Rocky alien worlds shrouded in hydrogen and helium could prove habitable for life as we’ve known it for billions of years, according to a new study, with key characteristics including temperate conditions and liquid water.
On Earth, there is life almost everywhere there is water. As such, the search for extraterrestrial life has often focused on worlds whose surfaces are temperate enough to host liquid water, which astronomers describe as habitable. Apart from this prerequisite, scientists remain uncertain to what extent exoplanets must resemble Earth in order to support life.
Previous work analyzing distant worlds suggests that most planetary systems may differ from ours. For example, super-Earths – rocky planets up to 10 times the mass of Earth – are absent from our solar system but very common elsewhere.
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“The exoplanets we detect are so different from planets in our solar system, and that’s a good argument to keep thinking outside the box when it comes to habitability,” said study lead author Marit Mol. Lous, an exoplanet researcher at the University of Zürich, told Space.com.
In the new study, the researchers investigated to what extent super-Earths could support life. So far, astronomers have only detected super-Earths orbiting near their stars because they are the easiest to spot. However, previous computer models of planet formation have suggested that many super-Earths may orbit far from their stars.
Super-Earths far from their stars can keep the hydrogen and the helium gas that made up most of their planetary systems in their youth. Together, hydrogen and helium make up 99.9% of the known normal matter in the universe; heat from a star would evaporate these gases from any nearby super-Earths, but super-Earths farther from a star may retain this primordial atmosphere.
In the new study, the researchers investigated whether a super-Earth shrouded in these gases could prove habitable. If such a world’s atmosphere is massive enough, its hydrogen can serve as a greenhouse gas, trapping the star’s heat despite the planet’s distance, the scientists noted.
Scientists developed computer simulations of super-Earths between one and 10 times the mass of Earth with hydrogen and helium-rich atmospheres that orbited sun-like stars at distances of 1 to 100 astronomical units (AU ). (An AU is the average distance between the earth and the sunthat’s about 93 million miles, or 150 million kilometers.)
The researchers found that super-Earths located at distances greater than 2 AU could host temperate conditions and liquid water on their surface for a period of up to 5 to 8 billion years. Such findings suggest that “we should also consider exotic habitats when studying habitability on other planets,” Mol Lous said.
These sorts of super-Earths could prove habitable even if they disappear”thug” – that is, to wander through space without being bound to a star. Recent work suggested that free planets may be common. The new study found that many rogue super-Earths coated in hydrogen and helium are too hot soon after they form to host liquid water, but among these wandering planets, those with more than five times Earth’s mass could possibly cool to host liquid water for more than 50 billion years if they have atmospheres about 10,000 times more massive than Earth’s.
However, don’t expect to be able to wander the surfaces of these planets. The atmospheres of these worlds must be 100 to 1,000 times thicker than Earth’s, resulting in “about 100 or even 1,000 times the pressure at Earth’s surface,” Mol Lous said. “They are comparable to what we find at the bottom of the ocean.”
Additionally, the surfaces of these planets likely receive negligible sunlight, due to their distance from their stars. As such, any life that might evolve on such worlds might have to rely on energy sources other than sunlight – for example, chemical reactionsnote the scientists.
“Whether life can arise on such a planet is indeed an important question to address, but a difficult one since we know so little about it,” Mol Lous said. “There are life forms on Earth living in extreme conditions that could be similar to the environments of the planets we’ve studied, so we don’t think that’s impossible. But again, many questions remain unanswered.”
Scientists have detailed their findings online Monday, June 27 in the journal Nature Astronomy.
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