Some planets may have more suitable conditions for life than Earth

Scientists have identified 24 planets outside our solar system that they say may have conditions more suitable for life than the Earth.

The study, published in the journal Astrobiology, details characteristics of potential "superhabitable" planets, that include those that are older, a little larger, slightly warmer and possibly wetter than Earth.

The researchers from the Washington State University (WSU) in the US noted life could more easily thrive on planets that circle more slowly changing stars with longer lifespans than our Sun.

The 24 top contenders for superhabitable planets are all more than 100 light-years away, but the study could help focus future observation efforts, such as from NASA's James Web Space Telescope, they said.

"With the next space telescopes coming up, we will get more information, so it is important to select some targets," said Dirk Schulze-Makuch, a professor at WSU and the Technical University in Berlin.

"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," Schulze-Makuch said.

The researchers identified superhabitability criteria and searched among the 4,500 known exoplanets beyond our solar system for good candidates.

Habitability does not mean these planets definitely have life, merely the conditions that would be conducive to life, they noted.

The researchers 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.

While the Sun is the centre of our solar system, it has a relatively short lifespan of less than 10 billion years, they said.

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, according to the researchers.

In addition to looking at systems with cooler G stars, they also looked at systems with K dwarf stars, which are somewhat cooler, less massive and less luminous than the Sun.

The researchers said 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, they said.

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, the researchers 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 gravity to retain an atmosphere over a longer time period, they said.

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

They noted that a slightly overall warmer temperature, a mean surface temperature of about 5 degrees Celsius greater than Earth, together with the additional moisture, would be also better for life.

Among the 24 top planet candidates, none of them meet all the criteria for superhabitable planets, but one has four of the critical characteristics, making it possibly much more comfortable for life than our home planet, the researchers added.