Can we detect plate tectonics on exoplanets?

As celebrated in this Ars Technica piece, the 2010s was ‘the decade of the exoplanet’. Largely thanks to the Kepler telescope, the past ten years has seen an explosion in exoplanet discoveries. More than 4000 planets have now been identified orbiting other stars, generally arranged in ways not at all like our own solar system. 

It’s very exciting, particular for a space junkie like me*, who grew up reading space books that discussed the possibility of the solar system being totally unique in the Universe. I have a hard time nowadays believing astronomers seriously thought this, but the point is that we had no evidence either way.

Nowadays, geologist me greedily wonders if we can look for signatures of active geology. And it turns out that maybe you can. This paper (see also the write-up here, and there is a version on arXiv if you have problems accessing the version of record) suggests if an exoplanet’s atmosphere is monitored, we could potentially detect the spike of sulphate aerosols injected into the stratosphere by large explosive eruptions (such as the 1991 Pinatubo eruption).

You might be wondering how we can know anything about the composition of an exoplanetary atmosphere, let alone detect a change. It depends on a technique known as ‘transit spectroscopy‘. When it passes in front of its star, gases in an exoplanet’s atmosphere (if it has one) will absorb some of the star’s light. Different gases absorb light at different frequencies, so we can get data on the composition of what is doing the blocking.

Detecting a volcanic eruption across light years would be a pretty amazing feat in itself, but on Earth the most common cause of large explosive eruptions is silica and gas-rich magmas generated at subduction zones – so this particular kind of volcanic signature is at least potentially a signal of plate-tectonic like recycling of the lithosphere specifically, and not just volcanism generally.

Unfortunately, the sensitivity required to detect a sulphate aerosol spike is beyond the capabilities of current telescopes. But good news! Imminently operational telescopes such as the James Webb Space Telescope (allegedly) and the Extremely Large Telescope would potentially be able to make such a detection. However, there is a small caveat: even these new telescopes will only be able to reliably make a detection on Earth-sized planets within about 30 light years of Earth. The authors estimate that there are likely to be less than 10 of those, which is not a great sample size. On the one hand, if we don’t detect anything, it doesn’t really tell us much definitive. On the other hand, if we do find an explosively volcanic planet on one of our first tries, it might indicate that geologically active Earth-ish planets, that turn themselves inside out, are relatively common. 

This would be very cool. Let’s hope that the next generation of telescopes gets lucky.

*If there are parallel universes, there is almost certainly a fairly adjacent one where I pursued my original science career goal of astronomy and astrophysics; if so, I’m pretty sure that alt-Chris is all about the (exo)planets.