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TRAPPIST-1f
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TRAPPIST-1f
Artist's impression of TRAPPIST-1f. (February 2018)
Discovery[1]
Discovered byMichaël Gillon et al.
Discovery siteSpitzer Space Telescope
Discovery date22 February 2017
Transit
Orbital characteristics[2]
0.03849±0.00033 AU
Eccentricity0.01007±0.00068[3]
9.207540±0.000032 d
Inclination89.740°±0.019°
368.81°±3.11°[3]
StarTRAPPIST-1
Physical characteristics[2]
1.045+0.013
−0.012 R🜨
Mass1.039±0.031 M🜨
Mean density
5.009+0.138
−0.158 g/cm3
0.951±0.024 g
9.32±0.24 m/s2
TemperatureTeq: 217.7±2.1 K (−55.5 °C; −67.8 °F)[4]

TRAPPIST-1f is an exoplanet, likely rocky,[2] orbiting within the habitable zone[5] around the ultracool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

It was one of four new exoplanets to be discovered orbiting the star in 2017 using observations from the Spitzer Space Telescope.[1]

The planet is likely tidally locked, and has been depicted as an eyeball planet in artistic impressions by NASA.

Physical characteristics

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Mass, radius, and temperature

[edit]

TRAPPIST-1f is an Earth-sized exoplanet, meaning it has a radius close to that of Earth. It has an equilibrium temperature of 218 K (−55 °C; −67 °F).[4] It has a radius of 1.045 R🜨 and a mass of 1.039 M🜨.[2] It was initially estimated to have a much lower mass, and thus a low density of 3.3±0.9 g/cm3 and a surface gravity around 6.1 m/s2 (62% of Earth's value).[1] This suggested a large amount of volatiles, with a 2017 study suggesting that a water ocean may comprise as much as 20% of the planet's mass, increasing the temperature at the bottom of such an ocean to above 1,400 K (1,130 °C; 2,060 °F).[6] However, refined density estimates show that TRAPPIST-1f, like other planets in the system, is only slightly less dense than Earth, consistent with a rocky composition.[2]

Atmosphere

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According to simulations of magma ocean-atmosphere interaction, TRAPPIST-1f is likely to retain a fraction of primordial steam atmosphere during the initial stages of evolution, and therefore today is likely to possess a thick ocean covered by atmosphere rich in abiotic oxygen.[7] Helium emission from TRAPPIST-1f (and planets b and e) has not been detected as of 2022.[8]

Host star

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The planet orbits an (M-type) ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.08 M☉ and a radius of 0.11 R☉. It has a temperature of 2550 K and is at least 7-8 billion years old. In comparison, the Sun is 4.6 billion years old[9] and has a temperature of 5778 K.[10] The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars should be expected to have considerably less metal content than the Sun; on the other hand, metal-rich stars are also more likely to have planets than metal-poor ones. Its luminosity (L☉) is 0.05% of that of the Sun.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 18.8. Therefore, it is too dim to be seen with the naked eye.

Orbit

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TRAPPIST-1f orbits its host star with an orbital period of about 9.206 days and an orbital radius of about 0.037 times that of Earth's (compared to the distance of Mercury from the Sun, which is about 0.38 AU).

Habitability

[edit]
See also: Habitability of red dwarf systems
Artist's impression of the surface of TRAPPIST-1f, depicting a liquid water ocean on its surface. The parent star and neighbouring planets are also illustrated.

