Improved precision on the radius of the nearby super-Earth 55 Cnc e^⋆

¹ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5C, 4000 Liège, Belgium
e-mail: michael.gillon@ulg.ac.be
² Department of Earth, Atmospheric and Planetary Sciences, Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
³ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁴ Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland

Received: 21 October 2011
Accepted: 28 December 2011

Abstract

We report on new transit photometry for the super-Earth 55 Cnc e obtained with Warm Spitzer/IRAC at 4.5 μm. An individual analysis of these new data leads to a planet radius of $\mathrm{2.2}{\mathrm{1}}_{-0.16}^{\mathrm{+}\mathrm{0.15}}$ R_⊕, which agrees well with the values previously derived from the MOST and Spitzer transit discovery data. A global analysis of both Spitzer transit time-series improves the precision on the radius of the planet at 4.5 μm to 2.20 ± 0.12 R_⊕. We also performed an independent analysis of the MOST data, paying particular attention to the influence of the systematic effects of instrumental origin on the derived parameters and errors by including them in a global model instead of performing a preliminary detrending-filtering processing. We deduce an optical planet radius of 2.04 ± 0.15 R_⊕ from this reanalysis of MOST data, which is consistent with the previous MOST result and with our Spitzer infrared radius. Assuming the achromaticity of the transit depth, we performed a global analysis combining Spitzer and MOST data that results in a planet radius of 2.17 ± 0.10 R_⊕ (13   820 ± 620 km). These results point to 55 Cnc e having a gaseous envelope overlying a rocky nucleus, in agreement with previous works. A plausible composition for the envelope is water which would be in super-critical form given the equilibrium temperature of the planet.