A recent burst of blogs and media reports excitedly announced the discovery of molecules in the atmosphere of exoplanet K2-18b that could have been produced, they say, only by living organisms.1 The two “biosignature” molecules, dimethyl sulfide and chloromethane, were detected by two infrared spectrometers (NIRS and NIRISS) on board the James Webb Space Telescope (JWST). Although speculation ran wild, evidence indicates a lack of credibility.

Characteristics of K2-18b
K2-18b is a sub-Neptune planet (one with a radius 1.4–3.0 times Earth’s radius and a mass greater than Earth’s but less than Neptune’s) orbiting an M3 red dwarf star 124 light-years away. K2-18b orbits its star every 32.9 days at a distance of just 40% of Mercury’s orbital distance from the Sun. However, because this exoplanet’s host star is so much smaller and colder than the Sun, it’s possible that K2-18b’s surface temperature permits the existence of liquid water there. 

K2-18b is 8.6 times more massive than Earth with a radius 2.6 times larger than Earth’s. Thus, the planet’s density amounts to only 2.67 grams per cubic centimeter. By comparison, Earth’s density is 5.5134 grams per cubic centimeter.

The proximity of K2-18b to its host star means that it’s either tidally locked (with one hemisphere always facing the host star) or, like Mercury, it orbits in a spin-orbit resonance (rotating on its axis 2 or 3 times for every 1 or 2 orbital revolutions).2 In either case, one hemisphere of K2-18b faces its host star for at least 16 Earth days at a time. 

What Level of Certainty?
Through the use of JWST’s spectrometers, a team of six astronomers led by Nikku Madhusudhan positively detected methane in K2-18b’s atmosphere.3 The methane signal was at the 5-sigma level (five standard deviations, or about five times greater than the background noise). In other words, the level of certainty that methane exists in the planet’s atmosphere is 99.99997%. In the physical science literature, a detection is recognized as valid only if the signal is at least 5 sigma, or 5 times above, the background noise.

Madhusudhan’s team also detected carbon dioxide in K2-18b’s atmosphere, but only at the 3-sigma level. Their search for water (H20), ammonia (NH3), carbon monoxide (CO), and hydrogen cyanide (HCN) revealed no evidence for the presence of any of these molecules. However, they were able to establish upper limits for the abundance of water, ammonia, carbon monoxide, and hydrogen cyanide, and they were low.

What generated all the excitement was the team’s “potential” detection of dimethyl sulfide (CH3SCH3) and chloromethane (CH3Cl) (see figure 1). In reality, the detections were marginal, at best. For dimethyl sulfide, the detection was only at the 1-sigma level, and for chloromethane it was even lower.

 

 

Figure 1: Spacefill Models of Dimethyl Sulfide (top) and Chloromethane (bottom)
Credit: public domain

The community of astronomical researchers remains highly skeptical of the Madhusudhan team’s “detection,” especially that of dimethyl sulfide. They’re reminded of the sensational claims in 2020 that phosphine (PH3) had been found in the upper atmosphere of Venus. That announcement generated more than 4,700 news stories around the world. Then, within just a few months, the claim had to be withdrawn in light of follow-up observations that failed to detect any phosphine at all.4 To their credit, Madhusudhan and his colleagues referred to their discovery as “tentative” and recommended more sensitive spectral measurements of K2-18b’s atmosphere.

Definitive Biosignatures?
Dimethyl sulfide is the simplest thioether. It is produced naturally on Earth by certain marine phytoplankton, algae, and corals. These marine organisms release about 10 million tons of dimethyl sulfide into Earth’s atmosphere annually. Industrial activity and chemistry occurring at the interface between the atmosphere and the ocean5 produce additional amounts of this molecule, but at trivial levels by comparison with the amount generated by marine organisms. Thus, astronomers figure that the detection of a large quantity of dimethyl sulfide in a planet’s atmosphere would be a reliable indicator of the likelihood that marine life exists on that planet.6   

On Earth, the quantity of dimethyl sulfide in the atmosphere is a benefit to advanced life. Sunlight in the atmosphere causes it to decompose into sulfur aerosols that serve as cloud condensation nuclei, the quantity and distribution of which determine the rate and location of rainfall and snowfall. Sulfur aerosols in Earth’s atmosphere also regulate Earth’s radiation balance.

Although most of the chloromethane in Earth’s atmosphere comes from organisms, this molecule does not serve as a reliable biomarker for life in other locations. Astronomers have detected chloromethane in the coma (surrounding cloud) of comet 67P/Churyumov-Gerasimenko and in the gas around the protostar IRAS 16293–2422, where physical and chemical conditions rule out the possibility of life.7 They have also demonstrated that there are abiotic chemical pathways for the production of chloromethane.

It has become increasingly clear to astronomers that they cannot rely on a single biosignature as proof of life’s presence. If life truly exists on an exoplanet, its presence will yield multiple biosignatures all at strong levels.

