These planets are just a little older, a little wetter, a little warmer and a little larger than Earth is, researchers wrote Sept. 18 in the journal Astrobiology (opens in new tab). All of these factors could mean that some of these planets are the best places to search for extraterrestrial life.
"We have to focus on certain planets that have the most promising conditions for complex life. However, we have to be careful to not get stuck looking for a second Earth because there could be planets that might be more suitable for life than ours," University of Washington astrobiologist Dirk Schulze-Makuch said in a statement (opens in new tab).
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Seeking superhabitable planets
Astronomers have discovered more than 4,000 exoplanets, or planets outside our solar system, so far. Most of these are not particularly conducive to life. For example, planet KELT-9b is so hot that its atmosphere is constantly melting. The darkest known planet, TrES-2b, has an atmospheric temperature of 1,800 degrees Fahrenheit (980 degrees Celsius). On the other end of the inhospitable spectrum is GJ 433 d, whose discoverers described it as the coldest Neptune-like planet ever discovered.
But there are also many planets within their star's habitable zone, or the "just-right" distance conducive to surface temperatures that aren't too hot or too cold for life as we know it to evolve. Schulze-Makuch and his colleagues aimed to identify exoplanets most likely to be "superhabitable," or not only in the habitable zone but also boasting other features that might make them a good place for life to blossom.
These features included a star of the right size and life span, especially considering that it took complex life 3.5 billion years to evolve on Earth, and 4 billion years for life as advanced as humans to appear. A large size could mean more space for landmass and habitat; a larger planet would also have higher gravity, which would make for a thicker atmosphere, something that could be beneficial for organisms that travel by flight, the researchers wrote. A planet slightly warmer than Earth would be more habitable, given a lack of largely barren polar regions, but that warmer planet would also need to be wetter than Earth so that deserts wouldn't dominate the landmasses. A more habitable planet might thus resemble Earth in the early Carboniferous, about 359 million years ago, when much of the world's landmass had the climate of a tropical rainforest. (Modern-day global warming isn't good for life on Earth both because the change is happening too quickly for many animals to adapt and because of the effects on human infrastructure due to rising sea levels; slightly warmer temperatures, however, aren't inherently bad for life.)
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A better version of Earth might also have a slightly larger moon, or a moon slightly closer to the planet, which would help stabilize its orbit and prevent life-disrupting wobbles, the researchers wrote.
The researchers came up with a set of parameters to use to meet all these criteria. According to these parameters, the perfect superhabitable planet would be in orbit around a K dwarf star, which is a relatively small star star that’s slightly cooler than our sun (which is considered a yellow dwarf); about 5 billion to 8 billion years old; about 10% larger than Earth; about 9 F (5 C) warmer than Earth, on average; moist with an atmosphere that is 25% to 30% oxygen, with scattered land and water. The perfect planet would also have plate tectonics or a similar geological process in order to recycle minerals and nutrients through the crust and to create diverse habitats and topography, and would have a moon between 1% and 10% of its size orbiting it at a moderate distance.
Looking for life
It's not possible to evaluate distant exoplanets on all these criteria. There is no way to calculate an exoplanet's landmass area, for example, much less how it's distributed.
But based on factors that can be measured, such as star type and planet radius, the researchers honed in on objects that seem to meet that criteria and have been spotted by the Kepler telescope; they found 24 Kepler Objects of Interest, which are objects that may or may not be planets. Two of the 24 have been confirmed as exoplanets (Kepler 1126 b and Kepler-69c). Some of the others may be false positives that don't turn out to be planets. Of the 24 objects, nine were orbiting around the proper type of star, 16 fell into the correct age range, and five were in the right temperature range. Only one candidate, KOI 5715.01, fell into the correct range for all three categories, but the planet's true surface temperature depends on the strength of the greenhouse effect in its atmosphere, the researchers wrote.
The 24 possible planets are all more than 100 light-years away, and some are probably too far to study right now even with the strongest telescopes. Kepler-69c, for example, is more than 2,000 light-years away, meaning astronomers probably won't be able to examine it more closely for signs of life anytime soon. However, pinpointing what makes a planet "superhabitable" is important, Schulze-Makuch and his colleagues wrote, because it's possible that one of these planets will be discovered within 100 light-years. If so, they wrote, that planet should be the first place Earthlings turn to find out if there is other life in the universe.
Originally published on Live Science.