere, a discovery made in 2007 with the help of the Spitzer Space Telescope. They made the finding by studying how light from the host star filters through the planet’s atmosphere. (courtesy: ESA/ NASA/ UCL (G. Tinetti))
"This is a crucial stepping stone to
eventually characterising prebiotic molecules on planets where life could
exist", said Mark Swain of NASA's Jet Propulsion Laboratory (JPL), USA, who led
the team that made the discovery. Swain is lead author of a paper to be
published in tomorrow's issue of Nature.
The discovery comes after
extensive observations in May 2007 carried out with Hubble's Near Infrared
Camera and Multi-Object Spectrometer (NICMOS). It also confirms the existence
of water molecules in the planet's atmosphere, a discovery originally made by
co-author Giovanna Tinetti in 2007 while she was an ESA fellow at Institute
d'Astrophysique de Paris, France, using NASA's Spitzer space
telescope.
"With this observation there is no question whether there is
water or not - water is present", said Swain.
The planet HD 189733b, now
known to have methane and water vapour, is located 63 light-years away in the
constellation Vulpecula, the little fox. HD 189733b, a hot Jupiter-type
extrasolar planet, is so close to its parent star that it takes just over two
days to complete an orbit. Hot Jupiters are the size of Jupiter but orbit
closer to their stars than Mercury, the innermost planet in our Solar System.
HD 189733b's atmosphere swelters at 900ºC, about the same temperature as
the melting point of silver.
The observations were made as the planet
passed in front of its parent star in what astronomers call a transit. As the
light from the star passed briefly through the atmosphere along the edge of the
planet, the gases in the atmosphere imprinted their unique signatures on the
light from the star HD 189733.
Giovanna Tinetti, now affiliated to
University College added, "Water alone could not explain all the spectral
features observed. The additional contribution of methane is necessary to fit
the Hubble data".
A wide field image of the region of sky in which HD 189733b is located. In this image we can see the asterism of the "Summer Triangle" a giant triangle in the sky composed of the three bright stars Vega (top left), Altair (lower middle) and Deneb (far left). HD 189733b is orbiting a star very close to the centre of the triangle. (courtesy: A. Fujii)
Methane, composed of carbon and hydrogen, is
one of the main components of natural gas, a product of petroleum. On Earth,
methane is produced by a variety of sources: natural sources such as termites,
the oceans and wetland environments, but also from livestock and manmade
sources such as waste landfills and as a by-product of energy
generation.
Tinetti is however quick to rule out any biological origin
of the methane found on HD 189733b. "The planet's atmosphere is far too hot for
even the hardiest life to survive - at least the kind of life we know from
Earth. It's highly unlikely that cows could survive here!"
The
astronomers were surprised to find that the planet has more methane than
predicted by conventional models for hot Jupiters. This type of hot planet
should have much more carbon monoxide than methane but HD 189733b
doesn't.
A wide star field image of the region around HD 189733b. The star HD 189733 is located in the centre, just to the left of the planetary nebula Messier 27. The field-of-view is approximately 2.7 x 2.8 degrees. (courtesy: NASA/ ESA/ Digitized Sky Survey 2, ESA/Hubble (Davide De Martin))
Tinetti adds: "A sensible explanation is
that the Hubble observations were more sensitive to the dark night-side of this
planet where the atmosphere is slightly colder and the photochemical mechanisms
responsible for methane destruction are less efficient than on the
day-side".
Although this star-hugger is too hot for life as we know it,
"this observation is proof that spectroscopy can eventually be done on a cooler
and potentially habitable Earth-sized planet orbiting a dimmer red dwarf-type
star", Swain said.
The ultimate goal of studies like these is to
identify prebiotic molecules in the atmospheres of planets in the habitable
zones around other stars, where temperatures are right for water to remain
liquid rather than freeze or evaporate away.
"These measurements are an
important step to our ultimate goal of determining the conditions, such as
temperature, pressure, winds, clouds, etc., and the chemistry on planets where
life could exist. Infrared spectroscopy is really the key to these studies
because it is best matched to detecting molecules", said Swain.
The
paper 'Methane present in an extrasolar planet atmosphere' by Mark Swain,
Gautam Vasisht and Giovanna Tinetti is being published in the 20 March issue of
the journal Nature.
Giovanna Tinetti was an ESA Research Fellow at
University College London until October 2007, where she is now
based.
The Hubble Space Telescope is a project of international
co-operation between ESA and NASA.
(source: ESA)
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