Is K2-18b a Viable Candidate to Host Life?
Several factors raise skepticism about K2-18b as a candidate for life. Previously posted articles explain why the characteristics of M-dwarf stars would rule out the possibility of life’s existence on any planet in their orbit.8

The Mudhusudhan team based their speculation about possible life on K2-18b on the hypothesis that a thin, hydrogen-dominated atmosphere overlays a thick ocean of liquid water. Such a thin atmosphere would mean that the atmospheric greenhouse effect would be too weak to turn any of K2-18b’s liquid water into steam. It also might explain why the team detected no water in the planet’s atmosphere. If K2-18b is, indeed, covered by a very thick ocean of liquid water several thousand kilometers deep, that could well account for the planet’s low bulk density. According to the Mudhusudhan team’s hypothesized scenario, K2-18b could be termed a “Hycean” planet—representing a whole new category of exoplanets claimed as prospective candidates to support life. (The word Hycean is a blend of hydrogen and ocean).

The determination that K2-18b has a thin, rather than a thick, hydrogen-dominated atmosphere is based on detection of an abundance of both methane and carbon dioxide in the exoplanet’s atmosphere, and an undetectably low (or nonexistent) quantity of ammonia and carbon monoxide. The team’s spectral observations established only that methane is abundant in K2-18b’s atmosphere. 

Given that water is the third most abundant molecule in the universe (after H2 and H3), I would agree that K2-18b could be a Hycean planet. However, to claim Hycean planets as candidates for life support is to ignore some significant realities. The greatest problem with the “oceanworld” model is this: the pressure at the bottom of an ocean as deep as several hundred kilometers or more is great enough to turn that liquid water into ice.9  

Figure 2 illustrates a cross section of the K2-18b Hycean model. Note that an ice barrier separates the planet’s liquid water from its rocky material. This barrier, which would be permanent, rules out the operation of chemical processes necessary for both the origin of life and the survival of life. An additional challenge emerges from research on ocean acidification. The team of Amit Levi and Dimitar Sasselov has demonstrated that the pH of such an ocean would range from 2 to 4, in other words, from the acidity of pure lemon juice to the acidity of pure tomato juice.10 Earth’s ocean water, by contrast, is slightly basic, with a pH of 7.5–8.4.

 

Figure 2: Hycean Planet (Waterworld) Cross Section
Credit: Hugh Ross    

One must also note the fact that K2-18b orbits so closely to its host star that its rotation period roughly equals its revolutionary period, equivalent to at least 16 Earth days. With such a lengthy rotation period, one hemisphere of K2-18b will be blisteringly hot while the opposite hemisphere remains frigidly cold. The day-night temperature difference would be several hundred degrees Celsius. Such a huge difference rules out the survival of any conceivable physical life form.

Life As We Know It?
Of course, I’m assuming, here, that extraterrestrial life must be like life as we know it on Earth. Some people assert that a consideration of life as we don’t know it could yield much more optimistic estimates for the probability of extraterrestrial life. However, my only assumption is that extraterrestrial physical life must be carbon-based, and this assumption is based on well-established research.

Decades ago, biochemists considered the theoretical possibility of physical life-forms based on silicon, boron, or arsenic, for example, as substitutes for carbon. Their study led to the conclusion that carbon is the only element in the periodic table possessing the chemical bonding stability and the chemical bonding complexity that even the simplest conceivable physical life-form would require. Therefore, all life constrained by the physics and dimensions of the universe must be carbon-based.

Hycean Worlds as Viable Candidates for Life?
So far, astronomers have discovered and determined the characteristics of 5,504 planets orbiting stars other than the Sun.11 Catalogs of such planets reveal that sub-Neptune planets orbiting close to their host stars are relatively common. Such planets comprise about 5% of the 5,504 known and measured exoplanets.

A sizable fraction of sub-Neptune planets may, indeed, be Hycean worlds. The quantity of liquid water on these planets leads astronomers and laypeople, alike, to view them as promising candidates to host life. What dashes this hope, however, is the fact that too much liquid water on a planet renders the planet uninhabitable. In view of its mass and distance from the Sun, Earth is habitable because it is appropriately water-poor. Its abundance of water is at the just-right (relatively low) level to support a broad diversity and abundance of life.

Enthusiasm for the possibility of life on Hycean worlds is largely driven by an unwarranted focus on the liquid water habitable zone within a planet’s orbit around its host star. This particular zone is the broadest of the dozen or more known habitable zones within a planetary orbit.12 For a planet to accommodate life of any complexity for any length of time it must reside simultaneously in all these known habitable zones. None of the sub-Neptune planets discovered to date resides in as many of three of these planetary habitable zones simultaneously. Of 5,512 known planets, only Earth resides in more than three.

