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.”