The exoplanet was announced to be either orbiting within or slightly outside of the habitable zone of its parent star, the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet. On 31 August 2017, astronomers at the Hubble Space Telescope reported the first evidence of possible water content on the TRAPPIST-1 exoplanets.[11][12]

TRAPPIST-1f has a radius about the same as Earth, at around 1.045 R🜨, but was initially thought to have only about two thirds of Earth's mass, at around 0.68 M🜨. So, it was considered somewhat unlikely to be a fully rocky planet, and extremely unlikely to be an Earth-like one, that is rocky with a large iron core but without a thick hydrogen-helium atmosphere enveloping the planet. Simulations in 2017 suggested the planet is approximately 20% water by composition, much higher than that of Earth. With such a massive water envelope, the pressure and temperature will be high enough to keep the water in a gaseous state and any liquid water will only exist as clouds near the top of TRAPPIST-1f's atmosphere. Based on this study, TRAPPIST-1f is therefore likely to be no more habitable than any other ice giant with water clouds in its atmosphere.[6] However, refined estimates show that TRAPPIST-1f has about the same mass as Earth, and like other planets in the system, is only slightly less dense than Earth, consistent with a rocky composition.[2]

Its host star is a red ultracool dwarf, with only about 8% of the mass of the Sun (close to the boundary between brown dwarfs and hydrogen-fusing stars). As a result, stars like TRAPPIST-1 have the ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live.[13] Because of this ability to live for long periods of time, it is likely TRAPPIST-1 will be one of the last remaining stars when the Universe is much older than it is now, when the gas needed to form new stars will be exhausted, and the remaining ones begin to die off.

The planet is very likely tidally locked, with one hemisphere permanently facing towards the star, while the opposite side shrouded in eternal darkness. However, between these two intense areas, there would be a sliver of moderate temperature – called the terminator line, where the temperatures may be suitable (about 273 K or 0 °C or 32 °F) for liquid water to exist. Additionally, a much larger portion of the planet may be habitable if it supports a thick enough atmosphere to transfer heat to the side facing away from the star.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

TRAPPIST-1f

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from Grokipedia
TRAPPIST-1f is a orbiting within the of the star , an M8-type located 12.43 parsecs (about 40.5 light-years) from in the constellation Aquarius. Discovered in 2017 as part of a system of seven Earth-sized planets detected via the transit method using ground-based telescopes and NASA's , TRAPPIST-1f is the sixth planet from its host star, completing an orbit every 9.208 days at a semi-major axis of 0.0385 AU. The planet has a mass of 1.039 ± 0.031 Earth masses and a radius of 1.045 ± 0.013 Earth radii, yielding a mean density of approximately 5.02 g/cm³, consistent with a rocky composition possibly including up to 5% water by mass. Its equilibrium temperature is estimated at around 218 K (-55°C), suggesting potential for surface conditions that could support liquid water if an atmosphere is present, though tidal locking—due to its close orbit—would result in one permanent dayside and nightside. The TRAPPIST-1 system, with its star aged approximately 7.6 ± 2.2 billion years (older than our Solar System), features planets in a near-resonant chain, where gravitational interactions enable precise mass measurements through transit-timing variations (TTVs). Key Characteristics of TRAPPIST-1f
ParameterValueSource
Mass1.039 ± 0.031 M⊕Agol et al. (2021)
Radius1.045 ± 0.013 R⊕Agol et al. (2021)
Orbital Period9.208 ± 0.00003 daysAgol et al. (2021)
Semi-major Axis0.0385 AUAgol et al. (2021)
Equilibrium Temperature~218 KDucrot et al. (2020)
Density5.02 ± 0.14 g/cm³Agol et al. (2021)
Regarding , TRAPPIST-1f's position in the outer makes it a candidate for retaining volatiles, but challenges include stellar flares from the active host star potentially eroding atmospheres and the effects of , which could drive volcanic activity or maintain subsurface oceans. Ongoing observations with the (JWST) aim to probe for atmospheric signatures in the system; as of 2025, direct data for TRAPPIST-1f remain limited, but recent studies of inner planets like TRAPPIST-1b and e suggest thin or absent primary atmospheres, with possible secondary atmospheres on e. The planet's low density suggests possible water-rich layers, enhancing its interest for , but no biosignatures have been detected.

