The snazziest addition to the Kepler mission’s stable of confirmed planets is kind of a tease. A bit bigger than Earth, planet Kepler-22b orbits a sunlike star at a distance where liquid water, and therefore life, can exist.
But Kepler-22b is unlikely to be habitable.
The planet’s radius, only 2.4 times Earth’s, probably makes it too big to be life-friendly, says planetary scientist Abel Mendez at the University of Puerto Rico in Arecibo. Mendez produces the Planetary Habitability Catalog, a newly released compilation of exo-objects, each graded for habitability. He used Kepler-22b’s radius to calculate possible masses and densities. Mendez says the most optimistic habitable scenario would be if Kepler-22b were a sort of water planet, with a global ocean and some clouds. But even that is only marginally life-friendly, he notes. “I’m not optimistic,” Mendez says. “But I would love to be wrong.”
Astronomer Sara Seager of MIT notes that the planet’s size suggests the presence of a massive atmosphere, complete with a massive greenhouse effect. “It’s going to be hot,” she says. “Too hot at the surface for life to survive.”
When announcing the planet’s discovery on December 5, scientists speculated that temperatures on Kepler-22b would hover around 72 degrees Fahrenheit. But that calculation was based on an atmosphere with Earthlike properties and warming effects. Scientists can’t determine whether the planet is truly Earthlike in composition without knowing its mass. If they knew how heavy Kepler-22b is, they could determine the planet’s composition, whether rocky, watery or gassy. Seager assumes that Kepler-22b is a gassy planet, a mini-Neptune, the likes of which are absent from Earth’s own solar system.
But it’s possible that Kepler-22b is just a dense, rocky planet. If it is really dense, with a thin atmosphere, then there’s a chance it could be habitable, Seager says. “However, Kepler has not found any massive planets,” she notes.
Assessing habitability is tricky, says David Morrison, director of the SETI Institute’s Carl Sagan Center for the Study of Life in the Universe, in Mountain View, Calif. There’s no easy definition for the term, and studying a class of planets that doesn’t exist in our solar system presents more questions than answers.
“It’s very exciting,” he says. “We don’t really know what habitable is. But it’s the right question to be asking.”