TRAPPIST-1: Seven Earth-Sized Worlds

A dim red dwarf 40 light-years away hosts seven roughly Earth-sized planets — the richest compact system known, and a prime hunting ground for habitability.

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In February 2017 NASA unveiled a system unlike any other: seven planets, all close to Earth's size, circling a single ultracool dwarf star barely larger than Jupiter, all packed closer to their star than Mercury is to the Sun.

Three or four of them fall in the habitable zone. The star is so dim that even those temperate worlds orbit in a matter of days, and the whole system would nest inside Mercury's orbit.

The seven move in a near-perfect chain of orbital resonances — a cosmic metronome — which let astronomers weigh them by their mutual tugs and made them the top small-planet targets for the James Webb Space Telescope.

TRAPPIST-1 (an M8 dwarf ~40 light-years away in Aquarius) hosts seven transiting planets, b through h, with radii from about 0.75 to 1.13 Earth. Discovered by ground-based transit surveys and Spitzer and announced in full in 2017, their masses came from transit-timing variations — the planets are far too faint for radial velocity.

The planets sit in a chain of first-order resonances, strong evidence they migrated inward together through the birth disk. Planets e, f, and g lie in the conservative habitable zone; all are likely tidally locked.

JWST is systematically probing their atmospheres. Early results point to the innermost worlds (b, c) being largely airless — the expected toll of red-dwarf flares — while the temperate outer planets remain the field's marquee test of whether the galaxy's most common habitable zones can hold air.

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