TRAPPIST-1e
TRAPPIST-1e is a terrestrial exoplanet orbiting the ultracool red dwarf star TRAPPIST-1, an M8-type star located approximately 39 light-years (12 parsecs) from Earth in the constellation Aquarius.[1] As the fourth planet from its host star in a compact system of seven Earth-sized worlds, TRAPPIST-1e resides within the habitable zone, receiving about 62% of the stellar flux that Earth does from the Sun, which suggests potential conditions for liquid surface water if an atmosphere is present.[1] Discovered in 2017 using the transit method with ground-based telescopes including the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) in Chile, it completes one orbit every 6.1 days at a semi-major axis of 0.029 AU, with a low eccentricity of approximately 0.01.[1][2] The planet's physical characteristics indicate a rocky composition similar to Earth, with a mass of 0.692 ± 0.024 Earth masses, a radius of 0.910 ± 0.031 Earth radii, and a mean density of about 5.1 g/cm³ (relative density 0.923 times Earth's), consistent with a silicate mantle and iron core lacking significant volatile envelopes.[2] Its equilibrium temperature is estimated at around 251 K (-22°C), though tidal locking—due to the close orbit—would create a permanent dayside and nightside, potentially leading to extreme temperature gradients without atmospheric heat transport.[2] The TRAPPIST-1 system itself is notable for its near-resonant orbital chain, where the planets' periods form ratios close to integers (e.g., 3:2 for e and d), stabilizing the configuration over billions of years despite the host star's age of 7.6 ± 2.2 billion years—roughly twice that of the Solar System.[1][3] Habitability assessments for TRAPPIST-1e highlight both promise and challenges: its position in the conservative habitable zone and rocky nature make it one of the most Earth-like exoplanets known, but the active host star emits frequent flares that could strip atmospheres through high levels of X-ray and ultraviolet radiation.[4] Models suggest the planet could retain a thin atmosphere over geological timescales, potentially supporting liquid water oceans on the dayside if protected by a magnetic field or specific compositions like nitrogen-dominated gases.[4] However, early studies indicated possible water loss from the outer planets, though TRAPPIST-1e, receiving moderate irradiation, fares better than inner siblings.[4] Recent observations with the James Webb Space Telescope (JWST) in 2025 have provided the first transmission spectra of TRAPPIST-1e, revealing no strong evidence for a thick atmosphere and weakly disfavoring CO₂-rich scenarios at pressures similar to Venus or Mars.[5] These data, spanning 0.6–5.5 μm, show significant contamination from stellar spots but rule out hydrogen-rich atmospheres with carbon dioxide or methane, while permitting bare-rock surfaces or thin, nitrogen-dominated atmospheres with trace gases.[6] Such findings narrow the possibilities for volatile retention, emphasizing TRAPPIST-1e's likely barren or minimally atmospheric state, though future JWST transits of other system planets may refine these constraints. Additional JWST observations, including fifteen more transits of TRAPPIST-1e, are ongoing as of late 2025 to further refine these constraints.[6]Discovery and nomenclature
Initial detection
TRAPPIST-1e was first detected as part of the TRAPPIST Ultra-cool Dwarf Transit Survey (TUDTS), a ground-based photometric program aimed at identifying Earth-sized exoplanets transiting nearby ultracool dwarfs within their habitable zones.[1] The survey utilized the 0.6-meter TRAPPIST (Transiting Planets and Planetesimals Small Telescope) robotic telescope installed at the La Silla Observatory in Chile, which monitored the ultracool dwarf star TRAPPIST-1—a late M8-type star located approximately 12 parsecs from Earth—for periodic brightness dips indicative of planetary transits.[1] Initial observations of TRAPPIST-1 began in late 2015, leading to the detection of three inner transiting planets (designated b, c, and d) announced in May 2016. To resolve ambiguities in the light curve and search for additional planets, follow-up observations were conducted using NASA's Spitzer Space Telescope, which provided continuous monitoring in the infrared to minimize atmospheric interference.[1] Starting in September 2016, Spitzer observed TRAPPIST-1 for nearly 500 hours over 20 days, revealing four additional shallow transit signals beyond the initial three planets.[7] Among these, the signal for TRAPPIST-1e was identified through detailed analysis of the transit timing variations and photometric data, confirming it as an Earth-sized planet orbiting in the habitable zone of the system.[1] The full seven-planet configuration, including TRAPPIST-1e as the innermost of the newly detected worlds, was announced by Michaël Gillon and collaborators on February 22, 2017, in a paper published in Nature.[1] This detection highlighted the potential of ultracool dwarfs as hosts for compact multi-planet systems amenable to transit surveys.[1]Confirmation and naming
The existence of TRAPPIST-1e, along with the other planets in the system, was confirmed through the extensive Spitzer observations conducted in late 2016, supplemented by ground-based photometry from the TRAPPIST telescope and the Very Large Telescope (VLT) at Paranal Observatory, which corroborated the initial signals and provided light curves to distinguish true planetary transits from stellar variability.[1] The planet was formally named TRAPPIST-1e in accordance with the International Astronomical Union's nomenclature for exoplanets, where letters 'b' through 'h' denote the planets in ascending order of orbital periods around the host star TRAPPIST-1, positioning 'e' as the fourth innermost world.[1] No informal or provisional names have been adopted for TRAPPIST-1e or its siblings. Early parameter estimates derived from these transit data indicated a radius of approximately 0.92 Earth radii (R⊕) for TRAPPIST-1e, establishing it as an Earth-sized planet, though no direct mass determination was possible at this stage due to the faint radial-velocity signal of the ultracool dwarf host star.[1] The comprehensive confirmation of the seven Earth-sized planets orbiting TRAPPIST-1, including TRAPPIST-1e, was detailed in a seminal paper by Gillon et al., published in Nature in 2017, marking a milestone in the study of compact multi-planet systems around low-mass stars.[1]The TRAPPIST-1 system
