Planets beyond our solar system, we now know, come in all shapes, sizes and consistencies. There are rocky planets, water worlds, gaseous planets, super-Earths, hot Jupiters, tidally locked planets, planets in orbital resonance with each other, and so much more.
A group of exoplanet researchers at the University of Cambridge have recently proposed a new category of planet, one that has seldom been considered even potentially habitable. They call them Hycean planets due to the presence of substantial hydrogen in the atmospheres and large oceans (hydrogen and ocean = Hycean) on their surfaces.
And in an article in The Astrophysical Journal, they make the case that under certain conditions, some Hycean planets could, indeed, be habitable.
“Hycean planets open a whole new avenue in our search for life elsewhere,” said Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research.
Many of the prime Hycean candidates identified by the researchers are bigger and hotter than Earth, but the researchers argue that they still have the characteristics to host large oceans that could support microbial life similar to that found in some of Earth’s most extreme watery environments.
Hycean planets, Madhusudhan said in a release, offer a new paradigm for the search for life beyond Earth.
“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,” he said. “But we think Hycean planets offer a better chance of finding several trace biosignatures.”
Co-author Anjali Piette, also from Cambridge, added: “It’s exciting that habitable conditions could exist on planets so different from Earth.”
There are no planets of this size and type in our solar system, but planets in the Hycean range are quite common in the galaxy.
Planets between the sizes of Earth and Neptune (the next largest planet in our solar system) and are often referred to as “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 over 1.6 times the size of Earth; smaller than Neptune but too big to have rocky interiors like Earth. Earlier studies of such planets have found that the pressure and temperature beneath their hydrogen-rich atmospheres would be too high to support life.
However, in their study of what had been identified as a mini-Neptune, K2-18b, Madhusudhan’s team found that in certain conditions these planets — on the boundary between super-Earths and mini-Neptunes — could have rocky cores and potentially could support life.
The result led to a detailed investigation into the full panoply of planetary and stellar properties which can tell researchers whether a planet is potentially habitable or definitely not, and whether or not any biosignatures might be observable.
The investigation led the researchers to identify their new category of planets — Hycean planets — as having massive planet-wide oceans beneath hydrogen-rich atmospheres. Hycean planets can be up to 2.6 times larger than Earth and have atmospheric temperatures up to nearly 200 degrees Celsius, depending on their host stars. But the researchers concluded their oceanic conditions could nonetheless be similar to those which allow for extremophile microbial life in Earth’s oceans.
The paper also reports that some of these planets may well be tidally locked — meaning that one side is always in daylight from the star it orbits and the other side is perpetually in the dark, like our moon. In this case, tidally locked Hycean planets would only be potentially habitable on one side.
Planets of this general size dominate the known exoplanet population, although they have not been studied in nearly as much detail as planets that are super-Earths and smaller. This subset of Hycean worlds are likely quite common just as the super-Earths and mini-Neptunes are common, the Cambridge team argued, meaning that the most promising places to look for life elsewhere in the galaxy may have been hiding in plain sight.
However, size alone is not enough to confirm whether a planet is Hycean. Numerous other features — including the planet’s mass and temperature, the nature of the host sun and atmospheric properties — are required for any potential confirmation.
And, of course, astrophysicists need to determine whether the planets lie within the habitable zones of their stars — at a distance from their suns where liquid water could at least sometimes be present.
Because of their size, these planets would offer a far wider habitable zone compared to Earth-sized planets. This means that they could still support life even though they lie significantly further from their host suns than a planet of Earth’s size.
The planet used in the initial computations in the Cambridge study is K2-18b, which is described as a mini-Neptune or a super-Earth. The exoplanet orbits its star, the red dwarf K2-18, located some 124 light-years from Earth.
According to the NASA Exoplanet Catalog, it was first discovered in 2015 through the Kepler Space Telescope. K2-18b, has a mass of about 8.92 times compared to Earth.
While it was initially described as a “potentially rocky world,” two independent studies in 2019 used data from the Kepler Space Telescope, the Hubble Space Telescope, and the now-retired Spitzer Space Telescope to confirm that it did indeed have a rocky core.
These studies revealed that this exoplanet is a water world, with vast amounts of water vapor in the atmosphere. One of the studies, published in the September 2019 Nature Astronomy, notes that “K2-18 b offers an unprecedented opportunity to gain insight into the composition and climate of habitable-zone planets.”
While K2-18b has been described as a mini-Neptune and now as a Hycean planet, Madhusudhan said in an email that most mini-Neptunes are not in that newly coined category of planet because they are too big or too hot. This is why the Hycean planets “straddle the boundary between super-Earths and mini-Neptunes,” he said.
In the abstract to their paper, the Cambridge teams writes that a number of trace biomarkers that, if present, could suggest an inhabited world would be readily detectable using modest observing time on the James Webb Space Telescope (JWST), scheduled to launch on a European Ariane rocket in late November or early December.
The paper identifies a sizable sample of nearby potential Hycean planets that would be “ideal targets” in the search for chemical biosignatures in their atmospheres.
The term “Hycean planets” was coined by Madhusudhan and his colleagues and does not have official backing from the astronomy community. But some day it certainly might.