Kepler-1649c

Kepler-1649c is a planet almost exactly the size of Earth, 1.06 times its radius, orbiting a small red dwarf about 300 light-years away in Cygnus. It circles every 19.5 days and receives about 75% of the starlight Earth gets from the Sun. When NASA announced it in April 2020, it said that no other planet found in a habitable zone was closer to Earth in both size and temperature together. For years it had sat in the Kepler mission’s pile of false alarms.

Found in the reject pile

The Kepler space telescope picked up so many possible signals of planets passing in front of their stars that a computer program, the Robovetter, sorted them, flagging the ones that did not look like real planets. It filed the 19.5-day signal from Kepler-1649 as a false positive.

The mistake came from the star’s motion. Kepler-1649 moves fast across our sky, 168.2 milliarcseconds a year, and over the years of observation the patch of pixels used to measure its light had drifted about half a pixel off target. Members of the Kepler False Positive Working Group, checking the computer’s rejects by hand, noticed that the signal was real. In 2020 Vanderburg and colleagues published it: an Earth-sized planet, validated statistically with a false-positive probability of about 0.2%.

A Venus and an Earth

Its star is a mid M dwarf, 0.23 times the Sun’s radius and only 3 240 K (2 970 °C), giving off about half a percent of the Sun’s light. Closer in orbits a second known planet, Kepler-1649 b, about the size of Earth but receiving more than twice Earth’s sunlight, which the discoverers likened to Venus. The two orbits are in an almost exact rhythm: the outer planet’s year is 2.248 times the inner one’s, only 0.08% short of the ratio 9:4. That hints at a third planet between them, with a 13-day orbit, too small or too tilted to have been seen.

Even the size of its orbit is uncertain. The archive lists 0.065 AU, taken from an older Kepler table. A 2021 study by Kane and colleagues derived 0.083 AU from the discovery paper’s star, and only that value matches the published 75% of Earth’s sunlight.

Could it hold water?

Nobody knows what its atmosphere is like, or whether it has one. Its equilibrium temperature is about 234 K (-39 °C), an estimate that leaves out any warming by air. In climate models with an atmosphere, Kane’s team found average surface temperatures between about 245 and 305 K (32 °C), with some liquid water in every case they ran. Whether its spin is locked to its star, with one side in endless day, is not known either: the models tried both locked and unlocked rotation. A 2025 study found that both planets could have kept atmospheres for billions of years. Red dwarfs often flare, which may make life difficult, but NASA notes that no flares have been seen from this star.