Gliese 832c. ¿Actividad Estelar o Exoplaneta? .

Abstract

Exoplanets are planets located outside our Solar System. The search of these objects have grown during the years due to the scientific interest and to the advances on astronomical instrumentation. There are many methods used to detect exoplanets, where one of the most efficient is the radial velocity (RV) method. But this technique accounts false positives as stellar activity can produce RV variation with an amplitude of the same order of the one induced by a planetary companion. In this thesis, we study Gliese 832, an M dwarf located 4.96 pc away from us. Two planets orbiting this star were found independently by the RV method: a gas-giant planet in a wide orbit, and a super Earth or mini-Neptune located within the stellar habitable zone. However, the orbital period of this latter planet is close to the stellar rotation period, casting doubts on the planetary origin of this RV signal. This motivated us to perform a rigours analysis on stellar activity in order to determine if this phenomenon is causing these RV variations. We re-calculated the period of the stellar rotation using Gaussian process (GP) regression on the S-index activity indicator. This resulted in a stellar rotation period of 35.76 +0.95 −0.26 days, in agreement with the reported value but reducing the errors by 89.78%. By performing a 1-planet Keplerian model, the Generalised Lomb Scargle (GLS) periodogram displayed the strongest signal around the reported planet in a wide orbit (planet b). The periodogram of the residuals showed a significant (FAP < 1%) signal near 184, while the signal of the inner planet (planet c) does not reach this level of confidence. By incorporating a GP trained on the S-index, these signals were absorbed. We subsequently made 2-planets Keplerian models including the signal of planet b plus the 35 and 184 days signal in different models, and then added a GP (stellar activity) to each model. By comparing our different models in a Bayesian framework, the favored model resulted to be 1-planet plus stellar activity model, updating in this way the orbital solutions of the Gl 832 system. Since the 35 days signal is attributable to stellar rotation, we conclude planet c is an artifact of stellar activity.