A search for transits of GJ 581\lowercase{e} and characterization of the host star variability using MOST space telescope photometry

TL;DR

This study used space-based photometry to search for transits of GJ 581e and analyze the star's variability, setting limits on possible planetary transits and observing stellar brightness stability.

Contribution

First to analyze GJ 581's stellar variability and search for transits of GJ 581e using MOST data, providing constraints on planetary transits and stellar activity.

Findings

Ruled out transits of GJ 581e for planets larger than 1.62 R⊕ at 2σ confidence.

Excluded transits of GJ 581b within certain impact parameters.

Detected stellar brightness stable within 1%, with a possible stellar signal at 5.586 days.

Abstract

The GJ 581 system has been amply studied since its discovery in 2005: the number of known planets in the system has increased and their orbital parameters are among the most precisely determined for radial velocity detected exoplanets. We have acquired MOST space-based photometry during 2007 and 2009, with the aims of measuring the stellar variability and searching for transits of GJ 581e, respectively. We quantify our sensitivity to shallow transit signals using Monte Carlo simulations, and perform a transit search within the 3$\sigma$ transit windows corresponding to both the circular and Keplerian orbit ephemerides. Our analysis rules out transits for a planet with an orbital period of 3.15 days (GJ 581 e) having a radius larger than 1.62 $R_{\oplus}$ (or a density lower than 2.39 g cm$^{-3}$ for an orbital inclination of 90$^{\circ}$) to 2$\sigma$ confidence. Thus, if the planet transits, we can exclude hydrogen, helium and water theoretical model compositions. The MOST photometry also allows us to rule out transits of GJ 581b within the Keplerian orbit-derived transit window for impact parameter values smaller than $\sim$0.4 and confirm previous results which exclude transits for this planet within the circular orbit-derived transit window, for all plausible interior compositions. We find that the stellar brightness of GJ 581 is stable to within 1%, a characteristic which is favourable to the development of life in the habitable zone of the system. In the 2009 photometry, we detect a stellar signal with a period of 5.586 $\pm$ 0.051 days, which is close to the orbital period of GJ 581b ($P=$5.37 days). However, further monitoring of the system is necessary to verify the nature of this variation.