There are exoplanets that might be better at supporting life - even human life - than the Earth, experts say.
These "superhabitable Earths" might have conditions more suitable for life than our own planet. Some of these "better Earths" orbit stars that could be better than our sun at creating the conditions for life, they say.
Habitability refers to the conditions conducive to life. It doesn't mean these planets have life, however.
These assumptions were put forward in a study by researchers from Washington State University published in the journal Astrobiology. The study spells out characteristics of potential superhabitable planets.
It includes planets that are older, a little larger, slightly warmer and possibly wetter than Earth. The study also argues these superhabitable planets might orbit more slowly.
The study was led by Dirk Schulze-Makuch, an adjunct professor at WSU School of Earth and Environmental Sciences and a professor at the Center for Astronomy and Astrophysics at the Technical University Berlin in Germany.
Schulze-Makuch and astronomers Rene Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University identified superhabitability criteria and searched the 4,500 known exoplanets beyond our solar system for superhabitability candidates.
The team identified 24 of these superhabitable planets with at least one of the conditions necessary for sustaining life better than on the Earth. All these planets are more than 100 light years away.
The study contends the sweet spot for life is a planet between 5 billion to 8 billion years old. The Earth is about 4.5 billion years old.
A planet 10% larger than the Earth should have more habitable land. One that is 1.5 times Earth's mass can be expected to retain its interior heat through radioactive decay longer. Its gravity will also be stronger, helping it retain an atmosphere over a longer period of time.
A planet wetter than the Earth might also be more conducive to life. A superhabitable planet will have a slightly overall warmer temperature. It should have a mean surface temperature of about 5 degrees Celsius (8 degrees Fahrenheit) higher than Earth's. This slightly warmer temperature, plus the additional moisture from water vapor, would be better for life.
This warmth and moisture preference is seen on Earth in tropical rain forests with their greater biodiversity compared with colder areas prone to snow and drier areas with vast deserts.
Schulze-Makuch said one only of 24 top candidates identified as superhabitable meet all the criteria. On the other hand, one planet has four of the critical characteristics - making it probably much more comfortable for life than Earth.
"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 lifeforms, 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."
