How to see mountains on exoplanets

Astronomers have worked out a technique that would make it possible to discover, for the first time, a mountain on a planet outside our solar system.

The discovery of planets orbiting other stars is among the most important and exciting in modern science. It turns out that our galaxy is teeming with these exoplanets — astronomers find them everywhere they look, thousands of them so far, with the certainty of many more to come.

And that has given rise to a new discipline devoted to investigating what exoplanets are like. The first breakthroughs have revealed the size of these planets, how quickly they orbit their parent suns, and whether they sit in the so-called habitable zone where liquid water could exist.

But astronomers want to know more and are planning another generation of powerful telescopes to find out. What will this new imaging technology reveal about our these distant worlds?

Today we get an answer of sorts thanks to the work of Moiya McTier and David Kipping at Columbia University in New York. They have developed a technique for spotting mountains and valleys on exoplanets and show how next-generation telescopes should start to reveal this geography and topography in the near future.

The technique is simple in principle. McTier and Kipping point out that mountains and valleys change the silhouette of a planet as it rotates. When a planet passes in front of its sun, these changes should be observable as small variations in the amount of light the planet blocks as it rotates. Over time, as the mountains rotate in and out of view, these variations should provide a record of the “bumpiness” of the planet’s surface.

In practice, this technique is extremely hard because the changes are tiny. But they become observable for planets orbiting smaller stars, say the astronomers.

McTier and Kipping first calculate the effect of an Everest-size mountain on an Earth-size planet, otherwise entirely smooth, that is orbiting a sun-like star. In that case, the mountain would change the light the planet blocks by one part in 100,000,000.

But if the same planet were orbiting a smaller white dwarf, the change would be one part in 10,000. That’s much more observable.

Of course, the surface of any real planet would be much more bumpy. But the proof-of-principle calculation shows that this bumpiness will be observable.

A number of factors will make these observations difficult. The most obvious is the thick atmosphere and clouds that obscure the surface of some exoplanets. But that would still allow the characterization of Mars-like planets with little or no atmosphere.

Among the telescopes currently in development that could achieve such observations are the Extremely Large Telescope, which has a 39-meter primary mirror and a five-meter secondary mirror with adaptive optics that can correct for atmospheric distortion. It is currently under construction on Cerro Armazone, a mountain in the Atacama Desert in Chile. Slated to be finished in 2024, it should be able to capture images 16 times sharper than those the Hubble Space Telescope is capable of.

McTier and Kipping calculate that this telescope will be able to characterize the surface topography — the bumpiness — of a Mars-like planet orbiting a nearby white dwarf with just a few hours of observational data. That would represent the discovery of the first mountain outside the solar system.

Such a discovery would provide important insights. Mount Everest on Earth formed by plate tectonics, while Mons Olympus on Mars formed through volcanic activity. So mountains are important clues about a planet’s internal processes.

Astronomers have ambitious plans to find out more. The way sunlight glints off water could provide clues about potential oceans on exoplanets, so the way this reflected light changes as the planet rotates and continents come into view will also be important.

On Earth, this produces an effect known as the “red edge,” because Earth’s continents are covered in a pigment that absorbs strongly in the red part of the spectrum (things that absorb in the red look green). So astronomers are working out how to see the red edge on exoplanets too.

And what about the prospect of seeing industrial pollution, the night lights from alien cities, and — perhaps more likely — orbiting exomoons and rings?

The study of exoplanets may be in its infancy, but it is clearly set to become a science rich in discovery and surprise.

Ref: arxiv.org/abs/1801.05814 : Finding Mountains with Molehills: The Detectability of Exotopography