Exoplanets discovered with better conditions for life than Earth

Life could also more easily thrive on planets that circle more slowly changing stars with longer lifespans than our sun.

The recent discovery of phosphine in the atmosphere of Venus has reignited excitement around the possibility that that microbial life may exist closer to home than expected. If proven correct, this could imply that the evolution of life is a more commonplace process in the galaxy than is currently believed.

The scientists said the 24 top contenders for superhabitable planets are all more than 100 light years away, but study leader Dirk Schulze-Makuch said the study could help focus future observation efforts, such as from Nasa’s James Web Space Telescope.

“With the next space telescopes coming up, we will get more information, so it is important to select some targets,” he said. “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 team tried to identify superhabitability criteria among the 4,500 known exoplanets beyond our solar system. Habitability does not mean these planets have life, merely that the conditions would be conducive to life.

The researchers selected planet-star systems with probable terrestrial planets orbiting within the host star’s liquid water habitable zone.

While the sun is the centre 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.

In addition to looking at systems with cooler G stars, the researchers also looked at systems with K dwarf stars, which are somewhat cooler, less massive and less luminous than our 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. Earth is around 4.5 billion years old, but the researchers argue that the sweet spot for life is a planet that is between 5 billion to 8 billion years old.

Size and mass also matter. A planet that is 10 per cent larger than the Earth should have more habitable land, they said. 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 gravitational field to retain an atmosphere over a longer time period.

Water is key to life and the authors argue that a little more of it would help foster life, especially in the form of moisture, clouds and humidity.

A slightly overall warmer temperature, a mean surface temperature of about 5°C greater than Earth, together with the additional moisture, would be also better for life. This warmth and moisture preference is seen on Earth with greater biodiversity in tropical rainforests than in colder, drier areas.

Among the 24 top planet candidates, none of them meet all the criteria for superhabitable planets. However, one has four of the critical characteristics, making it possibly much more comfortable for 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 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.”