“These are utterly different worlds compared to our own Earth,” said Harvard University astronomer Li Zeng 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, that is, planets featuring radii about two to four times that of Earth, are likely to be water worlds, and not gas dwarfs surrounded by thick atmospheres as conventionally believed.
Some of these planets, Zeng said, 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.
In 2016, for example, Kepler astronomers discovered planets that are unlike anything in our solar system –a “water world” planetary system orbiting the star Kepler-62. This five-planet system has two worlds in the habitable zone — their surfaces completely covered by an endless global ocean with no land or mountains in sight.
In stark contrast, Earth has an obvious surface, with water compositions ranging between 25 to 50 percent of the planet’s total mass, these objects would be completely water-logged. They “may or may not have a well-defined surface,” said Li, and they “could be fluid all the way down—all the way down, to great depth.”
Could these ocean worlds support life? Perhaps even intelligent life? “There may be life there,” says Lisa Kaltenegger, Director of the Carl Sagan Institute at Cornell. “But could it be technology-based like ours? Life on these worlds would be under water with no easy access to metals, to electricity, or fire for metallurgy. Maybe life’s inventiveness to get to a technology stage will surprise us.”
Could alien waters worlds at some point evolve life as we know it on Earth? “Purely ocean worlds (without land on the surface),” writes Avi Loeb, chairman of Harvard’s astronomy department in an email to dailygalaxy.com, “are not likely to develop the diversity of life as we know it because they will be depleted of essential nutrients for life, such as phosphorous and molybdenum.”
“We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live,” explains astrophysicist Natalie Hinkel Senior Research Scientist at the Southwest Research Institute and a Co-Investigator for the Nexus for Exoplanet System Science (NExSS) research network at Arizona State University. “However, a planet that is a water world, or one that doesn’t have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.”
“I think it could be dangerous just thinking about everything in an Earth-mindset,” says Ramses Ramirez at the Tokyo Institute of Technology. “You might be missing out on other possibilities.”
New research suggests that about 35% of all known exoplanets which are bigger than Earth should be water-rich. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterize their atmospheres with important implications for the search of life in the Milky Way.
“It was a huge surprise to realize that there must be so many water-worlds”, said Zeng.
The oceans of Earth itself became the conduit for evolution, says Peter Godfrey-Smith in Other Minds. Earth’s ocean-dwelling cephalopods – octopuses, squids and nautiluses, Godfrey-Smith writes – “are an island of mental complexity in the sea of invertebrate animals”, he writes, having developed on a different path from us, “an independent experiment in the evolution of large brains and complex behavior.”
“If we can make contact with cephalopods as sentient beings, it is not because of a shared history, not because of kinship, but because evolution built minds twice over,” says Godfrey-Smith. “This is probably the closest we will come to meeting an intelligent alien.”
This article was originally published as “Unfathomable Abodes of Life?” –Water Worlds of the Milky Way posted on Apr 30, 2019