17 October 2021

University of Cambridge: “New class of habitable exoplanets ‘a big step forward’ in search for life”

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.

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.

Nikku Madhusudhan, Anjali A. A. Piette & Savvas Constantinou

Exciting to think that scientists could detect potential signs of life on one of these planets over the course of the next decade! Exciting with a touch of sadness, as humans will never (at least not in my lifetime) be able to visit these planets and examine their strange life forms up close. At least I can console myself with science-fiction novels, where such worlds were imagined already – the closest analog that comes to mind is Poseidon in Alastair Reynolds’ Poseidon’s Wake.

The Hycean Habitable Zone
Figure 3: The Hycean Habitable Zone. Cyan, dark-red and purple regions show the HZs for regular, Dark (nightside), and Cold (nonirradiated) Hycean planets, respectively. The terrestrial HZ from the literature is shown in teal (Kopparapu et al. 2013). Black circles denote known planets with Rp < 3 R, Mp < 10 M, Teq < 600 K and whose host stars have J-band magnitudes below 13. We additionally circle the planets that are presented in Table 1 as promising Hycean candidates.

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