The star cluster Messier 67 in the constellation of Cancer
Wide-field view of the open star cluster Messier 67
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Six-year search with HARPS finds three new planets in Messier 67
Astronomers have used ESO's HARPS planet
hunter in Chile, along with other telescopes around the world, to
discover three planets orbiting stars in the cluster Messier 67.
Although more than one thousand planets outside the Solar System are now
confirmed, only a handful have been found in star clusters. Remarkably
one of these new exoplanets is orbiting a star that is a rare solar twin
— a star that is almost identical to the Sun in all respects.
Planets orbiting stars outside the Solar System are now known to be
very common. These exoplanets have been found orbiting stars of widely
varied ages and chemical compositions and are scattered across the sky.
But, up to now, very few planets have been found inside star clusters [1].
This is particularly odd as it is known that most stars are born in
such clusters. Astronomers have wondered if there might be something
different about planet formation in star clusters to explain this
strange paucity.
Anna Brucalassi (Max Planck Institute for Extraterrestrial Physics,
Garching, Germany), lead author of the new study, and her team wanted to
find out more. “In the Messier 67 star cluster the stars are all
about the same age and composition as the Sun. This makes it a perfect
laboratory to study how many planets form in such a crowded environment,
and whether they form mostly around more massive or less massive
stars.”
The team used the HARPS planet-finding instrument on ESO's 3.6-metre telescope at the La Silla Observatory. These results were supplemented with observations from several other observatories around the world [2]. They carefully monitored 88 selected stars in Messier 67 [3]
over a period of six years to look for the tiny telltale motions of the
stars towards and away from Earth that reveal the presence of orbiting
planets.
This cluster lies about 2500 light-years away in the constellation of
Cancer (The Crab) and contains about 500 stars. Many of the cluster
stars are fainter than those normally targeted for exoplanet searches
and trying to detect the weak signal from possible planets pushed HARPS
to the limit.
Three planets were discovered, two orbiting stars similar to the Sun
and one orbiting a more massive and evolved red giant star. The first
two planets both have about one third the mass of Jupiter and orbit
their host stars in seven and five days respectively. The third planet
takes 122 days to orbit its host and is more massive than Jupiter [4].
The first of these planets proved to be orbiting a remarkable star —
it is one of the most similar solar twins identified so far and is
almost identical to the Sun (eso1337) [5]. It is the first solar twin in a cluster that has been found to have a planet.
Two of the three planets are “hot Jupiters” — planets comparable to
Jupiter in size, but much closer to their parent stars and hence much
hotter. All three are closer to their host stars than the habitable zone
where liquid water could exist.
“These new results show that planets in open star clusters are
about as common as they are around isolated stars — but they are not
easy to detect,” adds Luca Pasquini (ESO, Garching, Germany), co-author of the new paper [6]. “The
new results are in contrast to earlier work that failed to find cluster
planets, but agrees with some other more recent observations. We are
continuing to observe this cluster to find how stars with and without
planets differ in mass and chemical makeup.”
Notes
[1] Star clusters come in two main
types. Open clusters are groups of stars that have formed together from a
single cloud of gas and dust in the recent past. They are mostly found
in the spiral arms of a galaxy like the Milky Way. On the other hand
globular clusters are much bigger spherical collections of much older
stars that orbit around the centre of a galaxy. Despite careful
searches, no planets have been found in a globular cluster and less than
six in open clusters. Exoplanets have also been found in the past two
years in the clusters NGC 6811 and Messier 44, and even more recently one has also been detected in the bright and nearby Hyades cluster.
[2] This work also used observations from the SOPHIE instrument at the Observatoire de Haute-Provence in France, the Swiss 1.2-metre Leonhard Euler Telescope at ESO's La Silla Observatory in Chile and the Hobby Eberly Telescope in Texas, USA.
[3] Most open clusters dissipate after a few tens of
million years. However, clusters that form with a higher density of
stars can stay together for much longer. Messier 67 is an example of such a long-lived older cluster and is one of the oldest and best-studied of such clusters close to the Earth.
[4] Mass estimates for planets observed using the
radial velocity method are lower estimates: if the planet's orbit is
highly inclined it could have a higher mass and create the same observed
effects.
[5] Solar twins, solar analogues and solar-type stars
are categories of stars according to their similarity to the Sun. Solar
twins are the most similar to the Sun, as they have very similar
masses, temperatures, and chemical abundances. Solar twins are very
rare, but the other classes, where the similarity is less precise, are
much more common.
[6] This detection rate of 3 planets in a sample of
88 stars in Messier 67 is close to the average frequency of planets
around stars that are not members of clusters.
More information
This research was presented in a paper
entitled “Three planetary companions around M67 stars“, by A. Brucalassi
et al., to appear in the journal Astronomy & Astrophysics.
The team is composed of A. Brucalassi (Max-Planck-Institut für
extraterrestrische Physik, Garching, Germany [MPE]; Sternwarte, Munich,
Germany), L. Pasquini (ESO, Garching, Germany), R. Saglia (MPE;
Sternwarte), M.T. Ruiz (Universidad de Chile, Santiago, Chile), P.
Bonifacio (GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot,
France), L. R. Bedin (INAF - Osservatorio Astronomico di Padova, Padova,
Italy), K. Biazzo (INAF - Osservatorio Astronomico di Catania, Catania,
Italy), C. Melo (ESO, Santiago, Chile), C. Lovis (Observatoire de
Geneve, Switzerland) and S. Randich (INAF - Osservatorio Astrofisico di
Arcetri, Florence, Italy).
ESO is the foremost intergovernmental astronomy organisation in
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sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope, the world's most advanced
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works in the infrared and is the world's largest survey telescope and
the VLT Survey Telescope is the largest telescope designed to
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E-ELT, which will become “the world's biggest eye on the sky”.
Links
- Research paper: A&A letter
- Photos of the ESO 3.6-metre telescope
Contacts
Anna BrucalassiMax Planck Institut for Extraterrestrial Physics
Garching, Germany
Tel: +49 89 30000 3022
Email: abrucala@mpe.mpg.de
Luca Pasquini
ESO
Garching, Germany
Tel: +49 89 3200 6792
Email: lpasquin@eso.org
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
