If there’s one thing the Kepler planet-hunting mission taught us, it’s that planets are common in our galaxy. That telescope alone has discovered more than 2,300 confirmed planets around other stars, with nearly twice as many awaiting confirmation. Included in that count are 30 worlds less than twice the size of Earth, located in the habitable zones of their respective stars.
Unfortunately, we don’t know what these small, rocky planets look like—even the largest telescopes currently in development won’t be powerful enough to resolve surface features. But as the instruments get more powerful still, one research team is wondering whether we might be able to detect mountains on an Earth-sized planet. Under the right circumstances, they argue, these large surface features should show up.
Moiya McTier, a Ph.D. student in astronomy at Columbia University, and her advisor David Kipping, initially were interested in how rough some of the worlds in our own solar system would appear from a great distance. They chose the four rocky planets (Mercury, Venus, Earth, and Mars) along with Earth’s moon. They weren’t surprised that Mars had the bumpiest surface—it does have the huge Olympus Mons volcano (three times higher than Everest) and the vast Valles Marineris canyon, after all. With its oceans, Earth appears pretty flat. Remove the oceans, though, and the planet is almost as bumpy as Mars. “The ocean hides a lot,” McTier says. Earth’s moon was the smoothest body—no surprise, given the ancient lava flows that sculpted its surface.
What the researchers call “exotopography” is a fairly new concept in exoplanet science. Lacking data from a real planet, they tried simulating a solar system where a Mars-like planet orbits a red dwarf, a star dimmer than our own sun (which makes planetary transits easier to spot). They found that the topography of Mars would only generate a small change in the light signal produced when the planet transits in front of the star—something like one part in 10 million or 100 million, McTier says. But if Mars was orbiting an even smaller and dimmer planet—a white dwarf—the signal would be closer to 10 parts per million.
That’s still a challenge to observe, but, as McTier says in the video above, it would be possible using the Overwhelmingly Large Observatory once proposed by the European Southern Observatory. That telescope would have had a planned aperture of 100 meters (328 feet), and could spot topography (not individual mountains, but general bumpiness) in just 20 hours of observing time, the researchers determined. Although that project has been shelved for now, ESO is already pouring resources into the 39-meter (128-foot) diameter European Extremely Large Telescope (ELT), which should be online in the 2020s. McTier says the measurement is still possible with ELT, “but it would just take a lot more time.”
That’s assuming we find enough planets around white dwarf stars in the first place. White dwarfs are the remnants of sun-like stars that have run out of hydrogen and helium to burn. Astronomers haven’t found many planets around these types of stars, but they haven’t spent much time looking, either.
Why search for mountains around other planets? It may be that planets with mountains—which are associated with plate tectonics and volcanism on Earth—are more conducive to life. These kinds of studies may therefore add another piece to the puzzle for scientists trying to identify other planets like our own.