Born Dry or Born Wet? A Palette of Water Growth Histories in TRAPPIST-1 Analogs and Compact Planetary Systems

TL;DR

The study addresses whether rocky planets in compact M-dwarf systems can accrete and retain volatiles by coupling time-resolved N-body accretion histories with a volatile growth model that tracks H2O, CO2, and N2 across atmosphere, mantle, and core. Using a Mercury6-based N-body framework and a volatile growth simulator, the authors quantify how collisions, atmospheric erosion, mantle–atmosphere exchange, and core sequestration shape final water inventories across TRAPPIST-1–like architectures, including the influence of an extended pre-main-sequence phase. The results reveal a strong radial gradient: inner planets (b–c) are predominantly water-poor, while outer planets (e–h) can retain substantial water; PMS heating markedly dehydrates inner planets, reducing water inventories by 1–2 orders of magnitude. The findings offer a formation-based explanation for JWST nondetections of atmospheres around TRAPPIST-1 b and c, underscore that many compact M-dwarf planets may form inherently volatile-depleted, and provide physically grounded initial conditions for exoplanet interior and atmospheric models, with broad implications for interpreting observations of compact planetary systems.

Abstract

It is still unclear whether exoplanets in compact multiplanet systems such as TRAPPIST-1 are able to accrete large quantities of volatiles, grow to sufficient mass, and maintain robust atmospheres and hydrospheres. Previous estimates of water content in M-dwarf systems have largely relied on population synthesis or atmosphere-interior evolution models, often treating impacts and atmospheric loss in isolation. In this work, we couple impact delivery, impact erosion, and mantle-atmosphere exchange within a model that tracks volatile evolution through stochastic collision histories. By explicitly including both planetesimal accretion and the prolonged luminous pre-main-sequence phase of M dwarfs, we find lower water inventories for the inner TRAPPIST-1 analogs (b-e), spanning only $10^{-4}$-$10^{-2} M_{\oplus,\rm ocn}$ across a wide range of disk structures and impact scenarios. By contrast, the outer planets (f-h analogs) frequently retain water inventories exceeding an Earth ocean mass. This systematic volatile gradient provides a physically motivated explanation for JWST's nondetections of atmospheres on TRAPPIST-1 b and c, implying an origin rooted in formation conditions rather than in post-formation escape. Our results suggest that many rocky planets in compact M-dwarf systems may form already depleted in volatile compounds, fundamentally limiting their capacity to sustain atmospheres or surface oceans. More broadly, our multistage framework for volatile tracking can help interpret future observations of compact systems and set more realistic initial conditions for exoplanet interior compositions and atmospheric models.

Figures (7)

• Figure 1: Schematic of our modeling framework, illustrating the connection between N-body accretion outcomes and volatile growth simulations. Each panel highlights the primary physical processes represented at each stage of the model. Terrestrial planet formation begins with planetesimals and planetary embryos assumed to emerge from an initial gas+dust disk. Their subsequent growth is explicitly simulated using an N-body accretion model that incorporates processes such as collision fragmentation. The outputs from each collision event and accretion phase then serve as inputs to a self-consistent volatile growth model, which evolves key volatile species such as H$_2$O, C, and N, based on cosmochemically informed initial compositions. Major processes contributing to volatile delivery and loss include impact erosion and mantle degassing. Roman numerals denote model sub-components: mantle (i), atmosphere (ii), core (iii), impactors (iv), and stellar EUV activity (v). Components (i)–(iii) constitute the three primary reservoirs of the growing protoplanet. The schematic layout is conceptually inspired by gu2024.
• Figure 1: Final atmospheric CO$_2$ (left) and N$_2$ (right) masses of each planet as a function of total planet mass (in units of $M_\oplus$) and orbital distance (in AU). Nitrogen content is expressed in units of $M_{\oplus,N_2,atm}$ , or the mass of N$_2$ in Earth's atmosphere. Carbon content is expressed in units of $M_{\oplus,CO_2,{\rm BSE}}$ , or the total mass of carbon in the BSE (Bulk Silicate Earth). The water mass fractions of accreted planetesimal and embryos assume to follow the step wise function given by Equations 1-3. Measured TRAPPIST-1 properties are indicated by black circles Agol+21.
• Figure 2: Snapshots of VGS-calculated surface water mass (in units of Earth’s ocean mass) for TRAPPIST-1 analogs, shown as a function of total planet mass ($M_\oplus$) and final orbital distance (AU). Panel (a) depict planets at the halfway point of accretion (i.e., when the protoplanet reaches half its final mass), while panels (b) show the final planet properties. Measured properties of the TRAPPIST-1 planets are indicated by black circles Agol+21. At mid-assembly, most inner planets are water-poor due to early impact-driven loss and higher irradiation. By the end of accretion, increased volatile delivery events from more water-rich materials and reduced erosion allow many planets, particularly beyond $\sim$∼$$0.03 AU,to accumulate and preserve large water reservoirs, leading to the strong radial gradient in water content.
• Figure 2: Timeseries of VGS-simulated surface-to-mantle volatile mass ratios for example simulations of TRAPPIST-1 b (a, b, c, d) and TRAPPIST-1 e (e, f, g, h). The panels are: H$_2$O wet planetesimal (a and e), CO$_2$ wet planetesimal (b and f), H$_2$O dry planetesimals (c and g), and CO$_2$ dry planetesimal (d and h). Refer to Sect. \ref{['sec:methods']} regarding how the water mass fractions are set in our model.
• Figure 3: Histogram of simulated TRAPPIST-1 analogs, binned by final atmospheric water content (in Earth ocean masses, $M_{\oplus,\mathrm{ocn}}$). Blue bars denote water-rich worlds capable of sustaining at least a shallow global ocean ($>10\%$ of Earth’s surface water), while red bars represent water-poor worlds ($<10\%$). Each panel corresponds to a specific TRAPPIST-1 planet, with the center panel showing the combined distribution for all seven. Across the ensemble, inner planets (e.g., b–c) are predominantly dry, whereas mid-to-outer planets (e.g., e–h) are skewed toward water-rich outcomes, reflecting the strong radial dependence of volatile retention identified in our simulations.