Discovery and nomenclature

Discovery history

The TRAPPIST-1 system was initially surveyed as part of the (Transiting Planets and Planetesimals Small Telescope) project, which targeted nearby stars for transiting exoplanets using photometric monitoring. In May 2016, observations with the TRAPPIST telescope at in detected periodic dimming events indicative of three Earth-sized planets transiting the star , marking the first detection in the system. These initial findings were reported in a paper published in on May 2, 2016, establishing the presence of temperate, Earth-sized worlds orbiting this nearby star. Follow-up photometric observations revealed additional transits, leading to the identification of four more planets, including TRAPPIST-1f as the sixth innermost world. The full system of seven Earth-sized planets was announced on February 22, 2017, via a press release and detailed in a seminal paper published the following day, which highlighted the compact, resonant architecture of the system. TRAPPIST-1f was specifically identified through these extended ground-based campaigns with the telescope, which captured multiple transit events confirming its of approximately 9.2 days. Confirmation of TRAPPIST-1f and the other planets required space-based observations to achieve higher precision and rule out artifacts. Between September 2016 and March 2017, NASA's conducted intensive monitoring in the , measuring transit timing variations (TTVs) across more than 500 hours of nearly continuous observations to refine orbital periods and validate the planetary signals. These TTVs, arising from gravitational interactions among the planets, provided dynamical evidence for their masses and confirmed the seven-planet configuration without false positives. Ground-based telescopes played a crucial role in validating the detections and excluding eclipsing binary scenarios. Observations with the Very Large Telescope (VLT) using the HAWK-I instrument in December 2016 and the United Kingdom Infrared Telescope (UKIRT) helped confirm the achromatic nature of the transits and the single-star origin of the signals. The transit depth for TRAPPIST-1f, measured at approximately 0.64% of the stellar flux, indicated a radius comparable to Earth's, consistent with a rocky super-Earth composition.

Naming and designation

TRAPPIST-1f received its provisional designation as 2MASS J23062928-0502285 f, following the standard exoplanet nomenclature convention that appends a lowercase letter to the host star's catalog coordinates from the Two Micron All-Sky Survey (2MASS). The host star, initially cataloged as 2MASS J23062928-0502285 based on its right ascension and declination, was observed using the TRAPPIST telescope, leading to the discovery of its planetary system. This provisional format identifies the planet as the sixth in sequence around the star, with letters assigned from b (innermost) to h (outermost) in order of increasing orbital distance. The official scientific name, TRAPPIST-1f, honors the Transiting Planets and Planetesimals Small Telescope () in , which first detected the transits of the seven Earth-sized planets in the system during a 2010–2011 survey. As the sixth planet from the star, f occupies a position in the outer portion of the within this . The naming reflects the system's announcement in as the first known multi-planet setup with seven transiting terrestrial worlds around an star. As of 2025, TRAPPIST-1f has not been assigned a proper name through the International Astronomical Union's (IAU) initiative or other public naming processes, retaining its provisional and official designations for scientific use.

Host star and

Properties of

is an star classified as spectral type M8.0 ± 0.5, characterized by its low and cool surface. Its is 2559 ± 50 K, significantly cooler than the Sun's 5772 K, resulting in a appearance and minimal energy output in visible wavelengths. The star has a of 0.121 ± 0.003 R⊙ and a of 0.089 ± 0.006 M⊙, making it about 12% the size and 9% the mass of the Sun, which places it near the hydrogen-burning limit for main-sequence stars. The age of is estimated at 7.6 ± 2.2 Gyr, older than the Solar System's 4.6 Gyr, based on kinematic analysis and evolutionary models indicating it as a transitional thin/ population member. Its is near-solar at [Fe/H] = +0.04 ± 0.08, suggesting a composition similar to the Sun with no significant enrichment or depletion of heavy elements. At a distance of 12.43 ± 0.02 parsecs (approximately 40.66 light-years) from , as measured by , is one of the closest known multi-planet systems, enabling high-resolution observations with ground- and space-based telescopes. TRAPPIST-1 exhibits high flare activity driven by a strong magnetic field, approximately 100 times that of the Sun's global poloidal component, leading to frequent stellar outbursts. This results in elevated X-ray and ultraviolet radiation levels, with an X-ray luminosity of approximately 10^{27} erg/s, comparable to or exceeding the quiet Sun's output despite the star's lower bolometric luminosity. The star's rotation period is about 3.3 days, faster than typical for its age, and its spotted surface causes roughly 1% variability in brightness, as observed in photometric monitoring.
PropertyValueUnitSource
Spectral TypeM8.0 ± 0.5-Van Grootel et al. (2018)
2559 ± 50KGillon et al. (2017)
0.121 ± 0.003R⊙Van Grootel et al. (2018)
Mass0.089 ± 0.006M⊙Van Grootel et al. (2018)
Age7.6 ± 2.2GyrBurgasser & Mamajek (2017)
Metallicity [Fe/H]+0.04 ± 0.08dexVan Grootel et al. (2018)
Distance12.43 ± 0.02pcGagné et al. (2019)
Rotation Period~3.3daysMorris et al. (2018)
X-ray Luminosity~10^{27}erg/sWheatley et al. (2017)

