The Himalayas distort Earth’s surface only about as much as a human hair would that of a billiard ball. Discerning such a minute effect on a planet orbiting another star might seem laughable—akin to perceiving the hair on the ball, with the ball on the moon. Nevertheless, two astronomers have proposed a way to detect mountains and other surface features on exoplanets. And as mind-blowing as that may seem, the researchers have even loftier goals in mind.
“It almost might be fair to say that this project by itself isn’t that exciting,” says Moiya McTier, a Columbia University graduate student who co-wrote the paper describing the proposal with Columbia assistant professor of astronomy David Kipping. “Because once you find mountains, then what? [But] that leads to answering the question that we've actually wanted to answer for the last few decades, which is: Can these planets hold life?”
Life on our planet apparently depends on the life of our planet, including plate tectonics that recycle carbon and regulate temperature as well as a magnetic field, created by the combination of the Earth’s rotation and molten core, which protects the planet from an otherwise life-threatening solar wind. So beyond being scenery, surface features like mountains and volcanoes can be seen as outward signs of our planet’s life-giving inner life.
But seeing them on exoplanets is an audacious idea. Although astronomers have now identified some 3,700 such planets so far, little is known about them—generally not much more than their size and mass. Most were detected by the so-called transit method, wherein astronomers measure a slight dimming of the light from a distant star when an orbiting planet passes in front of it. McTier and Kipping’s strategy builds on that method and likely requires huge telescopes with diameters greater than 70 meters. Several of these have been proposed, but are probably decades away from first light.
Even those monsters won’t see mountains on exoplanets directly. The astronomers’ insight is that a rotating, mountainous planet creates a changing silhouette when passing its star, such that the resulting transit light curve will exhibit not only the telltale dip proving the planet’s presence, but bumps within that trough that reflect the body’s fluctuating cross-sectional area. By modeling the transits of well-mapped bodies such as Earth, Mars and the moon, McTier and Kipping derived an approximate relationship between the transit curve’s bumpiness and the bumpiness of the body, which by their measure is about 500 meters for Earth and 3,000 meters for Mars (Mars has taller mountains, and Earth’s oceans have a smoothing effect). Although the method does not promise to resolve individual features, it gives a sense of the overall ruggedness of a planet.
Even that is easier said than done. To achieve a sufficient signal-to-noise ratio with foreseeable telescopes, the astronomers narrow their focus to the optimal situation of particularly bumpy planets orbiting the smallest possible stars—white dwarfs, the Earth-size remnants left over after larger stars die. Although no exoplanets have yet been found orbiting such stars, Kipping is optimistic they exist, because evidence of planetlike material nearby has shown up in a large fraction of their atmospheres; theories also suggest they could harbor habitable planets. Based on the population of white dwarfs in our galaxy, and a conservative assumption of the number with planets, the astronomers figure some should be within roughly 225 light-years of us, and find that Mars-level bumpiness could be measured with confidence by a 74-meter telescope observing transits for about 20 hours spread out over five and a half months. A tall order, given that currently the world’s largest optical telescopes are around 10 meters in diameter, although the so-called European Extremely Large Telescope now under construction in Chile will weigh in at 39 meters.
And there are other complications. Kipping’s biggest concern is mountain-cloaking clouds. Nicolas Cowan, an astronomer at McGill University who studies exoplanet atmospheres, agrees clouds are “a huge bugaboo,” but also worries that even without them, atmospheric absorption, scattering and refraction of light could spoil the view. “I suspect,” he says, “that in order for that method to work for a planet, it'll probably need to be airless.” The Columbia researchers, though, think they can mitigate these effects by observing different wavelengths of light.
Even if astronomers do manage to measure a planet’s bumpiness, they will need additional data to interpret its implications for habitability. A mixture of land and water on a planet’s surface seems the best bet for life, indicating that a moderate level of bumpiness, somewhere between the smoothness of a water-covered world and the ruggedness of Mars is ideal. But bumpiness alone will not distinguish a partially water-covered world from one with rolling hills—although paired with specular reflections indicating oceans it could. Nor can bumpiness distinguish mountains from volcanoes, although partnered with spectroscopic measurements of CO2 and sulfur in the planet’s atmosphere, it might. And mountains do not necessarily confirm the type of tectonic activity required by Earth’s carbon cycle, but they could strongly hint at it when combined with estimates of the concentration of relevant metals in the planet’s crust. “No single piece of information is going to solve it,” Kipping notes.
And although Cowan says the idea of detecting exomountains “is a little bit out there,” Kipping maintains it is worthwhile, not least for motivating the design of bigger telescopes. “These types of missions take decades to plan,” he says. “And if there is no one thinking about these questions, then we’ll just be sort of bumping around.”