有五到七颗行星。
Richest Planetary System Discovered
Up to seven planets orbiting a Sun-like star
24 August 2010
Astronomers using ESO’s world-leading HARPS instrument have discovered a
planetary system containing at least five planets, orbiting the Sun-like star
HD 10180. The researchers also have tantalising evidence that two other
planets may be present, one of which would have the lowest mass ever found.
This would make the system similar to our Solar System in terms of the number
of planets (seven as compared to the Solar System’s eight planets).
Furthermore, the team also found evidence that the distances of the planets
from their star follow a regular pattern, as also seen in our Solar System.
“We have found what is most likely the system with the most planets yet
discovered,” says Christophe Lovis, lead author of the paper reporting the
result. “This remarkable discovery also highlights the fact that we are now
entering a new era in exoplanet research: the study of complex planetary
systems and not just of individual planets. Studies of planetary motions in
the new system reveal complex gravitational interactions between the planets
and give us insights into the long-term evolution of the system.”
The team of astronomers used the HARPS spectrograph, attached to ESO’s 3.6-
metre telescope at La Silla, Chile, for a six-year-long study of the Sun-like
star HD 10180, located 127 light-years away in the southern constellation of
Hydrus (the Male Water Snake). HARPS is an instrument with unrivalled
measurement stability and great precision and is the world’s most successful
exoplanet hunter.
Thanks to the 190 individual HARPS measurements, the astronomers detected the
tiny back and forth motions of the star caused by the complex gravitational
attractions from five or more planets. The five strongest signals correspond
to planets with Neptune-like masses — between 13 and 25 Earth masses [1] —
which orbit the star with periods ranging from about 6 to 600 days. These
planets are located between 0.06 and 1.4 times the Earth–Sun distance from
their central star.
“We also have good reasons to believe that two other planets are present,”
says Lovis. One would be a Saturn-like planet (with a minimum mass of 65 Earth
masses) orbiting in 2200 days. The other would be the least massive exoplanet
ever discovered, with a mass of about 1.4 times that of the Earth. It is very
close to its host star, at just 2 percent of the Earth–Sun distance. One “
year” on this planet would last only 1.18 Earth-days.
“This object causes a wobble of its star of only about 3 km/hour— slower
than walking speed — and this motion is very hard to measure,” says team
member Damien Ségransan. If confirmed, this object would be another example
of a hot rocky planet, similar to Corot-7b (eso0933).
The newly discovered system of planets around HD 10180 is unique in several
respects. First of all, with at least five Neptune-like planets lying within a
distance equivalent to the orbit of Mars, this system is more populated than
our Solar System in its inner region, and has many more massive planets there
[2]. Furthermore, the system probably has no Jupiter-like gas giant. In
addition, all the planets seem to have almost circular orbits.
So far, astronomers know of fifteen systems with at least three planets. The
last record-holder was 55 Cancri, which contains five planets, two of them
being giant planets. “Systems of low-mass planets like the one around HD 1018
0 appear to be quite common, but their formation history remains a puzzle,”
says Lovis.
Using the new discovery as well as data for other planetary systems, the
astronomers found an equivalent of the Titius–Bode law that exists in our
Solar System: the distances of the planets from their star seem to follow a
regular pattern [3]. “This could be a signature of the formation process of
these planetary systems,” says team member Michel Mayor.
Another important result found by the astronomers while studying these systems
is that there is a relationship between the mass of a planetary system and
the mass and chemical content of its host star. All very massive planetary
systems are found around massive and metal-rich stars, while the four lowest-
mass systems are found around lower-mass and metal-poor stars [4]. Such
properties confirm current theoretical models.
The discovery is announced today at the international colloquium “Detection
and dynamics of transiting exoplanets”, at the Observatoire de Haute-Provence
, France.
Notes
[1] 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.
[2] On average the planets in the inner region of the HD 10180 system have 20
times the mass of the Earth, whereas the inner planets in our own Solar System
(Mercury, Venus, Earth and Mars) have an average mass of half that of the
Earth.
[3] The Titius–Bode law states that the distances of the planets from the Sun
follow a simple pattern. For the outer planets, each planet is predicted to
be roughly twice as far away from the Sun as the previous object. The
hypothesis correctly predicted the orbits of Ceres and Uranus, but failed as a
predictor of Neptune’s orbit.
[4] According to the definition used in astronomy, “metals” are all the
elements other than hydrogen and helium. Such metals, except for a very few
minor light chemical elements, have all been created by the various
generations of stars. Rocky planets are made of “metals”.
More information
This research was presented in a paper submitted to Astronomy and Astrophysics
(“The HARPS search for southern extra-solar planets. XXVII. Up to seven
planets orbiting HD 10180: probing the architecture of low-mass planetary
systems” by C. Lovis et al.).
The team is composed of C. Lovis, D. Ségransan, M. Mayor, S. Udry, F. Pepe,
and D. Queloz (Observatoire de Genève, Université de Genève, Switzerland),
W. Benz (Universität Bern, Switzerland), F. Bouchy (Institut d’Astrophysique
de Paris, France), C. Mordasini (Max-Planck-Institut für Astronomie,
Heidelberg, Germany), N. C. Santos (Universidade do Porto, Portugal), J.
Laskar (Observatoire de Paris, France), A. Correia (Universidade de Aveiro,
Portugal), and J.-L. Bertaux (Université Versailles Saint-Quentin, France)
and G. Lo Curto (ESO).
ESO, the European Southern Observatory, is the foremost intergovernmental
astronomy organisation in Europe and the world’s most productive astronomical
observatory. It is supported by 14 countries: Austria, Belgium, 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 VISTA, the world’s largest survey
telescope. ESO is the European partner of a revolutionary astronomical
telescope ALMA, the largest astronomical project in existence. ESO is
currently planning a 42-metre European Extremely Large optical/near-infrared
Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”
.
Contacts
Christophe Lovis
Observatoire de l’Université de Genève
Switzerland
Cell: +41 787 280 354
Email: christophe.lovis@unige.ch
Damien Ségransan
Observatoire de l’Université de Genève
Switzerland
Tel: +41 223 792 479
Email: damien.segransan@unige.ch
Francesco Pepe
Observatoire de l’Université de Genève
Switzerland
Tel: +41 223 792 396
Email: francesco.pepe@unige.ch
Richard Hook
La Silla, Paranal, E-ELT & Survey Telescopes Press Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org |
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