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

TL;DR

This study refines the radius measurement of super-Earth 55 Cnc e using new and reanalyzed transit data from Spitzer and MOST, supporting the presence of a water-rich gaseous envelope over a rocky core.

Contribution

It provides improved radius estimates by combining multiple datasets and carefully accounting for systematic effects, confirming the planet's gaseous envelope and potential water composition.

Findings

Refined planet radius to 2.17±0.10 Rearth

Consistent results across infrared and optical data

Supports a water-rich gaseous envelope hypothesis

Abstract

We report on new transit photometry for the super-Earth 55 Cnc e obtained with Warm Spitzer/IRAC at 4.5 microns. An individual analysis of these new data leads to a planet radius of 2.21-0.16+0.15 Rearth, in good agreement 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 microns to 2.20+-0.12 Rearth. 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 Rearth 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 Rearth (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.