Research

Planet Occurrence Around Evolved Stars

In 2016, I began a search for giant planets orbiting giant stars using data from the NASA K2 Mission. So far, our search has identified two new planets, K2-97b and K2-132b, whose discoveries we published here and here. Both of these planets are gas giants, 1.3 times the size and half the mass of Jupiter, orbiting their host stars approximately every 9 days. These targets were identified and accurately and precisely characterized with asteroseismology, the study of oscillations of stars. We find that among stars that are 3 to 8 times the size of our Sun, it seems that planets larger than the size of Jupiter at orbital periods of 10 days or less are significantly more common than such planets are around main sequence, Sunlike stars. With the launch of TESS in early 2018, we will soon be sensitive to these types of systems all over the sky, revolutionizing our understanding of planetary evolution and tidal dynamics. These two poorly understood processes are crucial to life everywhere. Check back in a few months for more updates!

Eccentric Giants Orbiting Giants

Studies have predicted that as main sequence stars evolve into red giants, any long-period, massive planets on eccentric orbits should be pulled closer and into more circular orbits. This should create a population of short-period, moderately eccentric giant planets orbiting giant stars. In 2018, analysis of radial velocity measurements from the Keck-I telescope revealed that both planets discovered by our survey of giant planets orbiting giant stars were on moderately eccentric orbits, suggesting that they may have migrated to their current locations from longer period, more eccentric orbits. A similar Kepler planetary system revealed a slightly more eccentric, longer-period planet orbiting a less evolved star, indicative of an evolutionary sequence.

Comparing the populations of all known planets with measured eccentricities reveals that close-in giant planets seem to be significantly more eccentric around giant stars than dwarf stars. Additional information about this study can be found by reading the paper here or by watching the Aloha Brief below.

Planet Re-Inflation

A large fraction of gas giant planets with temperatures above 1000 K are much larger than 1.2 times the size of Jupiter, the maximum size models predict for a self-gravitating sphere of hydrogen and helium. Though these inflated giant planets have been known for over twenty years, the mechanism responsible for their inflation remains unclear. However, if we were to observe an inflated planet receiving a moderate amount of radiation from a red giant host star, such that the planet would have been too cool to inflate until the host star became a red giant, this would provide evidence for a planet inflation mechanism where the stellar irradiation deposited into the planet's interior causes it to expand. In December 2016, I discovered K2-97b, an inflated planet orbiting a red giant star every 8.4 days, and discovered its cosmic twin a year later. These planets' incident flux history indicate they were too cool to inflate until their host stars became red giants, and thus provide the first evidence that planets may be inflated directly by incident stellar radiation rather than by delayed loss of heat from formation. Assuming that gas planets begin their lives inflated due to heat from formation, K2-97b and K2-132b are likely the first known planets to be currently re-inflated. For more information, check out the original discovery papers: K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star and Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars.