We've become very adept at finding extrasolar planets, but we're still working on the trickier task of figuring out what those planets' atmospheres look like. Now astronomers have found a way to test whether an Earth-like exoplanet has active volcanoes.
Currently, our atmospheric tests for exoplanets only work on Jupiter-sized gas giants, leaving the handful of smaller rocky planets we find a tantalizing mystery. In a sense, this new method is no different from the one used on the gas giants - it's all about identifying what gases are present in huge quantity in the atmosphere. Of course, while a gas giant is naturally completely covered in thick layers of gas, a rocky planet requires a massive volcanic eruption to throw up detectable quantities of gas into its atmosphere.
Specifically, astronomers are on the lookout for sulfur dioxide. A sufficiently huge eruption would create enough of this compound for us to detect it on Earth, and the compound takes a long time to break down in the atmosphere. That would give astronomers a much better chance of spotting the remains of a volcanic eruption on a distant world.
Of course, it isn't just any volcanic eruption that would produce this effect. In fact, any alien astronomers searching for signs of volcanic activity on Earth would only be able to detect a single eruption in all of recorded history: the 1815 Tambora Event, which was still barely powerful enough to register on this scale.
Still, we can hope Earth is on the low end of the volcanic scale, as astronomer Wade Henning argues:
"A Tambora-sized eruption doesn't happen often here but could be more common on a younger planet, or a strongly tidally active planet - analogous to Io. Once you detected one eruption, you could keep watch for further ones, to learn if frequent eruptions are common on other planets."
Using the James Webb Telescope, the most powerful exoplanet-hunting telescope yet built, we could detect volcanic eruptions - and thus signs of larger geological activity - on planets within thirty light-years. It isn't much, but it establishes a toehold in the complex task of studying exoplanet atmospheres, and with further technological advances here we might be able to detect fainter eruptions further away.