Seven Earth-sized worlds, packed tighter than Mercury's orbit.
TRAPPIST-1 is an ultra-cool red dwarf, roughly 9% of the Sun's mass and barely larger than Jupiter. All seven of its known planets orbit closer to it than Mercury orbits our Sun. The system is about 40.7 light-years away and older than our own Solar System; current age estimates put it at roughly 7.6 billion years, versus the Sun's 4.6 billion.
How did we actually find seven planets around a star this faint?
The star itself was catalogued in 1999 during a survey of nearby ultra-cool dwarfs, long before anyone knew it had planets. In 2016, a team led by Michaël Gillon at the University of Liège, using the TRAPPIST telescope (Transiting Planets and Planetesimals Small Telescope) at Chile's La Silla Observatory, caught the telltale dimming of two planets passing directly in front of the star and announced TRAPPIST-1b and c. Follow-up observations revealed five more planets, the largest number of roughly Earth-sized planets known around any single star.
Because TRAPPIST-1 is far too dim and small for the usual method to pin down planet masses (radial-velocity, or measuring a star's own wobble), astronomers used something more elegant. The seven planets are locked in a real, unbroken chain of orbital resonances. Their orbital periods form near-exact whole-number ratios (8:5, 5:3, 3:2, 3:2, 4:3, and 3:2 between each successive pair), so each planet's gravity measurably tugs at its neighbors' transit timing. Modeling those tiny timing shifts let researchers extract every planet's mass directly. It's the same basic idea behind the Laplace resonance of Jupiter's largest moons, just applied to an entire exoplanet system.
What do we know now, and what's still an open question?
The star's habitable zone is primarily occupied by planets e, f, and g, with planet d also qualifying depending on the climate model used. Planets b and c are almost certainly too hot to be considered habitable. Whether any of them actually holds liquid water depends entirely on atmospheres nobody has directly confirmed yet. That's exactly what the James Webb Space Telescope has been probing since 2022. JWST's thermal-emission measurements have found TRAPPIST-1b consistent with having little to no atmosphere at all, and specifically ruled out a thick, Venus-like or CO2-rich atmosphere on TRAPPIST-1c. The rest remain genuinely unresolved as of the most recent published observations, and a tentative eighth, unconfirmed planet candidate was floated in 2025 from JWST transit data, though it may simply be an artifact of b and c briefly overlapping in the sky.
Where do this app's numbers come from? Every planet's orbital distance, diameter, and relative position here are drawn from the transit-timing-variation solutions published by Agol et al. (2021) and Grimm et al. (2018). These are the same measurements aggregated on the NASA Exoplanet Archive. Nothing about this system needed stylizing or exaggerating to be worth exploring!