The HARPS spectrograph on the 3.6-metre telescope at ESO's La Silla
Observatory in Chile is the world's most successful planet finder [1].
The HARPS team, led by Michel Mayor (University of Geneva, Switzerland),
have announced the discovery of more than 50 new exoplanets orbiting
nearby stars, including sixteen super-Earths [2]. This is the largest
number of such planets ever announced at one time [3]. The new findings
are being presented at a conference on Extreme Solar Systems where 350
exoplanet experts are meeting in Wyoming, USA.
"The harvest of discoveries from HARPS has exceeded all expectations
and includes an exceptionally rich population of super-Earths and
Neptune-type planets hosted by stars very similar to our Sun. And even
better -- the new results show that the pace of discovery is
accelerating," says Mayor.
In the eight years since it started surveying stars like the Sun
using the radial velocity technique HARPS has been used to discover more
than 150 new planets. About two thirds of all the known exoplanets with
masses less than that of Neptune [4] were discovered by HARPS. These
exceptional results are the fruit of several hundred nights of HARPS
observations [5].
Working with HARPS observations of 376 Sun-like stars, astronomers
have now also much improved the estimate of how likely it is that a star
like the Sun is host to low-mass planets (as opposed to gaseous
giants). They find that about 40% of such stars have at least one planet
less massive than Saturn. The majority of exoplanets of Neptune mass or
less appear to be in systems with multiple planets.
With upgrades to both hardware and software systems in progress,
HARPS is being pushed to the next level of stability and sensitivity to
search for rocky planets that could support life. Ten nearby stars
similar to the Sun were selected for a new survey. These stars had
already been observed by HARPS and are known to be suitable for
extremely precise radial velocity measurements. After two years of work,
the team of astronomers has discovered five new planets with masses
less than five times that of Earth.
"These planets will be among the best targets for future space
telescopes to look for signs of life in the planet's atmosphere by
looking for chemical signatures such as evidence of oxygen," explains
Francesco Pepe (Geneva Observatory, Switzerland), the lead author of one
of the recent papers.
One of the recently announced newly discovered planets, HD 85512 b,
is estimated to be only 3.6 times the mass of Earth [6] and is located
at the edge of the habitable zone -- a narrow zone around a star in
which water may be present in liquid form if conditions are right [7].
"This is the lowest-mass confirmed planet discovered by the radial
velocity method that potentially lies in the habitable zone of its star,
and the second low-mass planet discovered by HARPS inside the habitable
zone," adds Lisa Kaltenegger (Max Planck Institute for Astronomy,
Heidelberg, Germany and Harvard Smithsonian Center for Astrophysics,
Boston, USA), who is an expert on the habitability of exoplanets.
The increasing precision of the new HARPS survey now allows the
detection of planets under two Earth masses. HARPS is now so sensitive
that it can detect radial velocity amplitudes of significantly less than
4 km/hour [8] -- less than walking speed.
"The detection of HD 85512 b is far from the limit of HARPS and
demonstrates the possibility of discovering other super-Earths in the
habitable zones around stars similar to the Sun," adds Mayor.
These results make astronomers confident that they are close to
discovering other small rocky habitable planets around stars similar to
our Sun. New instruments are planned to further this search. These
include a copy of HARPS to be installed on the Telescopio Nazionale
Galileo in the Canary Islands, to survey stars in the northern sky, as
well as a new and more powerful planet-finder, called ESPRESSO, to be
installed on ESO's Very Large Telescope in 2016 [9]. Looking further
into the future also the CODEX instrument on the European Extremely
Large Telescope (E-ELT) will push this technique to a higher level.
"In the coming ten to twenty years we should have the first list of
potentially habitable planets in the Sun's neighbourhood. Making such a
list is essential before future experiments can search for possible
spectroscopic signatures of life in the exoplanet atmospheres,"
concludes Michel Mayor, who discovered the first-ever exoplanet around a
normal star in 1995.
Notes
[1] HARPS measures the radial velocity of a star 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.
[2] Planets with a mass between one and ten times that of Earth are
called super-Earths. There are no such planets in our Solar System, but
they appear to be very common around other stars. Discoveries of such
planets in the habitable zones around their stars are very exciting
because -- if the planet were rocky and had water, like Earth -- they
could potentially be an abode of life.
[3] Currently the number of exoplanets stands at close to 600. In
addition to exoplanets found using radial velocity techniques, more than
1200 exoplanet candidates have been found by NASA's Kepler mission
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 planets discovered by this
transit method are very distant from us. But, in contrast, the planets
found by HARPS are around stars close to the Sun. This makes them better
targets for many kinds of additional follow-up observations.
[4] Neptune has about seventeen times the mass of Earth.
[5] This huge observing programme is led by Stephane Udry (Geneva Observatory, Switzerland).
[6] 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.
From a statistical point of view, this minimum mass is however often
close to the real mass of the planet.
[7] So far, HARPS has found two super-Earths that may lie within the
habitable zone. The first one, Gliese 581 d, was discovered in 2007
(eso0722). HARPS was also recently used to demonstrate that the other
candidate super-Earth in the habitable zone around the star Gliese 581
(Gliese 581 g) does not exist.
[8] With large numbers of measurements, the detection sensitivity of
HARPS is close to 100% for super-Earths of ten Earth-masses with orbital
periods of up to one year, and even when considering planets of three
Earth masses with a one-year orbit, the probability of detection remains
close to 20%.
[9] ESPRESSO, the Echelle SPectrograph for Rocky Exoplanet and Stable
Spectroscopic Observations, is to be installed on the ESO Very Large
Telescope. Currently undergoing preliminary design, it is scheduled to
start operating in 2016. ESPRESSO will feature radial velocity precision
of 0.35 km/h or less. For comparison, Earth induces a 0.32 km/h radial
velocity on the Sun. This resolution should thus enable ESPRESSO to
discover Earth-mass planets in the habitable zone of low-mass stars.
More information
The results are being presented on 12 September 2011 at the
conference on Extreme Solar Systems held at the Grand Teton National
Park, Wyoming, USA.
A summary is presented in the following paper (in preparation): "The
HARPS search for southern extra-solar planets, XXXIV -- Occurrence, mass
distribution and orbital properties of super-Earths and Neptune-type
planets" to appear in the journal Astronomy & Astrophysics.
The team is composed of M. Mayor (Observatoire de Geneve [OAUG],
Switzerland), M. Marmier (OAUG), C. Lovis (OAUG), S. Udry (OAUG), D.
Segransan (OAUG), F. Pepe (OAUG), W. Benz (Physikalisches Institut
Universitat Bern, Switzerland), J. L. Bertaux (Service d'Aeronomie,
Paris, France), F. Bouchy (Institut d'Astrophysique de Paris, Universite
Pierre & Marie Curie, France and Observatoire de
Haute-Provence/CNRS, France), X. Dumusque (OAUG), G. LoCurto (ESO,
Germany), C. Mordasini (Max Planck Institute for Astronomy, Germany), D.
Queloz (OAUG), N. C. Santos (Centro de Astrofisica da Universidade do
Porto, Portugal and Departamento de Fisica de Astronomia, Faculdade de
Ciencias da Universidade do Porto, Portugal), D. Queloz (OAUG).
— ScienceDaily (Sep. 12, 2011)
— ScienceDaily (Sep. 12, 2011)
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