Host star characteristics
TRAPPIST-1 is an ultracool dwarf star of spectral class M8V, classified as a late-type M dwarf due to its low effective temperature and small size. Located in the constellation Aquarius at a distance of 40.5 light-years (12.4 parsecs) from Earth, it hosts a compact system of seven Earth-sized planets.[8][9] The star's fundamental parameters are as follows:| Parameter | Value | Source |
|---|---|---|
| Mass | 0.0898 ± 0.0023 M⊙ | Agol et al. (2021)[10] |
| Radius | 0.1192 ± 0.0013 R⊙ | Agol et al. (2021)[10] |
| Effective temperature | 2566 ± 26 K | Agol et al. (2021)[10] |
| Bolometric luminosity | (5.5 ± 0.3) × 10^{-4} L⊙ | Agol et al. (2021)[10] |
| Age | 7.6 ± 2.2 Gyr | Burgasser & Mamajek (2017)[11] |
System architecture and planets
The TRAPPIST-1 system comprises seven rocky, Earth-sized planets designated b through h, which orbit their ultracool dwarf host star in a tightly packed, near-resonant chain, enabling detailed characterization through transit observations. This compact architecture, with all planetary orbits confined within approximately 0.06 AU of the star, facilitates the detection of transit timing variations (TTV) that have been crucial for estimating planetary masses.[13] TRAPPIST-1e occupies the fourth position in this sequence, lying within the system's habitable zone and receiving about 0.65 times the average stellar insolation that Earth experiences from the Sun. The planets span a narrow range of sizes, with radii between roughly 0.76 and 1.13 Earth radii, and masses estimated via TTV analyses from approximately 0.33 to 1.37 Earth masses, positioning TRAPPIST-1e as one of the more Earth-like members in both dimensions.[14]Physical properties
Size, mass, and density
TRAPPIST-1e has a radius of 0.920 ± 0.012 Earth radii, determined from the depth of its transits across the host star as observed by the Spitzer Space Telescope and other facilities.[15] This measurement reflects the planet's size relative to Earth, placing it among the terrestrial worlds in the TRAPPIST-1 system. The planet's mass is 0.692 ± 0.022 Earth masses, derived from detailed analysis of transit-timing variations (TTVs) that capture gravitational interactions among the planets, refining earlier estimates from initial discoveries.[15] These TTVs, combined with N-body simulations, provide constraints on the orbital dynamics and bulk properties. From the mass and radius, the mean density is calculated as 4.885 ± 0.18 g/cm³, a value consistent with a predominantly rocky composition similar to Earth's.[15] The surface gravity on TRAPPIST-1e is approximately 0.817 g, or 8.01 m/s², computed using the formula g = \frac{GM}{r^2}, where M is the planetary mass and r is the radius.[15] This lower gravity compared to Earth's arises from the planet's reduced mass despite its near-Earth size.Composition and internal structure
TRAPPIST-1e is characterized as a rocky planet, with its bulk density of approximately 4.89 g/cm³ indicating a differentiated interior dominated by refractory materials.[16] This high density supports the presence of an iron-rich core comprising about 25–28% of the planet's total mass, overlaid by a silicate mantle that constitutes roughly 65–70% of the mass, and potentially a thin crust.[16][17] The core-mantle boundary is inferred from interior structure models that account for the planet's mass and radius constraints, suggesting a core radius of around 0.4–0.5 times the planet's radius under Earth-like compositional assumptions depleted in iron (approximately 21 wt% Fe).[16] The planet's density effectively rules out a substantial hydrogen-helium envelope, with models limiting any gaseous layer to less than 1% of the total mass, as thicker envelopes would reduce the bulk density below observed values.[16] Instead, the interior is consistent with a volatile-poor to moderately volatile-enriched rocky composition, without evidence for extended gas layers.[17] Interior structure models further indicate the potential for a water or ice layer, with volatile mass fractions estimated at 5–20% depending on formation scenarios and atmospheric escape histories.[18] These models, incorporating multi-phase water layers (including supercritical, liquid, and condensed ice phases), predict that such a hydrosphere could overlie the silicate mantle, though confirmation awaits direct observational constraints on the planet's full mass-radius profile.[18]Orbital dynamics
Key orbital parameters
TRAPPIST-1e orbits its ultracool dwarf host star at a close distance, completing one revolution in approximately 6.1 days, placing it within the system's habitable zone alongside planets f and g. This short orbital period results from the planet's proximity to the star, with key parameters derived from extensive transit timing variations (TTVs) and photometric observations using telescopes such as Spitzer and ground-based facilities. The orbit is nearly circular, as indicated by a low eccentricity value, and highly inclined relative to the line of sight, enabling frequent transits observable from Earth. The following table summarizes the primary orbital parameters for TRAPPIST-1e:| Parameter | Value | Unit | Source |
|---|---|---|---|
| Semi-major axis | 0.02925 ± 0.00016 | AU | Agol et al. (2021)[15] |
| Orbital period | 6.101013 ± 0.000035 | days | Agol et al. (2021)[15] |
| Eccentricity | 0.00510 ± 0.00038 | - | Agol et al. (2021)[15] |
| Inclination | 89.75° ± 0.05° | degrees | Ducrot et al. (2020)[19] |