Counterpoint: What Makes a Habitable Planet?

We all know how the story goes. 

A mysterious spaceship is detected in the atmosphere. Humans try to communicate with the aliens on it. Aliens are hostile and attempt to conquer Earth. Pandemonium ensues.

The “alien invasion” trope and extraterrestrial beings in general have been parts of movies, books, and other media for decades, from H. G. Wells’s The War of the Worlds to the cult classic film Independence Day to everyone’s favorite quarantine video game, Among Us. The idea of encountering aliens has captured our imaginations. However, in scientific communities, the search for extraterrestrial life has yet to find success.

Traditionally, scientists have looked towards planets with conditions like ours in their search for life. Whether a planet has appropriate conditions for liquid water has been a primary concern. These planets can neither be too close nor too far from the star they orbit: this famed “Goldilocks” region is usually considered to be the habitable zone for a star. An additional constraint is that the models used to predict the bounds of this region assume a small, rocky planet with an Earth-like atmosphere filled with nitrogen gas, oxygen gas, and carbon dioxide. However, two recent studies tell us that we may not be looking in the right places.

Nikku Madhusudhan and his team at the University of Cambridge proposed a new type of potentially habitable planet. These planets, known as “Hycean worlds,” are composed of massive oceans with surrounding atmospheres made mostly of hydrogen gas. Madhusudhan’s team first explored the range of masses and radii that Hycean worlds can take on and then determined the range of temperatures (and, by extension, distances from various stars) that allow for habitable Hycean surfaces. 

Madhusudhan’s team found that Hycean planets offer several advantages over Earth-like ones when it comes to the search for life. Hycean worlds can be much larger than rocky, terrestrial ones, and their thick atmospheres provide insulation that allows for liquid water far away from a star: some “Cold Hycean” planets may not need any stellar irradiation at all, with their only heat source being internal. The increased range of sizes and distances from a star that Hycean planets have could mean that scientists can broaden their search for extraterrestrial life. 

Meanwhile, Noah Tuchow and Jason Wright of Penn State questioned the habitability of planets in the traditionally defined habitable zone. They noted that, while the traditional definition considers whether liquid water could exist under current conditions, a planet’s habitability is dependent on whether it has existed in the habitable zone ever since life there began. Planets currently observed in a star’s habitable zone may have entered the zone relatively recently, either due to changes in a star’s luminosity or planetary migration. These “belatedly habitable” planets are unlikely to gain the ability to host life: if Venus somehow took Earth’s spot in our solar system, entering the “habitable zone,” it would never regain liquid water. 

Identifying the “belatedness” of a planet’s habitability is a difficult task. It requires knowledge of both a star’s life history as well as when and how planetary formation occurs. However, while no simple model can tell us which planets we can ignore, Tuchow and Wright’s research will guide future extraterrestrial exploration. Considering belated habitability for planets may change how we approach future mission design, as many planets found in habitable zones will merely be belatedly habitable.

These two studies are challenging our traditional ideas of what makes a planet habitable. Our current definition of the habitable zone, centered around the possibility of finding liquid water on Earth-like planets, ignores other types of potentially habitable planets and fails to consider the impact of stellar history on habitability. These studies teach us that our initial conceptions about science are often false: life in the universe need not look like life on Earth. Our current definition for “habitable zone” may be less useful than we once thought, and it may be time to reconsider it. Perhaps applying a new definition will help us find those aliens we’ve fantasized about for so long—let’s just hope they aren’t as hostile as those in all the movies.