There are stunning mountain ranges on Earth, Mars and even Pluto. But what about worlds further afield?
at Columbia University in New York presented her research into the embryonic field of exotopography on 11 January at a meeting of the American Astronomical Society. She says that by analysing the dip in a star’s light as a planet passes in front of it, we might be able to discern actual details about the planet’s landscape.
None of the rocky planets in our solar system are perfectly round: there are mountains, canyons, craters and other features that carve elevations high and low across a surface. So why shouldn’t there be similar features on planets orbiting other stars?
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McTier took US Geological Survey maps of our four terrestrial planets and the moon to determine what their light curve around the sun would look like. She also factored in Earth with and without the ocean. She then analysed the data to look at the shape of the shadow as it passed in front of the star to see if large telescopes could detect these variations.
She found that landscape features might show up as bumps in the data, which would correspond to sharp rises or drops in elevation, revealing large scale features on the surface of the world.
It is, however, a few generations from becoming testable. There’s no current instrument that has enough resolution to draw out that kind of detail. The upcoming large telescopes, like the 30 Meter Telescope or the European Extremely Large Telescope, will be just barely up to the task of judging whether some of the bumps in the data are simply natural artifacts of light. On theoretical next generation telescopes, it would take 20 hours of transit data to distinguish features.
But with smaller telescopes, McTier says, “It would just take a lot longer to determine that we had detected bumpiness and not just photometric scatter.”
Finding exo-Alps
But the generation of telescopes after that, ones nearer 100 metres in diameter, will be up to the task. To best separate signal-from-noise, the exoplanet and star should be similar in size. The ideal target for such a test is a white dwarf star with a Mars-sized planet. White dwarfs are around the size of a terrestrial planet and are the remains of a star like the sun after they become a red giant.
“It would require extremely precise measurements,” says , an exoplanet researcher with the SETI Institute who was not involved in the study. “Even if you had a really big telescope, stars are not perfect – they’re variable.”
Flares, star spots or exomoons could throw in random artifacts that mimic a small mountain or crater. Oceanic events and atmospheric distortion could also throw in a few curveballs. Or there could be a seismological event – an alien earthquake – that could alter the planet’s shadow as it passes its star.
But if we can make these measurements, we could have rough topographic maps of distant worlds, learn more about exo-oceans, or even use repeating surface features to study the rotation rate of some of these distant planets – by figuring out how long a day is, it could bolster the case for whether or not a planet is habitable.
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