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Hycean planets: Cambridge astronomers say our best hope for finding extraterrestrial life may have been hiding in plain sight




The search for life outside our Solar System could be accelerated following the discovery of a new class of habitable exoplanet by University of Cambridge researchers.

Dubbed ‘Hycean planets’, these ocean-covered worlds have hydrogen-rich atmospheres and it is believed they could support microbial life similar to that found in some of Earth’s most extreme aquatic environments.

An artist's impression of a Hycean planet. Picture: Amanda Smith
An artist's impression of a Hycean planet. Picture: Amanda Smith

Thought to be more numerous and observable than Earth-like planets, the discovery means our best hope of finding extraterrestrial life may have been hiding in plain sight.

And it is hoped that the launch of the James Webb Space Telescope this year could greatly improve our chances of detecting tell-tale biosignatures on one in the coming years.

“Hycean planets open a whole new avenue in our search for life elsewhere,” said Dr Nikku Madhusudhan from Cambridge’s Institute of Astronomy, who led the research.

“A biosignature detection 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.”

Since the first planet outside our Solar System was identified in 1995 by Didier Queloz - now a University of Cambridge professor who shared a Nobel Prize for the feat - thousands more have been identified.

The vast majority lie between the size of Earth and Neptune and are, accordingly, known as super-Earths or mini-Neptunes. They may be predominantly rocky, or ice giants with hydrogen-rich environments, or lie in between.

In their search for extraterrestrial life, astronomers have primarily focused on exoplanets of a similar size, mass, temperature and atmospheric composition to Earth.

Many prime Hycean candidates identified by Cambridge researchers are bigger and hotter than Earth, but have a much wider ‘Goldilocks zone’ of habitability compared to Earth-like planets.

The Institute of Astronomy's Dr Nikku Madhusudhan, third right, receives the 2019 Pilkington Prize in recognition of innovative and passionate lectures and for going beyond the call for duty in helping students
The Institute of Astronomy's Dr Nikku Madhusudhan, third right, receives the 2019 Pilkington Prize in recognition of innovative and passionate lectures and for going beyond the call for duty in helping students

Their discovery came when Dr Madhusudhan’s team at the Institute of Astronomy studied a mini-Neptune called K2-18b.

These types of planet are typically more than 1.6 times the size of our own, meaning they are smaller than Neptune but too large to have rocky interiors like Earth.

Earlier studies suggested the pressure and temperature beneath their hydrogen-rich atmospheres would be too high to support life.

But the Cambridge researchers found that in certain conditions these planets could support life, so began a detailed investigation of the full range of planetary and stellar properties that made these conditions possible. They sought to discover which known exoplanets may satisfy such conditions and whether their biosignatures would be observable.

This led them to identify the Hycean planets, which 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.

However, their oceanic conditions could be similar to those that support microbial life in Earth’s oceans.

Bizarrely, such planets also include tidally-locked ‘dark’ Hycean worlds that may have habitable conditions only on their permanent night sides, and ‘cold’ Hycean worlds that are receive little radiation from their stars.

It is believed such worlds are common, since planets of this size dominate the known exoplanet population.

But to date they have not been studied in nearly as much detail as super-Earths.

“It’s exciting that habitable conditions could exist on planets so different from Earth,” said Anjali Piette, also from Cambridge, who was a co-author of the study published in The Astrophysical Journal.

To confirm a planet is Hycean, astronomers need to know not only its size, but features such as mass, temperature and atmospheric properties.

To determine what conditions are like on distant exoplanets, they first must determine whether it lies within the habitable zone of its star, then look for molecular signatures that indicate its atmospheric and internal structure, which give rise to the surface conditions and govern whether oceans are present.

The presence of oxygen, ozone, methane and nitrous oxide, which are all present on Earth, are biosignatures of potential life.

Other biomarkers such as methyl chloride and dimethyl sulphide that are less abundant on Earth can still be promising indicators of life on planets with hydrogen-rich atmospheres, where oxygen or ozone may not be as abundant.

Dr Madhusudhan explained: “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. But we think Hycean planets offer a better chance of finding several trace biosignatures.”

Spectroscopic observations of several such trace terrestrial biomarkers expected to be present in Hycean atmospheres would be readily detectable in the near future, the researchers say.

In fact, thanks to their larger sizes, higher temperatures and abundance of hydrogen, the atmospheric signatures of Hycean planets should prove to much more detectable than Earth-like planets.

Already, the Cambridge astronomers have found a sizeable number of potential Hycean worlds orbiting red dwarf stars between 35-150 light years away, which is relatively close. These are prime candidates for detailed study with the James Webb Space Telescope (JWST) and other next-generation telescopes.

The most promising candidate, K2-18b, is already due to be studied by JWST, the testing of which was completed towards the end of August, meaning it will soon be on its way to the launch site.

That could yet lead to the detection of biosignature molecules that would suggest we are not alone in this universe.

Read more

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