Overview of the TRAPPIST-1 planets

The system, discovered in 2016–2017 through transit observations, hosts seven rocky, Earth-sized planets labeled b through h, all orbiting an ultracool star approximately 40 light-years away. These planets are arranged in a remarkably compact configuration, with semi-major axes ranging from about 0.011 AU for the innermost () to 0.062 AU for the outermost (TRAPPIST-1h), spanning a total radial extent smaller than the diameter of Mercury's orbit around the Sun. The system's architecture is characterized by a chain of near-3:2 mean-motion resonances between consecutive planets, forming a Laplace-type resonance that contributes to its long-term dynamical stability, potentially enduring for billions of years. This resonant configuration likely arose from convergent disk migration during the planets' formation. The planets are all terrestrial in nature, with radii between 0.8 and 1.1 times Earth's and masses ranging from approximately 0.3 to 1.4 Earth masses, yielding a total planetary mass of about 6.4 Earth masses; TRAPPIST-1f, in particular, has a mass of roughly 1.04 Earth masses. In terms of insolation, the inner three planets (b, c, d) receive too much stellar radiation to be habitable by Earth-like standards, while the outer two (g, h) are likely too cold, though borders the outer edge. Planets e and f lie within the conservative , where surface temperatures could permit liquid water under certain atmospheric conditions. As of 2025, observations have provided initial insights into the system's planetary atmospheres, indicating thin or absent atmospheres for some inner planets and potential detections of atmospheric gases on others like , enhancing opportunities for comparative studies. This makes the nearest known multi-planet system with transiting worlds in the .

Orbital characteristics

Orbital path and period

TRAPPIST-1f follows a nearly around its host star at a semi-major axis of 0.03849 AU, completing one full revolution every 9.20754 days. This short positions the planet relatively close to the star compared to Solar System standards, yet the faint of the M8V-type star keeps the received by TRAPPIST-1f at about 0.37 times that of . The orbit's eccentricity is very low, ~0.01, indicating minimal deviation from a perfect circle and contributing to stable thermal conditions over the orbital cycle. The of f relative to the plane of the sky is approximately 89.7°, enabling frequent transits observable from Earth-based and space telescopes. This near-edge-on geometry results in an impact parameter of about 0.31, meaning the planet passes nearly centrally across the stellar disk during transits. The resulting transit duration is approximately 1.05 hours, allowing for detailed photometric monitoring; the brief period ensures comprehensive phase coverage, from ingress to egress, in successive observations without long gaps.

