Ocean Worlds With Hydrogen-Rich Atmospheres Could be the Perfect Spots for Life

In the last decade, extrasolar planet searches have grown exponentially. There have been 4,514 confirmed exoplanets in 346 planetary systems. Another 7,721 candidates are still being confirmed. Astrobiologists are currently focusing on finding exoplanets with similar size, mass and atmospheric composition to Earth. It is Earth-like.
Astrobiologists are interested in finding exotic life. Hycean planets. These discoveries could have important implications for exoplanet research and the field of Astrobiology.

Dr. Nikku Madhusudhan (a Reader in Astrophysics & Exoplanetary Science at the University of Cambridge's Institute of Astronomy, IoA) led the research. Anjali Piette, a Ph.D. student in Astrophysics (Dr. Madhusudhan was her supervisor), and Dr. Savvas Constantinou, a fellow IoA member, joined him. The Astrophysical Journal published the study they did, Habitability and Biosignatures of Hycean Worlds.

Artist's conception of the commonity of exoplanets in the Milky Way Galaxy. Image credit: Wikipedia

Little Ice Giants: What is it like to live?

The vast majority of exoplanets discovered over the past 30 year have either been rocky planets many times larger than Earth (Super-Earths), or ice giants with hydrogen rich atmospheres (mini Neptunes). Super-Earths make up about 30% (1.383) of all exoplanets found to date. Mini-Neptunes, however, account for 34% (1.531).

Mini-Neptunes can be between 1.7 and 3.9 times larger than Earth. They are thought to have interiors made of ice, rocks, and oceans of volatile element. Studies of these planets have shown that their atmospheres are too hot and cold to sustain life. In a previous study Nikku Madhusudhan's team discovered that these planets can support life under certain conditions.

They focused on the exoplanet K2-18b. This was a mini-Neptune, which attracted a lot of attention in 2019, when two teams reported finding water vapor in its hydrogen rich atmosphere. These results led Dr. Madhusudhan's team to explore the entire range of stellar and planetary properties that could make mini-Neptunes habitable.

They were able to identify a new group of planets they called Hycean. This is a portmanteau of hydrogen and ocean. However, Hycean's atmospheres would be dominated by hydrogen, consistent with the gas giants. This atmospheric hydrogen would create a greenhouse effect that would keep the oceans liquid.

Artists impression of K2-18b Credit: Amanda Smith


These planets would have a diameter of about 2.6 times Earth's and an atmospheric temperature of up to 200C (382F) depending on their star and their proximity to it. This is similar to the conditions scientists believe Earth was like billions of years ago when single-celled microbial species first emerged.

These results, Dr. Madhusudhan stated in a University of Cambridge news release, could indicate that exoplanets are more abundant than Earth-like ones and support life. This would make it easier for astrobiologists find them.

The Hycean planets offer a new way to search for life in other places. We have focused on Earth-like planets when searching for molecular signatures. This is a good place to start. We believe Hycean planets have a greater chance of discovering trace biosignatures.

The team also identified prime Hycean candidates to be followed-up by their study. These are much larger and more hot than Earth, but they can still be covered by large oceans that provide the conditions necessary for microbial life. These life forms would be found in extreme environments like hydrothermal at ocean-mantle border, which is similar to what we have seen here on Earth.

This exoplanet could also include a subclass tidally locked dark Hycean planets. These planets are only habitable on their permanent night side. The planets host star's side would be too hot for water to remain in liquid form. It could transfer heat from the dark side via oceanic or atmospheric convection. Also, there is the possibility that cold Hycean planets could exist. These worlds receive very little radiation from their stars but have icy shells.

Artist's impression of an eyeball world, a planet where the sun can keep a liquid-water ocean. Copyright and Credit: DeviantArt/ eburacum45

Astrobiology: Implications

These planets are the most common of all known exoplanet populations, although they haven't been explored in as great detail as super-Earths. Their commonality means that many of the best places to search for life in the Galaxy might be hidden from view. These planets also allow for a wider circumsolar habitable area than Earth-like planets.

It is not enough to determine the size of these exoplanets from the statistically significant Super-Earth or mini-Neptune population. Before a candidate can confidently be classified as Hycean, they must be assessed on other aspects, such as temperature, mass, and atmospheric properties. Astronomers must take a close look at potential Hyceans in order to determine if they have biosignatures.

Anjali Piette said that it is exciting to think that there could be habitable conditions on planets different than Earth. It is also exciting to note that potential Hycean biosignatures identified by Dr. Madhusudhan's team will be easier to detect with spectroscopic observations. Because of their larger size, higher temperatures and more hydrogen-rich atmospheres, any atmospheric signatures will be easier to detect than with Earth-like planets. Said Madhusudhan:

The discovery of biosignature would revolutionize our understanding of the universe. As nature continues to amaze us, we need to be honest about where life might be found and in what form.

Another possible biosignatures are organic compounds such as methyl chloride or dimethyl sulfide. These organic compounds, although less common on Earth, could indicate life on other planets with high levels of oxygen and hydrogen. This is in line with the low-hanging fruit approach where we look for biomarkers that are required or produced by living things as we know them.

The large collection of potential Hycean planets that the Cambridge team has prepared will provide excellent opportunities for further observations when next-generation telescopes are available. These planets orbit M-type (red dwarf), stars 35 to 150 light years from the Solar System. There are already plans to study K2-18b, the most promising candidate, using the next-generation James Webb Space Telescope.

The JWST will launch in November or early December of this year. Astronomers will now be able conduct Direct Imaging studies on exoplanets using the near- and mid-infrared wavelengths. They can also obtain spectra from their atmospheres. In 2025, the Nancy Grace Roman Space Telescope will be followed by direct imaging studies. It will use its advanced system, optics, coronographs and spectrometers.

These results show how exoplanet science has evolved and changed over the past few years. The process of exoplanet discovery and characterization has improved with the availability of thousands upon thousands of confirmed exoplanets. Astronomers have the ability to place greater constraints on how the planetary environments will look with improved instruments and methods.

Scientists can also test their theories on the conditions in which life might exist in the cosmos, all while this is happening. The ultimate goal is to discover life in all its glory and diversity. Although we are confined to the low-hanging fruits right now, we may one day be able climb the tree to find the exotic fruit that grows farthest from earth.

Further Reading: University of Cambridge. The Astrophysical Journal