'Hycean' exoplanets may not be able to support life after all

Sep 11, 2022
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Yet another nail in the coffin of the hypothesis that life must be abundant in the universe (and that to think otherwise is "parochial" or "arrogant"). No, dear Trekkies, it is not. A series of extreeeemely improbable accidents had to occur for this planet to be hosting complex, multicellular life for billions of years un-interrupted. The odds against it happening were literally astronomical, the odds against a Starship being built by a pink or green-skinned Elon Musk anywhere are super-exponentially more astronomical. The trillion-year-old kid setting up this experiment for their Junior Science Fair designed it that way, or that is just the way it fell out as a consequence from unfathomable (to us) objectives. Whether or not a spacefaring civilization exists in some distant galaxy is immaterial. We will never meet or interact with them.*

*But if we do, I say nuke them from orbit. It's the only way to be sure.
"Although no Hycean worlds have been confirmed to exist, the massive exoplanet survey by NASA's Kepler mission identified several candidate worlds that, based on estimates of their size and density, might be Hycean planets."

Good to have models, better when observations and measurement using nature show the models are true. Exoplanet studies are moving fast and the very definition of a planet's habitable zone is changing too.

WHERE TO LOOK FOR LIFE: HOMING IN ON THE HABITABLE ZONE, https://skyandtelescope.org/astronomy-news/where-to-look-for-life-homing-in-on-the-habitable-zone/

"The so-called habitable zone is the region around a star where, if a planet had an atmosphere, its surface temperature would be just right for liquid water to exist. Yet even though the habitable zone is where many astronomers focus their efforts, its very definition is still being refined. In a study to appear in Astrophysical Journal Letters (preprint available here), Cassandra Hall (University of Georgia) and colleagues identify in what part of the habitable zone (HZ) photosynthesis can occur. This region is the most likely to host planets capable of producing the key signs of life known as biosignatures, Hall and colleagues write, which will aid in efforts to home in on habitability. A HABITABLE ZONE FOR PHOTOSYNTHESIS All life as we know it depends on photosynthesis, directly or indirectly..."

ref - A New Definition of Exoplanet Habitability: Introducing the Photosynthetic Habitable Zone, https://arxiv.org/abs/2301.13836, 12-April-2023.

From the conclusion of the paper. "5. CONCLUSION We have demonstrated the existence of a photosynthetic habitable zone (PHZ). It is the distance from the host star where the habitable zone overlaps with where photosynthesis is possible...The PHZ becomes smaller with increasing atmospheric attenuation (i.e., more dense atmospheres), and so may make life less likely on super-Earths, since their larger gravitational field can hold onto more atmosphere. The PHZ also becomes smaller as the conditions for life become less favourable, which we describe as respiration rate relative to maximum possible photosynthetic rate, increasing. We therefore conclude that the parameter space for signs of life is far narrower than the standard HZ...We identify five planets, Kepler-452 b, Kepler-1638 b, Kepler-1544 b and Kepler-62 e and Kepler-62 f, that are consistently in the PHZ in a variety of environments. For Kepler-452 b, we calculate
that it should have a rotation period of 11 hours. The other four planets are estimated to have rotation periods between 9 and 11 hours. We suggest the search for signs of life elsewhere in the Universe should begin in earnest on the candidate planets we have identified."
My note, I calculated 1.65 km/s rotation rate for Kepler-452 b using the properties shown here, http://exoplanet.eu/catalog/kepler-452_b/, and the conclusion of the PDF report; calculated temperature 265 K, 273 K is freezing of water. Mass is not shown and atmosphere composition is not known too. The radius for Kepler-452 b is 1.63 earth radii. Using 3.8 g cm^-3 mean density indicates the exoplanet could be at least 3 earth masses and surface gravity stronger than Earth too, 1.1289E+00 with possible escape velocity 15.1679878 km/s. When we encounter various unknown properties for exoplanets that may look good for biological life to exist there, I always double check a few parameters :)
Nov 22, 2019
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Thanks for that great article, Dr. Sutter. I enjoyed that and learned something at the same time. The research that you reported on goes beyond the usual simplistic understanding of habitable zones and helps clarify our ideas on the habitability of Super Earths, which would now seem to be quite a bit rarer than previously thought.

We seem to be getting ever more evidence that seems to confirm the hypothesis that stars like our Sun and planets like our Earth are truly the best places for complex life to thrive.

While it's true that many icy moons and planets could have complex life swimming in oceans buried under icy crusts that are many miles thick, how would such life ever be able to gaze up at the stars and learn of the vast universe beyond their home world, let alone have an environment like an oxygen atmosphere that allows fire, metallurgy, and the building of telescopes and spaceships? They will likely never venture among the stars, and never send signals to or receive signals from other civilizations. SETI programs like ours will likely never find them. Surface-dwelling complex life is probably quite rare.
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