© ESO Artist rendering of HD69830 and its three planets detected by the improved HARPS instrument measurements.
Artist rendering of HD69830 and its three planets detected by the improved HARPS instrument measurements.
Astronomers have discovered more than 180 extrasolar planets in the last decade, 17 of which are multi-planet systems. However, most of these systems have included at least one “gas giant”–a planet that is about the same size as Jupiter, or about 320 times as big as the Earth. Now, Christophe Lovis and colleagues have found a new extrasolar system that is different because it contains three planets that are much smaller at just 10.2, 11.8, and 18.1 times as massive as Earth, or around the same size as Neptune.2
One way of detecting an extrasolar planet is by the Doppler method where astronomers measure the “wobble” (or change in velocity) the planet causes in the star it orbits around. This method is good at uncovering heavier planets because such bodies produce larger stellar wobbles that can be more easily detected.
Lovis and co-workers have improved on this technique, thanks to the sensitivity of the European Southern Observatory's HARPS3 instrument, and have so been able to detect small radial-velocity variations of the star HD69830, due to smaller planets orbiting around it. The detected velocity variations are between just 2 and 3 m/s, which is about 9 km/h. Normally, such tiny signals cannot be distinguished from simple “noise” by most of today's available spectrographs.
HD69830 is about 41 light years away, located towards the constellation of Puppis. “Among the known planetary systems, HD69830 is quite similar to our own solar system because it contains three non-giant planets located on nearly circular orbits within 1 Astronomical Unit (AU),4 like Mercury, Venus and Earth, but 10 to 20 times more massive,” explains team member François Bouchy.5 It also contains an asteroid belt, like our solar system. The three planets have orbital periods (the time it takes to orbit HD69830) of 8.67, 31.6, and 197 days respectively.
Extensive theoretical simulations indicate that the planet closest to the star consists of a rocky core, with a possible tiny gaseous envelope or atmosphere. The outermost planet and the middle one both contain a central rocky core (containing rock and ice) and a huge gaseous envelope. More importantly, the outermost planet appears to lie within the star's “habitable zone” in which liquid water–often thought to be a prerequisite for life as we know it–could exist on the planet's surface. However, the researchers have calculated that this planet's water would not be able to support life because it is at high pressure and temperature, a state called supercritical.6
Although the outermost planet is also unlikely to be Earth-like because of its size, its discovery opens the way to the detection of even smaller planets in the near future. From a theoretical point of view, this system could also put some strong constraints on planet formation models and will therefore improve our understanding of planet formation as a whole.
“We now expect to detect similar planets with transit surveys like 'Corot,”7 says Bouchy. A complementary method called the “transit” technique–where a planet causes its host star to “blink” as it passes in front of it–would allow the astronomers to measure the size of these future planets. Combining these methods would allow the mass, and therefore density, of the planets to be determined.
“Moreover, the SOPHIE spectrograph,8 which aims to reach the same precision as HARPS, is expected to detect and characterize tens of such exoplanetary systems in the coming years,” adds Bouchy.
1. The team comprises French, Swiss, and Portuguese researchers.
2. C. Lovis et al., “Discovery of an extrasolar planetary system with three Neptune-Mass Planets,” Nature. 441: 305. 2006.
3. The High Accuracy Radial velocity Planet Searcher (HARPS), at the ESO La Silla 3.6 m telescope, is dedicated to the discovery of extrasolar planets.
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4. One AU is the average distance between the Earth and the Sun.
5. Institut d'astrophysique de Paris (CNRS / Paris-VI joint lab).
6. Y. Alibert et al., “Formation and structure of three Neptune mass planets system around HD69830,” Astronomy and Astrophysics Letters. in press.
7. The French-led mission Corot is the first mission to search for rocky planets around stars. It will be launched in October 2006.
8. The SOPHIE echelle spectrograph, operational in September 2006, is being built at the Haute Provence Observatory.