The search for planets outside of our solar system (extrasolar planets) has increased by leaps and bounds over the past decade. A total of 4,514 exoplanets in 3,346 planetary systems have been confirmed, and a further 7,721 candidates are awaiting confirmation. Currently, astrobiologists are primarily focused on the “low-hanging fruit” approach, which looks for exoplanets that are similar in size, mass, and atmospheric composition to Earth (also known as “Earth-like”).
However, astrobiologists are also interested in finding examples of “exotic life” that arose under conditions that are not “Earth-like”. For example, a team of astronomers from Cambridge University recently carried out a study that showed how life could arise on ocean-covered planets with hydrogen-rich atmospheres (also known as “hycean” planets). These results could have a significant impact on exoplanet studies and the field of astrobiology.
The research was carried out by Dr. Nikku Madhusudhan, an astrophysics and exoplanetary science reader from the Institute of Astronomy (IoA) at Cambridge University. He was graduated from Ph.D. Astrophysics student Anjali Piette (Dr. Madhusudhan is her supervisor) and IoA member Dr. Savvas Constantinou. The study describing its results, entitled Habitability and Biosignatures of Hycean Worlds, recently appeared in the Astrophysical Journal.
An artist’s idea of how common exoplanets are in the Milky Way. Image source: Wikipedia
Life on small ice giants?
Of all exoplanets discovered in the last 30 years, the vast majority were either predominantly rock planets with multiple Earth masses (“super-earths”) or ice giants with a hydrogen-rich atmosphere (“mini-neptunes”) or somewhere in between. While super-earths make up about 30% (1,383) of all exoplanets discovered so far, mini-Neptunes are the most numerous with 34% (1,531).
Most mini-Neptunes are over 1.7 and 3.9 times the size of Earth and are believed to have interiors made up of ice, rocks, and oceans with volatile elements. Previous studies of such planets have shown that the pressure and temperature conditions under their hydrogen-rich atmospheres are too great to allow life. However, in an earlier study, Nikku Madhusudhan and his team found that these planets can support life under certain conditions.
In particular, they examined the exoplanet K2-18b, a mini-Neptune that was the focus of attention in 2019 when two different teams reported evidence of water vapor in its hydrogen-rich atmosphere. The results of this study led Dr. Madhusudhan and his team to study the full spectrum of planetary and stellar properties that would make it possible to make mini-Neptune potentially habitable.
This led them to identify a new class of planets that they named “Hycean,” which is a combination of the words “hydrogen” and “ocean”. Like the so-called “water worlds”, the worlds of the Hycea would be covered by oceans all over the world, but would have atmospheres dominated by hydrogen (in line with gas giants). The presence of this atmospheric hydrogen would enable a greenhouse effect that would help keep the surface oceans fluid.
Artist’s impression of K2-18b. Photo credit: Amanda Smith
Planets falling into this classification would be about 2.6 times the diameter of the Earth and have atmospheric temperatures of up to 200 ° C (392 ° F), depending on the nature of their host star and the proximity of the planet to it. This is akin to what scientists thought the Earth was billions of years ago when the first unicellular microbial organisms appeared.
Like Dr. Madhusudhan summarized in a recent University of Cambridge press release, these results could mean that there is a class of exoplanets more abundant than “Earth-like” planets that support life, making them much easier for astrobiologists to find :
“Hycea planets open a whole new avenue in our search for life elsewhere. Essentially, in our search for these various molecular signatures, we have focused on Earth-like planets, which is a reasonable place to start. But we believe that Hycea planets offer a better chance of finding multiple traces of biosignatures. “
In addition, the team identified several top-rated Hycean candidates for follow-up in their study. Many of these are larger and hotter than Earth, but can still be covered in large oceans with conditions that could support microbial life. This life would likely be concentrated in extreme environments such as hydrothermal at the ocean-mantle boundary, similar to what has been observed here on Earth.
This type of exoplanet could include a subclass of tidal-locked “dark hycean” planets where habitable conditions exist only on their permanent night sides. The side facing the planet’s parent star would be too hot to hold water in liquid form indefinitely and transfer heat to the dark side through oceanic and atmospheric convection. There is also the possibility of “cold Hycean” worlds that receive little radiation from their stars and have icy shells.
Artist’s impression of an “eyeball” planet, a water world in which the side facing the sun can support an ocean with liquid water. Picture credits and copyright: eburacum45 / DeviantArt
Implications for Astrobiology
Planets of this size are the most common among the known exoplanet population, although they have not been studied in nearly as much detail as super-earths. But what they have in common means that some of the most promising places to look for life elsewhere in the galaxy may be hidden within sight. In addition, these planets allow for a much wider circumolar habitable zone than Earth-like planets.
To locate these exoplanets among the statistically significant super-earth and mini-Neptune populations, one would not simply have to determine their size. Other aspects such as mass, temperature, and atmospheric properties must also be investigated before a candidate can be safely called a Hycean. But most of all, astronomers need to take a close look at these potential Hyceans to see if there is any evidence of biosignatures.
“It’s exciting that habitable conditions can exist on planets so different from Earth,” said Anjali Piette. Also exciting is the fact that the potential Hycea biosignatures that Dr. Madhusudhan and his team have identified it will be easier to spot with spectroscopic observations. Their larger sizes, higher temperatures, and hydrogen-rich atmospheres mean that all atmospheric signatures are much more detectable than those of Earth-like planets. Madhusudhan said:
“Recognizing biosignatures would change our understanding of life in the universe. We have to be open about where we expect life and what form it could take, since nature often surprises us in unimaginable ways. “
Other possible biosignatures are organic compounds such as methyl chloride and dimethyl sulfide, which are less common on Earth but could be indicators of life on planets with hydrogen-rich atmospheres and low levels of oxygen or ozone. Again, this is in line with the “low hanging fruit” approach, in which we look for biomarkers that are required for or produced by life as we know it.
When next generation telescopes become available in the near future, the wide range of potential Hycea worlds the Cambridge team has prepared will be an excellent opportunity for follow-up observation. In addition, by cosmic standards, these planets are all relatively close and orbit M (red dwarf) -type stars that are 35 to 150 light-years from the solar system. There are already plans to study K2-18b (the most promising candidate) with the next generation James Webb Space Telescope (JWST).
When JWST kicks off in November (or early December) this year, astronomers will be able to conduct direct imaging studies of exoplanets in the near to mid-infrared and extract spectra directly from their atmosphere. The Nancy Grace Roman Space Telescope (RST) will follow in 2025 and will also perform direct imaging studies with its advanced system of optics, coronographs and spectrometers.
These results help illustrate how exoplanet research has grown and changed over the past few years. With thousands of confirmed exoplanets now available for study, the process has evolved from discovery to characterization. With improved tools and methods, astronomers can constrain the planetary environments more.
In the midst of it all, scientists can test their theories about the conditions under which life could exist in our cosmos. After all, the point is not just to find “life as we know it”, but as it exists in all its diversity and splendor. While we are now just looking for the low-hanging fruit, a day may come when we can climb the tree of life and find out which exotic fruit grows farthest from the earth.
Further reading: University of Cambridge, The Astrophysical Journal