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Register a new userThe TRAPPIST-1 system: Orbital evolution, tidal dissipation, formation and habitability
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We study the dynamical evolution of the TRAPPIST-1 system under the influence of orbital circularization through tidal interaction with the central star. We find that systems with parameters close to the observed one evolve into a state where consecutive planets are linked by first order resonances and consecutive triples, apart from planets c, d and e, by connected three body Laplace resonances. The system expands with period ratios increasing and mean eccentricities decreasing with time. This evolution is largely driven by tides acting on the innermost planets which then influence the outer ones. In order that deviations from commensurability become significant only on $Gy$ time scales or longer, we require that the tidal parameter associated with the planets has to be such that $Q > sim 10^{2-3}.$ At the same time, if we start with two subsystems, with the inner three planets comprising the inner one, $Q$ associated with the planets has to be on the order (and not significantly exceeding) $10^{2-3}$ for the two subsystems to interact and end up in the observed configuration. This scenario is also supported by modelling of the evolution through disk migration which indicates that the whole system cannot have migrated inwards together. Also in order to avoid large departures from commensurabilities, the system cannot have stalled at a disk inner edge for significant time periods. We discuss the habitability consequences of the tidal dissipation implied by our modelling, concluding that planets d, e and f are potentially in habitable zones.
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The Kepler-186 system consists of five planets orbiting an early-M dwarf. The planets have physical radii of 1.0-1.50 R$_oplus$ and orbital periods of 4 to 130 days. The $1.1~$R$_oplus$ Kepler-186f with a period of 130 days is of particular interest. Its insolation of roughly $0.32~S_odot$places it within the liquid water habitable zone. We present a multi-faceted study of the Kepler-186 system. First, we show that the distribution of planet masses can be roughly reproduced if the planets accreted from a high-surface density disk presumably sculpted by an earlier phase of migration. However, our simulations predict the existence of 1-2 undetected planets between planets e and f. Next, we present a dynamical analysis of the system including the effect of tides. The timescale for tidal evolution is short enough that the four inner planets must have small obliquities and near-synchronous rotation rates. Tidal evolution of Kepler-186f is slow enough that its current spin state depends on a combination of its dissipation rate and the stellar age. Finally, we study the habitability of Kepler-186f with a 1-D climate model. The planets surface temperature can be raised above 273 K with 0.5-5 bars of CO$_2$, depending on the amount of N$_2$ present. Kepler-186f represents a case study of an Earth-sized planet in the cooler regions of the habitable zone of a cool star.
We perform numerical simulations of the TRAPPIST-1 system of seven exoplanets orbiting a nearby M dwarf, starting with a previously suggested stable configuration. The long-term stability of this configuration is confirmed, but the motion of planets is found to be chaotic. The eccentricity values are found to vary within finite ranges. The rates of tidal dissipation and tidal evolution of orbits are estimated, assuming an Earth-like rheology for the planets. We find that under this assumption the planets b, d, e were captured in the 3:2 or higher spin-orbit resonances during the initial spin-down but slipped further down into the 1:1 resonance. Dependent on its rheology, the innermost planet b may be captured in a stable pseudosynchronous rotation. Non-synchronous rotation ensures higher levels of tidal dissipation and internal heating. The positive feedback between the viscosity and the dissipation rate -- and the ensuing runaway heating -- are terminated by a few self-regulation processes. When the temperature is high and the viscosity is low enough, the planet spontaneously leaves the 3:2 resonance. Further heating is stopped either by passing the peak dissipation or by the emergence of partial melt in the mantle. In the post-solidus state, the tidal dissipation is limited to the levels supported by the heat transfer efficiency. The tides on the host star are unlikely to have had a significant dynamical impact. The tides on the synchronized inner planets tend to reduce these planets orbital eccentricity, possibly contributing thereby to the systems stability.
The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized terrestrial planets, three of which have equilibrium temperatures that may support liquid surface water, making it a compelling target for exoplanet characterization. TRAPPIST-1 is an active star with frequent flaring, with implications for the habitability of its planets. Superflares (stellar flares whose energy exceeds 10^33 erg) can completely destroy the atmospheres of a cool stars planets, allowing ultraviolet radiation and high-energy particles to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet flux when quiescent, raising the possibility of frequent flares being necessary for prebiotic chemistry that requires ultraviolet light. We combine Evryscope and Kepler observations to characterize the high-energy flare rate of TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire Southern sky in g every two minutes. Evryscope observations, spanning 170 nights over 2 years, complement the 80-day continuous short-cadence K2 observations by sampling TRAPPIST-1s long-term flare activity. We update TRAPPIST-1s superflare rate, finding a cumulative rate of 4.2 (+1.9 -0.2) superflares per year. We calculate the flare rate necessary to deplete ozone in the habitable-zone planets atmospheres, and find that TRAPPIST-1s flare rate is insufficient to deplete ozone if present on its planets. In addition, we calculate the flare rate needed to provide enough ultraviolet flux to power prebiotic chemistry. We find TRAPPIST-1s flare rate is likely insufficient to catalyze some of the Earthlike chemical pathways thought to lead to RNA synthesis, and flux due to flares in the biologically relevant UV-B band is orders of magnitude less for any TRAPPIST-1 planet than has been experienced by Earth at any time in its history.
The discovery of potentially habitable planets around the ultracool dwarf star Trappist-1 naturally poses the question: could Trappist-1 planets be home to life? These planets orbit very close to the host star and are most susceptible to the UV radiation emitted by the intense and frequent flares of Trappist-1. Here we calculate the UV spectra (100 - 450 nm) of a superflare observed on Trappist-1 with the K2 mission. We couple radiative transfer models to this spectra to estimate the UV surface flux on planets in the habitable zone of Trappist-1 (planets $e$, $f$, and $g$), assuming atmospheric scenarios based on a pre-biotic and an oxygenic atmosphere. We quantify the impact of the UV radiation on living organisms on the surface and on a hypothetical planet ocean. Finally, we find that for non-oxygenic planets, UV resistant lifeforms would survive on the surface of planets f and g. Nevertheless, more fragile organisms (i.e. textit{E. coli}) could be protected from the hazardous UV effects at ocean depths greater than 8m. If the planets have an ozone layer, any lifeforms studied here would survive in the HZ planets.
The planetary system of TRAPPIST-1, discovered in 2016-2017, is a treasure-trove of information. Thanks to a combination of observational techniques, we have estimates of the radii and masses of the seven planets of this very exotic system. With three planets within the traditional Habitable Zone limits, it is one of the best constrained system of astrobiological interest. I will review here the theoretical constraints we can put on this system by trying to reconstruct its history: its atmospheric evolution which depends on the luminosity evolution of the dwarf star, and its tidal dynamical evolution. These constraints can then be used as hypotheses to assess the habitability of the outer planets of the system with a Global Climate Model.
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