55 Cancri e is an exoplanet with an identity crisis.
Following its 2004 discovery in a scorching close orbit around a star 40 light-years away, astronomers dubbed the planet a “super-Earth.” At just under eight times the mass and twice the size of our own world, 55 Cancri e is a welterweight that straddles the hazy boundary between terrestrial and gas-giant planets. Nothing like it exists in our solar system. For years researchers have thought the best explanation for its intermediate size is a rocky, Earth-size core smothered beneath a thick, steamy atmosphere. Now, however, a new map of the planet—the first of its kind for such a small world—suggests instead that 55 Cancri e might have no atmosphere at all, and could be an airless, half-molten ball of slag.
Using 80 hours of observing time on NASA’s infrared Spitzer Space Telescope, a team led by Brice-Olivier Demory of the University of Cambridge has crudely mapped the planet’s thermal “phase curve”—variations in its brightness as it circles its star. The planet is tidally locked to its star, meaning that the world rotates just once per orbit, so that one hemisphere is eternally bathed in sunlight while the other languishes in darkness. The team’s measurements suggest the planet’s nightside would actually exhibit a ruddy glow because its temperature appears to be in excess of 1,000 degrees Celsius—as hot as a blast furnace. The real surprise is that 55 Cancri’s dayside is twice as scalding, with its hottest point shifted eastward from its expected position directly beneath the star overhead. The findings appear in Nature. (Scientific American is part of Springer Nature.)
The extreme difference between day- and nightside temperatures suggests 55 Cancri has no appreciable atmosphere, because winds should redistribute heat between the two hemispheres. And yet, the shifted dayside hotspot suggests something is redistributing heat anyway, sweeping it eastward away from the “substellar point” on the planet’s surface where the star’s radiance is most intense. “For the first time, we have used remote sensing to map the temperature distribution of a super-Earth,” says study co-author Nikkhu Madhusudhan of the University of Cambridge. “That’s a great result, but what it’s telling us is quite peculiar: One part of the data says this planet can’t have an atmosphere, but another part says there is something there that must flow.”
That “something,” according to Demory, is probably molten rock. “55 Cancri e’s hotspot, if it has no atmosphere, would mean we are seeing a bare rocky planet with a patch of flowing lava at its surface partially circulating the heat away from the day side,” Demory says. The dayside would be covered in seas of circulating lava, with the lightest, least viscous lavas flowing toward the night side, where they could cool, grow sluggish and crystallize as solid rock.
Alternatively, he says, the planet could have an atmosphere, albeit a very strange one filled with clouds of evaporated rock. Prevailing winds could sweep the clouds eastward with the planet’s rotation, creating the shifted hotspot, but the rock vapor would have to very quickly freeze out and fall to the surface after crossing into the nightside. “The problem is finding a mixture of compounds that would behave like this,” Demory says. “We need something funny that could be gaseous on the dayside yet still condense out on the nightside. We’ve looked, but so far we haven’t found any substance to fit these criteria, so our favored explanation is lava until proven otherwise.”
“It’s tremendously exciting that we have a phase curve of a super-Earth,” says Sara Seager, an astronomer and planetary scientist at the Massachusetts Institute of Technology. “It shows that astronomers are working their best to optimize techniques to work on smaller and smaller planets, and that nature has once again delivered on a fascinating planet orbiting a bright nearby star.”
A hard target
Not everyone is convinced that 55 Cancri e is a lava world, or that the planet’s phase curve from Spitzer is even valid at all. Nicolas Cowan, an astronomer at McGill University who has used Spitzer data to create thermal maps of gas-giant exoplanets, is not sure the aging telescope is up to the task of investigating smaller worlds. “Raw telescope data often exhibit variations of a few percent that have nothing to do with the planet in question or even its host star,” Cowan says. By contrast, the changes claimed for 55 Cancri e amount to shifts in brightness measured in parts per million. Such exquisite measurements are presently only possible at all because the planet’s host star is very bright and thus offers more photons for astronomers to work with. In addition, 55 Cancri e transits its star, meaning it crosses the star’s face as seen from Earth, casting a shadow that astronomers can data-mine for information about the planet’s possible atmosphere and surface.
“If real, this is an exciting result that marks the collision of exo-geology and exo-climatology,” Cowan continues. “My suspicion is that the signal is instrumental rather than astrophysical in nature. Spitzer performs better than advertised, but we routinely push it orders of magnitude beyond spec. We extract science by carefully modeling all the ways in which the spacecraft and the instruments themselves could have caused the apparent brightness of a planetary system to change over time… We are pretty sure we can trust our models of Spitzer down to about a part in 10,000; we are in uncharted territory as far as detector behavior is concerned.”
Heather Knutson, an astronomer at California Institute of Technology who pioneered Spitzer’s thermal mapping of giant exoplanets, is similarly skeptical. “The billion-dollar question here is whether or not 55 Cancri e is, in fact, a rocky planet with little or no atmosphere,” she says.
Knutson is part of a team led by her postdoc Björn Benneke that is now using the Hubble Space Telescope to monitor the planet for signs of an atmosphere. In particular, they are looking at the planet as it transits, seeking a telltale broadening of its planetary shadow due to starlight being absorbed by a hydrogen-rich atmosphere. Already, a competing team has used publicly available data from Benneke's observational program to claim the detection of an atmosphere on 55 Cancri e, but Knutson doubts those results, too. “This planet has been the target of multiple large observing campaigns with both Hubble and Spitzer, and yet we are still trying to figure out the answers to basic questions about the composition of its interior and—if it has one—its atmosphere,” Knutson says. “This speaks to the difficulty of observing such small planets.”
If 55 Cancri e is in fact a lava world, it could open a new frontier in studying exoplanets. Of the thousands of exoplanets now known, most are warm super-Earths close to their stars, suggesting that this variety of planet may be the most common in our galaxy. Launching in 2017, NASA’s Transiting Exoplanet Survey Satellite (TESS) and ESA’s Characterizing Exoplanets Satellite (CHEOPS) should collectively discover and study many more worlds like 55 Cancri e circling nearby stars.
The CHEOPS team is already planning extensive observations of Cancri 55 e, Demory says, to look for plumes or other outbursts linked with possible volcanic eruptions. And NASA’s James Webb Space Telescope, planned for launch in 2018, he says, could study the chemical composition of vaporized material sputtered off the planet by the intense radiation of its star, sniffing out signs of rocks like pyroxene and olivine to track geochemical cycles on the distant world. Alternatively, it could dismiss the notion that Cancri 55 e harbors lava oceans once and for all.
“People might wonder why this sort of planet is interesting when it is so uninhabitable and so unlike Earth,” says study co-author Vlada Stamenkovic, a geoscientist at Caltech. “But in order to understand Earth-like planets we have to somehow understand their chemistry and their interiors, which govern things important for life, like oceans and atmospheres. Partially molten planets like 55 Cancri e could let us learn more about what is inside them and how they are made.”
“These planets are helpful, but they are hellish—you wouldn’t enjoy living there,” Stamenkovic adds. “But they will be enormously helpful for one day understanding those planets where, if we could go there, we would want to sit down, pour a drink and enjoy the sunset.”