We’re entering the era of the ‘super-Earths,’ when rocky planets larger than our own will pepper the lists of new discoveries. These smaller worlds will occasionally make a transit of their star, as does CoRoT-7b, and that’s when things really get interesting. After all, we know that secondary eclipses, in which a transiting exoplanet swings behind its star as seen from Earth, can be used to study distant atmospheres. The method collects light from both star and planet and, when the planet is hidden, subtracts the starlight to get the planetary signature.

Now Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and colleagues Wade Henning and Dimitar Sasselov are advancing the idea that we can use near-term instrumentation like the James Webb Space Telescope to spot volcanic eruptions using these same methods. Their model is based on eruptions on an Earth-like planet, extrapolating from what happens on our world to suggest that sulfur dioxide from a major volcanic event on an exoplanet is measurable because it can be produced in huge quantity and is slow to wash out of the air.

Mount Pinatubo, which erupted in the Philippines in 1991, accounted for 17 million tons of sulfur dioxide blown into the atmosphere in a layer between 10 and almost 50 kilometers above the Earth’s surface. But we also have the example of the Tambora eruption of 1815, which was as much as ten times more powerful than Pinatubo. Tambora is the largest observed eruption in recorded history, an explosion that could be heard 2600 kilometers away. Fine ash particles stayed in the atmosphere for a period of years, and the summer of 1816 became known as ‘the year without a summer’ in the northern hemisphere , a climatic anomaly evidently related to the release of vast amounts of sulfur.

Eruptions of this magnitude don’t occur frequently on Earth, but there is no reason to think that young, rocky exoplanets would not be more volcanically active than our more mature world. And ponder the effects of tidal heating, not only on planets but also on their moons. From the paper:

Tidal heating may contribute significantly to volcanism for eccentric exomoons or eccentric planets in heliocentric periods of 20-30 days or less… Such tidal heating has the potential to generate from thousands to millions of times more internal heating than in the modern Earth. However, at the upper end of this heat range, magma is more likely to escape in a non-explosive pattern, or may simply emerge into lava lakes or magma oceans partly sustained by the high insolation values near stars, such as the magma ocean suggested for Corot 7b. Such extreme worlds may also be more likely to have a reducing atmosphere. Significant tidal activity can be stimulated in multi-body systems by secular perturbations, secular resonance crossings, or a deep and stable mean motion resonance, analogous to the Galilean moon system.

So the most extreme heating may well work against detectable volcanic activity, but tidal effects may be pronounced in more moderate scenarios:

Modest tidal heating cases may easily supply some exoplanets with both a 10x increase in the size and frequency of large eruptions, while simultaneously enhancing nonexplosive activity.

And that, of course, is only part of the picture. Volcanic activity may also be keyed to planetary age, with younger planets expected to have more residual accretion heat and higher radionuclides, while plate tectonic activity can increase the frequency of explosive volcanoes. The paper is careful to examine other mechanisms for heat escape in non-explosive events and notes that we don’t know whether hotter planets would necessarily show more explosive eruptions. Nonetheless, an Earth-like world less than thirty light years from the Sun should be a fair candidate for JWST studies that can help us put constraints on some of these variables.

Although it’s true that secondary eclipse studies give us a relatively crude picture of a planetary atmosphere, they do help us find particularly abundant molecules and provide us with a basic model that can be developed over time. This new work shows that in the best case scenario, volcanic features become visible at values between 1 to 10 times the Pinatubo eruption — the probability of observing an eruption of Pinatubo class is about 1 percent if four Earth-like planets are observed for one year, while a Tambora-size event could be detected with a 10 percent probability by observing roughly 50 such planets for two years. The paper concludes:

These observations becomes a very interesting option to characterize rocky planets, especially if one assumes larger, and/or more frequent eruptions than on Earth, or smaller host stars, where a planet in the HZ orbits closer to their stars, increasing the transit probability., or longer SO2 residence times than on Earth.

The paper is Kaltenegger et al., “Detecting Volcanism on Extrasolar Planets,” in press at The Astrophysical Journal (preprint).