Earth Scores a B In Potential Habitability—And That’s a Good Thing!
published during a waxing gibbous moon.

An artist’s rendition of Kepler-452b, the most Earth-sized planet yet found orbiting in the habitable zone of a sun-like star. But twinship of Earth may not make a world the most likely to be habitable, according to a new index. Credit: NASA

A new method of ranking worlds identified not only by NASA’s Kepler Space Telescope but also the forthcoming James Webb Space Telescope (JWST) would only give Earth an 82 out of 100. The reason? Our planet sits just a bit too close to the sun for comfort. But never fear—the low grade may well be good news. The scale wasn’t made to rank Earth, so this could mean good news for other worlds.

Under the current motif, planets are deemed potentially habitable if they lie in a region scientists call the ‘habitable zone’, the area around a star where a rocky world could hold onto water. The size of the zone depends on the light and heat from its parent star. Whether or not a planet holds up is a yes or no question: either it’s in the zone or it’s out of bounds. For planets orbiting in this precious path, there’s no way to effectively compare which one might be more promising. With the wide collection of planets identified by NASA’s Kepler Space Telescope, and the wealth of worlds anticipated from the upcoming TESS and PLATO missions, astronomers Rory Barnes, Victoria Meadows, and Nicole Evans, all at the University of Washington, decided that there had to be a better way.

As a world passes between its star and Earth, scientists on our planet can measure the very slight dimming in the light of the star, known as the transit method. When measured multiple times, the transit method can reveal how long it takes the planet to go around the sun, the length and amount of dimming that occurs, and the radius of the planet. Using information about the star, they can also determine the temperature on the world and the gravity of the host star.

Barnes and his colleagues used this information to create their habitability index, which will be published in the Astrophysical Journal. According to their research, a potentially habitable planet is one that is mostly rock, with a small atmosphere, and with the right energy structure and temperature that will allow liquid water to sit on the surface for extensive periods of time. While other features can contribute to the potential habitability of a planet, the scientists wanted to create a method that could rely on current observations.

As a result, a handful of worlds identified by Kepler actually score higher in potential habitability than Earth. This doesn’t mean that they’re in the class of “super-habitable” planets, worlds that are potentially even better for life to evolve than our own, the authors cautioned. Instead, it means that an Earth-twin orbiting a solar-twin won’t have the highest probability of being habitable.

This is good news, since to date only one such world has been identified by Kepler. In fact, out of over 4700 objects identified by the space telescope, only one confirmed planet and three potential planets have a significant probability of being habitable, according to the new index. Of those four, KOI 5554.01, a roughly Earth-sized world with a 362-day orbit, is the highest priority target for James Webb.

But just because only one Kepler world has made the cut doesn’t mean all hope is lost. TESS and PLATO will both being looking for more planets, and should find worlds that JWST can follow up on. And the rebooted Kepler mission, K2, is currently scanning the heavens for more planets as we speak.

As those worlds are identified, a process for prioritizing them should help scientists decide which ones to pursue first in the hunt for the holy grail of exoplanet science—a world that scientists can point to and definitively say, “This world could host life as we know it.”