This rainy exoplanet could be ripe for life

A nearby exoplanet with rainclouds in its atmosphere may have habitable conditions at its surface, researchers report today. The planet, dubbed K2-18b, is 124 light-years away and 2.6 times the radius of Earth. Last year, astronomers detected clouds of liquid water in the planet’s hydrogen-rich atmosphere, a first for such a small planet.

K2-18b, which is midway in size between Earth and Neptune, is in the habitable zone of its star, so liquid water on its surface is possible; but no one knows what its surface is like. Researchers can’t say for sure whether it has a rocky exterior and thin atmosphere, like Earth, or a dense hydrogen atmosphere above a high-pressure water-ammonia ocean and metallic core, like Neptune—conditions not at all friendly to life.

Now, a team of researchers in the United Kingdom describe in The Astrophysical Journal Letters how they calculated a range of possible atmospheres for the planet, based on its mass, size, and previously measured spectra of light that passed from K2-18b’s star through its atmosphere. (Molecules in the planet’s atmosphere absorb certain frequencies of light, so if the starlight passes through it on its way to Earth, the light’s spectrum can reveal those molecules.) They then used those possibilities to limit what conditions could exist in the planet’s interior. Their conclusion: The heart of K2-18b could be anything from a ball of almost pure iron with a hefty hydrogen atmosphere, to something more Neptune-like, to a water world with a lighter atmosphere and ocean conditions similar to Earth (artist’s conception above).

The researchers conclude that if such a massive planet could still be habitable, seekers of life beyond our solar system might want to look beyond their usual Earth-size suspects—at worlds far larger than our small rock.