Searching for stinky planets

Artist's conception of a volcanic moon

Artist's conception of an extremely volcanic moon orbiting a gas giant planet in another star system.

  • Exoplanet atmospheres can now be analysed
  • Super-volcanoes could emit copious sulphur gases
  • Could be detected by next generation space telescope

Astronomers think they’ve found a way to detect the presence of sulphur-spewing volcanoes on planets orbiting stars beyond our Solar System.

Volcanoes display the awesome power of Nature like few other events. Earlier this year, ash from an Icelandic volcano disrupted air travel throughout much of northern Europe. Yet this recent eruption pales next to the fury of Jupiter’s moon Io, the most volcanic body in our Solar System.

And now that astronomers are finding rocky worlds orbiting distant stars, they’re asking the next logical questions: Do any of those worlds have volcanoes? And if so, could we detect them?

Work by theorists at the Harvard-Smithsonian Centre for Astrophysics suggests that the answer to the latter is a qualified “Yes.”

“You would need something truly earthshaking, an eruption that dumped a lot of gases into the [planet’s] atmosphere,” said Smithsonian astronomer Lisa Kaltenegger.

“Using the James Webb Space Telescope, we could spot an eruption 10 to 100 times the size of Pinatubo for the closest stars,” she added.

Jupiter's moon Io

Jupiter's moon Io is the most volcanically active body in the Solar System.

Astronomers are decades away from being able to take images of the surface of alien worlds, or “exoplanets”. However, in a few cases they’ve been able to detect the atmospheres of “gas giant” planets known as “hot Jupiters.”

A volcanic eruption would emit fumes and various gases, so volcanic activity on a rocky exoplanet might leave a telltale chemical signature in the planet’s atmosphere.

Sniffing out volcanoes on other world

To work out which volcanic gases might be detectable, Kaltenegger and her Harvard colleagues, Wade Henning and Dimitar Sasselov, developed a computer model for eruptions on an Earth-like exoplanet based on the present-day Earth.

They found that sulphur dioxide from a very large, explosive eruption is potentially measurable because a lot is produced and it is slow to wash out of the air.

Sulphur dioxide has a sharp, acrid smell, sometimes described as similar to the smell of a just lit match.

To look for volcanic sulphur dioxide, astronomers would rely on a technique known as a secondary eclipse, when the exoplanet goes behind its star as seen from Earth.

By collecting light from the star and planet, then subtracting the light from the star alone (while the planet is hidden in the eclipse), astronomers are left with the light spectrum from just the planet. They can examine that spectrum for signs of particular chemical molecules.

Artist's concept of an exoplanet orbiting a star

Artist's concept of an exoplanet orbiting a star. By measuring the light both when the planet is in front of and behind (eclipse) by the star, astronomers can isolate the planet's light and gain information about it's atmospheric gases.

“Our first sniffs of volcanoes from an alien Earth might be pretty rank!” Kaltenegger said. “Seeing a volcanic eruption on an exoplanet will show us similarities or differences among rocky worlds.”

Catching one in the act

The 1991 eruption of Mount Pinatubo in the Philippines spewed about 17 million tonnes of sulphur dioxide into the stratosphere—a layer of air 10 to 45 miles above Earth’s surface. The largest volcanic eruption in recorded history, the 1815 Tambora event, was about 10 times more powerful.

Such gigantic eruptions are infrequent, so astronomers would have to monitor many rocky, Earth-sized exoplanets for years to catch one in the act. However, if alien worlds are more volcanically active than Earth, success might be more likely.

“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,” said Henning. “Once you detected one eruption, you could keep watch for further ones, to learn if frequent eruptions are common on other planets.”

Due to its proximity, a hypothetical Earth or super-Earth orbiting the nearby star Alpha Centauri would offer a best-case scenario for a sun-like star.

But any Earth-like planet less than 30 light-years away could show faint signs of volcanism when studied with the James Webb Space Telescope, the next generation optical telescope due to be launched in the middle of this decade.

Adapted from information issued by CfA / Wade Henning / NASA.

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