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EI2GYB > ASTRO 02.09.21 13:05l 102 Lines 5004 Bytes #999 (0) @ WW
BID : 14018_EI2GYB
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Subj: Astronomers identify a new class of habitable planet
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Sent: 210902/1202Z @:EI2GYB.DGL.IRL.EURO #:14018 BPQ6.0.22
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Astronomers identify a new class of habitable planet
These giant, ocean worlds are likely to be significantly more abundant than
Earth-like exoplanets.
In the search for potentially habitable worlds, astronomers have focused on
Earth-like planets orbiting Sun-like stars.
These rocky worlds have a similar radius and mass to Earth and must be warm
enough to allow liquid water to exist on the surface.
But although astronomers have found a number of promising candidates-for
example Trappist-1d or Proxima Centauri b, which both orbit nearby stars--
the necessary conditions are stringent and this significantly limits their
number.
Now Nikku Madhusudhan and colleagues at the University of Cambridge in the U.K.
have identified an entirely new class of habitable planets that are likely to
be considerably more abundant than Earth-like planets, and much easier to study.
These new candidates are ocean worlds significantly larger than Earth but
smaller than Neptune and enveloped in a hydrogen-rich atmosphere.
Crucially, they must have a temperature at their surface that allows liquid
water to flow, so in this respect they are like Earth.
Hycean worlds
Madhusudhan and colleagues call these worlds "Hycean," a portmanteau from the
words hydrogen and ocean. They say these worlds can have a radius up to 2.6
that of Earth's and be up to ten times as massive.
That's significantly larger than any planets previously considered habitable.
The researchers also study the Hycean habitable zone-the range of distances
from the parent star that allow for liquid water on the surface.
And tt turns out that the Hycean habitable zone is significantly bigger than
for terrestrial-like planets.
That's partly because the equilibrium temperature on these planets can be as
high as 440 degrees Fahrenheit (227 degrees Celsius), much higher than for
terrestrial planets. Under these conditions, liquid water could exist if the
planet were tidally locked with the same side always facing the sun.
In that case, one half of the planet would be too hot to support life while
the other could be temperate.
Another reason for the much broader zone is that "cold" Hycean worlds could
support liquid water by virtue of their internal heating.
Such planets would require little or no radiation from the parent star and so
could orbit at vast distances.
"Our results show that the [habitable zone] for Hycean planets is considerably
wider than the terrestrial [habitable zone]," they say.
Alien biosignature
Should these planets support life, an important question is whether they would
display measurable biosignatures.
Madhusudhan and colleagues investigate this question in detail and say that
the main biosignatures on Earth, such as oxygen, ozone and methane, are
likely to be produced on Hycean worlds by processes unrelated to life.
So they cannot be considered biosignatures on these worlds.
However, metabolic processes on Earth produce a number of secondary compounds
that are likely to be more easily detected in hydrogen rich atmospheres.
These include dimethysulfide (DMS), dimethyldisulfide (DMDS), methanethiol
(CH3SH), and carbonylsulfide (OCS).
What's more, because Hycean worlds are larger than terrestrial-like planets,
they should be easier to study.
Astronomers have already spotted a number of candidates, such as K2-18 b,
an exoworld with a mass about eight times that of Earth which orbits a red
dwarf some 124 light-years away. This planet has already been earmarked for
study by the James Webb Space Telescope, Hubble's replacement which is due
for launch later this year.
Madhusudhan and colleagues say that this investigation is promising.
"We predict that the approved Cycle 1 JWST observations of K2-18 b, a
candidate Hycean planet, will be able to detect these biomarkers if present
at the quantities considered in this work," they say.
So if K2-18 b and other Hycean worlds display biosignatures, we may see the
first signs of life elsewhere in the Universe in the next few years.
Fingers crossed for an uneventful launch of James Webb Space Telescope in
the coming months.
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