WATERWORLDS

How Astronomers Could Discover Oceans on Distant Planets

Planets too far away to photograph could yield some clues to whether water—and maybe even life—could exist.

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Earth, Venus, Mars, the moon, and Pluto are very different worlds, but they have something in common: mountains. In fact, mountains occur on so many different bodies in the solar system that astronomers are pretty sure many exoplanets—planets orbiting other stars—also have them. And like planets and moons close to home, those mountains can tell us a lot about what’s going on with exoplanets. They might even help us discover how habitable these far-off worlds are.

But first, we have to see exoplanetary mountains. In a new paper to be published in the prestigious journal Monthly Notices of the Royal Astronomical Society, Columbia University astronomers Moiya A.S. McTier and David M. Kipping figured out what it might take to detect mountains on a world too far away to photograph even with our most powerful telescopes.

The trick is to see their shadows as the planet passes in front of its host star, a phenomenon known as “transiting.” During these brief eclipses, the planet cuts off some of the host star’s light, which lets astronomers measure the size of the planet and how quickly it orbits. McTier and Kipping showed that if they observe multiple transits, astronomers might be able to see smaller fluctuations in the light when mountains are on the “horizon.”

“My paper is, to my knowledge, the first work that’s ever been done to come up with a method for finding mountains on planets outside our solar system,” McTier told The Daily Beast. “We’ve found mountains inside our solar system, on Earth and on other planets like Mars. But we’ve never found [mountains] outside of our solar system, even though we’ve found thousands of these types of planets out there.”

Many exoplanets are gas giants like Jupiter or Neptune, without a solid surface to have any mountains at all. However, “super-Earths” are another extremely common type: planets bigger and more massive than Earth, but still made of rock. Since no planet in the solar system is like that type of planet, we don’t know much about them yet, including whether they could have mountains or make oceans of water. Though they’re harder to detect, Earth- or Mars-sized exoplanets are probably even more common.

Even a large mountain on a super-sized super-Earth won’t block out that much extra light during a transit. So, instead of trying to see individual topographic features, McTier worked out a way to measure the overall mountainousness of an exoplanet.

“We really wanted bumpiness—as we called it—to be a measure of how much an average feature sticks out from the surface of the planet,” she said. That’s better than looking for big standalone mountains, like Mauna Kea on Earth or Olympus Mons on Mars. “This method could find anything that sticks out from the surface, so it could be mountain ranges, it could be single mountains, it could be volcanoes.”

We could learn about the length of its day, which is really exciting and currently extremely difficult to do for these small rocky planets orbiting stars hundreds of light years away.
Moiya A. S. McTier

Here’s how the method works. As an exoplanet orbits its host star, it turns relative to astronomers on Earth. During that turning, we get a bit of a sunrise or sunset effect as the star’s light is blocked or unblocked by mountains, over the course of its transit. It’s not a huge effect, so McTier calculated we might need to see hundreds of transits to get an accurate measurement.

But the potential payoff is big: “If we’re able to actually detect bumpiness, then we could potentially learn something about oceans on an exoplanet, and whether or not it has tectonic plate movement,” McTier said.

Surface oceans are a particularly exciting possibility. Earth is the only world we know that has them, and we don’t know whether it’s a coincidence that we’re also the only world known to have life. (Icy moons like Europa and Enceladus have subsurface oceans, which are another intriguing possibility for life.)

Mars probably once had oceans, but doesn’t any more. That absence makes the planet “bumpier” than Earth: The difference between its highest peaks and deepest valleys aren’t hidden by water. Measuring bumpiness may allow us to distinguish between a watery world and a dry one. Saturn’s moon Titan, which has liquid (and very frigid!) hydrocarbon lakes, has a natural “sea level” like Earth—a potentially common feature of planets with surface liquids.

Similarly, Earth has plate tectonics, which makes our big mountain ranges, but doesn’t grow volcanoes as large as the ones on Mars.

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“If it’s really bumpy (meaning there are a lot of features), we can do work from there to figure out what those features are,” McTier said.

The biggest bumps aren’t the mountains: They’re making the observations. Finding mountains is easiest for a very large planet orbiting a very small star, because transiting data depends on the ratio of the planet’s size to the star’s size. For that reason, McTier’s calculated example was a Mars-sized planet orbiting a white dwarf—the burned-out remnant of a star. A typical white dwarf is about the size of Earth, but with a mass comparable to the Sun, so the paper doesn’t describe a normal exoplanet system.

To measure bumpiness of a super-Earth orbiting a red dwarf star (one of the most common systems we see), McTier estimated we would need a huge telescope like the Colossus, which is still in the design stages.

To McTier, the potential science is worth the wait. “We could learn about the length of its day, which is really exciting and currently extremely difficult to do for these small rocky planets orbiting stars hundreds of light years away.”

Habitability is what gets people excited about exoplanet science,” she added. “If we can overcome those challenges, we can learn exciting things about the planet.”