Writing in the journal Astrobiology, a group of researchers led by Washington State University scientist Dirk Schulze-Makuch have identified two dozen “superhabitable” planets outside the solar system that are older, larger, warmer and possibly wetter than Earth.
Even though these planets are all more than 100 light years away from us, the study could prove useful in managing future observation efforts, such as from NASA’s James Web Space Telescope, the LUVIOR space observatory, and the European Space Agency’s PLATO space telescope.
“With the next space telescopes coming up, we will get more information, so it is important to select some targets,” said Schulze-Makuch. “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.”
The study looked at the 4,500 known exoplanets to identify planet-star systems with suns cooler than our own, which would allow more time for life to develop, and systems with K dwarf stars, which are cooler, less massive and less luminous than the Sun, yet have much greater life-spans.
Very old planets were considered unfavourable to life as they would likely have exhausted their geothermal heat and would lack protective geomagnetic fields. The sweet spot for life was set at roughly 5 billion to 8 billion years (the Earth is around 4.5 billion years old).
Another criteria used in the study was size and mass – planets about 1.5 times larger than Earth were considered ideal as they would be expected to retain their interior heating through radioactive decay longer and would have stronger gravity to retain an atmosphere over a longer period of time.
Lastly, researchers looked at water (especially in the form of moisture, clouds and humidity) and surface temperature – slightly more of each, as compared to Earth, was considered more conducive to the emergence and development of life.
None of the 24 identified candidates meet all of the specified criteria, yet one has as many as four, which could mean that it is much more comfortable to life than our home planet.
“It’s sometimes difficult to convey this principle of superhabitable planets because we think we have the best planet,” said Schulze-Makuch. “We have a great number of complex and diverse life-forms, and many that can survive in extreme environments. It is good to have adaptable life, but that doesn’t mean that we have the best of everything.”