The CoRoT-7 planetary system: two orbiting super-Earths

Abstract

We report on an intensive observational campaign carried out with HARPS at the 3.6 m telescope at La Silla on the star CoRoT-7. Additional simultaneous photometric measurements carried out with the Euler Swiss telescope have demonstrated that the observed radial velocity variations are dominated by rotational modulation from cool spots on the stellar surface. Several approaches were used to extract the radial velocity signal of the planet(s) from the stellar activity signal. First, a simple pre-whitening procedure was employed to find and subsequently remove periodic signals from the complex frequency structure of the radial velocity data. The dominant frequency in the power spectrum was found at 23 days, which corresponds to the rotation period of CoRoT-7. The 0.8535 day period of CoRoT-7b planetary candidate was detected with an amplitude of 3.3 m s −1 . Most other frequencies, some with amplitudes larger than the CoRoT-7b signal, are most likely associated with activity. A second approach used harmonic decomposition of the rotational period and up to the first three harmonics to filter out the activity signal from radial velocity variations caused by orbiting planets. After correcting the radial velocity data for activity, two periodic signals are detected: the CoRoT-7b transit period and a second one with a period of 3.69 days and an amplitude of 4 m s −1 . This second signal was also found in the pre-whitening analysis. We attribute the second signal to a second, more remote planet CoRoT-7c . The orbital solution of both planets is compatible with circular orbits. The mass of CoRoT-7b is 4.8 ± 0.8 (M ⊕ ) and that of CoRoT-7c is 8.4 ± 0.9 (M ⊕ ), assuming both planets are on coplanar orbits. We also investigated the false positive scenario of a blend by a faint stellar binary, and this may be rejected by the stability of the bisector on a nightly scale. According to their masses both planets belong to the super-Earth planet category. The average density of CoRoT-7b is ρ = 5.6 ± 1.3 g cm −3 , similar to the Earth. The CoRoT-7 planetary system provides us with the first insight into the physical nature of short period super-Earth planets recently detected by radial velocity surveys. These planets may be denser than Neptune and therefore likely made of rocks like the Earth, or a mix of water ice and rocks.

The HARPS search for southern extra-solar planets

“…The velocity jitter is barely correlated with log R HK , or with the FWHM and the bisector of the CCF. The correlation between the FWHM and log R HK is different from the observations made for example in the case of the spotted star CoRoT-7, for which those two values are rather anti-correlated (Queloz et al 2009). Also, the apparent correlation between log R HK and the radial velocity jitter is different from the picture seen in the cases of the active stars HD 166435 (Queloz et al 2001), GJ 674 (Bonfils et al 2007) or HD 189733 (Boisse et al 2009).…”

“…Bonfils et al 2007;Forveille et al 2009), or transiting planets (e.g. Bouchy et al 2008;Queloz et al 2009). The essential Based on observations made with HARPS spectrograph on the 3.6-m ESO telescope at La Silla Observatory, Chile, under the programs ID 072.C-0488, 074.C-0364 and 078.C-0044.…”

Transiting exoplanets from the CoRoT space mission

“…This instrument was designed to discover and study in detail exoplanets for which the transits are difficult or impossible to detect from the ground. This concerns specifically small planets or "Super-Earths" (Léger et al 2009;Queloz et al 2009), i.e., planets orbiting more active stars, e.g., CoRoT2b, ) and with longer periods, e.g., CoRoT-4b, (Aigrain et al 2008;Moutou et al 2008). It is relatively easy for CoRoT to detect larger planets, similar in size to Jupiter, and a number of these have also been reported The CoRoT space mission, launched on December 27, 2006, has been developed and is being operated by CNES, with the contribution of Austria, Belgium, Brazil, ESA, The Research and Scientific Support Department of ESA, Germany and Spain.…”

“…It is therefore capable of detecting transiting planets with periods in excess of 50d. With the launch of CoRoT, which provides essentially uninterrupted and long time sequences, the detection of small planets with radii only a few times the radius of the Earth has become possible (Léger et al 2009;Queloz et al 2009). For exoplanets, the visual magnitude range observable by CoRoT is about 11-16.5 and the photometric precision is close to the photon noise in the upper half of this range (Auvergne et al 2009).…”

The silicate and carbon-rich models of CoRoT-7b, Kepler-9d and Kepler-10b

“…Its obtained mass combined with radial velocity measurements is M = 4.8 ± 0.8 M ⊕ (Queloz et al 2009). …”