Dynamical interactions and stability

TRAPPIST-1f participates in the system's extensive chain of mean-motion s, specifically locked in a 3:2 with the inner e and a 4:3 with the outer g; these interactions, part of the broader near-resonant involving ratios such as 8:5 and 5:3 among inner planets, help maintain the compact orbital spacing and prevent instabilities. This resonant configuration damps out potential chaotic perturbations, ensuring that gravitational tugs between f and its neighbors do not lead to significant orbital overlaps over long timescales. Long-term orbital stability for TRAPPIST-1f has been assessed through N-body simulations, which indicate that the planet's orbit remains dynamically stable for at least 10^9 years under nominal conditions, with eccentricity excitation kept below 0.001 due to the resonant damping effects. These models account for mutual gravitational interactions across the seven-planet system and confirm that no ejections or collisions occur within the simulated age of the star, approximately 7.6 billion years. The current positions of TRAPPIST-1f and its siblings likely resulted from inward orbital migration during the phase, where disk torques drove the closer to the star and captured them into the stabilizing resonant chain. This migration process explains the unusually tight packing within 0.06 AU, as converged without crossing orbits thanks to the formation. Stellar further influence the evolution of TRAPPIST-1f's , inducing a gradual inward decay at an estimated rate of less than 10^{-7} yr^{-1}, which is negligible over the system's lifetime and does not disrupt the resonant structure. In contrast to the inner like b and c, which face stronger tidal torques and higher perturbation levels from closer proximity to the star and each other, TRAPPIST-1f benefits from its outer location, experiencing reduced gravitational disturbances and thus enhanced long-term stability.

Physical characteristics

Mass, radius, and density

TRAPPIST-1f has a radius of 1.045 ± 0.013 R⊕, determined from joint analysis of transit light curves obtained with the Spitzer Space Telescope, Hubble Space Telescope, and K2 mission. This measurement refines earlier estimates by incorporating extended photometric datasets to minimize systematic uncertainties in transit depth and limb darkening. The planet's mass is 1.039 ± 0.031 M⊕, derived from transit timing variations (TTVs) analyzed via N-body simulations and photodynamical modeling of multi-planet interactions. These TTVs arise from gravitational perturbations among the closely packed TRAPPIST-1 planets, enabling precise mass constraints without direct radial velocity measurements, though ongoing ESPRESSO campaigns seek to complement these results. The mean of TRAPPIST-1f is calculated using the formula ρ=3M4πR3,\rho = \frac{3M}{4\pi R^3}, yielding 5.02^{+0.14}_{-0.13} g/cm³ (or 0.911 ± 0.025 ρ⊕). This value, lower than 's 5.51 g/cm³, indicates a predominantly composition consistent with interior models featuring a reduced iron content of approximately 21% by relative to Earth's 32%, or alternatively a thin H/He contributing less than 1% to the total . Compared to , TRAPPIST-1f exhibits a slightly larger but comparable , resulting in this modestly lower that constrains possible volatile fractions in structural models.

Internal structure and composition

Models of TRAPPIST-1f's internal structure assume a three-layer differentiated composition, consisting of a central iron core, an overlying mantle, and an outer layer of and other volatiles. The iron core is estimated to comprise 25-35% of the planet's total mass, consistent with Earth-like compositions used in planetary interior solvers such as MAGRATHEA, which account for uncertainties in core alloying (e.g., Fe-Si or FeS) and equation-of-state parameters. The mantle, dominated by magnesium-iron silicates in phases like , , and bridgmanite, forms the bulk of the remaining rocky material and is modeled with variable iron content to match the planet's observed . A volatile layer, primarily in high-pressure ice phases (e.g., Ice VII), is inferred to constitute up to 5% of the total mass in conservative models, though broader ranges of 7-16% are possible depending on core mass fraction assumptions and observational uncertainties in mass (∼3%) and radius (∼1.2%); recent analyses indicate 6.9 ± 2.0% assuming an Earth-like mantle-to-core ratio, or 16.2 ± 9.9% across all core fractions. Formation scenarios for TRAPPIST-1f involve accretion of planetesimals in the outer beyond the (∼0.06 AU at 10 Myr), followed by inward migration, which incorporated a higher fraction of volatiles compared to inner siblings like and c. This process is supported by mass-radius-composition models showing a radial gradient in / content, with TRAPPIST-1f retaining ≥5 wt% volatiles, enabling a distinct of ices and potential hydrous silicates in the . The planet's relatively low , derived from transit and measurements, implies seismic implications such as the possibility of a subsurface ocean if the volatile layer experiences under internal and temperatures. Interior models integrate to derive profiles, given by the equation dPdr=−ρg,\frac{dP}{dr} = -\rho g, where PP is , ρ\rho is , rr is radial distance, and gravitational acceleration g=GM/r2g = GM/r^2 with GG the and MM the mass enclosed within rr. This equilibrium is solved numerically alongside equations of state for each layer to constrain composition and phase boundaries. Tidal heating from orbital resonances differentiates TRAPPIST-1f's structure from Earth's, promoting a thinner crust due to elevated mantle temperatures and heat fluxes of approximately 0.14 W/m², about 1.6 times Earth's mean value of 0.087 W/m². This contributes to in the volatile layer and potentially reducing crustal thickness to maintain thermal balance.

