Kepler-438 b was announced in 2015 as one of the two planets most similar to Earth found to that date, judging by its size and the light it receives. It is about 1.1 times Earth's width and orbits a red dwarf about 640 light-years away every 35.2 days, receiving about 1.4 times the light Earth gets from the Sun.
A team led by Guillermo Torres confirmed it, as candidate KOI-3284.01, among twelve small Kepler planets in habitable zones, the range of distances where liquid water could last on a surface. Its signal in the star's wobble is too small to measure, so the team showed statistically, with a method called BLENDER, that a planet explains the dips in starlight far better than any false alarm. They singled out Kepler-438 b and Kepler-442 b as the planets most similar to Earth when size and incident light are considered together. Their imaging also found a fainter star close to Kepler-438 on the sky.
A year later a team led by David Armstrong took a closer look at the star and found a problem. Over four years of Kepler data, Kepler-438 produced seven significant flares, large ones roughly every 200 days, some of them superflares far more energetic than the Sun's. A planet orbiting this close to such a star could have its atmosphere stripped by repeated outbursts. Whether it survives depends on the planet's magnetic field and mass, neither of which has been measured. The team concluded that the high rate of strong flares must be taken into account when considering Kepler-438 b as an Earth-like planet.