Kepler-442b

On 6 January 2015 NASA announced a batch of small planets found by the Kepler telescope in the habitable zones of their stars. One of them, Kepler-442b, stood out. It is about a third wider than Earth, orbits an orange star a little smaller and cooler than the Sun every 112 days, and receives about 70 percent of the sunlight Earth does.

“More habitable than Earth”?

Later that year Rory Barnes, Victoria Meadows and Nicole Evans published a habitability index, a way of ranking planets by how likely they are to support liquid water from their size, orbit and starlight. Kepler-442b scored 0.836. Earth, measured the same way, scored 0.829. The authors warned that this does not mean the planet is more habitable than Earth, only that its measured properties sit very comfortably in the range that could allow surface water. As of 2018 it was described as the most habitable known planet that is not tidally locked.

How it was confirmed

Kepler-442b was confirmed statistically, by ruling out the other things that could mimic its signal. Guillermo Torres and colleagues examined twelve small candidates in habitable zones, with orbits of 34 to 207 days. They modelled the shape of each transit with a technique called BLENDER and combined it with spectroscopy, adaptive-optics and speckle imaging to look for nearby stars, and checks on whether the dip in light shifted position on the sky. Kepler-442b came out at least 99.73 percent likely to be a real planet. The study named it and Kepler-438b the most Earth-like planets found so far when size and starlight are taken together. Both stars lie in the direction of the constellation Lyra. NASA’s announcement put Kepler-442b about 1 100 light-years away; the current estimate is about 1 190.

Enough light for plants?

A 2021 study led by Giovanni Covone asked whether known planets get enough of the right light for oxygen-producing photosynthesis on the scale of Earth’s biosphere. Of the potentially habitable planets they examined, only Kepler-442b came close to receiving the light a large biosphere would need. The authors argue that planets around red dwarfs, stars about a third of the Sun’s temperature, would not get enough energy to drive photosynthesis at all.

What we don’t know

Almost everything else is unknown. Its mass has never been measured: about 2.4 Earth masses is an estimate from its size, with a wide margin. At 1.34 times Earth’s width it sits near the border between rocky super-Earths and small gas-rich mini-Neptunes, so it is probably rocky but not certainly. It orbits just outside the distance at which its star would lock its rotation, so its days may last weeks or months. And at about 1 200 light-years away, its atmosphere is far beyond the reach of any telescope we have.