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Nearest star's wobbles could reveal Earth's twin
05:08 29 February 2008
NewScientist.com news service
Stephen Battersby
Another Earth may be orbiting the star next door, and we
could detect its presence within a few years, a new study
argues. A telescope trained permanently on Alpha Centauri
should be able to pick up the slight stellar wobbles induced
by a small, rocky, Earth-like planet.
Alpha Centauri lies just over 4 light years away and is the
closest star system to the Sun. It appears to be a triple
system, with two Sun-like stars orbiting each other relatively
closely (about 23 times the Earth-Sun distance). The two
stars have high concentrations of heavy elements, which is
characteristic of stars that are born surrounded by dusty,
planet-forming discs.
Previous computer simulations suggested terrestrial planets
probably formed around one or both stars. That is borne out
by the work of Javiera Guedes at the University of California,
Santa Cruz (UCSC), US, and colleagues, who have gone a
step further and worked out how to detect such planets.
Enlarge image
Though it looks like a single star, Alpha Centauri (the
bright spot left of centre) is actually a triplet (Image:
Claus Madsen/ESO)
"If our understanding of terrestrial planet formation is at
all correct, then there should definitely be terrestrial
planets orbiting both members of the Alpha Centauri
binary pair," team member Greg Laughlin of UCSC told
New Scientist.
What's more, any such planets might boast the
conditions thought to be necessary to support life. In
the team's simulations of planet formation around the
smaller star, Alpha Centauri B, an Earth-like world often
coalesced in or near the star's habitable zone, where
liquid water could exist on the planet's surface.
Finding these planets could be time-consuming, but it
does not require any new techniques, they say. They
suggest using the "radial velocity" method, which looks
for spectral signs that a star is wobbling due to
gravitational tugs from an orbiting planet.
Calm atmosphere
The method has discovered most of the 228 known
exoplanets. But until now, it has turned up only giant Jupiter-like planets, which produce relatively large wobbles
in their host stars.
"Our aim is to find rocky planets by muscling up the same technique that has been so successful in finding
more massive planets," says team member Debra Fischer of San Francisco State University in California, US.
Laughlin realised that Alpha Centauri B was an exceptionally good target for this method, in part because it is a
calm star. The atmospheres of most stars of its type churn more violently, which would obscure the slight
movement caused by orbiting Earth-like planets.
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Nearest star's wobbles could reveal Earth's twin - space - 29 Februar...
http://space.newscientist.com/article.ns?id=dn13393&print=true
And because it is so near to Earth, Alpha Centauri B is very bright. That means astronomers can rapidly capture
a precise spectrum of its light, which is ideal for measuring small Doppler shifts due to terrestrial planets.
Faint signal
Even so, the researchers think they will need several years of data to smooth out random noise in their
observations to be able to spot the faint signal of another Earth. That's because a terrestrial planet would cause
Alpha Centauri B to wobble at speeds of only about 10 centimetres per second.
Laughlin and his team will start to monitor Alpha Centauri in May, using a 1.5-metre telescope at the Cerro Tololo
Inter-American Observatory in Chile. As well as searching for planets, their observations will be used to analyse
the stars' natural oscillations, which could reveal details about their internal structures.
Not all astronomers are convinced by the simulations that Alpha Centauri should host terrestrial planets. "I tend
to be sceptical of planet-formation models," says Sara Seager of MIT in Cambridge, US, who did not take part in
the study.
But Seager is impressed with the second part of the paper, demonstrating that these planets should be
detectable. "It is tremendously exciting that we can search for an Earth cousin in a habitable zone of a nearby
star with current technology," she told New Scientist.
Astrobiology - Learn more in our out-of-this-world special report.
Journal reference: Astrophysical Journal (forthcoming)
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Weblinks
Guedes et al. abstract
http://arxiv.org/abs/0802.3482
Greg Laughlin, UCSC
http://www.ucolick.org/~laugh/homepage.html
Debra Fischer, San Francisco State University
http://www.physics.sfsu.edu/~fischer/Welcome.html
Sara Seager, MIT
http://eapsweb.mit.edu/people/person.asp?position=Faculty&who=seagers
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