It’s hard to remember now, but there was a long period of time in which scientists weren’t sure how many planets might actually exist throughout the galaxy, much less how likely it might be to find a rocky planet. Thanks to missions like Kepler, we now know there are likely at least 100 billion planets in our galaxy. While many of these aren’t capable of supporting life, we now know a solid handful of planets exist that meet even conservative estimates for where life might survive. We’ve rounded up some of the most promising examples below.
Before we review the data, we should discuss how exoplanets are typically labeled. Unlike television shows like Star Trek, which typically refer to a planet using Roman numerals to represent its position relative to its host star and the host star designation (Cestus III, Ceti Alpha V), astronomers in the real world use the alphabet to denote when an exoplanet was discovered around its host star. The letter “a” is reserved; planetary designations begin with “b.” In the first example we’ll be discussing, the star Kepler-62 is the 62nd star identified by Kepler. The designation Kepler-62f means we’re discussing the fifth planet detected in the Kepler-62 system. It does not necessarily mean that Kepler-62f is the fifth planet in orbit around Kepler-62.
In situations where a star has a common name, this may also be incorporated into the planet designation. The International Astronomical Union gives the example of 51 Pegasi b, which corresponds to the first planet discovered around 51 Pegasi, which is a star in the constellation Pegasus. You can read more about naming conventions on the IAU’s website.
Kepler-62f has several qualities that make it likely to host life. It orbits a K-type star, which is a type of star similar to the Sun, but with a longer expected lifetime. Its orbital distance (0.7 AU), size (1.41x larger than Earth) and mass (at least 2.8x higher than Earth) means that it’s a rocky planet and could sustain a global ocean.
Researchers have identified multiple scenarios that would result in Kepler-62f being habitable, depending on the thickness of its atmosphere and the amount of liquid water present on the surface. It’s possible that Kepler-62f could be covered in a global ocean, which would dramatically reduce its chances of supporting life. At 1,200 light years away, even next-generation telescopes won’t be able to check for the presence of an atmosphere, and SETI has found no evidence of radio signals, but its position and characteristics still make Kepler-62f a leading habitable candidate.
Kepler-186f orbits a red dwarf star and is the first planet we’ve found with a radius similar to Earth’s in the habitable zone of its star. Kepler-186f has a radius ~1.11x larger than our own planet. One major problem with planets orbiting red dwarf stars is that they tend to become tidally locked, with one side of the planet blasted by eternal sunlight while the far side of the planet freezes solid. Theoretically it might be possible to live on the terminator of such planets, but the vast difference in atmospheric temperature would create constant storms as air circulated between the hot and cold sides of the planet.
But Kepler-186f has a higher orbit than other planets around Kepler-186, making it less likely to be tidally locked. Scientists estimate the planet has a 50 percent chance of rotating normally. No signals from Kepler-186f have been detected by SETI, though they would have to be quite powerful to reach us (10x more powerful than signals from Arecibo) and to have left the home system nearly 500 years ago. To put this in perspective, we’ve been broadcasting signals other civilizations could’ve received for less than a century ourselves.
Kepler-442b is a super-earth orbiting a K-type star with a radius 1.34x larger than Earth’s and a mass estimated at 2.34x of Earth. This would make surface gravity roughly 30 percent stronger than on our own planet — a significant difference, but not enough to prevent alien life from leaving the surface.
Kepler-442b’s close position to its host star (0.4x AU, or roughly as close to its star as Mercury is to the Sun) doesn’t preclude it as a habitable planet. K-type stars are not as luminous as our own and Kepler-442b only receives about 70 percent of the sunlight the Earth does. The planet is estimated to lie just outside the region where interactions with Kepler-442 would have tidally locked it. At 1,120 light years away, Kepler-442b is also too small and dim to be directly imaged with next-generation telescopes, though the Square Kilometer Array would be able to make much more detailed observations of the star.
As Kepler continues its mission and telescopes like the James Webb Space Telescope come online, our knowledge of stars and their host planets will only increase. Every potentially habitable planet we detect fills in our knowledge of the cosmos and offers exciting opportunities for future research.
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