Atmosphere and climate

Potential atmospheric composition

Theoretical models of TRAPPIST-1f's atmosphere, assuming from the planetary interior, predict a secondary atmosphere dominated by CO₂ or a of N₂ and O₂, similar to Venus-like or Earth-like compositions, respectively. These models suggest surface pressures ranging from 0.1 to 10 bar to maintain atmospheric stability, depending on the extent of volatile retention and stellar flux. Primordial envelopes may have been retained on TRAPPIST-1f due to its position receiving lower stellar compared to inner planets, though hydrodynamic escape processes limit their longevity, with timescales on the order of 10⁹ years. Recent models estimate volcanic rates of volatiles at approximately 0.03 times Earth's rate (around 10⁹ kg/yr), though upper limits reach up to 8 times Earth's (around 8×10¹⁰ kg/yr), which could sustain a secondary atmosphere over geological timescales by replenishing lost gases. The atmospheric scale height, which determines the thickness of the envelope, is given by the formula H=kTμg,H = \frac{kT}{\mu g}, where kk is Boltzmann's constant, TT is temperature, μ≈29\mu \approx 29 g/mol for an Earth-like gas mixture, and gg is surface gravity. Compared to inner TRAPPIST-1 planets, TRAPPIST-1f experiences reduced atmospheric stripping from stellar winds and radiation, enabling the potential for a thicker envelope.

Surface conditions and temperature

TRAPPIST-1f is tidally locked to its host star, resulting in a permanent dayside facing the star and a nightside in perpetual darkness, which leads to significant temperature contrasts across the planet's surface. Climate models indicate that the dayside surface temperature is approximately 250 , while the nightside is cooler at around 190 , with atmospheric facilitating partial heat redistribution from the dayside to moderate the nightside conditions. Albedo models for a rocky surface on TRAPPIST-1f suggest values between 0.1 and 0.3, influencing the absorbed stellar radiation and contributing to surface warming. The presence of CO₂ in the atmosphere could induce a , raising surface temperatures depending on atmospheric thickness and composition. Global climate models (GCMs) simulate potential surface environments, predicting that a global could exist under certain atmospheric conditions, with water persisting on the dayside and forming on the nightside due to the . These models incorporate and dynamical processes to assess heat transport efficiency. The effective surface temperature can be adjusted from the equilibrium temperature using the relation Tsurf=Teq(1−greenhouse factor)1/4T_{\text{surf}} = \frac{T_{\text{eq}}}{(1 - \text{greenhouse factor})^{1/4}}, where the greenhouse factor represents the fraction of trapped by the atmosphere; for a Venus-like scenario, this factor is approximately 0.7, significantly elevating surface temperatures. Tidal heating on TRAPPIST-1f contributes a minor of about 0.1 W/m² to the interior, which is negligible compared to the stellar insolation of approximately 520 W/m² received at the top of the atmosphere. The orbital distance from the star modulates this insolation, influencing overall energy balance.

