Sunday, February 5, 2012

A Disintegrating Planet

Planets that orbit their host stars with periods of less than one Earth day are entirely possible and a number of these planets have already been discovered. An example is the planet 55 Cancri e which takes just 17.8 hours to orbit around its host star. However, there is considerably less emphasis on the search for such planets because signals that might be indicative of short-period planets tend to be false positives that are associated with the highly diluted light from background binary stars that happen to be located along the line-of-sight to the subject foreground star. Furthermore, giant planets that orbit too close to their host stars can get destroyed when intense stellar radiation causes the planet to puff up and loose material.

A paper by Saul Rappaport, et al. (2012) entitled “Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548” describes the discovery of a possible super-Mercury sized planet that orbits a star slightly cooler than the Sun. Periodic oscillations in the overall brightness of the star KIC 12557548 have been detected by NASA’s Kepler space telescope and these oscillations occur with a periodicity of 15.685 hours. Kepler detects the presence of planets around other stars by looking for small periodic dips in a star’s brightness when a planet crosses in front of its host star. However, the shape of the light-curve representing the dimming of KIC 12557548 is inconsistent with what should be produced by a single transiting planetary body.

KIC 12557548 is a K-type orange dwarf star which is both smaller and cooler than the Sun. With a short period of just 15.685 hours, whatever that is orbiting KIC 12557548 must be in a very close-in orbit. Rappaport et al. suggest that the periodic dimming of KIC 12557548 is caused by a stream of macroscopic dust particles from a disintegrating planet that is larger in size than Mercury. This involves a planet with 10 percent the mass of the Earth (1.8 times the mass of Mercury) and half the diameter of the Earth (1.3 times the diameter of Mercury) orbiting the host star KIC 12557548 at a distance of just 3 stellar radii from the surface of the star. Being so close to its parent star, the planet is expected to be intensely radiated with an estimated peak dayside temperature of 2100 degrees Kelvin. Such a temperature is sufficient to vaporize rock material on the planet’s surface. Since rocks are agglomerations of different individual grains, the vaporized rock material is expected to carry with it a fair amount of still solid grain material.

The solid grains are then carried off the surface of the planet and into space by a thermal Parker-type wind. How such a thermal wind works in when gas drag from the vaporized rock material accelerates the solid grains. At some distance from the planet’s surface, these solid grains are accelerated to speeds exceeding the local escape velocity and escape the planet. An upper limit to the mass loss rate can exist when the outflow becomes too thick, leading to insufficient light from the star reaching the planet’s surface to heat it. Once the solid grains escape the planet and the gas density becomes sufficiently rarefied, the solid grains will decouple from the gas and form a comet-like tail behind the planet. The shape of this comet-like tail is defined by a combination of Coriolis forces and stellar radiation pressure acting on the solid grains. The transit of such a dust tail in front of KIC 12557548 is consistent with the observed dimming characteristics.

In the case for KIC 12557548, a planet that is more massive than the Earth is highly unlikely because the gravity of such a planet will be too strong to allow the dust grains to be accelerated into space. The efficiency of driving dust grains into space via a thermal wind decreases dramatically as the gravity of the planet increases. Future observations can help further confirm the existence of a disintegrating super-Mercury sized planet around the star KIC 12557548. Observations of KIC 12557548 with shorter cadence can look for asymmetries in the ingress and egress portions of the light-curve which might reveal the morphology of the dust tail extending from the disintegrating planet. Additionally, spectral observations using the Hubble Space Telescope can determine if the disintegrating planet is largely made of heavy elements like the Earth and Mercury by searching for absorption line features of metallic elements in the outflow.