Born Dry in the Photo-Evaporation Desert: Kepler's Ultra-Short-Period Planets Formed Water-Poor

Understand

Recent surveys have uncovered an exciting new population of ultra-short-period (USP) planets with orbital periods less than a day.

• These planets typically have radii <1.5 Earth radii, indicating that they likely have rocky compositions.
• This stands in contrast to the overall distribution of planets out to ~100 days, which is dominated by low-density sub-Neptunes above 2 Earth radii, which must have gaseous envelopes to explain their size.
• However, on ultra-short-period orbits, planets are bombarded by intense levels of photo-ionizing radiation and consequently gaseous sub-Neptunes are extremely vulnerable to losing their envelopes to atmospheric photo-evaporation.