StarTRAPPIST-1 system
TRAPPIST-1
A tiny ultracool dwarf at the dim end of the stellar family, 40 light-years away and circled by seven Earth-sized planets locked into a rare resonant chain.
- Star type
- M8V ultracool dwarf
- Distance
- 40.66 light-years · 12.47 parsecs
- Mass
- 0.0898 solar masses
- Mean radius
- 0.119 solar radii · about 1.16 Jupiter radii
- Luminosity
- About 0.00055 solar luminosities
- Surface temperature
- About 2,566 K
- Rotation
- About 3.3 days
- Age
- About 7.6 billion years
A star near the bottom of the chart
TRAPPIST-1 sits just above the boundary between the smallest stars and brown dwarfs. It holds about 9% of the Sun's mass, yet its radius is only slightly larger than Jupiter's. It was first catalogued by the 2MASS survey and later studied in detail by the small TRAPPIST telescope, which gave the system its name.
The star lies in the constellation Aquarius. It is far too faint to see by eye; at visible wavelengths its brightness is only around magnitude 18. Most of its light comes out in the infrared, so infrared telescopes have been essential for studying both the star and its planets.
Red, dim, and long-lived
With a surface temperature of about 2,566 K, TRAPPIST-1 glows a deep red. Its total light output is roughly one five-thousandth of the Sun's. Like other very low-mass stars, it burns hydrogen slowly and will remain on the main sequence for trillions of years, far longer than the current age of the Universe.
A resonant chain of seven worlds
TRAPPIST-1's planets orbit far closer than Mercury does to the Sun, all seven fitting inside about 0.06 AU. Their periods form a near-resonant chain, with neighbouring planets completing orbits in simple ratios. That orderly structure is a clue to how the system migrated inward from a wider, gas-rich birth environment.
The planets were found by the transit method. TRAPPIST spotted the first signals, the Spitzer Space Telescope confirmed and timed them, and Kepler's K2 mission added long-baseline observations. The chain was announced in 2017 and remains the largest known set of Earth-sized transiting planets around a single star.
How we study it
Because all seven worlds cross in front of the star, their sizes are known, and transit-timing variations give their masses and densities. The James Webb Space Telescope is now measuring the heat and potential atmospheres of individual planets by watching them pass behind the star, while precise radial velocities continue to constrain their masses and dynamics.
- Spitzer provided the long photometric baseline that established the seven-planet architecture.
- JWST measures thermal emission and searches for atmospheres on the inner planets.
- Ground-based photometry continues to refine transit times and detect potential additional planets.
Key publications
| Year | Paper | Venue | Link |
|---|---|---|---|
| 2016 | Gillon et al., Temperate Earth-sized planets transiting a nearby ultracool dwarf star | Nature 533, 221 | DOI |
| 2017 | Gillon et al., Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1 | Nature 542, 456 | DOI |
| 2021 | Agol et al., Refining the transit-timing and photometric analysis of TRAPPIST-1 | Planetary Science Journal 2, 1 | DOI |
| 2023 | Greene et al., Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST | Nature 618, 39 | DOI |
Values compiled from the NASA Exoplanet Archive and the papers listed above.
Worth knowing
It is the only known star with seven Earth-sized transiting planets.
Its radius is only 12% of the Sun's, barely larger than Jupiter.
All seven planets orbit closer than Mercury does to the Sun.