TRAPPIST-1

Planetary transits of TRAPPIST-1 over a period of 20 days from September to October, recorded by the Spitzer Space Telescope in 2016.

The orbital motions of the TRAPPIST-1 planets form a complex chain with three-body Laplace-type resonances linking every member. The relative orbital periods (proceeding outward) approximate whole integer ratios of 24/24, 24/15, 24/9, 24/6, 24/4, 24/3, and 24/2, respectively, or nearest-neighbor period ratios of about 8/5, 5/3, 3/2, 3/2, 4/3, and 3/2 (1.603, 1.672, 1.506, 1.509, 1.342, and 1.519). This represents the longest known chain of near-resonant exoplanets, and is thought to have resulted from interactions between the planets as they migrated inward within the residual protoplanetary disk after forming at greater initial distances.[39][36]

Most sets of orbits similar to the set found at TRAPPIST-1 are unstable, causing one planet to come within the Hill sphere of another or to be thrown out. But it has been found that there is a way for a system to migrate into a fairly stable state through damping interactions with, for example, a protoplanetary disk. After this, tidal forces can give the system a long-term stability.[17]

The tight correspondence between whole number ratios in orbital resonances and in music theory has made it possible to convert the system's motion into music.[18][51]

According to Ormel et al. previous models of planetary formation do not explain the formation of the highly compact TRAPPIST-1 system. Formation in place would require an unusually dense disk and would not readily account for the orbital resonances. Formation outside the frost line does not explain the planets' terrestrial nature or Earth-like masses. The authors proposed a new scenario in which planet formation starts at the frost line where pebble-size particles trigger streaming instabilities, then protoplanets quickly mature by pebble accretion. When the planets reach Earth mass they create perturbations in the gas disk that halt the inward drift of pebbles causing their growth to stall. The planets are transported by Type I migration to the inner disk, where they stall at the magnetospheric cavity and end up in mean motion resonances.[52] This scenario predicts the planets formed with significant fractions of water, around 10%, with the largest initial fractions of water on the innermost and outermost planets.[53]

It is suggested that all seven planets are likely to be tidally locked into a so-called synchronous spin state (one side of each planet permanently facing the star),[39] making the development of life there much more challenging.[15] A less likely possibility is that some may be trapped in a higher-order spin-orbit resonance.[39] Tidally locked planets would typically have very large temperature differences between their permanently lit day sides and their permanently dark night sides, which could produce very strong winds circling the planets. The best places for life may be close to the mild twilight regions between the two sides, called the terminator line. Another possibility is that the planets may be pushed into effectively non-synchronous spin states due to strong mutual interactions among the seven planets, resulting in more complete stellar coverage over the surface of the planets.[54]

Tidal heating is predicted to be significant: all planets except f and h are expected to have a tidal heat flux greater than Earth's total heat flux.[36] With the exception of planet c, all of the planets have densities low enough to indicate the presence of significant H
2O in some form. Planets b and c experience enough heating from planetary tides to maintain magma oceans in their rock mantles; planet c may have eruptions of silicate magma on its surface. Tidal heat fluxes on planets d, e, and f are lower, but are still twenty times higher than Earth's mean heat flow. Planets d and e are the most likely to be habitable. Planet d avoids the runaway greenhouse state if its albedo is ≳ 0.3.[55]

Bolmont et al. modelled the effects of predicted far ultraviolet (FUV) and extreme ultraviolet (EUV/XUV) irradiation of planets b and c by TRAPPIST-1. Their results suggest that the two planets may have lost as much as 15 Earth oceans of water (although the actual loss would probably be lower), depending on their initial water contents. Nonetheless, they may have retained enough water to remain habitable, and a planet orbiting further out was predicted to lose much less water.[25]

However, a subsequent XMM-Newton X-ray study by Wheatley et al. found that the star emits X-rays at a level comparable to our own much larger Sun, and extreme ultraviolet radiation at a level 50-fold stronger than assumed by Bolmont et al. The authors predicted this would significantly alter the primary and perhaps secondary atmospheres of close-in, Earth-sized planets spanning the habitable zone of the star. The publication noted that these levels "neglected the radiation physics and hydrodynamics of the planetary atmosphere" and could be a significant overestimate. Indeed, the XUV stripping of a very thick hydrogen and helium primary atmosphere might actually be required for habitability. The high levels of XUV would also be expected to make water retention on planet d less likely than predicted by Bolmont et al., though even on highly irradiated planets it might remain in cold traps at the poles or on the night sides of tidally locked planets.[56]

