An Extreme Planetary System around a Hot Star

KIC 10001893 is one of 19 subdwarf B (sdB) stars observed by NASA’s Kepler space telescope during its primary mission. An sdB star is a type of very hot star, typically containing ~0.5 times the Sun’s mass. It represents one of the final stages in the evolution of some stars. An sdB star forms when a red giant star loses its outer hydrogen layers before helium in its core starts fusing into carbon. As a consequence, an sdB star is almost entirely comprised of helium, with only a thin outer layer of hydrogen.

R. Silvotti et al. (2014) report the detection of three Earth-size planet candidates in very close-in orbits around KIC 10001893. The three planet candidates have orbital periods of 5.273, 7.807 and 19.48 hours. KIC 10001893 is a very hot star with an estimated surface temperature around 26,700 K. For comparison, the Sun’s effective surface temperature is only 5,778 K. Being so close to such a hot star, the day sides of the three planet candidates are expected to be heated to extraordinary temperatures, possibly up to several thousand degrees K.

Figure 1: Artist’s impression of a superheated planet.

The extreme planetary system around KIC 10001893 resembles the two Earth-size planets previously detected by S. Charpinet et al. (2011), dubbed Kepler-70b and Kepler-70c, in tight orbits around Kepler-70, another sdB star similar to KIC 10001893. Like the two planets around Kepler-70, the planetary system around KIC 10001893 probably formed when the star evolved into a red giant star and engulfed three massive gas giant planets, each having a few times the mass of Jupiter.

Once inside the red giant star, drag removes energy from the orbits of the three giant planets and they spiral towards the core of the star. At the same time, the gaseous envelopes of each of the three giant planets are stripped away, leaving behind their rocky cores. The spiral-in of the giant planets deposits orbital energy in the outer layers of the red giant star, aiding in the expulsion of these layers. Eventually, all that remains is a hot sdB star hosting three small remnant rocky cores in very close-in orbits.

Figure 2: (a) The two planets around Kepler-70 were initially massive gas giant planets orbiting much farther from their host star. (b) When the host star expanded to become a red giant star, its outer layers engulfed the two planets. This process stripped away the outer gaseous layers of the two planets and at the same time caused the orbits of the two planets to spiral inwards. (c) The two in-spiralling planets probably deposited sufficiently energy within the red giant star to aid the star in expelling its outer layers. The end result is a hot sdB star with two rocky remnants of the initial planets in extremely tight orbits. S. Charpinet et al. (2011).

Alternatively, based on a study by Bear & Soker (2012), the planetary system around KIC 10001893 could also have formed from the destruction of a single massive gas giant planet. The spiral-in of the giant planet within the red giant star releases orbital energy which unbinds the outer layers of the red giant star, leaving behind the core which then becomes a hot sdB star. The spiral-in also brings the giant planet close enough to the sdB star to be tidally destructed where its gaseous envelope is completely stripped away and its dense rocky core is broken up into several Earth-size fragments, 3 of which still circle KIC 10001893.

References:
- R. Silvotti et al. (2014), “Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC10001893”, arXiv:1409.6975 [astro-ph.EP]
- S. Charpinet et al. (2011), “A compact system of small planets around a former red-giant star”, Nature, 480, 496
- Bear & Soker (2012), “A tidally destructed massive planet as the progenitor of the two light planets around the sdB star KIC 05807616”, arXiv:1202.1168v1