What does TRAPPIST-1 f’s low density tell us?
TRAPPIST-1 f has about 1.04 times Earth’s mass and 1.045 times its radius. Its average density, calculated from these values, is about 5.02 g/cm³, roughly 9% lower than Earth’s. A large amount of water inside could lower the density this much, but the number alone does not prove that an ocean exists. A rocky planet with less iron, or one whose iron is bound to oxygen and spread through the mantle, can have the same mass and size.
- About 9.21 days Time required to orbit the star
- About 1.045 Earth radii Radius calculated from the amount of starlight blocked
- About 1.04 Earth masses Mass calculated from changes in transit times
- About 5.02 g/cm³ Average density calculated from mass and size
How do we measure TRAPPIST-1 f’s size and mass?
When f crosses in front of its star, the starlight falls by about 0.65%. A larger planet blocks more light, so this decrease gives the size ratio between the planet and star. Applying the star’s actual size then gives f’s radius.
Mass is found through the gravity of neighboring planets. The seven planets pull one another, making each transit occur slightly earlier or later than expected. In 2021, researchers combined four years of Spitzer data with observations from ground telescopes, Hubble and K2, and analyzed 447 transit times. Large timing changes with a cycle of about 490 days, especially among the outer five planets, allowed them to calculate f’s mass with an uncertainty of about 3%.
Average density is mass divided by volume. A planet’s volume depends on its radius multiplied by itself three times, so even a small error in radius has a larger effect on density. Average density also represents the whole planet with one value. It does not show where the core, mantle and any water layer begin or end.
What internal structures can produce the same density?
The first possibility is a nearly dry rocky planet with little iron. In a model assuming that all seven TRAPPIST-1 planets formed from similar material, an iron share of about 21% of the total mass explains the measured densities. This is lower than Earth’s iron share of about 32%, so the average density can be low without a thick water layer.
A second possibility is a planet where iron is not gathered in a metal core but is bound to oxygen and mixed through the mantle. The same elements can make a larger planet when arranged differently inside. A third possibility is a light layer of water or ice above Earth-like rock and iron. If the water layer is very deep, pressure could turn the lower water into solid forms of ice not found in Earth’s oceans.
Average density cannot determine how much water lies inside. The calculated amount of water changes with the assumed size of the iron core.
How was TRAPPIST-1 f’s water fraction estimated?
In 2025, researchers entered the measured mass and radius into several models of the planet’s interior. When they fixed the mantle-to-core ratio near Earth’s, water made up 6.9±2.0% of the total mass. When the core size was allowed to vary more freely, the result was 16.2±9.9%. The answers differ so much because a telescope did not measure water directly; each calculation began with a different assumed interior.
These calculations contain values that have not been measured directly, such as how much rock and iron compress under pressure, the temperature of each layer and the size of the core. Even an estimate of the total amount of water cannot say whether it is surface liquid, ice or water vapor in the atmosphere. Low density leaves open the possibility of a water-rich planet, but water is not the only explanation.
What more could its atmosphere tell us about water?
Hubble observations made a large, puffy, hydrogen-rich atmosphere like Neptune’s unlikely on f. It is still unknown whether the planet has a thin atmosphere of heavier gases such as nitrogen, carbon dioxide or water vapor. Without its pressure and greenhouse effect, the surface temperature cannot be calculated precisely even though f receives about 37% as much starlight as Earth.
According to NASA’s January 2026 summary, Webb has observed f but the atmospheric analysis is still under way. Researchers must first separate changes caused by spots and eruptions on the star, then check whether marks from molecules such as carbon dioxide repeat in several observations. Once the pressure and composition of the atmosphere are known, they can better calculate whether water could exist as liquid, ice or vapor.
- Decrease in starlight The size ratio of the planet and star gives the planet’s radius.
- Changes in transit times The mutual pull of the seven planets gives the planet’s mass.
- Average density Dividing mass by volume narrows the possible materials inside.
- Atmosphere observations Pressure and composition would show which states of water could exist.
Does being in the habitable zone mean f has water?
Planet f lies in the habitable zone. This means it receives enough starlight for surface liquid water to be possible if atmospheric conditions are suitable. With almost no atmosphere, the surface could be cold and the temperature difference between day and night large. With a thick atmosphere and a strong greenhouse effect, the same starlight could produce a much warmer surface.
Low density alone is therefore not enough to call f an ocean planet. Researchers must also confirm whether an atmosphere exists, its pressure and composition, marks from water molecules and the range of surface temperatures. What is firmly known today is the mass, radius and orbit. An ocean, ice layer and atmosphere remain unconfirmed.
The ocean and ice shown on this page are an illustration, not an observed photograph. The water figures of 6.9% and 16.2% were calculated under particular assumptions about the interior and do not mean that water or an ocean has been discovered.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 f
- The Planetary Science Journal — Masses, Radii, Densities and Dynamics
- Unterborn et al. — Uncertainties in the Interior of TRAPPIST-1 f
- NASA JPL — Possible Interiors of the TRAPPIST-1 Planets
- NASA Science — Hubble Constraints on TRAPPIST-1 f
- NASA Science — Webb Results for the TRAPPIST-1 System
- Nature Astronomy — The Resonant Chain of TRAPPIST-1