How can we measure the masses of the TRAPPIST-1 planets?
TRAPPIST-1 has seven planets roughly the size of Earth. From our viewpoint, every one passes in front of the star, so the amount of starlight blocked reveals each planet’s size. Their masses are measured another way. When the planets pull on one another, a transit can occur earlier or later than expected. Researchers calculate the timing differences of all seven planets together to find the masses that produced them.
This method works well because the seven planets orbit in a compact space and the orbital periods of neighbours are close to simple numerical ratios. Each time similar arrangements repeat, the effects of their mutual gravity accumulate. This is called a resonant chain. A single planet’s transit record therefore contains information about the mass and orbit of its neighbours as well as its own motion.
- About 40 light-years Distance from the Solar System to the TRAPPIST-1 system
- 7 confirmed planets Named b through h in order of distance from the star
- About 1.51–18.77 days Orbital periods from innermost b to outermost h
- All inside Mercury’s orbit Every planet is closer to its star than Mercury is to the Sun
What can we learn from the starlight a planet blocks?
When a planet passes in front of its star, the star dims by an amount related to the area the planet covers. If the star’s size is known, the fraction of light lost can be used to calculate the planet’s radius. Repeated observations also establish the orbital period and the expected time of the next transit. All seven TRAPPIST-1 planets were confirmed through these transits.
The loss of starlight alone does not reveal mass. Two planets of the same size can have different masses if one contains more iron and the other more water or lighter materials. For some exoplanets, mass is found by measuring how much the planet makes the star wobble. TRAPPIST-1 is faint and active with spots and flares, while the wobbles from seven planets overlap. In this system, changes in the planets’ transit times are therefore especially important for finding their masses.
Why does the time when a planet crosses its star change?
Without the influence of other planets, each planet would pass in front of the star at perfectly regular intervals. In reality, a neighbouring planet pulling from ahead can make it arrive slightly early, while a pull from behind can make it arrive late. The difference between the predicted time and the observed transit is called a transit timing variation, or TTV.
A more massive planet changes its neighbour’s orbit more strongly, but one late transit does not give the mass directly. Researchers vary each planet’s mass, orbital period and orbit shape, calculate the next transits, and search for the combination that best matches the observations. Changing one planet’s mass changes the calculated transits of several neighbours, so the records of all seven must be fitted together.
- Measure how much starlight is lost Use the amount and duration of the dip to find the planet’s size and orbital information.
- Collect the transit times Combine years of records from ground-based telescopes, Spitzer, Hubble and K2.
- Compare them with predicted times Calculate how early or late each planet arrived relative to a regular schedule.
- Calculate all seven planets together Adjust masses and orbits until a model including their mutual gravity matches the full record.
- Calculate average density Use mass and size together, while leaving several possible mixtures of materials inside the planet.
How does the resonant chain amplify timing differences?
For the two innermost planets, b completes about eight orbits while c completes about five. The orbit counts of c and d are close to 5:3, those of d and e to 3:2. The ratios are not locked at exact integers, but the planets return to similar arrangements in a regular sequence. The gravitational effect between one pair can therefore carry into the transit timing of the next planet.
Agol’s team added ground-based, Hubble and K2 records to four years of transit times from Spitzer. They then calculated the motions of all seven planets together to obtain more precise masses. Resonance makes a small gravitational effect repeat many times, amplifying it into a timing difference that can be measured.
When one planet pulls on a neighbour, the time it crosses the star changes. By fitting the timing differences of all seven planets together, researchers calculate each planet’s mass.
What does the resonant chain reveal about how the planetary system formed?
Such a long resonant chain does not fit well with a picture in which all seven planets formed independently at their present locations. When planets were forming, a disk of gas and dust surrounded the star. As this material altered their orbits, several planets could have moved inward and then travelled together with orbital periods close to fixed ratios. After the disk disappeared, the mutual gravity of star and planets could have shifted those ratios slightly.
The present resonance alone cannot determine exactly where the planets formed or the one sequence in which they moved. Different starting disks and gravitational interactions can produce similar arrangements. We did not observe the past directly, but any calculation of the system’s formation must be able to explain the resonant chain seen today.
Can mass and radius alone tell us about a planet’s environment?
Knowing both mass and size gives a planet’s average density. The values for all seven planets are consistent with worlds made mainly of rock. Average density alone, however, cannot separate how much iron lies in the core, what kinds of rock are present, or how much water, ice or atmosphere exists. Different interiors can produce the same mass and size.
Saying that e, f and g are in the habitable zone classifies them by the amount of starlight they receive. It does not confirm liquid water on their surfaces or conditions suitable for life. To assess their actual environments, researchers must separately observe whether each planet has an atmosphere, which gases it contains, its pressure, and how heat moves between day and night.
According to NASA’s December 2025 summary, Webb had observed all seven planets, but detailed published analyses then focused on b, c, d and e. No evidence of thick atmospheres appeared for b or c, and d and e did not appear to be planets with thick hydrogen atmospheres. Analysis of data for f, g and h is continuing. These results do not show that all seven planets lack atmospheres or life.
What can we learn by comparing all seven planets?
All seven planets orbit the same star but at different distances, so they receive different amounts of light and heat. Within one system, researchers can compare how distance, mass and the pull of neighbouring planets correspond to different atmospheres and surface environments. There is less need to account for differences between host stars than when comparing planets from unrelated systems.
The shared star also complicates every observation. When TRAPPIST-1’s spots and flares change its light, those effects can mix with signatures left by a planet’s atmosphere. NASA notes that deciding whether an atmosphere exists may require hundreds of transits observed over several years, depending on the planet. The importance of this system is not that all seven worlds are Earth-like. It is that seven planets orbiting the same star can be compared side by side.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 System
- NASA Science — What Is Webb Revealing About the TRAPPIST-1 System?
- Nature — Seven Temperate Terrestrial Planets Around TRAPPIST-1
- Nature Astronomy — A Seven-planet Resonant Chain in TRAPPIST-1
- The Planetary Science Journal — Masses, Radii, Densities, Dynamics, and Ephemerides
- Astronomy & Astrophysics — TTVs and Origin of the Resonant Chain
- NASA Science — TRAPPIST-1 System and Planets