The Search for Habitable Worlds Just Got a Boost

Scientists have long been on the hunt for exoplanets—planets far from our solar system—that have the potential to support life. And the search just got a little boost, reports Matt Williams at Science Alert. A new study suggests that more planets than previously thought could hold the right conditions for life.

The study, published in the journal Astrophysical Research, presents a new model for examining atmospheric circulation of planets, helping researchers sort out potential candidates that could support life. Unlike previous one-dimensional models, the newest calculations simulate atmospheric conditions in three dimensions, which allows researchers to examine the effects of large-scale circulation on the planet's surface from afar.

“Using a model that more realistically simulates atmospheric conditions, we discovered a new process that controls the habitability of exoplanets and will guide us in identifying candidates for further study,” says Yuka Fujii of NASA’s Goddard Institute for Space Studies, in a release.

This new study opens up doors to identifying habitable worlds beyond what Williams calls the “low-hanging fruit.” These planets have conditions similar to Earth, which sits within the so-called Goldilocks Zone of its star—close enough for the star’s glow to produce liquid water, but far enough away that it doesn't broil the planet's surface. The planet must also have hung out in that temperature zone for billions of years, long enough for a form of life to arise out of the primordial alien soup.

But the newest findings tweak the parameters of the Goldilocks Zone, suggesting that some planets can maintain liquid water even if they orbit relatively dim parent stars at a distance previously thought to be "too close."

Exoplanets orbiting close to their parent stars may become tidally locked, which means the star’s gravitational pull on the planet is so strong that it slows the planet’s rotation so much so that it zips around its star with the same side facing inward. In this situation, one side experiences perpetual daylight and the other descends into never-ending darkness. A thick layer of clouds forms on the star-facing side as the planet's oceans are slowly boiled away. This collection of water vapor in the upper atmosphere plunges the planet into what is known as a moist greenhouse state.

Past models suggest that planets in this condition are far too hot to support life. But the new model suggests that isn't necessarily the case. If the star is emitted a certain type of radiation called near-infrared radiation (NIR), it would heat the water vapor and humidify the planet’s stratosphere without boiling away the oceans. This is especially relevant for cooler, low-mass stars, which emit more of this type of radiation. For planets orbiting these cooler, NIR-emitting stars, the habitable zone could be much closer to the star, with the planet experiencing temperatures around those of Earth's tropics.

Low-mass stars are the most common type in the galaxy, so the study marks a significant step in the search for other planets where life could arise. As Bill Steigerwald writes in the NASA statement, “their sheer numbers increase the odds that a habitable world may be found among them.”

Scientists are already hot on the tracks seeking out these lukewarm worlds. Last year the planet-hunting Kepler space telescope spied over 1,000 potentially habitable planets, adding several hundred more this year. In February, researchers announced the discovery of the TRAPPIST-1 system, which is composed of seven earth-sized planets circling a dwarf star.

But as this latest study suggests, there may be even more habitable planets out there. And there's no guarantee that what we find will look like Earth. There's even a possibility one could exist that's even better than our home planet, astrobiologist Louisa Preston, speculates in her book, Goldilocks and the Water Bears. “There may conceivably be superhabitable worlds out there that are even better suited than the Earth to support life,” she writes.