No Conclusive Evidence for Transits of Proxima b in MOST Photometry

Abstract

The analysis of Proxima Centauri's radial velocities recently led Anglada-Escudé et al. (2016) to claim the presence of a low mass planet orbiting the Sun's nearest star once every 11.2 days. Although the a-priori probability that Proxima b transits its parent star is just 1.5%, the potential impact of such a discovery would be considerable. Independent of recent radial velocity efforts, we observed Proxima Centauri for 12.5 days in 2014 and 31 days in 2015 with the MOST space telescope. We report here that we cannot make a compelling case that Proxima b transits in our precise photometric time series. Imposing an informative prior on the period and phase, we do detect a candidate signal with the expected depth. However, perturbing the phase prior across 100 evenly spaced intervals reveals one strong false-positive and one weaker instance. We estimate a false-positive rate of at least a few percent and a much higher false-negative rate of 20-40%, likely caused by the very high flare rate of Proxima Centauri. Comparing our candidate signal to HATSouth ground-based photometry reveals that the signal is somewhat, but not conclusively, disfavored (1-2 σ) leading us to argue that the signal is most likely spurious. We expect that infrared photometric follow-up could more conclusively test the existence of this candidate signal, owing to the suppression of flare activity and the impressive infrared brightness of the parent star.

Searching for the Transit of the Earth-mass Exoplanet Proxima Centauri b in Antarctica: Preliminary Result

“…GP regression has been used to model the systematics in a nonparametric way for planetary transit and RV observations (e.g., Gibson et al 2012;Rajpaul et al 2015;Kipping et al 2017). Furthermore, GP regression is an interpolation, as well as a , where σ s =4.6 mmag is the average precision of a single photometric datum.…”

“…Using the RV solution of Anglada-Escudé et al (2016), Kipping et al (2017) derived the time of inferior conjunction T IC =2,456,678.78±0.56 HJD at an original epoch and the orbital period P=11.1856± 0.0013 days. We adopt the transit ephemeris in Kipping et al (2017) to predict the time of transit window in the BSST light curve. The first observing night is partially covered by the 1σ window of interest (T IC =2,457,629.56 ± 0.67 HJD for the …”

“…Note that the fit is just nominal, as correlated noise is not considered fully. We also attempt to fit the observed flux directly with a full model (Gibson et al 2012;Kipping et al 2017), which is a planetary transit model joined with a GP to model the correlated noise. However, the parameters of the transit model are not converged.…”

“…ProximaCentauri is a moderately active star and has a rotation period of ∼83 days (Christian et al 2004;Kiraga & Stepien 2007;Davenport et al 2016). Time-correlated structure and long-term trends are pronounced in its light curve (Kipping et al 2017). Considering that the expected transit duration of ProximaCentaurib is about 1 hr, the MOST light curve, with discrete sampling windows with width of <50 minutes but separated by >50 minutes, is not optimal for disentangling the potential transit signals from time-correlated noise.…”

“…The first attempt was made by Kipping et al (2017) using archival data of the Microvariability and Oscillation of Stars (MOST) space telescope observed in 2014 (for 12.5 days) and in 2015 (for 31 days). ProximaCentauri was observed for a fraction (<50%) of each MOST 101 minute orbit when it was observable, with an average sampling rate of about 1 minute.…”