While I would not claim that the Bible rules out the possibility of God’s creating life on other planets,13 I can affirm that astronomers’ extensive observations of galaxies, stars, and planets indicate the unlikelihood that God did. As astrophysicist Neil de Grasse Tyson famously declared, once one ventures beyond Earth, “The universe is a deadly place.”14

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Endnotes

1. Some examples include Ariana Garcia, “NASA Scientists May Have Discovered Planet That Smells Like the Beach,” Microsoft News (September 22, 2023); Julia Robinson, “Explainer: Has Life Been Discovered on an Exoplanet?” Chemistry World (September 15, 2023); Jackie Appel, “We Just Found a Molecule on Another World . . . and Only Living Organisms Can Produce It,” Popular Mechanics (September 15, 2023).
2. Mark A. Wieczorek et al., “Mercury’s Spin-Orbit Resonance Explained by Initial Retrograde and Subsequent Synchronous Rotation,” Nature Geoscience 5 (January 2012): 18–21, doi:10.1038/ngeo1350.
3. Nikku Madhusudhan et al., “Carbon-Bearing Molecules in a Possible Hycean Atmosphere,” (September 11, 2023), arXiv:2309.05566v1. The paper has been accepted for publication in the Astrophysical Journal but the peer review process is still underway.
4. Hugh Ross, “Life Signature in Venus’s Atmosphere: Genuine or Not?” Today’s New Reason to Believe (blog), Reasons to Believe, March 29, 2021; I. A. G. Snellen et al., “Re-analysis of the 267 GHz ALMA Observations of Venus. No Statistically Significant Detection of Phosphine,” Astronomy and Astrophysics: Letters 644 (December 20, 2020): id. L2, doi:10.1051/0004-6361/202039717.
5. Gordon A. Novak and Timothy H. Bertram, “Reactive VOC Production from Photochemical and Heterogeneous Reactions Occurring at the Air-Ocean Interface,” Accounts of Chemical Research 53, no. 5 (May 5, 2020): 1014–1023, doi:10.1021/acs.accounts.0c00095.
6. Sara Seager, W. Bains, and R. Hu, “Biosignature Gases in H2-Dominated Atmospheres on Rocky Planets,” Astrophysical Journal 777, no. 2 (November 10, 2013): id. 95, doi:10.1088/0004-637X/777/2/95.
7. Edith C. Fayolle et al., “Protostellar and Cometary Detections of Organohalogens,” Nature Astronomy 1 (October 2017): 703–708,  doi:10.1038/s41550-017-0237-7.
8. Hugh Ross, “‘Electric Wind’ Becomes 9th Habitable Zone,” Today’s New Reason to Believe (blog), Reasons to Believe, July 4, 2016; Hugh Ross, “Overlap of Habitable Zones Gets Much Smaller,” Today’s New Reason to Believe (blog), Reasons to Believe, December 27, 2016; Hugh Ross, “Inhabitability of Planets Orbiting Red Dwarfs,” Today’s New Reason to Believe (blog), Reasons to Believe, July 30, 2017; Hugh Ross, “Tiny Habitable Zones for Complex Life,” Today’s New Reason to Believe (blog), Reasons to Believe, March 4, 2019; Hugh Ross, “Do Superhabitable Planets Exist?” Today’s New Reason to Believe (blog), Reasons to Believe, November 2, 2020; Hugh Ross, “Red Sky Paradox Points to Rarity of Earth’s Life,” Today’s New Reason to Believe (blog), Reasons to Believe, August 23, 2021.
9. A. Levi, D. Sasselov, and M. Podolak, “Structure and Dynamics of Cold Water Super-Earths: The Case of Occluded CH4 and Its Outgassing,” Astrophysical Journal 792, no. 2 (September 10, 2014): id. 125, doi:10.1088/0004-637X/792/2/125; A. Levi and D. Sasselov, “Partitioning of Atmospheric O2 into High-Pressure Ice in Ocean Worlds,” Astrophysical Journal 926, no. 1 (February 10, 2022): id. 72, doi:10.3847/1538-4357/ac4500.
10. A. Levi and D. Sasselov, “A New Desalination Pump Helps Define the pH of Ocean Worlds,” Astrophysical Journal 857, no. 1 (April 10, 2018): id. 65, doi:10.3847/1538-4357/aab715.
11. Exoplanet TEAM, “Extrasolar Planet Catalog,” The Extrasolar Planets Encyclopaedia, accessed September 21, 2023. 
12. Hugh Ross, Designed to the Core (Covina, CA: RTB Press, 2022), 133–148, 171–181.
13. Hugh Ross, “Does the Bible Say We’re Alone in the Universe?” Today’s New Reason to Believe (blog), Reasons to Believe, May 2, 2022.
14. “Neil deGrasse Tyson, (caught on camera): The Universe Is Trying to Kill You,” interview outtake, Big Think Mentor, June 27, 2013.