Using data collected by NASA’s Kepler space telescope, a team of researchers have reported the discovery of an Earth-sized planet that whips around its parent star (KIC 8435766) in an 8.5-hour orbit. The planet was found as part of an effort to search for planets with very short orbital periods. This newfound planet is designated KIC 8435766b - basically adding a “b” to the back of the parent star’s designation. Some well-documented planets with orbits shorter than 1 day include Kepler-10b (20.1 hour orbit), Kepler-42c (10.9 hour orbit), COROT-7b (20.5 hour orbit) and 55 Cancri e (17.8 hour orbit). All these planets are smaller than twice the size of Earth.
Figure 1: Artist’s impression of a rocky planet crossing in front of its parent star. Credit: Darke Max Macedo.
Compared to small rocky planets, giant planets with orbits shorter than 1 day are less common and two such planets include WASP-43b (19.5 hour orbit) and WASP-19b (18.9 hour orbit). The rarity of giant planets with orbits shorter than 1 day suggests that giant planets are vulnerable to effects such as tidally-induced decay of their orbits and evaporation of their hydrogen-helium envelops. Since small rocky planets are less susceptible to these effects, they are expected to be more common than giant planets at the shortest orbital periods.
Because of its extreme proximity to its parent star, KIC 8435766b lies deep within the star’s gravitational well. As a result, the planet whizzes around in its tight orbit at a terrific speed of ~300 km/s. Every 8.5 hours, it crosses in front of its parent star and blocks a tiny fraction of the star’s light. This small but periodic dip in the star’s brightness was the telltale signature that led to the planet’s discovery. KIC 8435766b is expected to be a tidally-locked planet with possibly no atmosphere at all. One side of the planet permanently faces its parent star while the other side stares away into the darkness of space.
KIC 8435766b’s orbit is so tight that the planet is merely 2 stellar radii above the fiery surface of its parent star. On the planet’s dayside, the conflagrant disk of the planet’s parent star would span 1/5th of the sky. Temperatures on the planet’s dayside are estimated to range from 2300 K to 3100 K. Such ferocious temperatures are high enough to melt and possibly even boil rock material. KIC 8435766b is so intensely irradiated by its parent star that as the planet’s dayside rotates in and out of view, the combined brightness of the star-planet system actually varies by an amplitude of ~4.3 ppm (parts-per-million). It is fortunate that the star around which KIC 8435766b orbits happens to be a relatively nearby Sun-like star. As a result, it is bright enough for robust follow-up observations to be performed to unravel more of the planet’s properties.
Figure 2: Upper panel: The final light curve (dots) and best-fitting model (red curve); where the large dips represent the passage of KIC 8435766b in front of its parent star. The inset panel is a scale illustration of the star-planet system, where the dashed line represents the orbital distance of the planet. Lower panel: Close-up of the illumination curve and occultation. The illumination curve is basically the variation in brightness as the planet’s dayside rotates in and out of view. The occultation has a depth of ~10.3 ppm and it corresponds to the planet passing behind the star. Here, the data have been binned to 4 minute intervals for clearer visual inspection. Credit: Sanchis-Ojeda et al. (2013).
Sanchis-Ojeda et al. (2013), “Transits and occultations of an Earth-sized planet in an 8.5-hour orbit”, arXiv:1305.4180 [astro-ph.EP]