When a star evolves into a red giant towards the end of its life, planets that orbit the star at sufficiently close distances are expected to be engulfed by the star. Two Earth-mass planets around the hot B subdwarf star KOI 55 could be remnants of the metallic core of a single massive gas-giant planet that spiralled into the stellar envelope of KOI 55 when the star ballooned into a red giant. As the gas-giant planet spiralled in, it released gravitational energy into the red giant which removed most of the star’s outer layers. This process transformed KOI 55 from a red giant star into an extreme horizontal branch (EHB) star, which is essentially the leftover core of the red giant star. Basically, KOI 55 consists of a helium burning core that is surrounded by a thin envelope of hydrogen and it is type of EHB star known as a hot B subdwarf.
The gas-giant planet that spiralled into KOI 55 had all of its gaseous outer layers tidally stripped from it and the leftover metallic core of the planet was probably further destructed into several Earth-mass bodies. For this reason, the two Earth-mass planets around KOI 55 are likely to be two of the surviving Earth-mass bodies. KOI 55 has half the mass of the Sun, one fifth the Sun’s diameter, 23 times the Sun’s luminosity and an effective surface temperature of 27,730 degrees Kelvin. Even though KOI 55 is smaller in size than the Sun, it is many times more luminous because of its very large effective surface temperature. In comparison, the Sun has an effective surface temperature of 5,778 degrees Kelvin.
Credit: Sami Mattila
The two Earth-mass planets around KOI 55 are denoted KOI 55.01 and KOI 55.02 respectively. KOI 55.01 and KOI 55.02 orbit at distances of 0.0060 and 0.0076 AU, with orbital periods of 5.7625 and 8.2293 hours respectively. 1 AU is a unit of measurement which denotes the mean Earth-Sun separation distance. The orbits of KOI 55.01 and KOI 55.02 are close to a 3:2 resonance where the outer planet orbits twice for every three orbits of the inner planet. Being situated in such close proximity to a hot and luminous star, the dayside temperature of KOI 55.01 is estimated to be 9100 degrees Kelvin while the dayside temperature of KOI 55.02 is estimated to be 8100 degrees Kelvin. This makes the dayside surfaces of both planets hotter than the surfaces of most stars.
| KOI 55.01 | KOI 55.02 | |
| Orbital Period (hour): | 5.7625 | 8.2293 |
| Orbital Distance (kilometre): | 896,980 | 1,137,490 |
| Orbital Distance (AU): | 0.0060 | 0.0076 |
| Mean Dayside Temperature (Kelvin): | 9100 | 8100 |
| Planet Radius (Earth = 1.0): | 0.759 | 0.867 |
| Planet Mass (Earth = 1.0): | 0.440 | 0.655 |
KOI 55 is so hot that it emits most of its energy in the form of ultraviolet radiation. Orbiting at such close proximity to KOI 55, the two Earth-mass planets are expected to be blow-torched by intense radiation. This can cause severe ionization and evaporation of the planets’ material, leading to the complete vaporization of both planets within a few tens of million years. However, both planets are likely to posses global magnetic fields that are many time stronger than the Earth’s. A strong magnetic field can substantially reduce the rate of evaporation by keeping the ionized gases from escaping the planet. This can allow both planets to survive through the EHB phase of KOI 55.
The future of these two Earth-mass planets will also depend directly on the evolution of KOI 55. As an EHB star, KOI 55 will eventually run out of helium to burn and settle into a white dwarf star. KOI 55.01 and KOI 55.02 orbit so close to the central star that any increase in the physical size of KOI 55 will destroy the planets prior to the formation of the white dwarf star. However, the size of KOI 55 is not expected to increase and both planets will continue to orbit KOI 55 long after it has turned into a white dwarf star.
References:
1. Charpinet, et al. (2011), “A compact system of small planets around a former red-giant star”, Nature, 480, 496.
2. Ealeal Bear and Noam Soker (2012), “A tidally destructed massive planet as the progenitor of the two light planets around the sdB star KIC 05807616”, arXiv:1202.1168v1