When I was a child, my friends and I would sometimes lie on the grass and watch billowing clouds traverse the sky. As they traversed the sky and changed shape, we would call out to one another what kinds of animals, plants, toys, machines, or symbols the shapes reminded us of.
There is no doubt that Earth’s clouds are beautiful and a source of wonder and entertainment. But they are also much, much more. Our very existence depends on clouds and on our planet’s clouds being exquisitely fine-tuned.
For more than just a few extremophilic microbes to possibly inhabit a planet, the planet must possess an atmosphere. This requirement is not a severe limitation, however, as the vast majority of planets do possess atmospheres. Nonetheless, as a new research study demonstrates, the kind of atmosphere and the form that kind of atmosphere takes are critical factors for habitability.
The most numerous planets in the Extrasolar Planets Encyclopedia are gas giant planets, otherwise known as Jupiters, and ice giant planets, otherwise known as Neptunes. These planets are 100–6,000 and 15–100 times the mass of Earth, respectively, and are characterized by atmospheres that are thousands of miles thick. Atmospheres this thick will not permit the passage of the kind of light needed for photosynthesis.
Our planet detection methods, however, strongly favor the discovery of Jupiters and Neptunes. Planet types that may be just as numerous, or even more numerous, are super-Earths and mini-Neptunes. Super-Earths are planets that range in mass from 1.1 to 10 times the mass of Earth. Mini-Neptunes range in mass from 10 to 15 times the mass of Earth. Where measurements permit the possible detection of the planet’s atmosphere, every super-Earth and mini-Neptune so measured indeed possesses an atmosphere.
New measuring techniques not only establish that super-Earths and mini-Neptunes possess atmospheres but are now determining what kind of atmospheres they possess.1 Recent studies unequivocally show that clouds exist in the atmospheres of super-Earths and mini-Neptunes.
Transmission spectrum measurements by instruments aboard the Hubble Space Telescope reveal an optically thick high-altitude cloud layer in the 6.5-solar-mass planet GJ 1214b.2 The 7.9-solar-mass planet HD 97658b has a flat transmission spectrum indicative of a thick layer of clouds.3Likewise, transmission spectra of other super-Earths and mini-Neptunes all support the presence of pervasive clouds or heavy hazes. The only known super-Earths and mini-Neptunes that lack obscuring clouds are those planets orbiting so close to their host stars that the host stars’ intense heat and radiation have evaporated away the entirety of the planets’ atmospheres.
The ubiquitous presence of obscuring clouds on super-Earth and mini-Neptune planets means that creatures on the planets’ surfaces would be unable to ascertain the position of stars or the planets’ host stars in their skies. Unless a planet’s sky is transparent for at least a part of that planet’s year, animals will be unable to use the position of the host star and other stars to regulate their biological clocks. That incapacity rules out the possible existence of large animals on super-Earths and mini-Neptunes.
There are other problems with clouds on super-Earths and mini-Neptunes. Without atmospheric condensates, clouds cannot form. For super-Earths and mini-Neptunes, atmospheric condensates span a wide range of composition.
Astronomers use two approaches to determine the composition of super-Earth and mini-Neptune atmospheres: equilibrium chemistry calculations and chemical kinetics calculations. These calculations reveal that super-Earths and mini-Neptunes with reducing atmospheres will generate clouds dominated by potassium chloride (KCl) and zinc sulfide (ZnS) condensates.4 For super-Earths and mini-Neptunes with oxidizing atmospheres, the dominant cloud condensates will be potassium sulfate (K2SO4) and zinc oxide (ZnO).5 Super-Earths and mini-Neptunes with carbon-rich atmospheres will generate graphite clouds.6 Super-Earths and mini-Neptunes with close-in orbits of their host stars will form clouds from a wide range of rock-forming and metallic condensates.7
Each of these super-Earth and mini-Neptune cloud compositions poses a problem for some or all of Earth’s life. Super-Earths’ and mini-Neptunes’ much thicker and obscuring atmospheres, compared to Earth’s, also pose a problem for compensating over a long time period for the increasing luminosity of their host stars. If the goal is to find an extrasolar planet that can support complex life, super-Earth and mini-Neptune planets are not candidates.
These recent research findings about the clouds on super-Earths and mini-Neptunes yield an important by-product. They demonstrate just how fine-tuned Earth’s atmosphere and Earth’s clouds must be throughout the entire 3.8-billion-year-long history of life for Earth to be able to presently sustain the 8.7 million different eukaryotic species of life, including birds, mammals, and humans.8 Thank God for our atmosphere, our clouds, and our highly improbable planet.9
- Rostom Mbarek and Eliza M.-R. Kempton, “Clouds in Super-Earth Atmospheres: Chemical Equilibrium Calculations,” Astrophysical Journal 827 (August 2016): id. 121, doi:10.3847/0004-637X/827/2/121.
- Laura Kreidberg et al., “Clouds in the Atmosphere of the Super-Earth Exoplanet GJ1214b,” Nature 505 (January 2014): 69–72, doi:10.1038/nature12888.
- Heather A. Knutson et al., “Hubble Space Telescope Near-IR Transmission Spectroscopy of the Super-Earth HD 97658b,” Astrophysical Journal 794 (October 2014): id. 155, doi:10.1088/0004-637X/794/2/155.
- Mbarek and Kempton, “Clouds in Super-Earth,” 2–3.
- Camilo Mora et al., “How Many Species Are There on Earth and in the Ocean?,” PLOS Biology 9 (August 2011): e1001127, doi:10.1371/journal.pbio.1001127.
- Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016).