Alien life may exist on planets that have been hiding in plan sight, previously dismissed as astronomers as being incapable of hosting life.
That’s the conclusion of a study published today in The Astrophysical Journal in which a team from the University of Cambridge identify a new class of habitable exoplanets they’ve dubbed “Hycean.”
“Hycean” planets aren’t rocky like Earth, instead having oceans and hydrogen-rich atmospheres.
It’s thought that the new classification could accelerate the search for life outside our Solar System for one simple reason: planets that fulfil the criteria to be “Hycean” already dominate the known exoplanet population, which is approaching 4,500.
Such planets are also larger than Earth and many are relatively close to us, so much easier to find and study than tiny Earth-like planets.
The researchers singled out one particular exoplanet, called K2-18b, which tops a list of exoplanets that they say should be investigated by the massive James Webb Space Telescope (JWST), which launches this October.
“Hycean planets open a whole new avenue in our search for life elsewhere,” said Dr. Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research. He thinks that we should be looking for trace gases in these planets’ atmospheres that have previously been overlooked, but could be as valid a “biosignature”—a suggestion of life—as the typically searched-for oxygen, ozone, methane and nitrous oxide.
Several of the leading candidates for these planets identified by scientists are larger and hotter than Earth.
However, it still has properties for hosting large oceans that can support microbial life similar to those found in some of the most extreme aquatic environments on Earth, scientists have suggested.
Planets also allow for a much wider habitable zone, compared to Earth-like planets. This means that it can still support life despite being outside the range in which a planet similar to Earth would have to be in order to be habitable.
Since the discovery of the first exoplanet 30 years ago, thousands of other planets have been discovered outside our solar system.
The vast majority of planets are between the sizes of Earth and Neptune, and are often referred to as super-Earths or mini-Neptunes. They can often be rocky or ice giants with hydrogen-rich atmospheres, or something in between.
Previous research suggested that the pressure on these worlds would be too high to support life as we know it. But last year, Madhusudhan and colleagues published a paper on the mini-Neptune K2-18b. They found a range of conditions under which this world could, in fact, be habitable after all.
This prompted a detailed investigation of the full range of planetary and stellar properties for which these conditions are possible.
It also looked at known exoplanets that might meet these conditions, and whether their biometric fingerprints could be observed in the near future.
The investigation led scientists to identify a new class of planets, the Hycean planets, with massive planet-wide oceans beneath the hydrogen-rich atmosphere.
Hycean worlds, Madhusudhan and his team found, can be up to 2.6 times the size of Earth, and up to 10 times Earth’s mass. They also have a much larger range of distances from their host star at which life could survive – what we call the habitable zone. Hycean exoplanets can be so close to their star that atmospheric temperatures reach nearly 200 degrees Celsius (nearly 400 degrees Fahrenheit), or at distances at which a rocky planet would be too icy.
Astronomers suggest that such planets also include Hycean worlds that may have habitable conditions only on their perpetual nocturnal sides.
Size alone is not sufficient to determine whether a planet is Hycean, and other aspects such as mass, temperature and atmospheric properties are required for confirmation.
“Essentially, when we’ve been looking for these various molecular signatures, we have been focusing on planets similar to Earth, which is a reasonable place to start,” Madhusudhan said in a press statement.
“But we think Hycean planets offer a better chance of finding several trace biosignatures.”
Often these bio-fingerprints 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 could be promising indicators of life on planets with hydrogen-rich atmospheres.
On these planets, life can be supported despite the lack of oxygen or ozone.
Co-author Anjali Payet, also from the University of Cambridge, said: “It is exciting that habitable conditions exist on planets completely different from Earth.”
The team has identified a large sample of potential Hycean worlds that are prime candidates for detailed study using next-generation telescopes, such as the James Webb Space Telescope (JWST), which is due to launch later this year.
All of these planets orbit red dwarf stars between 35-150 light-years away, close to astronomical standards.