Planets covered entirely with water – sometimes to depths of hundreds of kilometres or more – may be more common than we thought, scientists say.
In fact, Earth’s own dry land may be the result of a lucky coincidence shortly before our solar system was born.
We may think of the Earth as wet, but that’s only because there’s a lot of water on its surface. By cosmic standards, our planet is actually quite dry, with water comprising only 0.02% of its total mass.
The reason for that, Michael Meyer told this week’s AbSciCon 2019 astrobiology conference in Bellevue, Washington, US, is that the early protoplanets from which the Earth and other planets formed were once rich in intensely radioactive isotopes such as aluminum-26.
Heat from the decay of these short-lived radionuclides was enough to raise these objects’ internal temperatures to above 1200 degrees Kelvin (about 925 degrees Celsius), says Meyer, an astronomy professor at the University of Michigan, Ann Arbor, US. “That would drive out most of the volatiles.”
Thankfully for those of us not wanting to be irradiated by our beer cans, aluminum-26 isn’t common in today’s solar system. It has a half-life of only 720,000 years, which means that something must, in Meyer’s words, have “polluted” our solar system’s environment with it, shortly before the planets began forming, about 4.5 billion years ago.
That “something,” he says, would have been a supernova explosion – caused by the death of a giant star somewhere in the star-forming cluster in which our solar system was born.
Such explosions aren’t super-common. And if one occurred, not all stars in a cluster would be close enough for much aluminum-26 to reach them.
Not that this means our solar system is unique. According to his team’s simulations, Meyer says, it appears that somewhere around 10 to 15% of newly forming star systems would end up with aluminum-26 levels comparable to those seen by our own.
But what of the others? These, he says, wouldn’t have planets formed from planetesimals that got hot enough to drive off enough water to form “dry,” rocky planets like the Earth, Mars, Venus and Mercury. Instead, they would have retained much of their primordial water (or ice).
“Most are very wet,” he says. “You get huge fractions of water.” In some cases, “upwards of 50%”.
Whether such worlds could be habitable is a more difficult question. Meyer paints them as covered by deep oceans, capped by impenetrable layers of ice. But that obviously depends on how far they are away from their suns.
It’s actually possible that water-worlds might be more habitable than terrestrial-style rocky planets. Yaoxuan Zeng of Peking University, China, notes that many of the known exoplanets are circling red dwarf stars.
In order to receive enough warmth from their suns to have liquid water, he says, such worlds have to huddle so close that tidal forces cause them to permanently turn the same face toward their stars, just as the Earth’s Moon always faces the Earth.
On one side of such a planet, it’s perpetual noon. On the other, it’s perpetual midnight.
On a rocky planet, that’s a climate disaster, with one side of the planet always scorching hot, while the other is perpetually frozen.
But on a water world, Zeng says, winds created by heating at the “sub-stellar” site would tend to blow eastward, producing ocean currents moving in the same direction. These currents, which he says could hit speeds of one metre per second, would carry warm water far around the planet. “These can warm the night side,” he says.
A more important problem is that worlds that didn’t heat up enough to lose their primordial water should also have retained their primordial carbon dioxide.
If that carbon dioxide wound up in their atmospheres, greenhouse warming would turn them into uninhabitable hothouses, says Nadejda Marounina of the University of Chicago, Illinois, US.
But it’s possible, she says, for water-worlds to sequester startling amounts of carbon dioxide beneath their surfaces.
Some of it would be dissolved in their ocean depths. More might be sequestered beneath the ocean in the form of clathrates (a type of pressurised ice that can trap gases such as carbon dioxide), or in solid carbon dioxide or a more exotic substance called carbonic acid monohydrate.
The formation of such compounds, Marounina says, would depend on whether the water-world’s early history allows massive amounts of carbon dioxide to be sequestered in this manner. “If conditions are favourable for carbon dioxide to sink and be trapped as solids, [such a world] is possibly habitable,” she says.