How was TRAPPIST-1 h’s orbital period determined?
TRAPPIST-1 h was first seen crossing its star once in 2016 data from the Spitzer Space Telescope. One transit cannot reveal an orbital period. Researchers first used the relationships among the inner planets’ orbital periods to calculate six possible periods. Earlier ground-based observations ruled out five of them, and a 79-day K2 observing run then found four transits spaced about 18.77 days apart and confirmed the period.
- About 18.77 days Orbital period confirmed by repeated transits
- 79 days Time K2 observed TRAPPIST-1 continuously
- 4 Additional transits of h found in the K2 data
- About 173 K Calculated temperature, equal to about −100°C
What can one transit tell us?
When a planet crosses its star, the decrease in starlight gives its radius and the duration of the dimming gives the transit duration. But without seeing the next transit, astronomers cannot determine how many days the planet takes to complete an orbit. During the gap in observations, it may have gone around once, twice or three times.
The discovery study used the timing and duration of the Spitzer transit to estimate a period of about 20 days. Yet the possible range was broad, from roughly 14 to 35 days. It was calculated by assuming sizes for the star and orbit, rather than measured from repeating transits.
How were the possible orbital periods narrowed down?
The six inner planets of TRAPPIST-1 orbit in stable relationships with one another. The numbers of orbits completed by neighboring planets over the same interval are close to ratios of small whole numbers. This relationship is called a resonance. Researchers assumed that f, g and h shared a similar relationship and calculated possible orbital periods for h.
The calculation produced six candidates ranging from 13.941 to 39.026 days. Researchers then revisited about 1,000 hours of ground-based data gathered before the Spitzer observation. If five of the candidates were correct, starlight should have decreased at particular times, but it did not. No data covered the transit time predicted by the 18.766-day candidate, so it was the only one left. At this stage, 18.766 days was merely the most likely prediction, not a value confirmed by repeated observations.
The planets’ resonant relationships narrowed the possibilities to six periods. The actual period was confirmed only after K2 observed transits repeating at the same interval.
Which period did K2 observations confirm?
K2 observed TRAPPIST-1 for 79 days from December 2016 to March 2017. The telescope entered safe mode, leaving a five-day gap in the data, and researchers also had to correct shaking caused by a failed attitude-control component. Once they examined the corrected brightness data, transits with a depth and shape similar to Spitzer’s appeared at all four times predicted at 18.766-day intervals.
The team also tested whether the result had simply been forced to fit the predicted times. After removing the known transits of planets b through g, they searched without setting h’s period in advance; the signal near 18.77 days was still the strongest. A different analysis method found the same four transits. Statistical comparison also showed that a seven-planet model including h explained the data far better than a model without h. Spitzer’s first transit and K2’s four transits together confirmed an orbital period of 18.767 days.
- Single Spitzer transit It revealed h’s size and transit time, but not its orbital period.
- Resonance calculation The periods of f and g produced six candidates that could fit h.
- Review of ground data Times when no transit occurred ruled out five candidates.
- Repeated K2 observations Four transits about 18.77 days apart confirmed the period.
What does TRAPPIST-1 h’s calculated temperature mean?
NASA’s catalog gives h a radius about 0.755 times Earth’s and a mass about 0.326 times Earth’s. It is the smallest and lightest of the seven planets and orbits about 9.27 million km from its star. The equilibrium temperature of 173 K reported in a 2017 study is about −100°C. This temperature assumes that the energy received from the star and the heat released by the planet balance each other; it was not measured with a thermometer on the surface.
The same study calculated that h receives about 200 watts of energy per square metre. In atmosphere models containing nitrogen, carbon dioxide and water vapor, this energy was insufficient to keep liquid water on the surface. Carbon dioxide could also freeze in cold regions, reducing the amount left in the atmosphere. But the actual reflectivity, air pressure, clouds and surface temperature have not been measured. The result therefore applies to a particular set of model conditions and does not confirm h’s real climate.
How can Webb separate changes in the star from an atmospheric signal?
According to NASA’s January 2026 summary, Webb has observed h, but the analysis has not yet been released. Dark patches and eruptions on the star’s surface alter its brightness differently at each wavelength. These changes can resemble marks from a planetary atmosphere, so the same molecular marks must be seen across several transits that occur about every 18.77 days.
Calculation and observation played different roles in finding h’s orbital period. Resonance predicted when to look, older data ruled out candidates that did not fit, and repeating K2 transits confirmed the period. Its atmosphere and surface environment, however, have not yet been confirmed by repeated observations.
The ice and clouds shown on this page are an illustration, not an observed photograph. Repeated transits confirmed the 18.77-day period, but 173 K is a value calculated with simple assumptions. No atmosphere, ice, liquid water or actual surface temperature has been confirmed.
Sources
- NASA Science — TRAPPIST-1 h Catalog
- Nature — Seven Temperate Terrestrial Planets
- Nature Astronomy — A Seven-planet Resonant Chain
- The Planetary Science Journal — Masses, Radii, Densities and Dynamics
- Astronomy & Astrophysics — Climate and Volatile Models
- NASA Science — Kepler and K2 Mission
- NASA Science — Webb Results for the TRAPPIST-1 System