Jeremy Leconte

The Milky Way is home to an estimated 22 billion rocky planets

that may be capable of sustaining life. These planets orbit their stars in the habitable zone—the region around a star where it's neither too hot nor too cold for liquid water to exist on the planet's surface. Now scientists are suggesting that the "Goldilocks zone" could be wider than expected, which they say could mean that an additional 1.5 billion planets might be hospitable to life.

The habitable zone was commonly thought to start at a distance of 0.99 astronomical units (AU) from a sun-like star and end at around 1.7 to 2.0 AU. For reference, one AU is the distance from the Earth to the sun; 0.99 AU is approximately 92 million miles, barely closer than we are to our star. Any closer than that, the thinking went, and the planet would experience a runaway greenhouse effect as the intense heat from the star would boil away all the planet's surface water. However, a study published this week in Nature suggests moving the inner boundary of the habitable zone to 0.95 AU, or about 3.7 million miles closer to the host star. If this is true, then perhaps desert planets such as Arrakis, from the sci-fi novel Dune might truly be capable of supporting life.

While the new boundary estimate doesn't overturn everything we thought we knew about exoplanets, it does continue the trend of pushing the limits of where scientists think life beyond Earth could exist. In our own solar system, for example, the icy moons Europa and Enceladus (which orbit Jupiter and Saturn, respectively) fall outside the traditional habitable zone, but scientists still consider the moons to be top contenders in the search for extraterrestrial life, because the gravitational tug of each moon's host planet generates heat that could allow vast oceans of water to exist beneath the icy surface. Other studies have proposed that life could exist in the hot atmospheres of Venus-like planets, or even on the starless rogue planets that wander the galaxy.

The paper's real innovation is that the new estimate relies on advanced computer modeling of the runaway greenhouse effect, thought to occur when a hot climate causes water to vaporize. The resulting clouds prevent heat from reflecting off the surface of the planet, causing a temperature feedback loop—the world grows hotter and hotter until all of its surface water evaporates.

To create the model, the team started with a 3D model that the Intergovernmental Panel on Climate Change uses to predict the impact of global warming. Then they added in astrophysical data to model planets that are much hotter than Earth. "The idea was to merge these tools that were available to create this climate model that is able to model very hot planets in very exotic contexts," says astrophysicist Jérémy Leconte of the Institut Pierre-Simon Laplace in Paris, a coauthor on the new paper.

Previous attempts to define the temperature threshold that starts a runaway greenhouse effect depended on simple, one-dimensional models that mostly looked at water's ability to absorb heat. "They ignore the effect of the clouds, and the dynamic effects at equator and poles," says Leconte. The 3D model that Leconte and his colleagues created incorporates factors such as cloud coverage and the way air circulates from hot areas to cold.

On Earth, air circulation patterns called Hadley cells form because of the differences in air temperature between the equator and the poles. The equator receives more sunlight, and so air there becomes becomes hotter. As that hot air rises, it moves toward the cooler poles. In the process, it cools and creates dry areas such as the Sahara, which help to stabilize Earth's climate and prevent the runaway greenhouse effect. Similar circulation patterns form on planetary bodies where the heat is uneven—places like Mars or Titan, which has one hot side and one cold side.

Because of the stabilizing effects of air circulation, Leconte's model shows the temperature threshold for runaway greenhouse to be higher than expected. That means planets can orbit closer to their stars and still be potentially habitable.

Penn State physicist Ravi Kopparapu, who was not involved in the study, says the results were not dramatically different from his own recent estimates, which placed the inner boundary of the habitable zone at 0.97 AU. But, he says, the study is the first of its kind and highlights the importance of using 3D models to understand the atmospheres and climates of Earth-like planets.

The definition of the habitable zone's inner boundary will be especially important in the years to come, Kopparapu added, because current planet-hunting telescopes are best at spotting exoplanets that orbit close to their stars, rather than being situated comfortably within the habitable zone.

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