Collaborative Research: Unveiling the Composition of Earth-sized Planets with the Keck Planet Finder

The majority of rocky planets that have both size and mass measured are much bigger than Earth, and studies suggest that an Earth-like composition may be common among them. However, the interior composition of truly Earth-sized planets remains largely unexplored. This proposal makes use of the newly-commissioned Keck Planet Finder (KPF) spectrograph to precisely measure the masses of 12 Earth-sized exoplanets whose size was already determined by transit measurements. These precise mass measurements are necessary in advance of more detailed characterization efforts. The project personnel will support outreach and educational activities: mentoring opportunities in Hawaii and lecture series in both English and Spanish for the general public in Southern California. Giant impact simulations predict that Earth-sized planets may exhibit greater diversity in their interior composition than super-Earths: they likely experience only a few giant impact collisions, whereas super-Earths undergo dozens. Hit-and-run collisions could make the remnant planet denser. If mass growth is mainly by pebble accretion, interior compositions could be size-independent but depend on the stellar composition. This program will test these hypotheses using one of the most advanced echelle spectrometers ever built: KPF is designed to achieve radial velocity precision of 30 cm/s. Coupled with the 10m Keck telescope, it is the most powerful and efficient system in the Northern Hemisphere for this work. Subsequent to this work, the planets may be suitable for follow-up measurements to characterize their atmospheres or the mineralogy of their bare rock surfaces. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.