New class of habitable exoplanets represent a big step forward in the search for life

Astronomers have discovered a new class, Hycean planets hot and ocean-covered planets with hydrogen rich atmospheres. This could be a significant step forward in our search for life elsewhere. Credit: Amanda Smith, University of Cambridge
Astronomers have identified a new class of exoplanets that are very different from our own but could support life. This could significantly accelerate the search for life beyond our Solar System.

Astronomers have mainly focused their search for life on other planets that are similar in size, mass and temperature to Earth. Astronomers at the University of Cambridge think there may be more exciting possibilities.

Researchers have discovered a new class in habitable planets. They are called 'Hycean', or ocean-covered, planets with hydrogen-rich atmospheres. These planets are more common and easier to observe than Earth-like ones.

According to the researchers, the results of the Astrophysical Journal could indicate that biosignatures of life beyond our Solar System are possible within the next two-three years.

"Hycean Planets open a whole new avenue for our search for life elsewhere," Dr. Nikku Madhusudhan, from Cambridge's Institute of Astronomy who led the research.

Researchers identified many prime Hycean candidates that are larger and more hotter than Earth but still have the characteristics to support large oceans that could sustain microbial life that is similar to what can be found in extreme aquatic environments on Earth.

These planets have a much wider habitable zone (or 'Goldilocks Zone') than Earth-like planets. They could support life even though they are outside of the range that a planet like Earth would need in order to become habitable.

Since the discovery of the first exoplanet nearly 30 years ago, thousands of planets have been found outside our Solar System. Most of these planets are between Earth and Neptune in size. They are sometimes called'super-Earths’ or'mini Neptunes'. These planets can be primarily rocky, ice giants with hydrogen rich atmospheres or anything in between.

Mini-Neptunes can be more than 1.6 times larger than Earth. They are smaller than Neptune, but much larger enough to support rocky interiors similar to Earth. Studies of these planets previously found that their atmospheres are too hot and cold to sustain life.

Madhusudhan's group recently conducted a study on the mini-Neptune K2-18b and found that these planets can support life under certain conditions. This result led to an in-depth investigation of the range of stellar and planetary properties that are possible. It also revealed which exoplanets might be capable of meeting those conditions and whether biosignatures could be observed.

Researchers discovered a new type of planet, Hycean planets. They have massive oceans that extend across the entire planet and are surrounded by hydrogen-rich atmospheres. Hycean planets are up to 2.6 times larger that Earth, with atmospheric temperatures of nearly 200 degrees Celsius. However, their oceanic conditions may be similar to Earth's. These planets may also contain tidally locked, 'dark' Hycean planets that are only habitable on their permanent night sides and 'cold" Hycean globes that receive very little radiation from their stars.

These planets are the largest known exoplanet populations, but they haven't been as thoroughly studied as super-Earths. Hycean worlds are quite common and may indicate that there might be some hidden places in the Galaxy where life is possible.

But, just the size of a planet does not prove it to be Hycean. Other aspects, such as temperature, mass and atmospheric properties, are needed for confirmation.

Astronomers must first determine if the planet is in the habitable zone. Then, they need to look for molecular signatures that can infer the planet’s atmospheric and internal structures. These signatures govern the surface conditions, presence and potential for life.

Astronomers also search for biosignatures that could indicate the possibility or existence of life. These biosignatures include oxygen, methane, ozone and nitrous oxide. All of these elements are present on Earth. Other biomarkers such as dimethyl sulphide and methyl chloride are also available on Earth, but they can still be very useful indicators of life on other planets that have hydrogen-rich atmospheres.

Madhusudhan stated that when looking for molecular signatures, the focus was on planets like Earth. This is a good place to begin. We believe Hycean planets have a greater chance of finding trace biosignatures.

"It's thrilling that habitable conditions could exist upon planets so different than Earth," stated co-author Anjali Piete, also from Cambridge.

Madhusudhan's team discovered that many trace terrestrial biomarkers, which are expected to be found in Hycean atmospherics, could be easily detected with spectroscopic observations in near future. Their atmospheric signatures are easier to detect than Earth-like planets due to their larger sizes, higher temperatures, and more hydrogen-rich atmospheres.

The Cambridge team discovered a large number of potential Hycean planets that are prime candidates to be studied with next-generation telescopes like the James Webb Space Telescope, which will launch later in the year. These planets orbit red dwarf stars around 35-150 lightyears away, which is close to astronomical standards. K2-18b, the most promising candidate for detection, is being observed by JWST. This could allow for the discovery of biosignature molecules.

Madhusudhan stated that a biosignature detection could transform our understanding about life in the universe. We need to be honest about where we think life will come from and what it could look like. Nature continues to amaze us in surprising ways.

Additional information: Habitability of Hycean Worlds and Biosignatures, Astrophysical Journal (2021). Information about the journal: Astrophysical Journal Habitability & Biosignatures Of Hycean Worlds (2021). doi.org/10.3847/1538-4357/abfd9c

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