The goal of this work is nothing less than the holy grail of astronomy: finding a habitable planet. Climate modeling isn’t limited just to planet Earth anymore. In recent decades, researchers have begun modeling the climates of other planets in the solar system and are now simulating conditions on faraway worlds orbiting other stars in the Milky Way galaxy. The goal of this work is nothing less than the holy grail of astronomy: finding a habitable planet that resembles Earth from among the likely billions of planets peppering the galaxy.

“These [climate] models have now been taken out of our solar system to study other planets orbiting other stars,” said Aomawa Shields, an exoplanet astronomer at the University of California, Irvine, at the 233rd Meeting of the American Astronomical Society (AAS), held last week in Seattle, Wash.

Mars and Beyond

Researchers first used climate models to study a planet other than Earth roughly 2 decades ago. They wanted to solve a Martian riddle: How had the Red Planet been warm enough in the past for liquid water to flow on its surface, as suggested by abundant physical evidence like dark surface features known as recurring slope lineae? An explanation, proposed in 1997, suggested that clouds of carbon dioxide in Mars’s atmosphere reflected infrared light back toward the planet’s surface, thereby warming Mars.

Shields and other scientists now are simulating the climate conditions of even more distant worlds. There’s no shortage of planets to study: Nearly 4,000 extrasolar planets (exoplanets) have already been found using such instruments as the Kepler Space Telescope. The team for NASA’s Transiting Exoplanet Survey Satellite (TESS), which was just launched last year, announced the discovery of its third exoplanet at the AAS meeting last week. And an increasing number of worlds are being found that resemble our home planet in size, Shields said. “We are now in the Earth-sized regime.”

Abiotic Oxygen

Climate models have shown that one of the signatures of life on Earth—oxygen expelled by photosynthesizing plants—might not be indicative of life elsewhere. That’s because the conditions on some planets might naturally yield oxygen abiotically. In one scenario, watery oceans on a distant world could be evaporated by the planet’s host star, lofting water vapor high into the atmosphere. There, the water molecules could be broken apart by ultraviolet radiation, resulting in the hydrogen—a light element—escaping into space and the subsequent buildup of oxygen in the atmosphere.

Shields is also studying the climatic effects of ice on exoplanets. Here on Earth, the majority of the sunlight that strikes the planet’s surface is in the form of visual light, which ice effectively reflects. But planets orbiting smaller, cooler stars known as M dwarfs—which are believed to be far more common in the Milky Way galaxy than Sun-sized stars—are bathed mostly in infrared light, which ice readily absorbs. As a result, planets orbiting M dwarfs were less likely to experience global ice coverage (i.e., a “snowball” stage), the researchers showed.

Tilted Worlds

Determining where in the universe these conditions come together to produce a habitable planet remains a challenge. That’s where climate models come in. The orbital characteristics of a planet, like its obliquity—its tilt—furthermore affect its climate by producing seasons. But planets that undergo constant changes in their obliquity—due to, for instance, gravitational tugs from other planets—may never experience planetwide glaciation because their poles never point away from the planet’s host star for sufficiently long periods of time for ice sheets to build up, Shields and a colleague noted in Physics Reports.

The necessary ingredients for life as we know it—a plentiful liquid such as water, bioessential elements like carbon and oxygen, and a source of energy—aren’t unique to Earth. But determining where in the universe these conditions come together to produce a habitable planet remains a challenge. That’s where climate models come in, said Shields. “There are many factors that must be explored that affect planet habitability.”

—Katherine Kornei ([email protected]; @katherinekornei), Freelance Science Journalist