In 2016, astronomers using data from NASA’s Kepler mission, discovered a planet unlike anything in our solar system –a “water world” planetary system orbiting the star Kepler-62 –a five-planet system with two worlds in the habitable zone — their surfaces completely covered by an endless global ocean with no land or mountains in sight.
“Utterly Different Worlds Than Earth”
“These are utterly different worlds compared to our own Earth,” said Harvard University astronomer Li Zeng in 2019 about the chances that water worlds are a common feature of the Milky Way, which was heightened by research using computer simulations showing that sub-Neptune-sized planets –planets featuring radii about two to four times that of Earth– are likely to be water worlds. Some of these planets, Zeng observed, have oceans deep enough to exert pressures equivalent to a million times our atmospheric surface pressure. Under those conditions, fluid water gets compressed into high-pressure phases of ice, such as Ice Seven or superionic ices. “These high-pressure ices are essentially like silicate-rocks within Earth’s deep mantle—they’re hot and hard,” he said.
“Unfathomable Abodes of Life?” –Water Worlds of the Milky Way
“It’s amazing to think that the enigmatic intermediate-size exoplanets could be water worlds with vast amounts of water. Hopefully atmosphere observations in the future–of thick steam atmospheres—can support or refute the new findings,” said Professor Sara Seager, Professor of Planetary Science at Massachusetts Institute of Technology, and deputy science director of the recently-launched TESS (Transiting Exoplanet Survey Satellite) mission, which will search for exoplanets.
New evaluations of data from the exoplanet-hunting Kepler Space Telescope and the Gaia mission indicates that many of the known exoplanets may contain as much as 50% water –much more than the Earth’s 0.02% (by weight) water content.
An Epoch in Planetary Evolution?
Perhaps some water worlds mark an early-stage epoch in planetary evolution. Science has just reported that ancient Earth was a water world, reports Paul Voosen for Science: “Evidence is mounting that some 3 billion to 4 billion years ago, the planet’s oceans held nearly twice as much water—enough to submerge today’s continents above the peak of Mount Everest. The flood could have primed the engine of plate tectonics and made it more difficult for life to start on land.”
“There’s clear geological evidence,” of ancient Earth as a water world says Benjamin Johnson, a geochemist at Iowa State University. “Titanium concentrations in 4-billion-year-old zircon crystals from Western Australia suggest they formed underwater,” reports Science. “And some of the oldest known rocks on Earth, 3-billion-year-old formations in Australia and Greenland, are pillow basalts, bulbous rocks that only form as magma cools underwater.”
Clear Evidence
Additional evidence of water-world Earth is found in “samples from a 3.24-billion-year-old chunk of oceanic crust left on Australia’s mainland were far richer in a heavy oxygen isotope than the present-day oceans, Johnson and Boswell Wing, a geobiologist at the University of Colorado, Boulder, concluded in a 2020 Nature Geoscience study. “Because water loses this heavy oxygen when rain reacts with the continental crust to form clays, its abundance in the ancient ocean suggests the continents had barely emerged by that point/ The finding doesn’t necessarily mean the oceans were larger,” Johnson notes, but, “It is easier to have submerged continents if the oceans are bigger.”
“It’s a very delicate system, the Earth,” says Junjie Dong, a graduate student in mineral physics at Harvard University who led the modelling published today in AGU Advances, referring to our ancient water as a reminder of how conditional Earth’s evolution is. Without continents, so essential for the planet’s life and climate, life as we know it would never have emerged. “Too much water, or too little, and it wouldn’t work.”
Maxwell Moe, NASA Einstein Fellow, University of Arizona, via Sciencemag.org
Image credit: With thanks to Domestika.org
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week probing the mysteries of the Universe at the Kitt Peak National Observatory. Max received his Ph.D in astronomy from Harvard University in 2015.