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  ASTRONOMY  -  HD69830 and its three Neptunes

It seems that a trio of planets the size of Neptune are orbiting a star that resembles our own Sun. This latest advance in the discovery of exoplanets is the culmination of two years of study by teams of European astronomers – assisted by the Harps spectograph fitted to one of the European Southern Observatory’s telescopes in La Silla (Chile) – and could prove invaluable in furthering our understanding of the universe.

Artist’s impression of star HD69830 © ESO
Artist’s impression of star HD69830. The star closest to its sun (about 10 Earth masses) completes its orbit in nine days, the middle star (12 masses) in 31 days and the outer planet (18 masses) in 197 days. Although an asteroid belt is also believed to exist, its location remains unknown.
© ESO
"The planetary system around star HD69830 represents a Rosetta stone in our understanding of how planets form. No doubt it will help us to understand the diversity we have observed since the first extra-solar planet was discovered 11 years ago,” explains Michel Mayor. It was in 1995 that, together with his colleague Didier Queloz, the director of the Geneva Observatory discovered the first extra-solar planet (51 Pegasi). Today he is one of the team of European scientists who have just revealed the existence of the ‘trio of Neptunes’ orbiting a star in the Stern constellation, 40 light years from our own solar system.

The study of exoplanets is one of the priorities of cosmologists who regard it as a tool to our understanding of the solar system, of the planet Earth and of the search for life in the universe. These distant giants, of between five and twenty times the mass of the Earth, revolve around a different sun to our own. Often lying very close to their sun, it is not unusual for them to have a very short orbiting cycle. Many of them are invisible and their existence must be deduced from the gravitational wobble they produce on their home star. In the case of HD69830, the very advanced technology of the Harps spectograph – "currently the world’s most precise planet hunter”, according to Mayor – made it possible to detect variations in the star’s velocity of just 9km/hr.

These three exoplanets have a mass that is close to that of Neptune (17 times that of the Earth) and constitute the distant solar system whose scale is closest to our own. Simulations enable us to situate them at distances from their star that are much less than the 150 million km that separates the Earth from the Sun. As to their structure, calculations based on planetary formation models tell us that the planet closest to HD69830 is essentially rocky, that the middle one is half-rocky and half-gaseous, and that the third consists of a gaseous envelope around a core of rock and ice. The latter also lies in the ‘habitable zone’ that is sufficiently distant from its star for liquid water to be present on the surface of a solid planet.  

"All this adds up to an exceptional system,” remarks Willy Benz of Bern University. “But the recent discovery by NASA’s Spitzer space telescope of the associated existence of a possible asteroid belt is the cherry on the cake.” This discovery, linked to strong infrared emissions attributable to numerous collisions between the second and third exoplanet, suggests that the system is evolving.


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