KELT-9b: A Case Study in Dynamical Planet Ingestion by a Hot Host Star

Nearly all of the known transiting extra-solar planets orbit stars with masses similar to, or less massive than, the Sun. Such stars typically do not evolve substantially over their hydrogen-fusion lifetime of roughly 10 billion years or more. In contrast, stars much more massive than the Sun evolve on much shorter timescales, and thus the planets they host represent important test cases for how such systems evolve as their parent stars expand -- perhaps engulfing their planets. Most planetary systems orbiting such massive stars have been found around stars that have already exhausted their core hydrogen, cooled, swelled, and likely erased the knowledge of their progenitor close-in planetary systems. In contrast, KELT-9b is a "hot Jupiter" orbiting a star with a mass of 2.2 Msun. The star is still unevolved and therefore still very hot (surface temperature ~ 10,000 K) and therefore the planet is extremely hot. The planet is on a near-polar orbit, likely resulting in orbital precession that will be detectable within a few years. Given the extreme planet temperature, large planet-to-star radius ratio, large planetary atmospheric scale height, and short orbital period, this system is an exceptionally good target for follow-up studies of the planet's atmosphere, which may exhibit unusual photochemistry due to the extreme amounts of high-energy radiation it receives.