Richard A Lovett
Ocean worlds that would put the Kevin Costner’s 1995 movie Waterworld to shame may be a dime a dozen elsewhere in the galaxy, scientists say.
In our own solar system, Earth is the only planet with liquid water on its surface. But in a study of more than 4000 exoplanets, a team led by Li Zeng, a planetary scientist at Harvard University in the US, found that fully 35% appear to contain up to half their mass as water.
To put this in perspective, the Earth’s water, which seems so abundant at the surface, is actually a mere 0.02% of the planet’s total mass. These worlds might have oceans thousands of kilometres deep, Zeng says.
The discovery came by calculating the densities of the target worlds, then attempting to figure out what mix of materials might account for it. It’s a three step-process. First, the size of the planet is determined by measuring how much of its star’s light it blocks as it passes in front of it. The bigger the dip, the bigger the world.
Then the planet’s mass is calculated by observing tiny Doppler shifts in its star’s spectral lines as the planet moves around it, tugging the star ever so slightly towards us or away from us in the process. “This is the most difficult part,” Zeng says.
Once the planet’s average density is obtained by comparing its size and mass, Zeng says, his team used models of planet formation to determine what mix of light and heavy materials might produce the observed result.
Among other things, these models took into account what is known about the composition of the protoplanetary disks from which solar systems are formed and how this affects the composition of the planets.
“Smaller planets are rocky; intermediate-sized planets are water worlds; larger planets contain more gas,” he says.
“This reflects the cosmic element hierarchy of three major planet-building materials: rock (plus metal), ice, and gas.”
In addition, there is a point in each planetary system, known as the snowline, which is the closest distance to the star at which water vapour will condense. In our own solar system, it lies in the Asteroid Belt, between Mars and Jupiter, but in other solar systems its location depends on the brightness of their suns.
“The snowline should act as a factory for these water worlds, or icy cores,” Zeng says. “No wonder why there are so many of them.”
In fact, four of our own solar system’s planets, Jupiter, Saturn, Uranus, and Neptune, appear to have been built around similar “icy cores,” but they also collected varying amounts of hydrogen and helium gas, swelling them into the giant planets we know today.
“We argue that these water worlds were formed in the same way, but for some reason did not collect the same amount of gas,” Zeng says.
Not that the water would be similar to what comes out of the tap … or what covers Antarctica.
“On these planets, the water would exist in different kinds of states,” Zeng says.
These worlds tend to be close enough to their suns that surface temperatures might be in the 200 to 500 degree Celius range, producing a deep shroud of water vapour. “In the deep interior, [the water] would be compressed into hot ice — hot, but solid,” he says.
But just beneath the surface, it could be liquid, opening the door to the possibility of life.
“The most important thing about this research is that it brings us this awareness that there are these water worlds, and they could harbour life,” Zeng says.
“In the inner solar system we think water is rare. But on the scale of our galaxy, water is very abundant. Oxygen is the third most abundant element. Hydrogen is the first. So whenever conditions are right, they combine to form water. Life could be a universal phenomenon.”
Zeng suggests that these planets formed in the same manner as the cores of our solar system’s giant planets, but migrated inward toward their present positions, closer to their stars, too quickly to have acquired significant amounts of hydrogen and helium gas.
Rory Barnes, an astrobiologist at the University of Washington, Seattle, US, who was not part of the study team, says it’s also possible these worlds might once actually have had gaseous envelopes but lost them when they moved too close to their stars.
“Close to their stars, stellar winds can be quite large,” he says, “and solar flares have enough energy to lift the hydrogen off the planet. You can strip away an atmosphere.”
That said, he notes that the details of Zeng’s research have not yet been published, making them difficult to assess. Not that he finds the basic result all that surprising. “There’s been indications for many years that there are going to be planets out there that are fairly rich in water,” he says.
Hal Levison, a planetary scientist from the Southwest Research Institute, Boulder, Colorado, US, agrees that these low-density exoplanets may well be water worlds, but notes that they could also be some “new type of planet” composed of rock and a thick hydrogen-helium atmosphere, without water.
“These authors claim to have insight into this based on some type of model,” he says. “I can’t say much more without learning about the modelling.”
The next step, says Sara Seager, a planetary scientist at Massachusetts Institute of Technology, in Cambridge, Massachusetts, also not a member of Zeng’s study team, will be to use a future generation of telescopes to study the atmospheres of these worlds in detail. “Hopefully, atmosphere observations in the future – of thick steam atmospheres – can support or refute the new findings,” she says.
Zeng’s research was presented earlier this month at the 2018 Goldschmidt conference in Boston, Massachusetts.
