Artist’s impression of a hot Jupiter exoplanet in the star cluster Messier 67
The star cluster Messier 67 in the constellation of Cancer
Wide-field view of the open star cluster Messier 67
Videos
Artist’s impression of hot Jupiter exoplanet in the star cluster Messier 67
An international team of astronomers have found that there are far more
planets of the hot Jupiter type than expected in a cluster of stars
called Messier 67. This surprising result was obtained using a number of
telescopes and instruments, among them the HARPS spectrograph at ESO’s
La Silla Observatory in Chile. The denser environment in a cluster will
cause more frequent interactions between planets and nearby stars, which
may explain the excess of hot Jupiters.
A Chilean, Brazilian and European team led by Roberto Saglia at the Max-Planck-Institut für extraterrestrische Physik, in Garching, Germany, and Luca Pasquini at ESO, has spent several years collecting high-precision measurements of 88 stars in Messier 67 [1]. This open star cluster is about the same age as the Sun and it is thought that the Solar System arose in a similarly dense environment [2].
The team used HARPS, along with other instruments [3],
to look for the signatures of giant planets on short-period orbits,
hoping to see the tell-tale “wobble” of a star caused by the presence of
a massive object in a close orbit, a kind of planet known as a hot Jupiters. This hot Jupiter signature has now been found for a total of three stars in the cluster alongside earlier evidence for several other planets.
A hot Jupiter is a giant exoplanet with a mass of more than
about a third of Jupiter’s mass. They are “hot” because they are
orbiting close to their parent stars, as indicated by an orbital period
(their “year”) that is less than ten days in duration. That is very
different from the Jupiter we are familiar with in our own Solar System,
which has a year lasting around 12 Earth- years and is much colder than
the Earth [4].
“We want to use an open star cluster as laboratory to explore the properties of exoplanets and theories of planet formation”, explains Roberto Saglia. “Here we have not only many stars possibly hosting planets, but also a dense environment, in which they must have formed.”
The study found that hot Jupiters are more common around stars in Messier 67 than is the case for stars outside of clusters. “This is really a striking result,” marvels Anna Brucalassi, who carried out the analysis. “The
new results mean that there are hot Jupiters around some 5% of the
Messier 67 stars studied — far more than in comparable studies of stars
not in clusters, where the rate is more like 1%.”
Astronomers think it highly unlikely that these exotic
giants actually formed where we now find them, as conditions so close to
the parent star would not initially have been suitable for the
formation of Jupiter-like planets. Rather, it is thought that they
formed further out, as Jupiter probably did, and then moved closer to
the parent star. What were once distant, cold, giant planets are now a
good deal hotter. The question then is: what caused them to migrate
inwards towards the star?
There are a number of possible answers to that question,
but the authors conclude that this is most likely the result of close
encounters with neighbouring stars, or even with the planets in
neighbouring solar systems, and that the immediate environment around a
solar system can have a significant impact on how it evolves.
In a cluster like Messier 67, where stars are much closer
together than the average, such encounters would be much more common,
which would explain the larger numbers of hot Jupiters found there.
Co-author and co-lead Luca Pasquini from ESO looks back on the remarkable recent history of studying planets in clusters: “No
hot Jupiters at all had been detected in open clusters until a few
years ago. In three years the paradigm has shifted from a total absence
of such planets — to an excess!”
Notes
[1] Some of the original sample of 88 were found to be
binary stars, or unsuitable for other reasons for this study. This new
paper concentrates on a sub-group of 66 stars.
[2] Although the cluster
Messier 67 is still holding together, the cluster that may have
surrounded the Sun in its early years would have dissipated long ago,
leaving the Sun on its own.
[3] Spectra from the High Resolution Spectrograph on the Hobby-Eberly Telescope in Texas, USA, were also used, as well as from the SOPHIE spectrograph at the Observatoire de Haute Provence, in France.
[4] The first exoplanet
found around a star similar to the Sun, 51 Pegasi b, was also a hot
Jupiter. This was a surprise at the time, as many astronomers had
assumed that other planetary systems would probably be like the Solar
System and have their more massive planets further from the parent star.
More Information
This research was presented in a paper entitled “Search for
giant planets in M67 III: excess of Hot Jupiters in dense open
clusters”, by A. Brucalassi et al., to appear in the journal Astronomy & Astrophysics.
The team consists of: A. Brucalassi (Max-Planck-Institut
für extraterrestrische Physik, Garching, Germany; University Observatory
Munich, Germany), L. Pasquini (ESO, Garching, Germany), R. Saglia
(Max-Planck-Institut für extraterrestrische Physik, Garching, Germany;
University Observatory Munich, Germany), M.T. Ruiz (Universidad de
Chile, Santiago, Chile), P. Bonifacio (GEPI, Observatoire de Paris,
CNRS, Univ. Paris Diderot, Meudon, France), I. Leão (ESO, Garching,
Germany; Universidade Federal do Rio Grande do Norte, Natal, Brazil),
B.L. Canto Martins (Universidade Federal do Rio Grande do Norte, Natal,
Brazil), J.R. de Medeiros (Universidade Federal do Rio Grande do Norte,
Natal, Brazil), 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, Sauverny, Switzerland) and S. Randich (INAF-Osservatorio
Astrofisico di Arcetri, Firenze, Italy).
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 16 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the
United Kingdom, along with the host state of Chile. 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 a major partner in ALMA, the largest astronomical
project in existence. And on Cerro Armazones, close to Paranal, ESO is
building the 39-metre European Extremely Large Telescope, the E-ELT,
which will become “the world’s biggest eye on the sky”.
Links
Contacts
Anna Brucalassi
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30000 3022
Email:
abrucala@mpe.mpg.de
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30000 3022
Email:
Luca Pasquini
ESO
Garching bei München, Germany
Tel: +49 89 3200 6792
Email: lpasquin@eso.org
ESO
Garching bei München, 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
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Hannelore Hämmerle
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30 000 3980
Email: hhaemmerle@mpa-garching.mpg.de
Max-Planck-Institut für extraterrestrische Physik
Garching bei München, Germany
Tel: +49 89 30 000 3980
Email: hhaemmerle@mpa-garching.mpg.de
Source: ESO
