A new class of exoplanets very different from ours, but that could support life, was identified by astronomers, which could greatly accelerate the search for life outside the Solar System.
In their search for life elsewhere, astronomers looked primarily for planets of Earth-like size, mass, temperature, and atmospheric composition. However, astronomers at the University of Cambridge (UK) believe there are more promising possibilities out there. His study was published in the journal The Astrophysical Journal.
Researchers have identified a new class of habitable planets, dubbed the “Hycean” planets—hot ocean-covered planets with hydrogen-rich atmospheres—that are more numerous and observable than Earth-like planets.
According to astronomers, the results could mean that finding biosignatures of life outside the Solar System in the next two or three years is a real possibility.
“The Hycean planets open up a whole new path in our search for life elsewhere,” said Dr. Nikku Madhusudhan, from the Institute of Astronomy, University of Cambridge, who led the research.
Many of the top Hycean candidates identified by researchers are larger and warmer than Earth, but still have the characteristics to host large oceans that could support microbial life similar to that found in some of our planet’s most extreme aquatic environments.
These exoplanets also allow for a much wider habitable zone compared to Earth-like planets. This means they can still support life, even though they are outside the range where an Earth-like planet would need to be to be habitable.
Thousands of planets outside the Solar System have been discovered since the first exoplanet was identified nearly 30 years ago. The vast majority are planets between the sizes of Earth and Neptune and are often called super-Earths or mini-Neptunes: they can be predominantly rocky or ice giants with hydrogen-rich atmospheres, or something in between.
Most mini-Neptunes are 1.6 times the size of Earth: they’re smaller than Neptune, but too big to have rocky interiors like Earth. Previous studies of these planets found that the pressure and temperature below their hydrogen-rich atmospheres would be too high to support life.
However, a recent study of mini-Neptune K2-18b by Madhusudhan’s team found that, under certain conditions, these planets could support life. The result led to a detailed investigation into the full range of planetary and stellar properties for which these conditions are possible, which known exoplanets can satisfy these conditions, and whether their biosignatures can be observable.
The investigation led researchers to identify a new class of planets: the Hyceans, with huge oceans covering the planet beneath hydrogen-rich atmospheres. Hycean planets may be up to 2.6 times larger than Earth and have atmospheric temperatures of up to nearly 200 degrees Celsius, but their oceanic conditions may be similar to those that lead to microbial life in Earth’s oceans. These planets also include “dark” Hycean worlds blocked by the tides, which may have habitable conditions only on their permanent nocturnal sides, and “cold” Hycean worlds that receive little radiation from their stars.
Planets of this size dominate the known exoplanet population, though they haven’t been studied in as much detail as super-Earths. Hycean worlds are probably quite common, meaning that the most promising places to look for life elsewhere in the galaxy may be hidden from view.
However, size alone is not enough to confirm whether a planet is Hycean: other aspects such as mass, temperature and atmospheric properties are needed for confirmation.
When trying to determine what conditions are like on a planet many light-years away, astronomers must first determine whether the planet is in the habitable zone of its star and then look for molecular signatures to infer the planet’s internal and atmospheric structure. , which govern surface conditions, the presence of oceans and the potential for life.
Astronomers also look for certain biosignatures that might indicate the possibility of life. Most of the time, they are oxygen, ozone, methane and nitrous oxide, all present on Earth. There are also several other biomarkers, such as methyl chloride and dimethyl sulfide, which are less abundant on Earth but could be promising indicators of life on planets with hydrogen-rich atmospheres, where oxygen or ozone may not be as abundant.
“Essentially, when we look for these various molecular signatures, we focus on Earth-like planets, which is a reasonable place to start,” said Madhusudhan. “But we think the Hycean planets offer a better chance of finding lots of biosignature traits.”
“It’s exciting that habitable conditions can exist on planets so different from Earth,” said co-author Anjali Piette, also from Cambridge.
Madhusudhan and his team found that several traces of terrestrial biomarkers that were supposed to be present in Hycean atmospheres would be readily detectable with spectroscopic observations in the near future. The larger sizes, warmer temperatures and hydrogen-rich atmospheres of the planets Hycean make their atmospheric signatures much more detectable than Earth-like planets.
The Cambridge team has identified a sizable sample of potential Hycean worlds that are prime candidates for detailed studies with next-generation telescopes such as the James Webb Space Telescope (JWST), which should be released later this year. All of these planets orbit red dwarf stars between 35 and 150 light-years away – close by astronomical standards. JWST’s planned observations of the most promising candidate, K2-18b, could lead to the detection of one or more biosignature molecules.
“A biosignature detection would transform our understanding of life in the universe,” said Madhusudhan. “We need to be open about where we hope to find life and what form that life can take, as nature continues to amaze us in ways that are often unimaginable.”
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