If a dense atmosphere like Earth's, with a protective ozone layer, exists on planets in the habitable zone of TRAPPIST-1, UV surface environments would be similar to present-day Earth. However, an anoxic atmosphere would allow more UV to reach the surface, making surface environments hostile to even highly UV-tolerant terrestrial extremophiles. If future observations detect ozone on one of the TRAPPIST-1 planets, it would be a prime candidate to search for surface life.[57]

Artistic representation of TRAPPIST-1 planets transiting their host star. Light passing through atmospheres of transiting exoplanets could reveal atmospheric compositions using spectroscopy.[58]

Because of the system's relative proximity, the small size of the primary and the orbital alignments that produce daily transits,[59] the atmospheres of TRAPPIST-1's planets are favorable targets for transmission spectroscopy investigation.[60]

The combined transmission spectrum of planets b and c, obtained by the Hubble Space Telescope, rules out a cloud-free hydrogen-dominated atmosphere for each planet, so they are unlikely to harbor an extended gas envelope, unless it is cloudy out to high altitudes. Other atmospheric structures, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectrum.[61]

Another study hinted at the presence of hydrogen exospheres around the two inner planets with exospheric discs extending up to seven times the planets' radii.[62]

In a paper by an international collaboration using data from space and ground-based telescopes, it was found that planets c and e likely have largely rocky interiors, and that b is the only planet above the runaway green-house limit, with pressures of water vapour of the order of 10¹ to 10⁴ bar.[48]

Observations by future telescopes, such as the James Webb Space Telescope or European Extremely Large Telescope, will be able to assess the greenhouse gas content of the atmospheres, allowing better estimation of surface conditions. They may also be able to detect biosignatures like ozone or methane in the atmospheres of these planets, if life is present there.[13][63][64][65] As of 2020, the TRAPPIST-1 is considered a most promising target for transmission spectroscopy using the James Webb Space Telescope.[66]

The K2 observations of Kepler revealed several flares on the host star. The energy of the strongest event was comparable to the Carrington event, one of the strongest flares seen on the Sun. As the planets in TRAPPIST-1 system orbit much closer to their host star than Earth, such eruptions could cause 10–10000 times stronger magnetic storms than the most powerful geomagnetic storms on Earth. Beside the direct harm caused by the radiation associated with the eruptions, they can also pose further threats: the chemical composition of the planetary atmospheres is probably altered by the eruptions on a regular basis, and the atmospheres can be also eroded in the long term. A sufficiently strong magnetic field of the exoplanets could protect their atmosphere from the harmful effects of such eruptions, but an Earth-like exoplanet would need a magnetic field in the order of 10–1000 Gauss to be shielded from such flares (as a comparison, the Earth's magnetic field is ≈0.5 Gauss).[7] The study in 2020 have found super-flare (defined as flare releasing at least 10²⁶ J - twice the Carrington event) rate of TRAPPIST-1 is 4.2^+1.9
_−0.2 year^-1, insufficient to permanently deplete ozone in the atmosphere of habitable-zone planets. Also, flare UV emission of TRAPPIST-1 is grossly insufficient to compensate the lack of quiescent UV emission and to power prebiotic chemistry.[67]

Hypothetically, if the conditions of the TRAPPIST-1 planetary system were to be able to support life, any possible life that had developed through abiogenesis on one of the planets would likely be spread to other planets in the TRAPPIST-1 system via panspermia, the transfer of life from one planet to another.[68] Due to the close proximity of the planets in the habitable zone with a separation of at least ~0.01 AU from each other, the probability of life being transferred from one planet to another is greatly enhanced.[69] Compared to the likelihood of panspermia from Earth to Mars, the likelihood of interplanetary panspermia in the TRAPPIST-1 system is thought to be about 10,000 times higher.[68]

