A new class of habitable exoplanets represents a big step forward in the search for life

A new class of exoplanets very different from ours, but which could harbor life, has been identified by astronomers, which could significantly speed up the search for life outside our solar system.

In the search for life elsewhere, astronomers have primarily looked for planets of similar size, mass, temperature, and atmospheric composition to Earth. However, Cambridge University astronomers believe there are more promising possibilities.

Researchers have identified a new class of habitable planets, called “Hycean” planets, hot planets covered by oceans with hydrogen-rich atmospheres, which are more numerous and observable than Earth-like planets.

The researchers say the findings, reported in The Journal of Astrophysics, could mean that finding biosignatures of life outside our solar system in the next two or three years is a real possibility.

“The Hycean planets are opening up a whole new avenue in our search for life elsewhere,” said Dr Nikku Madhusudhan of the Cambridge Institute of Astronomy, who led the research.

Many of the main Hycean candidates identified by the researchers are larger and hotter than Earth, but still have the characteristics of hosting large oceans that could support microbial life similar to that found in some of the most extreme aquatic environments in the world. Earth.

These planets also allow for a much larger habitable zone, or “Goldilocks zone,” compared to Earth-like planets. This means that they could still support life even if they are outside the range where an Earth-like planet would have to be to be habitable.

Thousands of planets outside of our solar system have been discovered since the first exoplanet was identified almost 30 years ago. The vast majority are planets between the sizes of Earth and Neptune and are often referred to as “super-Earths” or “mini-Neptunes”: they can be primarily rock or ice giants with hydrogen-rich atmospheres, or something else. thing in between.

Most mini-Neptunes are over 1.6 times the size of Earth: smaller than Neptune but too large to have rocky interiors like Earth. Previous studies of such planets have shown that the pressure and temperature under their hydrogen-rich atmospheres are too high to support life.

However, a recent study on the 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 may satisfy these conditions, and whether their biosignatures may be observable.

The investigation led the researchers to identify a new class of planets, the Hycean planets, with massive planet-wide oceans under hydrogen-rich atmospheres. Hycean planets can be up to 2.6 times larger than Earth and have atmospheric temperatures of up to nearly 200 degrees Celsius, but their ocean conditions could be similar to those conducive to microbial life in the oceans of Earth. Such planets also include tidal locked “dark” Hycean worlds which may have habitable conditions only on their permanent night sides, and “cold” Hycean worlds which receive little radiation from their stars.

Planets of this size dominate the known exoplanet population, although they have not been studied in as much detail as super-Earths. Hycean worlds are probably quite common, which means that the most promising places to seek life elsewhere in the Galaxy may have been hidden in plain sight.

However, size alone is not enough to confirm whether a planet is Hycean: other aspects such as mass, temperature, and atmospheric properties are needed to confirm.

When trying to determine what the conditions are on a planet several light years away, astronomers must first determine whether the planet is in the habitable zone of its star, then look for molecular signatures to infer the atmospheric structure and internal planet, which govern the surface conditions, the presence of the oceans and the potential for life.

Astronomers are also looking for certain biosignatures that might indicate the possibility of life. Most often, these are oxygen, ozone, methane, and nitrous oxide, all of which are present on Earth. There are also a number of other biomarkers, such as methyl chloride and dimethyl sulfide, which are less abundant on Earth but may be promising indicators of life on planets with hydrogen-rich atmospheres where l oxygen or ozone may not be as abundant.

“Essentially, when we researched these various molecular signatures, we focused on Earth-like planets, which is a reasonable starting point,” Madhusudhan said. “But we think the Hycean planets offer a better chance of finding more traces of biosignatures.”

“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 discovered that a number of traces of terrestrial biomarkers that are expected to be present in Hycean atmospheres would be easily detectable with spectroscopic observations in the near future. The larger sizes, higher temperatures, and hydrogen-rich atmospheres of Hycean planets make their atmospheric signatures much more detectable than those of Earth-like planets.

The Cambridge team has identified a large sample of potential Hycean worlds that are prime candidates for detailed study with next-generation telescopes, such as the James Webb Space Telescope (JWST), due to be launched later this year. . These planets all orbit red dwarf stars between 35 and 150 light years away: close by astronomical standards. Planned JWST 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,” Madhusudhan said. “We need to be open about where we expect to find life and what form that life might take, as nature continues to surprise us in ways that are often unimaginable.”