Artist’s impression of the planet around Alpha Centauri B
PR Image eso1241b
Artist’s impression of the planet around Alpha Centauri B (Annotated)
Alpha Centauri in the constellation of Centaurus (The Centaur)
The bright star Alpha Centauri and its surroundings
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A journey to Alpha Centauri
A fly-through of the Alpha Centauri system
ESO’s HARPS instrument finds Earth-mass exoplanet orbiting Alpha Centauri B
European astronomers have discovered a
planet with about the mass of the Earth orbiting a star in the Alpha
Centauri system — the nearest to Earth. It is also the lightest
exoplanet ever discovered around a star like the Sun. The planet was
detected using the HARPS instrument on the 3.6-metre telescope at ESO’s
La Silla Observatory in Chile. The results will appear online in the
journal Nature on 17 October 2012.
Alpha Centauri is one of the brightest stars in the southern skies
and is the nearest stellar system to our Solar System — only 4.3
light-years away. It is actually a triple star — a system consisting of
two stars similar to the Sun orbiting close to each other, designated
Alpha Centauri A and B, and a more distant and faint red component known
as Proxima Centauri [1].
Since the nineteenth century astronomers have speculated about planets
orbiting these bodies, the closest possible abodes for life beyond the
Solar System, but searches of increasing precision had revealed nothing.
Until now.
“Our observations extended over more than four
years using the HARPS instrument and have revealed a tiny, but real,
signal from a planet orbiting Alpha Centauri B every 3.2 days,”
says Xavier Dumusque (Geneva Observatory, Switzerland and Centro de
Astrofisica da Universidade do Porto, Portugal), lead author of the
paper. “It’s an extraordinary discovery and it has pushed our technique to the limit!”
The
European team detected the planet by picking up the tiny wobbles in the
motion of the star Alpha Centauri B created by the gravitational pull
of the orbiting planet [2].
The effect is minute — it causes the star to move back and forth by no
more than 51 centimetres per second (1.8 km/hour), about the speed of a
baby crawling. This is the highest precision ever achieved using this
method.
Alpha Centauri B is very similar to the Sun but slightly
smaller and less bright. The newly discovered planet, with a mass of a
little more than that of the Earth [3],
is orbiting about six million kilometres away from the star, much
closer than Mercury is to the Sun in the Solar System. The orbit of the
other bright component of the double star, Alpha Centauri A, keeps it
hundreds of times further away, but it would still be a very brilliant
object in the planet’s skies.
The first exoplanet around a
Sun-like star was found by the same team back in 1995 and since then
there have been more than 800 confirmed discoveries, but most are much
bigger than the Earth, and many are as big as Jupiter [4].
The challenge astronomers now face is to detect and characterise a
planet of mass comparable to the Earth that is orbiting in the habitable
zone [5] around another star. The first step has now been taken [6].
“This
is the first planet with a mass similar to Earth ever found around a
star like the Sun. Its orbit is very close to its star and it must be
much too hot for life as we know it,” adds Stéphane Udry (Geneva Observatory), a co-author of the paper and member of the team, “but
it may well be just one planet in a system of several. Our other HARPS
results, and new findings from Kepler, both show clearly that the
majority of low-mass planets are found in such systems.”
“This
result represents a major step towards the detection of a twin Earth in
the immediate vicinity of the Sun. We live in exciting times!” concludes Xavier Dumusque.
ESO will hold an online press conference offering journalists the
opportunity to discuss the result and its impact with the scientists. To
participate please read our media advisory.
Notes
[1] The components of a multiple star
are named by adding uppercase letters to the name of the star. Alpha
Centauri A is the brightest component, Alpha Centauri B is the slightly
fainter second star and Alpha Centauri C is the much fainter Proxima
Centauri. Proxima Centauri is slightly closer to Earth than A or B and
hence is formally the closest star.
[2] HARPS measures the radial velocity of a star —
its speed towards or away from the Earth — with extraordinary precision.
A planet in orbit around a star causes the star to regularly move
towards and away from a distant observer on Earth. Due to the Doppler
effect, this radial velocity change induces a shift of the star’s
spectrum towards longer wavelengths as it moves away (called a redshift)
and a blueshift (towards shorter wavelengths) as it approaches. This
tiny shift of the star’s spectrum can be measured with a high-precision
spectrograph such as HARPS and used to infer the presence of a planet.