In February 2017, Seth Shostak, senior astronomer for the SETI Institute, noted, "[T]he SETI Institute used its Allen Telescope Array [in 2016] to observe the environs of TRAPPIST-1, scanning through 10 billion radio channels in search of signals. No transmissions were detected."[21] Additional observations with the more sensitive Green Bank Telescope did not show evidence of transmissions.[70]

One study using the CAPSCam astrometric camera concluded that the TRAPPIST-1 system has no planets with a mass at least 4.6 M_J with year-long orbits and no planets with a mass at least 1.6 M_J with five-year orbits. The authors of the study noted, however, that their findings left areas of the TRAPPIST-1 system, most notably the zone in which planets would have intermediate-period orbits, unanalyzed.[71]

Stephen R. Kane, writing in The Astrophysical Journal Letters, notes that TRAPPIST-1 planets are unlikely to have large moons.[72][73] The Earth's Moon has a radius 27% that of Earth, so its area (and its transit depth) is 7.4% that of Earth, which would likely have been noted in the transit study if present. Smaller moons of 200–300 km (120–190 mi) radius would likely not have been detected.

At a theoretical level, Kane found that moons around the inner TRAPPIST-1 planets would need to be extraordinarily dense to be even theoretically possible. This is based on a comparison of the Hill sphere, which marks the outer limit of a moon's possible orbit by defining the region of space in which a planet's gravity is stronger than the tidal force of its star, and the Roche limit, which represents the smallest distance at which a moon can orbit before the planet's tides exceed its own gravity and pull it apart. These constraints do not rule out the presence of ring systems (where particles are held together by chemical rather than gravitational forces). The mathematical derivation is as follows:

${\displaystyle R_{H}=a_{p}{\sqrt[{3}]{\frac {M_{p}}{3M_{s}}}}}$

${\displaystyle R_{H}}$ is the Hill radius of the planet, calculated from the planetary semi-major axis ${\displaystyle a_{p}}$, the mass of the planet ${\displaystyle M_{p}}$, and the mass of the star ${\displaystyle M_{s}}$. Note that the mass of the TRAPPIST-1 star is approximately 30,000 M_⊕ (see data table above); the remaining figures are provided in the table below.

${\displaystyle R_{R}\approx 2.44R_{p}{\sqrt[{3}]{\frac {\rho _{p}}{\rho _{m}}}}}$

${\displaystyle R_{R}}$ is the Roche limit of the planet, calculated from the radius of the planet ${\displaystyle R_{p}}$, and the density of the planet ${\displaystyle \rho _{p}}$.

Planet	${\displaystyle M_{p}}$ (Earth masses)	${\displaystyle R_{p}}$ (Earth radii)	${\displaystyle \rho _{p}}$ (Earth density)	${\displaystyle a_{p}}$ (AU)	${\displaystyle R_{H}}$ (milliAU)	${\displaystyle R_{R}}$ (milliAU)	${\displaystyle R_{H}/R_{R}}$
TRAPPIST-1b	1.017	1.121	0.726	0.0116	0.261	0.127	2.055
TRAPPIST-1c	1.156	1.095	0.883	0.0158	0.372	0.133	2.797
TRAPPIST-1d	0.297	0.784	0.616	0.0223	0.334	0.084	3.976
TRAPPIST-1e	0.772	0.910	1.024	0.0293	0.603	0.116	5.198
TRAPPIST-1f	0.934	1.046	0.816	0.0385	0.845	0.124	6.815
TRAPPIST-1g	1.148	1.148	0.759	0.0469	1.101	0.132	8.341
TRAPPIST-1h	0.331	0.773	0.719	0.0619	0.961	0.087	11.046

Kane notes that moons near the edge of the Hill radius may be subject to resonant removal during planetary migration, leading to a Hill reduction factor roughly estimated as 1/3 for typical systems and 1/4 for the TRAPPIST-1 system; thus moons are not expected for the planets where ${\displaystyle R_{H}/R_{R}}$ is less than four. Furthermore, tidal interactions with the planet can result in a transfer of energy from the planet's rotation to the moon's orbit, causing a moon to leave the stable region over time. For these reasons, even the outer TRAPPIST-1 planets are believed to be unlikely to have moons.