Volcanoes on a Super-Earth?

Observations of nearby super-Earth 55 Cancri e reveal huge, as-yet unexplained changes in the exoplanet’s infrared emission. Volcanoes are one possible cause.

Artist's concept of 55 Cancri e

An artist's portrayal of the star 55 Cancri and its innermost planet.
University of Texas / NASA / NSF

The exoplanet 55 Cancri e, a super-Earth 46 light-years away, is having hot flashes. Over the course of a year, the planet’s infrared emission brightened almost four-fold, a huge and unusual variation that implies a temperature change of 1300°C between 2012 and 2013.

Brice-Olivier Demory (University of Cambridge, UK) and colleagues dug out this find from years of Spitzer Space Telescope observations. They have a compelling idea that might explain these odd observations: volcanoes.

Demory and his colleagues used Spitzer to measure 55 Cancri e’s thermal output for a total of 85 hours spread out between 2011 and 2013. The observations covered the planet as it passed in front of (transited) and passed behind (was occulted by) its host star multiple times. During each occultation, the star blocks the planet’s thermal emission (as well as its reflected light), which astronomers can then measure by its absence.

What Demory’s team found was a change of almost 300% in thermal emissions between occultations measured in 2012 and 2013. That’s no small blip — it signifies a corresponding temperature change from 1400 to 2700 Kelvin (or from 2100 to 4400°F). “This is a really exciting result,” says Laura Kreidberg (University of Chicago).

At more than seven times Earth’s mass and double its density, 55 Cancri e is probably rocky but not the most pleasant place for life. Circling its host star in just 18 hours, its snug but stable orbit makes the planet hot enough to melt metal — so hot, in fact, that some scientists have speculated that it could be a diamond-rich planet, though that’s being debated.

Because 55 Cancri e’s average temperature is so high, the planet’s crust and mantle are probably weakened, if not wholly molten, the authors write. So the surface of this alien world may well host magma oceans as well as volcanic activity.

Alien World, Volcanic Plumes

55 Cancri e, without and with volcanic plumes

55 Cancri e, a super-Earth with a molten surface (left) might occasionally be covered by giant volcanic plumes (right), which dim the planet's infrared emission.
NASA / JPL-Caltech / R. Hurt

Volcanism might explain how the exoplanet became dim and cool to begin with. Volcanic ejecta from this dense world would balloon up in plumes before falling back to the surface. Depending on the composition of the atmosphere and the volcanic ejecta (neither of which can be measured from these observations), a plume could climb anywhere between 100 and 300 miles. Plumes that are high enough and wide enough could mask some of the planet's heat and thereby lower its infrared emissions.

But the planet’s temperature could also vary if a donut-shaped ring of material around the planet (or perhaps the star) hides some of the planet’s infrared emission. A similar ring surrounds the Jovian moon Io.

“It's hard to say [which is the] likelier possibility. This planet is so completely unlike anything we have in the Solar System that we don't know what to expect,” says Kreidberg.

In fact, both scenarios may be at play. Combining the two “has the potential to explain all the observations we have about this planet,” Demory says, “especially a new set of observations we are currently working on.”

But ultimately, pinpointing the cause of 55 Cancri e’s thermal variations will require near-future instruments such as the James Webb Space Telescope, scheduled to launch in 2018. This infrared telescope should provide the high spectral resolution and broader wavelength coverage needed to determine the nature of the material around 55 Cancri e, and future observations could even shed light on the planet’s surface composition.

Brice-Olivier Demory et al., “Variability in the super-Earth 55 Cnc e.” arXiv.org. 10 March 2015.

Alien worlds are weird! Read more on alien weather reports in our May 2014 issue of Sky & Telescope.

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