Astronomers using the future James Webb Space Telescope could potentially detect volcanic activity on distant Earth-like exoplanets, say Harvard-Smithsonian Centre for Astrophysics theorists.

As astronomers are finding more and more planets around distant stars, in some cases imaging raging storms in their atmosphere, questions are turning to the state of geological activity on the rocky planet contingency. Since large volcanic eruptions expel gaseous fumes, these signatures could leave their mark in the planet's atmosphere. For example, the 1991 eruption of Mount Pinatubo in the Philippines spewed about 17 million tons of sulphur dioxide into Earth's atmosphere.

“You would need something truly earthshaking, an eruption that dumped a lot of gases into the atmosphere,” says Smithsonian astronomer Lisa Kaltenegger, “but using the James Webb Space Telescope we could spot an eruption 10 to 100 times the size of Pinatubo for the closest stars.”

Modelling volcanic eruptions for Earth-sized exoplanets, based on present-day Earth, the astronomers found that sulphur dioxide from a very large, explosive eruption could be detected because of the large volume of gas produced, and by the fact it lingers in the atmosphere. “Our first sniffs of volcanoes from an alien Earth might be pretty rank!” jokes Kaltenegger.

To detect this gaseous emission, astronomers would have to rely on a technique known as secondary eclipse, whereby the exoplanet crosses in front of and behind its star as seen from Earth. This enables astronomers to detect light from both the star, and the star and planet, and by subtracting the light from the star astronomers are left with the signal from the planet alone. Scientists can then analyse the signal for signatures of particular chemical molecules, which each have their own 'fingerprint'.

But large eruptions happen relatively rarely – the largest volcanic eruption in Earth's history was the 1815 Tambora event, which was ten times more powerful than Pinatubo – so astronomers would have to monitor Earth-like exoplanets for many years in order to catch one in the act. However, a younger planet would likely be more volcanically active.

“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,” says Wade Henning. Io, one of Jupiter's moons, is the most volcanically active body in our Solar System. “Once you detected one eruption, you could keep watch for further ones, to learn if frequent eruptions are common on other planets.”

This research will be published in The Astrophysical Journal.