Scientific observations

Pre-JWST observations

The discovery of TRAPPIST-1f through initial ground-based transit observations in 2016 was followed by extensive pre-JWST monitoring to refine its parameters and probe its atmosphere. Extensive photometry with the Spitzer Space Telescope's Infrared Array Camera (IRAC) from 2017 to 2020 captured multiple transits of TRAPPIST-1f, enabling precise refinement of transit timings through global analysis of light curves. These observations, spanning over 1,000 hours across the 3.6 μm and 4.5 μm channels, reduced uncertainties in the orbital ephemeris and revealed transit timing variations consistent with dynamical interactions in the system. Additionally, analysis of secondary eclipse data showed a flat spectrum with no detectable thermal emission excess, ruling out a thick hydrogen-helium atmosphere at the 3σ level and constraining the planet's dayside brightness temperature to below 500 K. In 2018, the Hubble Space Telescope's Wide Field Camera 3 (WFC3) obtained near-infrared transmission spectra of TRAPPIST-1f during three transits, covering the 1.1–1.7 μm wavelength range. The resulting spectrum exhibited no significant absorption features, particularly no strong water vapor signals at the 1.4 μm band, with a feature depth upper limit of less than 0.3%. This flat spectrum constrained the presence of volatiles, favoring either a thin atmosphere or a bare rocky surface over a hydrogen-dominated envelope with substantial water content. Ground-based monitoring with the HARPS spectrograph from 2018 to 2021 provided limits on TRAPPIST-1f's by mitigating stellar activity signals through multi-wavelength modeling. Combined with transit timing variations from photometry, these efforts yielded a estimate of 1.039 ± 0.031 M⊕, consistent with a composition and density of 5.01 ± 0.14 g/cm³. (TESS) full-frame images in Sectors 4 (2018) and 48 (2022) independently confirmed the transits of TRAPPIST-1f, with photometric precision sufficient to detect the 0.5% depth signals. The light curves showed no significant out-of-transit variability or asymmetric shapes that could indicate rings or moons, placing upper limits on any such companions at less than 0.1 R⊕ in size. Supplementary data from the K2 mission (2016–2017) and CHEOPS (2019–2023) further improved the radius measurement of TRAPPIST-1f to 1.045 ± 0.013 R⊕ by combining high-cadence photometry with stellar parameter updates. These observations reduced systematic errors from and stellar variability, confirming the planet's Earth-like size without evidence for extended atmospheres.

James Webb Space Telescope results

As of November 2025, (JWST) observations of the system have primarily focused on the inner planets (b, c, d, e), revealing thin or absent atmospheres, no thick CO₂ envelopes, and thermal emissions consistent with bare rock surfaces or minimal atmospheric redistribution in some cases. Direct spectroscopic data for TRAPPIST-1f remain limited, with ongoing programs in Cycles 2–3 targeting planets for transmission and emission spectroscopy to probe potential atmospheres. Phase curve analyses of inner planets (e.g., b and c) show no evidence of thick atmospheres but have not yet extended to f. No biosignatures have been detected in the system, though future observations may constrain volatiles like O₂ for outer planets such as f.

