Inferring the Composition of Disintegrating Planet Interiors from Dust Tails with Future James Webb Space Telescope Observations

Abstract

Disintegrating planets allow for the unique opportunity to study the composition of the interiors of small, hot, rocky exoplanets because the interior is evaporating and that material is condensing into dust, which is being blown away and then transiting the star. Their transit signal is dominated by dusty effluents forming a comet-like tail trailing the host planet (or leading it, in the case of K2-22b), making these good candidates for transmission spectroscopy. To assess the ability of such observations to diagnose the dust composition, we simulate the transmission spectra from 5-14 µm for the planet tail assuming an optically-thin dust cloud comprising a single dust species with a constant column density scaled to yield a chosen visible transit depth. We find that silicate resonant features near 10 µm can produce transit depths that are at least as large as those in the visible. For the average transit depth of 0.55% in the Kepler band for K2-22b, the features in the transmission spectra can be as large as 1%, which is detectable with the JWST MIRI low-resolution spectrograph in a single transit. The detectability of compositional features is easier with an average grain size of 1 µm despite features being more prominent with smaller grain sizes. We find most features are still detectable for transit depths of ∼ 0.3% in the visible range. If more disintegrating planets are found with future missions such as the space telescope TESS, follow-up observations with JWST can explore the range of planetary compositions.

LBT Reveals Large Dust Particles and a High Mass Loss Rate for K2-22 b

“…This is just a lower limit set by the longest wavelength (2.16 µm), so the median particle size could be larger for all compositions. Observations with JWST NIRSpec and MIRI can place valuable new constraints on the particle size distribution as well as the composition through mid-infrared solid state features (Bodman et al 2018;Okuya et al 2020b).…”

“…The dust debris can then be traced to either the core, mantle or crust material of a terrestrial planet. Quartz could indicate crust material, silicates could indicate mantle material, while iron can indicate core material (Bodman et al 2018;Okuya et al 2020a). If the planet is a coreless body (Elkins-Tanton & Seager 2008), it may also produce substantial crystalline fayalite (Fe 2 SiO 4 ) dust (Okuya et al 2020a).…”

Expecting the unexpected in the search for extraterrestrial life

“…JWST would be able to analyse the disintegration (via transmission spectroscopy of the planet's dust tail), thereby providing information on the interior of the planet. This may be a rare opportunity to ‘see’ the interior of a rocky planet (Bodman et al ., 2018).…”

“…This may be a rare opportunity to 'see' the interior of a rocky planet. (Bodman et al 2018) These options are all low risk, in the sense that it is vanishingly unlikely that they would deliver no results of interest. And they all offer potentially quite high rewards: the information we would expect to acquire in each case would be extremely valuable.…”

Star-planet interactions VI. Tides, stellar activity and planet evaporation

“…WASP-121b Delrez et al 2016;Bourrier et al 2020) and even to the partial or total loss of the planetary atmosphere (Ehrenreich et al 2015). Interestingly, in the future, using the JWST (James Webb Space Telescope), observations of the planet evaporation process through transmission spectroscopy during transits might allow inferring the composition of the interior of small rocky planets (Bodman et al 2018). Some observed features, such as the existence of the radius valley and of the Neptunian desert in the plane of planet radius vs. orbital period (see e.g.…”