Of all the recent discoveries reported of late from worlds outside our solar system, a new find may be the most extraordinary yet: A world known as 55 Cancri e, just 40 light-years from Earth, might be volcanically active.
When astronomers first began discovering exoplanets back in the 1990s, they figured it would take a bigger, more powerful successor to the Hubble to study them in any detail. They were wrong. Even with the launch of the James Webb Space Telescope still several years in the future, exoplanet experts have managed to probe the atmospheres of these distant worlds; determine whether they’re gaseous, like Jupiter, or rocky, like Earth; and find what appears to be gigantic storms whipping across them.
It’s not the volcanoes themselves that have astronomers excited; it’s getting a better look at what exoplanets are actually like. The ultimate goal is to find a planet that supports life, and the more scientists can say about conditions on individual worlds, the better they can target their searches. This is one more step along that road—and, says Madhusudhan, “in our wildest imaginations, we couldn’t have guessed we’d be making discoveries like this as early as 2015.”
The discovery isn’t iron-clad yet, however, cautions Madhusudhan. “We don’t want to jump to conclusions,” he says. What he and his colleagues actually see with the Spitzer Space Telescope is a dramatic variation in the planet’s temperature—from about 1880°F at the “cooler” end to nearly 5,000°F at the hotter. “It’s the first time we’ve seen such a huge level of variability,” said Brice-Olivier Demory, also of Cambridge and the paper’s lead author.
Even at the cool end, 55 Cancri e, which is about twice as big as Earth and nearly eight times as massive, is far too hot to support life. If the surface is made of rock, as astronomers believe, it’s either partly or entirely molten. But the wide swings—from the lower temperature in 2012 to the higher in 2013—make it clear that something is going on. It could be variations in the star itself, say the scientists, but those would tend to be random. It could be an illusion of sorts: maybe the planet is being eclipsed every so often by a cloud of gas or dust orbiting the parent star. That could make the 55 Cancri e dimmer, which would make it seem cooler to observers.
Volcanism, however, could also dim the planet, and the astronomers consider that to be more likely. “We know it happens all over our own solar system,” says Madhusudhan. “We see evidence for it on Venus, Mars, [Jupiter’s moon] Io, all kinds of solid bodies. That’s the natural explanation.”
The idea is that powerful eruptions would spew massive plumes of gas and dust into the planet’s atmosphere, masking the hot surface and making the planet appear cooler. The eruptions would have to be more powerful than anything ever seen, but there's a chance that's possible. 55 Cancri e orbits so close to its parent star, with a “year” lasting just 18 hours, that it it could be flexing under tidal forces generated by the star’s gravity. The heat from that flexing could keep the planet’s interior molten, which might drive the eruptions.
“Could” is the operative word here. “We'll need more data to be able to tell for sure whether the variability is due to volcanoes,” says Laura Kreidberg, an expert on exoplanet atmospheres at the University of Chicago, who was not involved in this research. “But it’s still really exciting. This is a prototype of the kinds of observations we can use to search for volcanic activity on other planets.”
Heather Knutson, a Caltech astronomer who also studies exoplanet atmospheres, calls it "a very provocative result.”
“I think we will have to wait for the launch of the James Webb Space Telescope in order to obtain a definitive picture of the state of this boiling hot planet's surface and atmosphere,” Knutson says.
That’s Madhusudhan’s attitude as well. “This is all done with the Spitzer telescope,” he says, “which is on its last legs. One can only imagine what we can do with the [Webb telescope].”