The Himalayas distort Earth's contour only about as much as a human hair would that of a billiard ball. Discerning such a minuscule bump on a planet orbiting a distant star might seem laughably impossible, but two astronomers have proposed a way to detect mountains and other surface features on exoplanets.
Finding mountains could help address another key question: Can these planets hold life? So says astronomy graduate student Moiya McTier of Columbia University, one of the co-authors of the proposal, which was published in April in Monthly Notices of the Royal Astronomical Society.
Life on Earth is apparently dependent on the inner life of the planet itself. Plate tectonics recycles carbon and regulates temperatures, and Earth's magnetic field provides a shield from dangerous solar winds. Mountains and volcanoes are signs that a planet has, or at least at one point had, such an inner life.
Astronomers have now identified some 3,700 planets, but little is known about most of them besides their size and mass. Most were detected by the so-called transit method, in which astronomers measure a slight dimming of the light from a distant star when a planet orbits in front of it. The strategy proposed by McTier and her Columbia colleague David Kipping builds on that method but will likely require huge telescopes that may not be completed for decades.
The astronomers' insight is that a rotating, mountainous planet presents a changing silhouette during transit, causing measurements of the dip in light to fluctuate. Based on conservative estimates, the scientists believe the “bumpiness” of planets as mountainous as Mars could be measured accurately by a 74-meter telescope observing transits for about 20 hours, spread out over roughly six months. That is still a tall order for today's telescopes, but larger ones are on the horizon.
One of Kipping's biggest concerns with this approach is mountain-cloaking clouds. Nicolas Cowan, an astronomer at McGill University who was not involved in the research, agrees. But even without clouds, he worries that atmospheric absorption, scattering and refraction of light could spoil the view. “I suspect that for that method to work for a planet, it'll probably need to be airless,” Cowan says. The Columbia researchers, though, think they can mitigate these effects by observing different wavelengths of light.
Even if astronomers manage to confirm a planet's bumpiness, they will need additional information—such as the presence of liquid water, tolerable temperatures and an atmosphere—to interpret the implications for habitability. “No single piece of information is going to solve it,” Kipping notes.