The mantra for the astrobiology community is “Follow the water”—for good reason. Without liquid water, there is no possibility of life. However, many in the astrobiological community take the mantra too far. They consider every planet on which there is the possibility that liquid water may exist on some part of the planet’s surface for some period in the planet’s history as habitable.
As I have written elsewhere,1 for a planet to be truly habitable, it must not only reside in the liquid water habitable zone but also simultaneously reside in eight other known habitable zones. In addition to these nine known habitable zones, there may be other habitable zones that have yet to be discovered. So far, of the more than 3,500 planets with measured characteristics, only one resides in all nine known habitable zones—the one all readers of this blog are living on.
New research now reveals that liquid water itself can be a problem for life. Scientists have acknowledged for some time that too little liquid water, or liquid water in too small a place, or liquid water that remains for too short a period of time, eliminates the possible origin of life and the possible existence of life. Now, new studies establish that too much liquid water likewise is catastrophic for life.
Several research efforts demonstrate that too much water is the norm. Water is, after all, the third most abundant molecule in the universe, right after hydrogen and helium. Interstellar molecular clouds are loaded with water, and these clouds are the maternity wards of stars and planets. This high abundance of water explains why comets are 80–85 percent water and some of the moons of Jupiter and Saturn are composed of more than 15 percent water. And, as I explained in my book Improbable Planet, our own Earth started off with a universal ocean that was thousands of miles thick.2
In a paper that has just been accepted for publication in the journal Astronomy & Astrophysics, two Swiss astronomers, Yann Alibert and Willy Benz, produced a set of planet formation models for M dwarf stars.3 These stars are the most abundant stars in the universe. Alibert and Benz’s models showed that “planets forming in long lived disks [the most likely scenario] all have a fraction of water larger than 10%.”4 By comparison, Earth’s fraction of water today is a little less than 0.03 percent. Alibert and Benz’s results complement earlier research by MIT astronomers whose planet formation models showed that planets ranging in size from 1 to 30 times Earth’s mass possess, at the end of their formation periods, water content ranging from 10 to 20 percent, implying very deep oceans and very thick atmospheres.5 Both the Swiss and MIT planet formation models showed one of two outcomes for planets in the liquid water habitable zone: either globally extensive, very deep oceans or total desiccation. In the one case where astronomers have been able to measure the water content of a planet—a planet that is 6.5 times Earth’s mass and orbits the star GJ 1214—the water content was measured to be 50 percent of the total mass of the planet.6
The norm for “cool” planets is to possess a whole lot of water, the equivalent of a global ocean thousands of miles of thick. In a paper published a year ago, a team of nine European astronomers explained the detriments that such water-rich planets pose for life.7
A liquid water ocean more than a thousand miles deep produces such extreme pressures on the ocean floor that the high pressure transforms the liquid water at this depth into high-pressure ice. This high-pressure ice forms a permanent barrier between the planet’s crust and its liquid ocean and, thus, its atmosphere also. This barrier eliminates the possibility of silicate in the crust being chemically transformed into carbonates and sand through the removal of carbon dioxide from the atmosphere. Without the ongoing removal of carbon dioxide from the atmosphere, there is no mechanism for compensating for the brightening of the host star. (All stars become brighter as they continue to fuse light elements into heavier elements in their nuclear furnaces.) Thus, the time window for the origin and survival of life on such planets is very brief.
The time window for life on water-rich planets becomes even briefer as a result of water evaporation. Water vapor, like carbon dioxide, is a greenhouse gas. As a water-rich planet’s host star brightens, the brightening causes more liquid water on the planet to evaporate. The greater amount of water vapor in the atmosphere traps more of the host star’s heat. So, not only does the planet heat up as the host star brightens, but it heats up faster as more and more water vapor is pumped into the planet’s atmosphere.
The time window for life on water-rich planets becomes even briefer owing to a negative carbon dioxide cycle. As the global ocean on a water-rich planet warms due to the brightening of its host star and the rising water vapor content in its atmosphere, carbon dioxide dissolved in the ocean is released into the planet’s atmosphere. The extra carbon dioxide in the atmosphere traps more heat from the host star, which warms the planet, which causes more of the carbon dioxide in the ocean to be released into the atmosphere.
We can draw several conclusions from all of these studies. The first is that for planets in the liquid water habitable zone, globally extensive, very deep oceans will be the norm. The second is that planets with globally extensive, very deep oceans are poor candidates for the origin and survival of unicellular life and are not candidates at all for more advanced life. Third, a planet like Earth that is neither water rich nor totally desiccated is a totally unexpected outcome.
I can suggest a fourth conclusion: thank God for all he did to design our planet with just the right amounts of water vapor, liquid water, and water ice to make possible our existence, our civilization, and our capacity to understand and respond to his gospel message. To read more about how God designed Earth for the redemption of billions of human beings, get a copy of my latest book, Improbable Planet, and after reading it, give it to a non-Christian friend.
- Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016), 78–93; Hugh Ross, “‘Electric Wind’ Becomes 9th Habitable Zone,” Today’s New Reason to Believe (blog), Reasons to Believe, July 4, 2016, https://www.reasons.org/blogs/todays-new-reason-to-believe/electric-wind-becomes-9th-habitable-zone.
- Ross, Improbable Planet.
- Yann Alibert and Willy Benz, “Formation and Composition of Planets around Very Low Mass Stars,” preprint, submitted October 11, 2016, https://arxiv.org/abs/1610.03460.
- Ibid., 5.
- Linda T. Elkins-Tanton and Sara Seager, “Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets,” Astrophysical Journal 685 (October 2008): 1237–46, doi:10.1086/591433.
- David Charbonneau et al., “A Super-Earth Transiting a Nearby Low-Mass Star,” Nature 462 (December 2009): 891–94, doi:10.1038/nature08679; Geoffrey Marcy, “Extrasolar Planets: Water World Larger Than Earth,” Nature 462 (December 2009): 853–54, doi:10.1038/462853a.
- Daniel Kitzmann et al., “The Unstable CO2 Feedback Cycle on Ocean Planets,” Monthly Notices of the Royal Astronomical Society 452 (October 2015): 3752–58, doi:10.1093/mnras/stv1487.