Water worlds, especially if they have no land on them, are unlikely to be home to life, or at least life we can detect. Some of the basic atmospheric and mineral cycles that make a planet habitable will be absent. Cool animation of such a world. (NASA)
Wherever we find water on Earth, we find life. It is a connection that extends to the most inhospitable locations, such as the acidic pools of Yellowstone, the black smokers on the ocean floor or the cracks in frozen glaciers. This intimate relationship led to the NASA maxim, “Follow the Water”, when searching for life on other planets.
Yet it turns out you can have too much of a good thing. In the November NExSS Habitable Worlds workshop in Wyoming, researchers discussed what would happen if you over-watered a planet. The conclusions were grim.
Despite oceans covering over 70% of our planet’s surface, the Earth is relatively water-poor, with water only making up approximately 0.1% of the Earth’s mass. This deficit is due to our location in the Solar System, which was too warm to incorporate frozen ices into the forming Earth. Instead, it is widely — though not exclusively — theorized that the Earth formed dry and water was later delivered by impacts from icy meteorites. It is a theory that two asteroid missions, NASA’s OSIRIS-REx and JAXA’s Hayabusa2, will test when they reach their destinations next year.
But not all planets orbit where they were formed. Around other stars, planets frequently show evidence of having migrated to their present orbit from a birth location elsewhere in the planetary system.
One example are the seven planets orbiting the star, TRAPPIST-1. Discovered in February this year, these Earth-sized worlds orbit in resonance, meaning that their orbital times are nearly exact integer ratios. Such a pattern is thought to occur in systems of planets that formed further away from the star and migrated inwards.
The TRAPPIST-1 worlds currently orbit in a temperate region where the levels of radiation from the star are similar to that received by our terrestrial worlds. Three of the planets orbit in the star’s habitable zone, where a planet like the Earth is most likely to exist.
However, if these planets were born further from the star, they may have formed with a high fraction of their mass in ices. As the planets migrated inwards to more clement orbits, this ice would have melted to produce a deep ocean. The result would be water worlds.
With more water than the Earth, such planets are unlikely to have any exposed land. This does not initially sound like a problem; life thrives in the Earth’s seas, from photosynthesizing algae to the largest mammals on the planet. The problem occurs with the planet itself.
The clement environment on the Earth’s surface is dependent on our atmosphere. If this envelope of gas was stripped away, the Earth’s average global temperature would be about -18°C (-0.4°F): too cold for liquid water. Instead, this envelope of gases results in a global average of 15°C (59°F).
Exactly how much heat is trapped by our atmosphere depends on the quantity of greenhouse gases such as carbon dioxide. On geological timescales, the carbon dioxide levels can be adjusted by a geological process known as the “carbon-silicate cycle”.
In this cycle, carbon dioxide in the air dissolves in rainwater where it splashes down on the Earth’s silicate rocks. The resulting reaction is termed “weathering”. Weathering forms carbonates and releases minerals from the rocks that wash into the oceans. Eventually, the carbon is released back into the air as carbon dioxide through volcanoes.
The rate of weathering is sensitive to temperature, slowing when he planet is cool and increasing when the temperature rises. This allows the Earth to maintain an agreeable climate for life during small variations in our orbit due to the tug of our neighboring planets or when the sun was young and cooler. The minerals released by weathering are used by all life on Earth, in particular phosphorous which forms part of our DNA.
However, this process requires land. And that is a commodity a water world lacks. Speaking at the Habitable Worlds workshop, Theresa Fisher, a graduate student at Arizona State University, warned against the effects of submerging your continents.
Fisher considered the consequences of adding roughly five oceans of water to an Earth-sized planet, covering all land in a global sea. Feasible, because weathering could still occur with rock on the ocean floor, though at a much reduced efficiency. The planet might then be able to regulate carbon dioxide levels, but the large reduction in freed minerals with underwater weathering would be devastating for life.
Despite being a key element for all life on Earth, phosphorus is not abundant on our planet. The low levels are why phosphorous is the main ingredient in fertilizer. Reduce the efficiency with which phosphorous is freed from rocks and life will plummet.
Such a situation is a big problem for finding a habitable world, warns Steven Desch, a professor at Arizona State University. Unless life is capable of strongly influencing the composition of the atmosphere, its presence will remain impossible to detect from Earth.
“You need to have land not to have life, but to be able to detect life,” Desch concludes.
However, considerations of detectability become irrelevant if even more water is added to the planet. Should an Earth-sized planet have fifty oceans of water (roughly 1% of the planet’s mass), the added weight will cause high pressure ices to form on the ocean floor. A layer of thick ice would seal the planet rock away from the ocean and atmosphere, shutting down the carbon-silicate cycle. The planet would be unable to regulate its surface temperature and trapped minerals would be inaccessible for life.
Add still more water and Cayman Unterborn, a postdoctoral fellow at Arizona State, warns that the pressure will seal the planet’s lid. The Earth’s surface is divided into plates that are in continual motion. The plates melt as they slide under one another and fresh crust is formed where the plates pull apart. When the ocean weight reaches 2% of the planet’s mass, melting is suppressed and the planet’s crust grinds to a halt.
A stagnant lid would prevent any gases trapped in the rocks during the planet’s formation from escaping. Such “degassing” is the main source of atmosphere for a rocky planet. Without such a process, the Earth-sized deep water world could only cling to an envelop of water vapor and any gas that may have escaped before the crust sealed shut.
Unterborn’s calculations suggest that this fate awaits the TRAPPIST-1 planets, with the outer worlds plausibly having hundreds of oceans worth of water pressing down on the planet.
So can we prove if TRAPPIST-1 and similarly migrated worlds are drowning in a watery grave? Aki Roberge, an astrophysicist at NASA Goddard Space Flight Center, notes that exoplanets are currently seen only as “dark shadows” briefly reducing their star’s light.
However, the next generation of telescopes such as NASA’s James Webb Space Telescope, will aim to change this with observations of planetary atmospheres. Intertwined with the planet’s geological and biological processes, this cloak of gases may reveal if the world is living or dead.