In the tireless search for life in the vast universe, astronomers often focus on those planets that share similar properties to Earth, be it in mass, size, temperature, and atmospheric composition. But a group of scientists at the University of Cambridge believes there are more promising possibilities beyond these characteristics that could speed up that search considerably.
These researchers have identified a new class of habitable planets called “Hycean” planets, which are covered by oceans with hydrogen-rich atmospheres that are more numerous and observable than Earth-like planets.
The results of a first investigation published in The Astrophysical Journal could mean that finding biosignatures of life outside our Solar System in the next few years is a real possibility.
“The Hycean planets open 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 leading Hycean candidates identified by the researchers are larger and hotter than Earth, but still have the characteristics to host large oceans that could support microbial life similar to that found in some of the most extreme aquatic environments. from the earth.
These planets also allow for a much wider habitable zone, or “Goldilocks zone,” compared to Earth-like planets. This means that they could still host life even though they are outside the range where an Earth-like planet would need to be to be habitable.
Thousands of planets have been discovered outside our Solar System 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 referred to as ‘super-Earths’ or ‘mini-Neptunes: they can be predominantly rock or ice giants with hydrogen-rich atmospheres, or something in between.
Most mini-Neptunes are more than 1.6 times the size of Earth: smaller than Neptune but too big to have rocky interiors like Earth. Previous studies of such planets have found that the pressure and temperature below their hydrogen-rich atmospheres would be too high to support life.
However, a recent study on mini-Neptune K2-18b by Madhusudhan’s team found that under certain conditions these planets could host life. The result led to a detailed investigation of the full range of planetary and stellar properties for which these conditions are possible, which known exoplanets can satisfy those conditions, and whether their biosignatures can be observable.
The research led researchers to identify a new class of planets, the Hycean planets, with massive oceans across the planet beneath 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, depending on their host stars, but their ocean conditions could be similar to those conducive to microbial life in the oceans of the earth.
Such planets also include tidal-locked “dark” Hycean worlds that may have habitable conditions only on their permanent night sides, and “cold” Hycean worlds that 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 likely fairly common, meaning that the most promising places to search for life in other parts of 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 required for confirmation.
When trying to determine what conditions are like on a planet many light-years away, astronomers must first determine if the planet is in the habitable zone of its star and then look for molecular signatures to infer the atmospheric and internal structure of the planet. that govern surface conditions, the presence of oceans, and the potential for life.
Astronomers also look for certain biological signatures that could indicate the possibility of life. Most often these are oxygen, ozone, methane, and nitrous oxide, which are all 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 maybe promising indicators of life on planets with hydrogen-rich atmospheres where oxygen or ozone may not be so abundant.
“Essentially, when we’ve been looking for these various molecular signatures, we’ve focused on Earth-like planets, which is a reasonable place to start, ” Madhusudhan said. “But we think the Hycean planets offer a better chance of finding various traces of biosignatures.”
“It’s exciting that habitable conditions exist on planets so different from Earth,” said co-author Anjali Piette, also from Cambridge.
Madhusudhan and his team discovered that a number of terrestrial biomarker traces expected to be present in Hycean’s atmospheres would be readily detectable with spectroscopic observations in the near future. The Hycean planets’ larger sizes, higher temperatures, and hydrogen-rich atmospheres make their atmospheric signatures much more detectable than Earth-like planets.
The Cambridge team identified a sizable 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 launch later this year.
All of these planets orbit red dwarf stars 35 to 150 light-years away. The planned JWST observations of the most promising candidate, K2-18b, could lead to the detection of one or more biological signature molecules.
“A biosignature detection would transform our understanding of life in the universe, ” Madhusudhan said. “We need to be open about where we hope to find life and what form that life might take, as nature continues to surprise us in ways often unimaginable.”