René Heller from the Max-Planck Institute, Ed Guinan from Villanova University and I searched through a database of more than 4.000 planets outside our Solar System, looking for candidates that might be “superhabitable”—meaning more habitable than Earth. We came up with a shortlist of 24 MVPs (Most Valuable Planets) that may fall into that category, and have published the results in the journal Astrobiology.
Could there really be a planet more habitable than our own? We think so. Earth’s amount of biomass and biodiversity has changed during the eons, and so has its degree of habitability. During the Permian time period more than 250 million years ago, for example, huge deserts existed on our planet that made it less habitable. A hundred million years earlier, during the Carboniferous period, extensive rain forests and swamps covered our planet (most of the coal and oil we use today was deposited then), and Earth would have been more hospitable to life than it is now. We identified various properties that would increase a planet’s habitability, such as being older and having slightly more mass, being slightly warmer and wetter, and having lots of shallow water and archipelagos.
While these factors may rely too much on the single example of Earth (the only world known to host life), stellar properties do not. So we also considered the host stars in our definition of superhabitability. We came to the conclusion that a K dwarf star, less luminous than our Sun (a G dwarf), would be more suitable to host habitable planets than either our own type of star or the even less luminous M dwarfs.
Early in their history, G dwarf stars emit lots of harmful radiation, which is likely to make the rise of life more difficult (although obviously not impossible, since we are here). They also have a short lifetime, which reduces the window for life to exist (our Sun will become uninhabitable for us in about a billion years). M dwarfs, on the other hand, have a habitable zone so close to the star that many otherwise promising planets would be tidally locked, meaning they always show the same face to their star and have greater imbalances of heat and cold. Global habitability under these circumstances is practically impossible. K dwarf stars fall into a comfortable middle zone. They also live longer and are more common that G dwarf stars (12 percent versus 8 percent of all stars).
Our analysis, considering both the planetary and stellar properties, yielded 24 candidate planets that might be superhabitable. Five of them are within 10 degrees Celsius of Earth’s average temperature (15o C). Only two of them, though—Kepler 1126 b (alias KOI 2162) and Kepler-69c (alias KOI 172.02)—have been validated as planets. The other objects are unconfirmed Kepler “Objects of Interest” (KOIs), some of which may turn out to be false positives. One of these, KOI 456.04 (see the video above), is especially intriguing because a preliminary assessment indicates that it’s practically a mirror image of our Sun-Earth system.
Unfortunately, none of these MVPs is closer than 100 light years, and so at present are inaccessible for further high-quality observations. But there may well be more superhabitable planets in the current list of known exoplanets. If such a world is found in our stellar neighborhood, it deserves a higher priority than Earth-like planets for follow-up observations.