A Search for short-period Rocky Planets around WDs with the Cosmic Origins Spectrograph (COS)

TL;DR

This study used archival Hubble Space Telescope data to search for transiting rocky planets around white dwarfs, setting limits on their presence and identifying potential candidates through UV and infrared observations.

Contribution

It is the first to utilize UV spectroscopy from HST to search for transiting planets around white dwarfs and to identify potential infrared excess candidates.

Findings

No transiting planets detected in the sample.

Stringent limits on planet sizes, down to sub-lunar radii.

Two infrared excess candidates suggest possible non-transiting planets.

Abstract

The search for transiting habitable exoplanets has broadened to include several types of stars that are smaller than the Sun in an attempt to increase the observed transit depth and hence the atmospheric signal of the planet. Of all spectral types, white dwarfs are the most favorable for this type of investigation. The fraction of white dwarfs that possess close-in rocky planets is unknown, but several large angle stellar surveys have the photometric precision and cadence to discover at least one if they are common. Ultraviolet observations of white dwarfs may allow for detection of molecular oxygen or ozone in the atmosphere of a terrestrial planet. We use archival Hubble Space Telescope data from the Cosmic Origins Spectrograph to search for transiting rocky planets around UV-bright white dwarfs. In the process, we discovered unusual variability in the pulsating white dwarf GD 133, which shows slow sinusoidal variations in the UV. While we detect no planets around our small sample of targets, we do place stringent limits on the possibility of transiting planets, down to sub-lunar radii. We also point out that non-transiting small planets in thermal equilibrium are detectable around hotter white dwarfs through infrared excesses, and identify two candidates.