Dusty Tails of Evaporating Exoplanets

Dusty tails of evaporating exoplanets. I. Constraints on the dust composition

Authors:

van Lieshout et al

Abstract:

Recently, two exoplanet candidates have been discovered, KIC 12557548b and KOI-2700b, whose transit profiles show evidence for a comet-like tail of dust trailing the planet, thought to be fed by the evaporation of the planet's surface. We aim to put constraints on the composition of the dust ejected by these objects from the shape of their transit light curves. We derive a semi-analytical expression for the attenuation of dust cross-section in the tail, incorporating the sublimation of dust grains as well as their drift away from the planet. This expression shows that the length of the tail is highly sensitive to the sublimation properties of the dust material. We compute tail lengths for several possible dust compositions, and compare these to observational estimates of the tail lengths of KIC 12557548b and KOI-2700b, inferred from their light curves. The observed tail lengths are consistent with dust grains composed of corundum (Al2O3) or iron-rich silicate minerals (e.g., fayalite, Fe2SiO4). Pure iron and carbonaceous compositions are disfavoured. In addition, we estimate dust mass loss rates of 1.7 +/- 0.5 M_earth/Gyr for KIC 12557548b, and greater than 0.007 M_earth/Gyr (1-sigma lower limit) for KOI-2700b.

KOI-2700b: An Exoplanet With a Comet-like Tail

KOI-2700b—A PLANET CANDIDATE WITH DUSTY EFFLUENTS ON A 22 hr ORBIT

Authors:

Rappaport et al

Abstract:

Kepler planet candidate KOI-2700b (KIC 8639908b), with an orbital period of 21.84 hr, exhibits a distinctly asymmetric transit profile, likely indicative of the emission of dusty effluents, and reminiscent of KIC 1255b. The host star has T eff = 4435 K, M sime 0.63 M ☉, and R sime 0.57 R ☉, comparable to the parameters ascribed to KIC 12557548. The transit egress can be followed for ~25% of the orbital period and, if interpreted as extinction from a dusty comet-like tail, indicates a long lifetime for the dust grains of more than a day. We present a semiphysical model for the dust tail attenuation and fit for the physical parameters contained in that expression. The transit is not sufficiently deep to allow for a study of the transit-to-transit variations, as is the case for KIC 1255b; however, it is clear that the transit depth is slowly monotonically decreasing by a factor of ~2 over the duration of the Kepler mission. We infer a mass-loss rate in dust from the planet of ~2 lunar masses per Gyr. The existence of a second star hosting a planet with a dusty comet-like tail would help to show that such objects may be more common and less exotic than originally thought. According to current models, only quite small planets with Mp lsim 0.03 M ⊕ are likely to release a detectable quantity of dust. Thus, any "normal-looking" transit that is inferred to arise from a rocky planet of radius greater than ~1/2 R ⊕ should not exhibit any hint of a dusty tail. Conversely, if one detects an asymmetric transit due to a dusty tail, then it will be very difficult to detect the hard body of the planet within the transit because, by necessity, the planet must be quite small (i.e., lsim 0.3 R ⊕).