http://www.sciencedaily.com/releases/2010/08/100824082222.htm
ScienceDaily (Aug. 24, 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 10180
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 was announced Aug. 24 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).
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