Are there 'superhabitable' planets with conditions better than Earth? Scientists say they may have found 24
One of the exoplanets called KOI 5715.01 has four of the critical characteristics, making it possibly much more comfortable for life than our home planet
A research team has identified two dozen "superhabitable" planets outside the Solar System which they say may have conditions more suitable for life than on Earth. The 24 top contenders are all more than 100 light years away and they include those that are older, little larger, slightly warmer and possibly wetter than Earth.
According to researchers, none of the top 24 planet candidates meet all the criteria for superhabitable planets. But one of them called KOI 5715.01 has four of the critical characteristics of a superhabitable planet, making it possibly much more comfortable for life than our home planet. Led by Washington State University (WSU) scientist Dirk Schulze-Makuch, the study emphasizes that Earth may not necessarily be the best planet in the universe.
"It's sometimes difficult to convey this principle of superhabitable planets because we think we have the best planet. We have a great number of complex and diverse lifeforms and many that can survive in extreme environments. It is good to have an adaptable life, but that doesn’t mean that we have the best of everything,” explains Schulze-Makuch, a professor with WSU and the Technical University in Berlin, in the study published in the journal Astrobiology.
The authors caution that habitability does not mean these exoplanets definitely have life, instead it refers to a planet’s potential to develop and maintain environments hospitable to life. "Thus, when we discuss the possibility of habitable or super habitable planets, we do not necessarily assume that these planets host life and thus do not address (at least directly) the notoriously difficult question on how, where and under what conditions life originates — conditions that might be very different from those under which life thrives,” they explain.
The team also includes experts from the Max Planck Institute for Solar System Research and Villanova University. To identify the "superhabitability criteria", they looked at the over 4,000 known exoplanets beyond the Solar System for good candidates. The investigators selected planet-star systems with probable terrestrial planets orbiting within the host star's "liquid water habitable zone" from the Kepler Object of Interest Exoplanet Archive of transiting exoplanets.
"The fact that Earth is teeming with life makes it appear odd to ask whether there could be other planets in our galaxy that may be even more suitable for life. Neglecting this possible class of superhabitable planets, however, could be considered anthropocentric and geocentric biases. We argue that there could be regions of astrophysical parameter space of star-planet systems that could allow for planets to be even better for life than our Earth," write scientists.
One factor of superhabitability may be the type of star the planets orbit. The analysis reveals some of these superhabitable planets orbit stars that may be better than even our Sun. According to the authors, life could also more easily thrive on planets that circle more slowly changing stars with longer lifespans than the Sun.
Besides exploring systems with cooler G stars (shorter-lived solar-type stars), researchers also looked at systems with K dwarf stars, which are slightly cooler, less massive and less luminous than the Sun. K stars have the advantage of long lifespans of 20 billion to 70 billion years. This would allow orbiting planets to be older as well as giving life more time to advance to the complexity currently found on Earth.
However, to be habitable, planets should not be so old that they have exhausted their geothermal heat and lack protective geomagnetic fields, explain scientists. Earth is around 4.5 billion years old, but the team argues that the sweet spot for life is a planet that is between 5 billion to 8 billion years old. "While the sun is the center of our Solar System, it has a relatively short lifespan of less than 10 billion years. Since it took nearly 4 billion years before any form of complex life appeared on Earth, many similar stars to our Sun, called G stars, might run out of fuel before complex life can develop," the findings state.
Water is key to life, and the authors emphasize that water content — especially in the form of moisture, clouds and humidity — on a warmer planet that is about 8 degrees Fahrenheit warmer than Earth, maybe more suitable for life, too. This warmth and moisture preference is seen on Earth with the greater biodiversity in tropical rain forests than in colder, drier areas, they add.
Size and mass also matter: a planet that is 10% larger than the Earth should have more habitable land. “One that is about 1.5 times Earth’s mass would be expected to retain its interior heating through radioactive decay longer and would also have a stronger gravity to retain an atmosphere over a longer period," says the study.
Since none of the planet candidates are closer than 100 light years, they are, therefore, inaccessible for high-quality observations from NASA's TESS mission. Researchers, however, point out that superhabitable planets might well be present in the exoplanet sample known today.
They suggest that should such a planet be discovered within about 100 light years in the near future, it would deserve higher priority for follow-up observations "in search of extrasolar life than the most Earth-like planets." The analysis could help focus 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, explain scientists.
"With the next space telescopes coming up, we will get more information, so it is important to select some targets. 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," writes Schulze-Makuch.