In the classic poem The Rime of the Ancient Mariner,1 the thirsty mariner looks out at the ocean surrounding him and declares, “Water, water every where, nor any drop to drink.”2 Similarly, astronomers studying exoplanets are recognizing that too much liquid water on a planet isn’t necessarily a good thing. In fact, it can present catastrophic problems for life.
As I explain in Improbable Planet, planets a little less to several times Earth’s mass that are the appropriate distance from their host stars for water to be in the liquid state on their surfaces will possess oceans at least a few hundred miles deep.3 Earth is the exception to this rule. Because of the dramatic nature of the Moon-formation event, Earth had its original deep ocean replaced by a shallow one.4
The conclusion that Earth-like planets must possess very deep oceans is not just based on planet-formation models. Where astronomers are able to measure the water content of an Earth-like exoplanet, that content ranges from 5% to 50% of the planet’s mass.5 This measure is not surprising since water is the third most abundant molecule in the universe, right after H2 and H3. By comparison, Earth presently possesses a water content slightly less than 0.03% of its total mass.
Without a shallow ocean there is no possibility of islands or continents. The lack of land above sea level obviously eliminates the possibility of land plants and land animals. It also eliminates the degree of silicate erosion that is needed to compensate for the brightening of the host star.
All stars stable enough to possibly sustain life on one of their planets gradually brighten as they age. For example, the Sun was 18–24% dimmer at the time of its origin than it is now.6 Life survived on Earth at that time because its atmosphere was loaded with far more heat-trapping greenhouse gases than it possesses today. Rain falling upon above-sea-level silicate landmasses generated a chemical reaction that, in the process of transforming silicates into carbonates and sand, removed carbon dioxide from the atmosphere.7 On Earth, the rate of carbon dioxide removal from the atmosphere perfectly compensated for the Sun’s gradual brightening so that life remained abundant on Earth throughout the past 3.8 billion years.
Now, a team of three astronomers has demonstrated that any kind of silicate erosion is ruled out for all planets that possess a liquid water mass fraction greater than 1%.8 Planets with this much liquid water will generate such high water pressures that a thick impenetrable ice layer will form on the ocean floor. This ice layer permanently blocks any possible contact between the liquid water ocean and interior silicates, thereby eliminating any possible silicate erosion and any possible removal of atmospheric carbon dioxide.
The same team of astronomers in the same research paper developed a suite of atmosphere-formation models for planets with liquid water mass fractions greater than 1%. In one set of models, where the ocean-atmosphere flux mechanism is dominated by polar sea ice, the planets possess atmospheres containing no less than 2 bars of carbon dioxide and probably several bars of carbon dioxide. In their second set of models, where wind-driven circulation is the dominant ocean-atmosphere flux mechanism, the astronomers found that planets possess tens of bars of carbon dioxide. For comparison, Earth’s total atmosphere adds up to 1.013 bars, of which carbon dioxide accounts for just 0.0004 bars.
Between two and several tens of bars of carbon dioxide in a planet’s atmosphere is a whole lot of carbon dioxide. With that much carbon dioxide, respiration of oxygen becomes impossible. Hence, these planets, at best, will be able to harbor only primitive life. That much carbon dioxide also will make the oceans on these planets highly acidic. The high acidity rules out the vast majority of primitive life-forms. Furthermore, the origin-of-life problem in high acidity environments becomes orders of magnitude more intractable for naturalistic models.
The buildup of carbon dioxide in a planet’s atmosphere from less than 0.001 bar to a few or tens of bars generates huge planetary temperature changes. The surface temperature will first rise rapidly (owing to the greenhouse effect of atmospheric carbon dioxide) and then fall rapidly as thick carbon dioxide clouds efficiently reflect away the host star’s light and heat. As atmospheric carbon dioxide builds up, less and less light from the host star will reach the planet’s surface. In addition, the planet’s surface chemistry will experience radical upheavals.
The planetary atmosphere models produced by the three astronomers reflect the norm for Earth-like planets. Earth is the unique exception. Earth’s extreme paucity of liquid water and atmospheric carbon dioxide and the fine-tuning of its quantities of atmospheric carbon dioxide throughout the past 3.8 billion years explains why Earth is home to such a wide diversity and abundance of life. Thank God for our terrestrial home with its tiny amount of liquid water perfectly measured out to support us and a great diversity of life. Or, as Coleridge wrote in the Rime of the Ancient Mariner, “All things both great and small; For the dear God who loveth us, He made and loveth all.”
Featured image credit: Lucianomendez
- Samuel Taylor Coolidge, “The Rime of the Ancient Mariner,” Part I, The Literature Network, http://www.online-literature.com/coleridge/646/.
- Hugh Ross, Improbable Planet (Grand Rapids: Baker, 2016): chapters 5–9.
- Ross, chapter 5.
- David Charbonneau et al., “A Super-Earth Transiting a Nearby Low-Mass Star,” Nature 462 (December 17, 2009): 891-94, doi:10.1038/nature08679; Geoffrey Marcy, “Extrasolar Planets: Water World Larger Than Earth,” Nature 462 (December 17, 2009): 853–54, doi:10.1038/462853a.
- Ross, chapter 12.
- For a detailed description of this chemical process, see Ross, chapter 12.
- A. Levi, D. Sasselov, and M. Podolak, “The Abundance of Atmospheric CO2 in Ocean Exoplanets: A Novel CO2 Deposition Mechanism,” Astrophysical Journal 838 (March 21, 2017): id. 24, doi:10.3847/1538-4357/aa5cfe.
Subjects: Atmosphere, Fine-Tuning, Extrasolar Planets, Life on Other Planets, Solar System Design