Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb

TL;DR

This study used the Hubble Space Telescope to search for transits of the Earth-mass exoplanet Alpha Centauri Bb, achieving high-precision photometry and ruling out its transiting nature at 96.6% confidence, while also detecting a potential new Earth-sized planet.

Contribution

First high-precision, continuous photometric search for transits of Alpha Centauri Bb with HST, demonstrating the telescope's capability for saturated star observations and constraining the planet's transiting status.

Findings

Alpha Centauri Bb does not transit at 96.6% confidence.

Detected a single transit-like event suggesting a possible additional Earth-sized planet.

HST can perform high-precision photometry on saturated stars over long durations.

Abstract

Results from exoplanet surveys indicate that small planets (super-Earth size and below) are abundant in our Galaxy. However, little is known about their interiors and atmospheres. There is therefore a need to find small planets transiting bright stars, which would enable a detailed characterisation of this population of objects. We present the results of a search for the transit of the Earth-mass exoplanet Alpha Centauri Bb with the Hubble Space Telescope (HST). We observed Alpha Centauri B twice in 2013 and 2014 for a total of 40 hours. We achieve a precision of 115 ppm per 6-s exposure time in a highly-saturated regime, which is found to be consistent across HST orbits. We rule out the transiting nature of Alpha Centauri Bb with the orbital parameters published in the literature at 96.6% confidence. We find in our data a single transit-like event that could be associated to another Earth-size planet in the system, on a longer period orbit. Our program demonstrates the ability of HST to obtain consistent, high-precision photometry of saturated stars over 26 hours of continuous observations.