‘Mini-Neptunes’ beyond solar system may soon yield signs of life

Cambridge astronomers identify new hycean class of habitable exoplanets, which could accelerate search for life

Signs of life beyond our solar system may be detectable within two to three years, experts have said after rethinking the kinds of planets that may be habitable.

Researchers have mostly looked for planets of a similar size, mass, temperature and atmospheric composition to Earth. But University of Cambridge astronomers believe there may be more promising possibilities after recent work suggested that a “mini-Neptune” more than twice the radius of Earth and more than eight times as massive may also be habitable.

They have now identified a new class of habitable exoplanets, called hycean planets – hot, ocean-covered and with hydrogen-rich atmospheres – which are more numerous and observable than Earth-like planets. The mini-Neptune, known as K2-18b, is one and there could be many more in existence.

“Hyceans are basically water worlds with hydrogen-rich atmospheres,” said Dr Nikku Madhusudhan, the lead author of the research from the University of Cambridge. He said focusing on hyceans could accelerate efforts to find life beyond Earth.

“We are saying that within two to three years we may see the first biosignature detection if these planets host life,” he said, adding that the James Webb space telescope – due for launch in November – could aid the search.

Madhusudhan said the small size of Earth-like planets relative to sun-like stars gave rise to weak atmospheric signatures, making it difficult to detect signs of life.

However, hyceans can be more than twice the radius of Earth for a planetabout 10 times the mass, and significantly hotter, with average atmospheric temperatures reaching almost 200C.

Larger planets are not only more common than Earth-sized ones but easier to find, and it is possible to detect their atmospheres more readily, Madhusudhan said.

What’s more, the makeup of hycean planets means it is possible to look for a wider array of molecules that may hint at the presence of life.

Writing in the Astrophysical Journal, the team said hycean planets were defined as having a hydrogen-rich atmosphere, a rocky core that accounts for at least 10% of the mass of the planet, and a water layer accounting for up to 90% of the planet’s mass.

Life may even be possible on the shady side of planets the team have called “dark hyceans”. These sit close to their star and are tidally locked, meaning the same side of the planet is always in the light – and potentially too hot to be habitable.

The team said any life on hyceans would necessarily be aquatic as the planets would, by definition, be covered by a water layer. But what form such life may take is an open question, according to Madhusudhan.

“At the bare minimum, microbial life should be possible,” he said, adding that this was how life started on Earth.

Prof Beth Biller, who studies exoplanets at the Institute of Astronomy at Edinburgh University and was not involved in the research, welcomed the work. “There is such diversity among the exoplanets discovered to date and it really makes sense to study as well the non-Earth-like planets that might conceivably host life,” she said.

But Biller added that searching for biosignatures on hycean planets may yet prove difficult. “Searches for biosignatures for Earth twins are already very tricky – most rely on proving an atmosphere is not in equilibrium by detecting a combination of different biosignature gases that shouldn’t exist together in an equilibrium state. This is possible in principle for hycean planets as well, but obviously without Earth as an example it is more difficult in any particular case to rule out false positives.”

Biller said that for both types of planets, data would be needed from a significant number of bodies to understand which may be habitable.

“[The James Webb space telescope] will indeed provide some very interesting spectra of hycean planets in the next few years, but hard to say yet whether such spectra will be conclusive,” she said.


Nicola Davis Science correspondent

The GuardianTramp

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