[3] Using the radial velocity method, astronomers can
only estimate a minimum mass for a planet as the mass estimate also
depends on the tilt of the orbital plane relative to the line of sight,
which is unknown. But, from a statistical point of view, this minimum
mass is often close to the real mass of the planet.
[4] NASA’s Kepler mission has found 2300 candidate
planets using an alternative method — searching for the slight drop in
the brightness of a star as a planet passes in front of it (transits)
and blocks some of the light. The majority of planet candidates detected
by this transit method are very distant from us. But, in contrast, the
planets found by HARPS are around stars close to the Sun — with the new
discovery being the closest yet. This makes them better targets for many
kinds of additional follow-up observations such as characterising the
planet’s atmosphere.
[5] The habitable zone is a narrow annular region
around a star in which water may be present in liquid form if conditions
are right.
[6] ESPRESSO, the Echelle SPectrograph for Rocky
Exoplanet and Stable Spectroscopic Observations, is to be installed on
the ESO Very Large Telescope. Currently undergoing final design, it is
scheduled to start operating in late-2016 or early-2017. ESPRESSO will
feature radial velocity precision of 0.35 km/hour or less. For
comparison, Earth induces a 0.32 km/hour radial velocity on the Sun.
This resolution should thus enable ESPRESSO to discover Earth-mass
planets in the habitable zone. The ESPRESSO consortium is led by team
members responsible for the current discovery.
More information
This research was presented in a paper “An
Earth mass planet orbiting Alpha Centauri B”, to appear online in the
journal Nature on 17 October 2012.
The team is composed of Xavier Dumusque (Observatoire de Genève,
Switzerland; Centro de Astrofisica da Universidade do Porto, Portugal),
Francesco Pepe (Observatoire de Genève), Christophe Lovis (Observatoire
de Genève), Damien Ségransan (Observatoire de Genève), Johannes Sahlmann
(Observatoire de Genève), Willy Benz (Universität Bern, Switzerland),
François Bouchy (Observatoire de Genève; Institut d’Astrophysique de
Paris, France), Michel Mayor (Observatoire de Genève), Didier Queloz
(Observatoire de Genève), Nuno Santos (Centro de Astrofisica da
Universidade do Porto) and Stéphane Udry (Observatoire de Genève).
The year 2012 marks the 50th anniversary of the founding of the
European Southern Observatory (ESO). ESO is the foremost
intergovernmental astronomy organisation in Europe and the world’s most
productive ground-based astronomical observatory by far. It is supported
by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark,
France, Finland, Germany, Italy, the Netherlands, Portugal, Spain,
Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious
programme focused on the design, construction and operation of powerful
ground-based observing facilities enabling astronomers to make
important scientific discoveries. ESO also plays a leading role in
promoting and organising cooperation in astronomical research. ESO
operates three unique world-class observing sites in Chile: La Silla,
Paranal and Chajnantor. At Paranal, ESO operates the Very Large
Telescope, the world’s most advanced visible-light astronomical
observatory and two survey telescopes. VISTA works in the infrared and
is the world’s largest survey telescope and the VLT Survey Telescope is
the largest telescope designed to exclusively survey the skies in
visible light. ESO is the European partner of a revolutionary
astronomical telescope ALMA, the largest astronomical project in
existence. ESO is currently planning the 39-metre European Extremely
Large optical/near-infrared Telescope, the E-ELT, which will become “the
world’s biggest eye on the sky”.
Links
Contacts
Xavier DumusqueObservatoire de l’Université de Genève
Switzerland
Tel: +41 22 379 22 64
Email: xavier.dumusque@unige.ch
Stéphane Udry
Observatoire de l’Université de Genève
Switzerland
Tel: +41 22 379 24 67
Email: stephane.udry@unige.ch
Willy Benz
Center for Space and Habitability
Bern, Switzerland
Email: willy.benz@space.unibe.ch
Francesco Pepe
Observatoire de l’Université de Genève
Switzerland
Tel: +41 223 792 396
Cell: +41 79 302 47 40
Email: francesco.pepe@unige.ch
Damien Ségransan
Observatoire de l’Université de Genève
Switzerland
Tel: +41 223 792 479
Email: damien.segransan@unige.ch
Nuno Santos
Centro de Astrofisica da Universidade do Porto
Porto, Portugal
Tel: +351 226 089 893
Email: Nuno.Santos@astro.up.pt
Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
