Dynamical Interactions and Habitability in the TOI-700 Multi-Planet System

TL;DR

The paper addresses whether the tightly packed TOI-700 system can harbor habitable conditions on TOI-700d and TOI-700e in the face of strong planet–planet interactions. It employs the modular VPLanet framework to model tidal evolution, obliquity damping, secular eccentricity variations, insolation evolution, and atmospheric water loss across a range of plausible system parameters, including different tidal dissipation values. The results show that dynamical interactions do not destabilize the planets' orbits or push them out of the habitable zone; both planets end up tidally locked with low obliquities and modest eccentricities, and water retention is possible if the initial water inventory is sufficiently large, though TOI-700e sits near the inner edge of the tidally locked HZ. These findings highlight the importance of atmospheric states and XUV history in assessing habitability and motivate more detailed climate modeling and further observations to refine the planets' parameters and posteriors.

Abstract

The discovery of a second earth sized planet (TOI-700e) interior to the habitable candidate TOI-700d has prompted further research into this system, as the additional planet makes the TOI-700 system a tightly packed multi-planet system with multiple planets in the habitable zone, like TRAPPIST-1. In this work, we use the planetary evolution code VPLanet to assess the potential habitability of TOI-700d and TOI-700e. We first examine their orbital dynamics to evaluate the influence of planet-planet interactions on the planet spin, obliquity, and eccentricity. We then investigate whether these interactions are sufficient to cause either TOI-700d or e to be perturbed out of a habitable state, and whether we expect either planet could retain surface oceans over Gyr timescales. Together, these analyses allow us to assess the long-term habitability prospects of both TOI-700d and TOI-700e. We find that multi-planet interactions in the TOI-700 system do not prevent either planet from potentially retaining habitable conditions; however, we find that TOI-700e is located very near the boundary of the tidally locked habitable zone (arXiv:1705.10362), suggesting further work is needed to determine whether it is truly habitable.

Figures (5)

• Figure 1: Incident flux in $S_\oplus$ received by planets orbiting M-dwarfs in systems where at least one planet resides in the habitable zone as a function of their host star's effective temperature. Each vertical line connects planets within the same system. Systems with at least one planet in the optimistic habitable zone (shaded orange) or conservative habitable zone (shaded green) are included. Multi-planet systems are labeled with the system name. The optimistic habitable zone (orange shading) is bounded by the Recent Venus and Early Mars 1D limits from Kopparapu2014. The conservative habitable zone (green shading) uses a more restrictive definition: its inner boundary is set by the stellar insolation shown by the 3D models of Kopparapu2017 to trigger a runaway greenhouse effect for tidally locked planets around M-dwarfs, while its outer boundary follows the maximum greenhouse limit assumed by Kopparapu2014. While the location of the inner edge was computed using 3D models, the outer edge relies solely on 1D models and may therefore be systematically offset from its true position Leconte2013. Tightly packed multi-planet systems with multiple planets in the habitable zone include TRAPPIST-1, GJ 3293, and TOI-700. Data obtained 6/7/2025 from the IPAC Exoplanet Archive Christiansen2025.
• Figure 2: Time evolution of planetary spin–orbit properties for planets TOI-700 b–e. Top panel: Rotation period as a function of time, with horizontal dotted lines indicating the present-day observed orbital periods for each planet. When the rotation and orbital periods for a planet are equal, the planet is considered tidally locked. Middle panel: Obliquity evolution, showing growth toward high obliquity followed by rapid damping to near-zero as each planet transitions into an equilibrium state (which occurs sooner for planets with shorter orbital periods). Bottom panel: Orbital eccentricity evolution, illustrating secular evolution of eccentricity.
• Figure 3: Eccentricity evolution of the four planets in the TOI-700 system over 10,000 years of the integration after 100 Myr of evolution. This plot is a zoom-in of a small range of bottom panel of Figure \ref{['fig:obl_evolution']}.. Additionally, all planets have low but non-zero eccentricity $0.03 < e<0.1$ over their entire evolution.
• Figure 4: The irradiation (in units of Earth insolation, solirad) ($S_{\oplus}$) over time on planets TOI-700e (red lines) and TOI-700d (blue lines). The dark grey region is the conservative habitable zone, where the light grey region is the optimistic habitable zone as found in the 1D modeling in Kopparapu2013. The bold lines represents the average of all of the simulations for each planet. The dotted blue line represents to the location of the tidally locked habitable zone boundary, which comes from the 3D modeling of Kopparapu2017. Using older 1D models, both planets reside solidly in the potential habitable zone, and the evolution of their surface fluxes due to eccentricity evolution and stellar evolution do not compromise that. However, TOI-700e is likely on the boundary of the tidally locked habitable zone, making its habitability prospects less clear. We assume the planets are at the equilibrium state found in Figure \ref{['fig:obl_evolution']} by 0.05 Gyr.
• Figure 5: Dependence of planetary water retention on the star's initial age (assumed to be the age of ocean formation on the planet) and starting water inventory. Each square corresponds to an independent simulation. Red squares indicate complete desiccation within 1 Gyr, while blue squares indicate retention of at least some initial water. The top panel shows results for TOI-700d, and the bottom panel for TOI-700e.