TRAPPIST-1e Has a Large Iron Core

Abstract

The TRAPPIST-1 system provides an exquisite laboratory for understanding exoplanetary atmospheres and interiors. Their mutual gravitational interactions leads to transit timing variations, from which Grimm et al. (2018) recently measured the planetary masses with precisions ranging from 5% to 12%. Combined with < 5% radius measurements on each planet, TRAPPIST-1 provides a unique opportunity to examine the range of permissible planetary interiors. Grimm et al. (2018) used their new masses and radii and compared them to those expected for planets comprised of pure silicate (no iron or volatiles). This revealed that planets b, d, f, g and h likely contain volatile layers to explain their properties, but c and e are compatible with being rocky. This is an example of a boundary condition comparison, first described in Kipping et al. (2013) in the context of planetary interiors, where the authors show how a minimum envelope height can be derived by comparison to pure water models. We briefly note that planets b through h all have a minimum envelope height compatible with zero when applying the method of Kipping et al. (2013) to the Grimm et al. (2018) masses and radii, to a confidence of ≥ 99.999%.Grimm et al. (2018)'s inference that planets c and e are consistent with a rock-iron composition is useful, but it is possible to go further and actually quantify the minimum and maximum size of an iron core using boundary condition arguments. Such an approach is laid out in our recent paper Suissa et al. (2018). In that work, we considered that the maximum core size is found by solving when the mass and radius of the planet equals that of an iron core surrounded by a light hydrogen/helium envelope. However, recent atmospheric studies by de Wit et al. (2016 exclude the possibility of such envelopes for planets b through f. Accordingly, we updated our model, hardCORE, such that the maximum core size corresponds to the next lightest layer plausibly found around the core, a water layer (where as in our original paper we use the Zeng & Sasselov (2013) interior model).Using the Grimm et al. (2018) posteriors, we are then able to derive a minimum and maximum core size for each planet (using the original maximum formulation for planets g and h). We find that the minimum core size is consistent with zero for all of the planets except e. In particular, for planet c, unlike the result of Grimm et al. (2018), we find that the radius of the planet if pure silicate would be 1.09 +0.04 −0.04 R ⊕ , which is consistent with the observed radius of 1.09 +0.03 −0.03 R ⊕ . The difference is likely a product of the different equations-of-state used in each model, that of Zeng & Sasselov (2013) used in our work, versus that of Connolly (2009) used by Grimm et al. (2018). We find that the probability of an iron core is modest at 57% and thus ambiguity remains regarding c's interior. For planet e, however, 99.3% of the posterior samples are consistent with a silicate-iron model indicating strong evidence for an iron core. Substituting the silicate ...

Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?

“…Recently, interior modeling of the TRAPPIST-1 planets predicts that they may contain water of 25 wt% (Quarles et al 2017;Dorn et al 2018;Grimm et al 2018;Suissa & Kipping 2018;Unterborn et al 2018a,b). Since the inner planets (1 b and 1 c) undergo a runaway greenhouse phase, water can be easily to be lost to space.…”

“…The low densities of the TRAPPIST-1 planets (see Table 1) may conceal substantial volatile content, e.g., water inside their cores. Monte Carlo studies on their internal compositions suggest that the inner planets have a small water content (Quarles et al 2017;Grimm et al 2018), which depends on their core-tomantle mass ratio (Dorn et al 2018;Suissa & Kipping 2018;Unterborn et al 2018a,b). Interior modeling of the TRAPPIST-1 planets using updated masses indicates that the mass fractions of water might be uniform or increase with semimajor axis ( 25 wt%) (Dorn et al 2018).…”

Limits on Clouds and Hazes for the TRAPPIST-1 Planets

“…This has motivated multiple investigations of the planets' habitability through the presence of surface liquid water and biomarkers (Barstow & Irwin 2016;Alberti et al 2017;Wolf 2017;Turbet et al 2018). Studies regarding the interior structure (Kislyakova et al 2017;Suissa & Kipping 2018) and bulk densities (Grimm et al 2018) of the TRAPPIST-1 planets also suggest terrestrial rather than gaseous worlds.…”

Disentangling atmospheric compositions of K2-18 b with next generation facilities

“…For all those planets, further observations are required to remove the remaining degeneracies and truly understand the nature of these worlds. Future observatories, such as JWST, Twinkle and Ariel, are needed to provide adequate observational constraints to the modelling effort inspired by these planets [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77].…”