Ocean covered exoplanets that are up to 2.6 times bigger than Earth should have microbial life which the James Webb Space Telescope should detect. The James Webb should launch this year. 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.
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. Already planned JWST observations of the most promising candidate, K2-18b, could lead to the detection of one or more biosignature molecules.
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.
Cambridge researchers have identified a new class of habitable planets, dubbed ‘Hycean’ planets – ocean-covered planets with hydrogen-rich atmospheres – which are more numerous and observable than Earth-like planets.
The researchers say the results, reported in The Astrophysical Journal, could mean that finding biosignatures of life outside our Solar System within the next few 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.
Astronomers say a new type of #exoplanet, dubbed 'Hycean' planets, could be our best chance of finding the signatures of life outside our Solar System, potentially in the next few years: https://t.co/Qdu6yyiENj@cambridge_astro @CambridgeIPLU @exomadhu #space
— Cambridge University (@Cambridge_Uni) August 26, 2021
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, depending on their host stars, 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.
We investigate a new class of habitable planets composed of water-rich interiors with massive oceans underlying H2-rich atmospheres, referred to here as Hycean worlds. With densities between those of rocky super-Earths and more extended mini-Neptunes, Hycean planets can be optimal candidates in the search for exoplanetary habitability and may be abundant in the exoplanet population. We investigate the bulk properties (masses, radii, and temperatures), potential for habitability, and observable biosignatures of Hycean planets. We show that Hycean planets can be significantly larger compared to previous considerations for habitable planets, with radii as large as 2.6 R⊕ (2.3 R⊕) for a mass of 10 M⊕ (5 M⊕). We construct the Hycean habitable zone (HZ), considering stellar hosts from late M to Sun-like stars, and find it to be significantly wider than the terrestrial-like HZ. While the inner boundary of the Hycean HZ corresponds to equilibrium temperatures as high as ∼500 K for late M dwarfs, the outer boundary is unrestricted to arbitrarily large orbital separations. Our investigations include tidally locked “Dark Hycean” worlds that permit habitable conditions only on their permanent nightsides and “Cold Hycean” worlds that see negligible irradiation. Finally, we investigate the observability of possible biosignatures in Hycean atmospheres. We find that a number of trace terrestrial biomarkers that may be expected to be present in Hycean atmospheres would be readily detectable using modest observing time with the James Webb Space Telescope (JWST). We identify a sizable sample of nearby potential Hycean planets that can be ideal targets for such observations in search of exoplanetary biosignatures.
SOURCES-Astrophysical Journal, Cambridge University
Written by Brian Wang, Nextbigfuture.com
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