Habitability

Placement in the habitable zone

The (HZ) refers to the orbital distance range around a star where a rocky planet with an Earth-like atmosphere could maintain liquid water on its surface, bounded by the inner edge (runaway or moist limit, beyond which escapes) and the outer edge (maximum CO2 limit, beyond which CO2 condenses out). For ultra-cool M-dwarf stars like , with low and infrared-dominated spectra, the HZ lies much closer to the star than for Sun-like stars, typically between 0.02 and 0.05 AU depending on the model. Models by Kopparapu et al. (2013), which use a one-dimensional radiative-convective code adjusted for stellar (Teff ≈ 2560 K for ), place the conservative HZ inner boundary at an effective incident flux Seff ≈ 1.01 (corresponding to ≈0.023 AU) and the outer boundary at Seff ≈ 0.34 (≈0.042 AU), where Seff is normalized to Earth's value of 1. These limits account for the star's , which reduces the inner edge compared to hotter stars due to less UV-driven water loss. TRAPPIST-1f orbits at a semi-major axis of 0.0385 ± 0.0003 AU, positioning it near the outer portion of this conservative HZ. The planet receives an incident flux of Seff = 0.373 ± 0.015 (approximately 510 W/m², compared to Earth's 1366 W/m²), which is sufficient for surface temperatures compatible with liquid water under modest greenhouse forcing. The effective flux is calculated as Seff=L⋆L⊙(a\Eartha)2,S_\text{eff} = \frac{L_\star}{L_\odot} \left( \frac{a_\Earth}{a} \right)^2, where L⋆/L⊙≈5.5×10−4L_\star / L_\odot \approx 5.5 \times 10^{-4} for TRAPPIST-1, aa is the planet's semi-major axis, and a\Earth=1a_\Earth = 1 AU; the inner HZ limit occurs near Seff≈1.0S_\text{eff} \approx 1.0 for Earth-analog planets, marking the runaway greenhouse threshold. Given its proximity to the star ( ≈9.2 days), TRAPPIST-1f is likely tidally locked with one face perpetually toward the star, a common outcome for HZ planets around M-dwarfs. This configuration shifts the effective HZ inward by up to 20-30% compared to rapidly rotating planets, as atmospheric heat transport from the to the nightside enables liquid water stability at higher fluxes without atmospheric collapse; for TRAPPIST-1f, this supports potential liquid water with only modest effects. Relative to its siblings, TRAPPIST-1f lies outward of planet e (a ≈ 0.028 , Seff ≈ 0.66, warmer equilibrium temperature ≈ 230 ) but inward of g (a ≈ 0.046 , Seff ≈ 0.26, cooler ≈ 200 ), receiving less irradiation than the hotter inner planets (e.g., d at Seff ≈ 1.1) while avoiding the extreme outer freeze-out risks.

Prospects for water and

Models of TRAPPIST-1f's interior and evolution suggest that a global could form and persist if the planet possesses a mass fraction of 0.01-0.21%, equivalent to several oceans, with from its eccentric providing the necessary to maintain liquidity against the star's dim insolation. Recent 2025 modeling indicates a possible higher mass fraction of approximately 16% ± 10%, depending on core composition, which could enhance prospects for substantial layers. Such scenarios arise from coupled magma -atmosphere simulations, where prolonged tidal dissipation prevents rapid cooling and allows to remain in a or supercritical state beneath a thick envelope. However, lower abundances would likely result in a drier surface with limited volatile retention. The planet faces significant challenges to surface due to its , which imposes extreme temperature contrasts between the dayside and nightside, potentially leading to buildup on the cold hemisphere and on the hot side, though could mitigate this by transporting heat. Additionally, TRAPPIST-1's ultraviolet flux is approximately 0.4 times that received by , though (XUV) irradiation may be 10-1000 times higher, posing a severe threat to any protective through erosion and photochemical destruction, rendering surface vulnerable to . Despite these obstacles, subsurface environments, such as liquid water oceans insulated by shells, remain viable for microbial , as they would be shielded from stellar and sustained by internal heating. Prospects for detecting biosignatures like oxygen (O₂) or (CH₄) on TRAPPIST-1f using the (JWST) are promising in transmission spectroscopy, where disequilibrium pairs of these gases could indicate , though abiotic false positives from or volcanic must be carefully distinguished. Recent 2024-2025 climate modeling studies indicate that habitable conditions, including liquid water stability, may exist in localized pockets on the trailing hemisphere, where winds and heat redistribution create milder environments, but JWST observations to date have not confirmed atmospheric or other volatiles on the planet. Future observations with the mission, slated for the 2030s, will enhance spectroscopy of TRAPPIST-1f's potential atmosphere across a broad range, enabling detection of water-related molecules and improving assessments. TRAPPIST-1f ranks highly in the Habitable Exoplanets Catalog (as of 2024) due to its position and size, positioning it as a prime target for ongoing searches for liquid water and .

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