Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions

Abstract

Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits [1,2], with reasonable mass estimates coming from transit timing variations [2, 3] and dynamical modeling [4]. The low bulk densities of the TRAPPIST-1 planets demand significant volatile content. Here we show using mass-radius-composition models, that TRAPPIST-1f and g likely contain substantial (≥ 50 wt%) water/ice, with b and c being significantly drier (≤ 15 wt%). We propose this gradient of water mass fractions implies planets f and g formed outside the primordial snow line whereas b and c formed inside. We find that compared to planets in our solar system that also formed within the snow line, TRAPPIST-1b and c contain hundreds more oceans worth of water. We demonstrate the extent and timescale of migration in the TRAPPIST-1 system depends on how rapidly the planets formed and the relative location of the primordial snow line. This work provides a framework for understanding the differences between the protoplanetary disks of our solar system versus M dwarfs. Our results provide key insights into the volatile budgets, timescales of planet formation, and migration history of likely the most common planetary host in the Galaxy.The derivation of a planetary composition from only its mass and radius is a notoriously difficult exercise because of the many degeneracies that exist. The geophysical and geochemical behavior of a planet is extremely sensitive to such factors as the size of the iron core, the mantle mineralogy, and the location of phase boundaries within any rock and ice layers [5,6]. For astrobiological applications it is crucial to constrain the exact amount of surficial water a planet contains. Yet current models assume only pure iron cores and an Earth-like composition for the mantles, and often assume either an iron core plus silicate mantle or a silicate planet plus ice mantle [7]. While useful for broadly constraining rocky versus volatile-rich composition, current mass-radius constraints often fail to meaningfully quantify the specific planetary composition [5,6]. For example, [4] constrained the masses of the TRAPPIST-1 planets using dynamical stability arguments, and found they were compatible with compositions between 0% and 100% water ice. The mass-radius fitting of [2] and [3] provided similar, but still uncertain, constraints. We argue that simultaneous mass-radiuscomposition fitting of all the TRAPPIST-1 planets, using the context from the planetary system as a whole, allows better quantification of the allowable structures and mineralogies given the mass-radius measurements and their uncertainties. We, therefore, analyzed the interior structures 1

The Pressure and Temperature Limits of Likely Rocky Exoplanets

“…They will not, however, provide many constraints on the interior mineralogy until the precision of mass and radius improve to ∼ 1% (Dorn et al, ; Unterborn et al, ). Similarly, both the abundance of light elements in a planet's core and the relative amount of FeO in an exoplanet's mantle lower the planet's density, thus mimicking the signal produced from an extended volatile envelope, limiting our ability to definitively determine the structure of a potentially water‐rich exoplanet (Unterborn, Desch, et al, ; Unterborn, Hinkel, et al, ).…”

“…Insight into the composition of an exoplanet is most frequently gained through measurement of both mass and radius of the planets (the “mass‐radius relationship”) from which mean density can be calculated and a planet's interior composition might be inferred (e.g., Seager et al, ; Valencia et al, ). However, planetary mass is rarely measurable to precision better than 20%, and planetary mean density is degenerate with respect to relative proportions of metallic core, silicate rock, and H 2 /He atmosphere or H 2 O layer (Dorn et al, ; Unterborn, Desch, et al, ; Unterborn, Hinkel, et al, ).…”

Holistic Approach for Studying Planetary Hydrospheres: Gibbs Representation of Ices Thermodynamics, Elasticity, and the Water Phase Diagram to 2,300 MPa

“…Looking beyond our solar system, the representation reported here encompassing all ice polymorphs and liquid water up to 2,300 MPa is also relevant for the study of exoplanets interiors and their potential habitability. This is especially true for water worlds like the ones proposed in the TRAPPIST-1 system (Grimm et al, 2018;Unterborn et al, 2018). The self-consistent thermodynamic properties for water and its ices provided by SeaFreeze can be used to accurately study the interior structure and evolution of watery exoplanets (Noack et al, 2016), computing of radius-mass curves (Sotin & Grasset, 2007;Unterborn et al, 2018), as well as studying their habitability and the effects of possible snow-ball regime on ocean words (Kite & Ford, 2018;Ramirez & Levi, 2018).…”

“…As a common molecular species in our cosmic neighborhood (Hanslmeier, ), water ice polymorphs at high pressures in planetary interiors could be the most abundant “mineral group” in the Universe. A focus on potentially habitable hydrospheres of icy moons, small bodies such as Pluto and Ceres, and ocean exoplanets (Sotin & Tobie, ; S. Vance & Brown, ; B Journaux et al, ; Baptiste Journaux et al, ; Noack et al, ; Kite & Ford, ; Unterborn et al, ; Hendrix et al, ) motivates an interest in thermodynamic properties of water and ices in the <200 MPa range. For example, the presence of an insulating layer of high‐pressure ice between the deep ocean and the underlying silicates on large water‐rich planetary bodies has been identified as a potential bottleneck for habitability, as it could limit nutrient transport (Léger et al, ; Noack et al, ; Baptiste Journaux et al, ; Kite & Ford, ).…”

Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds

“…The Earths bulk water content, ∼0.08wt%, is small compared to many other bodies in our own solar system, such as carbonaceous chondrites (up to 13wt% water: Alexander et al 2012 (Raymond et al 2004). In fact, the TRAPPIST-1 system likely contains several nearly Earth-mass planets with ≥ 8wt% H 2 O, corresponding to ≥ 250 Earth oceans of water (Gillon et al 2016;Unterborn et al 2018). Therefore, H 2 O mass fractions of up to several percent must be considered likely outcomes of planet formation.…”