October 2017
Carlos Saffe
Rocky Planet Engulfment Explains
Stellar Odd Couple
To date, astronomers have found more than 3,600 planets orbiting
around stars in the solar neighborhood. Nevertheless, current
observational techniques challenge those searching for possible
planets engulfed by their host star. By using Gemini North + GRACES
high-resolution spectra in the Fast Turnaround mode, we have found
a notable difference in the chemical pattern between the stars of the
HAT-P-4 binary system, which could be attributed to the ingestion of
at least ~10 M Earth of rocky material onto the primary star.
The story begins with the star HAT-P-4, which hosts a giant planet detected by the Hungar-
ian Automated Telescope Network (HATNet) transit survey (Kovács et al., 2007). This plane-
tary companion, designated HAT-P-4b, has a mass of 0.68 M JUP (between the mass of Jupiter
and Saturn), and orbits the star at a distance of only 0.04 astronomical units — about ten
times closer than the distance between Mercury and the Sun. The planet’s estimated densi-
ty of ~0.4 grams per cubic centimeter (g/