TRAPPIST-1
TRAPPIST-1, also designated 2MASS J23062928-0502285, is an ultra-cool red dwarf star with a radius slightly larger than the planet Jupiter, while having 84 times Jupiter's mass. It is located from the Sun in the constellation Aquarius. Seven temperate terrestrial planets have been detected orbiting the star, a larger number than detected in any other planetary system except Kepler-90. A study released in May 2017 suggests that the stability of the system is not particularly surprising if one considers how the planets migrated to their present orbits through a protoplanetary disk.
A team of Belgian astronomers first discovered three Earth-sized planets orbiting the dwarf star in 2015. A team led by at the University of Liège in Belgium detected the planets using transit photometry with the Transiting Planets and Planetesimals Small Telescope at the La Silla Observatory in Chile and the Observatoire de l'Oukaïmeden in Morocco. On 22 February 2017, astronomers announced four additional exoplanets around TRAPPIST-1. This work used the Spitzer Space Telescope and the Very Large Telescope at Paranal, amongst others, and brought the total number of planets to seven, of which three are considered to be within its habitable zone. The others could also be habitable as they may possess liquid water somewhere on their surface. Depending on definition, up to six could be in the optimistic habitable zone, with estimated equilibrium temperatures of. In November 2018, researchers determined that planet e is the most likely Earth-like ocean world and "would be an excellent choice for further study with habitability in mind."
Discovery and nomenclature
The star at the center of the system was discovered in 1999 during the Two Micron All-Sky Survey. It was entered in the subsequent catalog with the designation "2MASS J23062928-0502285". The numbers refer to the right ascension and declination of the star's position in the sky and the "J" refers to the Julian Epoch.The system was later studied by a team at the University of Liège, who made their initial observations using the TRAPPIST–South telescope from September to December 2015 and published their findings in the May 2016 issue of the journal Nature. The backronym pays homage to the Catholic Christian religious order of Trappists and to the Trappist beer it produces, which the astronomers used to toast their discovery. Since the star hosted the first exoplanets discovered by this telescope, the discoverers accordingly designated it as "TRAPPIST-1".
The planets are designated in the order of their discovery, beginning with b for the first planet discovered, c for the second and so on. Three planets around TRAPPIST-1 were first discovered and designated b, c and d in order of increasing orbital periods, and the second batch of discoveries was similarly designated e to h.
Stellar characteristics
TRAPPIST-1 is an ultra-cool dwarf star of spectral class that is approximately 8% the mass of and 11% the radius of the Sun. Although it is slightly larger than Jupiter, it is about 84 times more massive. High-resolution optical spectroscopy failed to reveal the presence of lithium, suggesting it is a very low-mass main-sequence star, which is fusing hydrogen and has depleted its lithium, i.e., a red dwarf rather than a very young brown dwarf. It has a temperature of, and its age has been estimated to be approximately. In comparison, the Sun has a temperature of and an age of about 4.6 Gyr. Observations with the Kepler K2 extension for a total of 79 days revealed starspots and infrequent weak optical flares at a rate of 0.38 per day ; a single strong flare appeared near the end of the observation period. The observed flaring activity possibly changes the atmospheres of the orbiting planets on a regular basis, making them less suitable for life. The star has a rotational period of 3.3 days.High-resolution speckle images of TRAPPIST-1 were obtained and revealed that the M8 star has no companions with a luminosity equal to or brighter than a brown dwarf. This determination that the host star is single confirms that the measured transit depths for the orbiting planets provide a true value for their radii, thus proving that the planets are indeed Earth-sized.
Owing to its low luminosity, the star has the ability to live for up to 12 trillion years. It is metal-rich, with a metallicity of 0.04, or 109% the solar amount. Its luminosity is 0.05% of that of the Sun, most of which is emitted in the infrared spectrum, and with an apparent magnitude of 18.80 it is not visible to the naked eye from the Earth.
Planetary system
On 22 February 2017, astronomers announced that the planetary system of this star is composed of seven temperate terrestrial planets, of which five are similar in size to Earth, and two are intermediate in size between Mars and Earth. Three of the planets orbit within the habitable zone.The orbits of the TRAPPIST-1 planetary system are very flat and compact. All seven of TRAPPIST-1's planets orbit much closer than Mercury orbits the Sun. Except for b, they orbit farther than the Galilean satellites do around Jupiter, but closer than most of the other moons of Jupiter. The distance between the orbits of b and c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth. A year on the closest planet passes in only 1.5 Earth days, while the seventh planet's year passes in only 18.8 days.
The planets pass so close to one another that gravitational interactions are significant, and their orbital periods are nearly resonant. In the time the innermost planet completes eight orbits, the second, third, and fourth planets complete five, three, and two. The gravitational tugging also results in transit-timing variations, ranging from under a minute to over 30 minutes, which allowed the investigators to calculate the masses of all but the outermost planet. The total mass of the six inner planets is approximately 0.02% the mass of TRAPPIST-1, a fraction similar to that for the Galilean satellites to Jupiter, and an observation suggestive of a similar formation history. The densities of the planets range from ~0.60 to ~1.17 times that of Earth, indicating predominantly rocky compositions. The uncertainties are too large to indicate whether a substantial component of volatiles is also included, except in the case of f, where the value "favors" the presence of a layer of ice and/or an extended atmosphere. Speckle imaging excludes all possible stellar and brown dwarf companions.
