How strong is gravity on TRAPPIST-1 g?
TRAPPIST-1 g has the largest radius among the seven planets, but not the greatest mass. Its radius is about 1.129 times Earth’s and its mass about 1.321 times Earth’s; the innermost planet b is slightly heavier than g. Surface gravity calculated from g’s mass and radius is about 1.04 times Earth’s. Size alone cannot tell us the surface gravity or whether an atmosphere is likely to remain.
- About 12.35 days Time required to orbit the star
- About 1.129 Earth radii Largest radius among the seven planets
- About 1.321 Earth masses Mass calculated from changes in transit times
- About 1.04 times Earth’s Surface gravity calculated from mass and radius
How are a planet’s size and surface gravity connected?
Radius comes from the amount of starlight blocked when the planet crosses its star. Mass is calculated from the way the seven planets pull one another and shift their transit times. A 2021 study found g’s mass to be 1.321±0.038 times Earth’s and b’s to be 1.374±0.069 times Earth’s. Thus g is the largest of the seven by radius, but slightly lighter than b.
Surface gravity grows with mass but weakens as radius grows. If a planet becomes both more massive and larger, its surface gravity may change little. For g, it is about 1.04 times Earth’s, while the escape speed needed to leave its gravity completely is calculated at about 12.1 km/s. Neither value was measured on the surface; both were derived from mass and radius.
NASA’s description of g as a “super-Earth” is also a classification based on size and mass. It means a rocky-type planet larger than Earth, not one with Earth-like land, oceans or air. The average density of about 5.07 g/cm³ suggests that rock is probably the main material, but does not determine the size of the core or the amount of water.
Planet g has the largest radius among the seven. Yet its surface gravity is almost the same as Earth’s, and an atmosphere has not been confirmed.
How can we tell whether an atmosphere remains?
Escape speed is the speed an object needs to leave a planet’s gravity completely. But this single number does not determine how long an atmosphere survives. The mass of gas molecules, temperature of the upper atmosphere, ultraviolet and X-ray light and particles from the star, a planet’s magnetic field and fresh gases supplied from its interior all matter.
g’s escape speed being slightly greater than Earth’s means gases would be harder to lose if every other condition were the same. Conditions are not the same: TRAPPIST-1 is a red dwarf with frequent powerful eruptions. Whether g has a magnetic field or receives a continuing supply of gases from inside has not been measured.
Hubble observations found that d, e and f lack thick, clear, hydrogen-rich atmospheres, but the data for g were not precise enough to make the same judgment. This does not mean a hydrogen atmosphere was found on g. The observations simply could not decide whether one existed.
What kind of atmosphere would liquid water require?
Planet g is about 7 million km from its star and receives about 26% as much energy as Earth receives from the Sun. It lies toward the outer part of the habitable zone, but its orbit alone cannot show that liquid water exists. With almost no atmosphere, most of the surface could freeze and the temperature difference between the side facing the star and the dark side could be very large.
Three-dimensional climate studies calculated cases in which enough carbon dioxide builds up to create a greenhouse effect and allow liquid water on part of the surface. The required pressure varied greatly, from several to tens of times Earth’s sea-level pressure. The answer changed with assumptions about clouds, surface reflectivity, nitrogen and how much daytime heat moves into the night. These figures are conditions in climate models, not atmospheric pressures measured on g.
If g always faces the star with the same side, the atmosphere matters even more. Enough air could move heat from the bright side to the dark side, while an atmosphere that is too thin would allow a large temperature difference. Being in the habitable zone alone therefore does not reveal g’s climate.
How can an atmospheric signal be confirmed?
According to NASA’s January 2026 summary, Webb has observed g but the analysis has not yet been released. Dark patches and eruptions on TRAPPIST-1 can make the amount of blocked starlight appear different at each wavelength. Because these changes can resemble a planetary atmosphere, the same marks must repeat at the same wavelengths even when the star’s condition changes.
If carbon dioxide is present, light near 4.3 μm could decrease clearly. If almost no differences appear among wavelengths, researchers must compare an airless planet, a very thin atmosphere and an atmosphere whose lower layers are hidden by high clouds. Observing the planet’s own heat and measuring the temperature difference between bright and dark sides could separately test whether an atmosphere carries heat.
- Decrease in starlight The amount of light blocked by g gave its radius.
- Changes in transit times The mutual pull of the seven planets gave its mass.
- Mass and radius These values gave its calculated surface gravity and escape speed.
- Future atmosphere observations They must test whether molecular marks repeat and whether daytime heat moves into the night.
How similar is TRAPPIST-1 g to Earth?
g’s radius, mass and surface gravity lie in a range similar to Earth’s. Its orbit could allow liquid water if it has the right atmosphere and enough greenhouse warming. Yet no one has confirmed whether an atmosphere exists, what its pressure and composition are, how much water is present or what the surface temperature is. Current measurements show that g deserves detailed study, not that it has an Earth-like environment.
The ocean, ice and clouds shown on this page are an illustration, not an observed photograph. Surface gravity and escape speed were calculated from mass and radius. No atmosphere, liquid water or life has been discovered.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 g
- NASA Science — TRAPPIST-1 g Catalog
- The Planetary Science Journal — Masses, Radii, Densities and Dynamics
- Nature Astronomy — Hubble Atmospheric Reconnaissance
- Astronomy & Astrophysics — Climate and Volatile Models
- MNRAS Letters — Habitable Scenarios for TRAPPIST-1 Planets
- NASA Science — Webb Results for the TRAPPIST-1 System