New class of exoplanets identified in search for extraterrestrial life
More than 25 years after the discovery of the first exoplanets, scientists have identified a new subclass of planets that are hot, have large oceans and thick hydrogen atmospheres, and may be habitable for microbial life.
Not present in our own solar system, these worlds are known as Hycean planets. They are typically up to 2.5 times larger than Earth, covered mostly or completely by oceans of liquid water, and have hydrogen-rich atmospheres.
Hycean planets appear to be the most common among the more than 4,000 exoplanets discovered since 1992. Their habitable zones are larger than those of Earth-like planets, on which most searches for extraterrestrial life have concentrated, because their oceans can support extremophiles, microbes that can survive in extreme conditions beyond those normally habitable for more advanced life forms.
In size and density, Hycean planets run the gamut from super-Earths to mini-Neptunes. They can be rocky worlds like the Earth or gaseous worlds with rocky interiors though gaseous worlds are less likely to be hospitable to life due to their high pressures and temperatures.
Surface temperatures on Hycean worlds can reach 392 degrees Fahrenheit (200 degrees Celsius). Some may orbit so close to their parent stars that they are tidally locked, with one side always facing their host star and the other always facing away from it.
Yet scientists believe microbial life could exist on these worlds, on the hemispheres facing away from the parent star, much like extremophiles on Earth that live in hydrothermal vents at the bottom of its oceans.
These worlds could be better targets in the search for extraterrestrial life than the Earth-like planets that scientists have prioritized. Some scientists think biosignatures, ingredients necessary for life, such as oxygen, ozone, nitrous oxide, methane, methyl chloride, and dimethyl sulfide, could be found on Hycean planets within the next two or three years.
“Essentially, when we’ve been looking for these various molecular signatures, we have been focusing on planets similar to Earth. But we think Hycean planets offer a better chance of finding several trace biosignatures,” Madhusudhan stated.
“A biosignature would transform our understanding of life in the universe. We need to be open about where we expect to find life and what form that life could take, as nature continues to surprise us in often unimaginable ways.”
Madhusudhan and his fellow research team members have selected known Hycean planets, all of which orbit red dwarf stars between 35 and 150 light years from Earth, for further study with the James Webb Space Telescope (JWST), which is scheduled to launch in the late fall. One of their targets is a gaseous mini-Neptune, K2-18b, that orbits a red dwarf 110 light years away.
In order to determine whether a planet is Hycean, scientists must identify its distance from its parent star, then measure its mass, atmosphere, and temperature. Spectroscopic observations should make it possible for researchers to detect biomarkers in planets’ atmospheres.
“It’s exciting that habitable conditions could exist on planets so different from Earth,” noted Anjali Piette, also of Cambridge University.
A paper on these findings has been published in The Astrophysical Journal.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.