"This is science at its thrilling and amazing best," says Didier Queloz, leader of the team who made the observations. "We did everything we could to learn what the object discovered by the CoRoT satellite looks like and we found a unique system."
In February 2009, a small exoplanet was discovered around a rather unremarkable star named TYC 4799-1733-1 by the French CNES CoRoT satellite. The star, known since as CoRoT-7, is located towards the constellation Monoceros (the Unicorn) at a distance of about 500 light-years. Slightly smaller and cooler than our Sun, CoRoT-7 is thought to be younger as well, with an age about 1.5 billion years. Every 20.4 hours, the planet eclipses a small fraction of the light of the star for a little over one hour by one part in 3,000 [1]. This planet, designated CoRoT-7b, is only 2.5 million kilometres away from its host star, or 23 times closer than Mercury is to the Sun. It has a radius of less than twice that of Earth.
The initial set of measurements, however, could not provide the mass of the exoplanet. Such a result requires extremely precise measurements of the velocity of the star, which is pulled a tiny amount by the gravitational tug of the orbiting exoplanet. The problem with CoRoT-7b is that these tiny signals are jumbled by stellar activity in the form of ’starspots’ (just like sunspots on our Sun), which are cooler regions on the surface of the star. Therefore, the main signal is linked to the rotation of the star, with makes one complete revolution in about 23 days.
To get an answer, astronomers had to call upon the best exoplanet-hunting device in the world, the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph attached to the ESO 3.6-metre telescope at the La Silla Observatory in Chile. "Even though HARPS is certainly unbeaten when it comes to detecting small exoplanets, the measurements of CoRoT-7b proved to be so demanding that we had to gather no less than 70 hours of observations on the star," says co-author François Bouchy. HARPS delivered, allowing the astronomers to tease out the 20.4-hour signal in the data. This figure led them to infer that CoRoT-7b has a mass of about five Earth masses, placing it in rare company as one of the lightest exoplanets yet found.
"Thanks to the fact that it is transiting it is the smallest mass ever precisely measured for an exoplanet [2]," says team member Claire Moutou. "Moreover, as we have both the radius and the mass, we can determine the density and get a better idea of the internal structure of this planet."
With a mass much closer to that of Earth than for example ice giant Neptune’s 17 Earth masses, CoRoT-7b belongs to the exoplanetary category of ’super-Earths’. About a dozen of these bodies have been detected, though in the case of CoRoT-7b, this is the first time that the density has been measured for such a small exoplanet. The calculated density is close to Earth’s, suggesting that the planet.s composition is similarly rocky.
"CoRoT-7b resulted in a ’tour de force’ of astronomical measurements. The superb light curves of the space telescope CoRoT gave us the best radius measurement, and HARPS the best mass measurement for an exoplanet. Both were needed to discover a rocky planet with the same density as the earth," says co-author Artie Hatzes.
CoRoT-7b earns another distinction as the closest known exoplanet to its host star, which also makes it the fastest - it orbits its star at a speed of 216 kilometres per second- .In fact, CoRoT-7b is so close that the place may well look like Dante’s Inferno. With a probable temperature well above 1000 degrees, the planet may have lava or boiling oceans on its surface. Such a ’sauna planet’ is definitively not a place for life to develop, says Queloz.
As a further testament to HARPS. sublime precision, the astronomers found from their dataset that CoRoT-7 hosts another exoplanet slightly further away than CoRoT-7b. Designated CoRoT-7c, it circles its host star in 3 days and 17 hours and has a mass about eight times that of Earth, so it too is classified as a super-Earth. Unlike CoRoT-7b, this sister world does not pass in front of its star as seen from Earth, so astronomers cannot measure its radius and thus its density.
Given these findings, CoRoT-7 stands as the first star known to have a planetary system made of two short period super-Earths with one that transits its host.
"These are very exciting times," concludes Queloz. "We may be very close to finding a sister planet to Earth, especially when new instruments, in particular the ESPRESSO spectrograph planned for ESO’s Very Large Telescope, will be online."
Notes [1] It is exactly the same effect in our Solar System when Venus transits the solar disc, as it did on 8 June 2004 (ESO PR 03/04). In the past centuries such events were used to estimate the Sun-Earth distance, with extremely useful implications for astrophysics and celestial mechanics. [2] Gliese 581e, also discovered with HARPS, has a minimum mass about 2 times the Earth’s mass (see ESO 15/09) but the exact geometry of the orbit is undefined making its real mass unknown.
(A) The team is composed of : D. Queloz, R. Alonso, C. Lovis, M. Mayor, F. Pepe, D. Segransan, and S. Udry (Observatoire de Genève, Switzerland), F. Bouchy, F. and G. Hebrard, G. (IAP, Paris, France), C. Moutou, M. Barbieri, P. Barge, M. Deleuil, L. Jorda, and A. Llebaria (Laboratoire d.Astrophysique de Marseille, France), A. Hatzes, D. Gandolfi, E. Guenther, M. Hartmann, and G. Wuchterl (Thüringer Landessternwarte Tautenburg, Germany), M. Auvergne, A. Baglin and D. Rouan (LESIA, CNRS, Observatoire de Paris-Meudon, France), W. Benz (University of Bern, Switzerland), P. Bordé, A. Léger and M. Ollivier (IAS, UMR 8617 CNRS, Université Paris-Sud, France), H. Deeg (Instituto de Astrofísica de Canarias, Spain), R. Dvorak (University of Vienna, Austria), A. Erikson and H. Rauer (DLR, Berlin, Germany), S. Ferraz Mello (IAG-Universidade de Sao Paulo, Brazil), M. Fridlund (European Space Agency, ESTEC, The Netherlands), M. Gillon and P. Magain (Université de Liège, Belgium), T. Guillot (Observatoire de la Côte d.Azur, CNRS UMR 6202, Nice France), H. Lammer (Austrian Academy of Sciences), T. Mazeh (Tel Aviv University, Israel), M. Pätzold (Köln University, Germany), and J. Schneider (LUTH, Observatoire de Paris-Meudon, France).
ESO Press Release CNRS Press release
Reference
The CoRoT-7 planetary system : two orbiting Super-Earths by D. Queloz et al. , Astronomy & Astrophysics
Contact
Michel Auvergne (Observatoire de Paris, LESIA, et CNRS)
Annie Baglin (Observatoire de Paris, LESIA, et CNRS)
Daniel Rouan (Observatoire de Paris, LESIA, et CNRS)
Jean Schneider (Observatoire de Paris, LUTH, et CNRS)