On 31 August 2017, astronomers using the Hubble Space Telescope reported the first evidence of possible water content on the TRAPPIST-1 exoplanets.
Between 18 February and 27 March 2017, a team of astronomers used the Spitzer Space Telescope to observe TRAPPIST-1 to refine the orbital and physical parameters of the seven planets using updated parameters for the star. Their results were published on 9 January 2018. Although no new mass estimates were given, the team managed to refine the orbital parameters and radii of the planets within a very small error margin. In addition to updated planetary parameters, the team also found evidence for a large, hot atmosphere around the innermost planet.
On 5 February 2018, a collaborative study by an international group of scientists using the Hubble Space Telescope, the Kepler Space Telescope, the Spitzer Space Telescope, and the ESO's SPECULOOS telescope released the most accurate parameters for the TRAPPIST-1 system yet. They were able to refine the masses of the seven planets to a very small error margin, allowing the density, surface gravity, and composition of the planets to be accurately determined. The planets range in mass from about to, with densities from 0.62 ρ⊕ to 1.02 ρ⊕. Planets c and e are almost entirely rocky, while b, d, f, g, and h have a layer of volatiles in the form of either a water shell, an ice shell, or a thick atmosphere. Planets c, d, e, and f lack hydrogen-helium atmospheres. Planet g was also observed, but there was not enough data to firmly rule out a hydrogen atmosphere. Planet d might have a liquid water ocean comprising about 5% of its mass—for comparison, Earth's water content is —while if f and g have water layers, they are likely frozen. Planet e has a slightly higher density than Earth, indicating a terrestrial rock and iron composition. Atmospheric modeling suggests the atmosphere of b is likely to be over the runaway greenhouse limit with an estimated to bar of water vapor.
Stellar spectrum study, performed in early 2020, has revealed the TRAPPIST-1 star rotational axis is well aligned with the plane of planetary orbits. The stellar obliquity was found to be degrees.
Planetary system data charts
| Companion | Stellar flux | Temperature | Surface gravity | Approximate orbital resonance ratio | Approximate orbital resonance ratio |
| b | 4.15 | ≥ | 0.812 | 1:1 | 1:1 |
| c | 2.211 | 0.966 | 5:8 | 5:8 | |
| d | 1.114 | 0.483 | 3:8 | 3:5 | |
| e | 0.645 | 0.930 | 1:4 | 2:3 | |
| f | 0.373 | 0.853 | 1:6 | 2:3 | |
| g | 0.252 | 0.871 | 1:8 | 3:4 | |
| h | 0.144 | 0.555 | 1:12 | 2:3 |
Orbital near-resonance
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 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. 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.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.
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.
Formation of the planetary system
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. 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.Tidal locking
It is suggested that all seven planets are likely to be tidally locked into a so-called synchronous spin state, making the development of life there much more challenging. A less likely possibility is that some may be trapped in a higher-order spin-orbit resonance. 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.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.With the exception of planet c, all of the planets have densities low enough to indicate the presence of significant 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.
Possible effects of strong X-ray and extreme UV irradiation of the system
Bolmont et al. modelled the effects of predicted far ultraviolet and extreme ultraviolet 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, 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.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.
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.
Spectroscopy of planetary atmospheres
Because of the system's relative proximity, the small size of the primary and the orbital alignments that produce daily transits, the atmospheres of TRAPPIST-1's planets are favorable targets for transmission spectroscopy investigation.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.
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.
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 to bar.
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. As of 2020, the TRAPPIST-1 is considered a most promising target for transmission spectroscopy using the James Webb Space Telescope.
Habitability and possibility of life
Impact of stellar activity on habitability
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.The study in 2020 have found super-flare rate of TRAPPIST-1 is 4.2 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.
Probability of interplanetary panspermia
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. 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. 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.Radio signal searches
In February 2017, Seth Shostak, senior astronomer for the SETI Institute, noted, "he SETI Institute used its Allen Telescope Array to observe the environs of TRAPPIST-1, scanning through 10 billion radio channels in search of signals. No transmissions were detected." Additional observations with the more sensitive Green Bank Telescope did not show evidence of transmissions.Other observations
Existence of undiscovered planets
One study using the CAPSCam astrometric camera concluded that the TRAPPIST-1 system has no planets with a mass at least with year-long orbits and no planets with a mass at least 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.Possibility of moons
, writing in The Astrophysical Journal Letters, notes that TRAPPIST-1 planets are unlikely to have large moons. The Earth's Moon has a radius 27% that of Earth, so its area is 7.4% that of Earth, which would likely have been noted in the transit study if present. Smaller moons of 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. The mathematical derivation is as follows:
is the Hill radius of the planet, calculated from the planetary semi-major axis, the mass of the planet, and the mass of the star. Note that the mass of the TRAPPIST-1 star is approximately ; the remaining figures are provided in the table below.
is the Roche limit of the planet, calculated from the radius of the planet, and the density of the planet.
| Planet | |||||||
| 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 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.