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EI2GYB > ASTRO 27.08.21 10:20l 143 Lines 7038 Bytes #999 (0) @ WW
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Subj: New class of habitable exoplanets
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Sent: 210827/0919Z @:EI2GYB.DGL.IRL.EURO #:13637 BPQ6.0.22
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New class of habitable exoplanets represent a
big step forward in the search for life
A new class of exoplanet very different to our own, but which could support
life, has been identified by astronomers, which could greatly accelerate
the search for life outside our Solar System.
In the search for life elsewhere, astronomers have mostly looked for
planets of a similar size, mass, temperature and atmospheric
composition to Earth. However, astronomers from the University of C
ambridge believe there are more promising possibilities out there.
The researchers have identified a new class of habitable planets,
dubbed 'Hycean' planets-hot, ocean-covered planets with hydrogen-rich a
tmospheres-which are more numerous and observable than Earth-like planets.
The researchers say the results, reported in The Astrophysical J
ournal, could mean that finding biosignatures of life outside our
Solar System within the next two or three years is a real possibility.
"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.
Many of the prime Hycean candidates identified by the researchers are
bigger and hotter than Earth, but still have the characteristics
to host large oceans that could support microbial life similar to that
found in some of Earth's most extreme aquatic environments.
These planets also allow for a far wider habitable zone, or
'Goldilocks zone', compared to Earth-like planets.
This means that they could still support life even though they lie
outside the range where a planet similar to Earth would need to be
in order to be habitable.
Thousands of planets outside our Solar System have been discovered
since the first exoplanet was identified nearly 30 years ago.
The vast majority are planets between the sizes of Earth and Neptune
and are often referred to as 'super-Earths' or 'mini-Neptunes':
they can be predominantly rocky or ice giants with hydrogen-rich
atmospheres, or something in between.
Most mini-Neptunes are over 1.6 times the size of Earth: smaller
than Neptune but too big to have rocky interiors like Earth.
Earlier studies of such planets have found that the pressure and
temperature beneath their hydrogen-rich atmospheres would be too
high to support life.
However, a recent study on the mini-Neptune K2-18b by Madhusudhan's
team found that in certain conditions these planets could support life.
The result led to a detailed investigation into the full range of
planetary and stellar properties for which these conditions are possible,
which known exoplanets may satisfy those conditions, and whether
their biosignatures may be observable.
The investigation led the researchers to identify a new class of
planets, Hycean planets, with massive planet-wide oceans beneath
hydrogen-rich atmospheres.
Hycean planets can be up to 2.6 times larger than Earth and have
atmospheric temperatures up to nearly 200 degrees Celsius, but their
oceanic conditions could be similar to those conducive for microbial
life in Earth's oceans.
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 receive little radiation from
their stars.
Planets of this size dominate the known exoplanet population, although
they have not been studied in nearly as much detail as super-Earths.
Hycean worlds are likely quite common, meaning that the most promising
places to look for life elsewhere in the Galaxy may have been hiding
in plain sight.
However, size alone is not enough to confirm whether a planet is
Hycean: other aspects such as mass, temperature and atmospheric
properties are required for confirmation.
When trying to determine what the conditions are like on a planet
many light years away, astronomers first need to determine whether
the planet lies in the habitable zone of its star, and then look for
molecular signatures to infer the planet's atmospheric and internal
structure, which govern the surface conditions, presence of
oceans and potential for life.
Astronomers also look for certain biosignatures which could indicate
the possibility of life.
Most often, these are oxygen, ozone, methane and nitrous oxide,
which are all present on Earth. There are also a number of other
biomarkers, such as methyl chloride and dimethyl sulphide, that are
less abundant on Earth but can be promising indicators of life on
planets with hydrogen-rich atmospheres where oxygen or ozone may not
be as abundant.
"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," said Madhusudhan.
"But we think Hycean planets offer a better chance of finding
several trace biosignatures."
"It's exciting that habitable conditions could exist on planets so
different from Earth," said co-author Anjali Piette, also from Cambridge.
Madhusudhan and his team found that a number of trace terrestrial
biomarkers expected to be present in Hycean atmospheres would be readily
detectable with spectroscopic observations in the near future.
The larger sizes, higher temperatures and hydrogen-rich atmospheres
of Hycean planets make their atmospheric signatures much more d
etectable than Earth-like planets.
The Cambridge team identified a sizeable sample of potential Hycean
worlds which are prime candidates for detailed study with next-generation
telescopes, such as the James Webb Space Telescope (JWST), which is
due to be launched later this year.
These planets all orbit red dwarf stars between 35-150 light years
away: close by astronomical standards.
Planned JWST observations of the most promising candidate, K2-18b,
could lead to the detection of one or more biosignature molecules.
"A biosignature detection would transform our understanding of life in
the universe," said Madhusudhan.
"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."
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