In the solar system’s dark hinterlands, well beyond the orbit of Pluto, lie Eris and Makemake—two primeval, Lilliputian icy worlds that see the sun as a diamantine speck. Each world’s surface temperature scarcely crests above absolute zero, and scientists long assumed their rocky core to be cold and dead. But when astronomers gazed at each world with the James Webb Space Telescope (JWST), they found evidence of planetary sorcery: fresh-looking methane, presumably produced by a recent flurry of icy volcanism. Somehow these worlds’ unquiet innards pumped out crust-puncturing plumes to lacquer their surface in a methane-rich frost.
To make this discovery, scientists used JWST’s keen infrared vision to determine what kind of hydrogen went into making up that frosty methane, which is a molecule composed of a carbon atom nestled in four hydrogen atoms. They found that the methane was deficient in deuterium—a heavier type of hydrogen mostly forged during the big bang—and had a surplus of the standard lighter hydrogen that’s more commonplace throughout the cosmos.
As reported by two recentstudies published in the journal Icarus, this suggests the methane on Eris and Makemake isn’t primordial but somewhat youthful—cooked up not at the dawn of time but rather by recent tempestuous geologic activity deep within these gelid orbs. In other words, despite the freezing cold that reigns so far from the sun, these dwarf planets may still have warm, beating hearts.
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“This is really cool,” says Jani Radebaugh, a planetary geologist at Brigham Young University, who wasn’t involved with either study. The evidence that the authors present is “really strong,” making it easy to picture Eris and Makemake as effervescing, ever-changing isles.
After decades of incremental discovery that have shown again and again that icy worlds of the outer solar system are adroit at sustaining liquid water seas and oceans, perhaps these latest data points aren’t so surprising. Their shocking implications for life’s potential cosmic prevalence, however, are hard to understate. “For life to be supported on a planet or moon, you need to have geologic activity,” says Christopher Glein, a planetary scientist at the Southwest Research Institute in San Antonio, Tex., and an author on both studies. And these fresh results from Eris and Makemake “motivate you to start thinking of these bodies as potentially habitable worlds.”
Both Eris and Makemake were discovered in 2005 during surveys at California’s Palomar Observatory. Makemake, with a radius about one ninth of Earth’s, is almost 50 times as far away from the sun as our own planet. Eris, a little smaller than Earth’s moon, is about 70 times further from the sun than Earth. Astronomers found hints of methane on their shiny surfaces early on—but aside from that, for the next decade, both worlds were little more than enigmatic smudges of light.
The Kuiper Belt—the donut-shaped torus of icy debris beyond Neptune—is full of cometlike icy objects, leftovers from the solar system’s creation. Astronomers presumed Eris and Makemake were essentially a bunch of ancient, methane-rich comets stuck together. “It’s a nice story. But we couldn’t really test it,” Glein says.
Then in 2015 NASA’s New Horizons spacecraft flew by the most famous Kuiper Belt object, Pluto—and the scientific community’s collective jaw crashed to the floor as a dynamic world swam into view, one with nitrogen icebergs, volcanolike mountains of alien ices and suspicious craters suggestive of subterranean liquid water oceans. Pluto wasn’t geologically comatose but alive and kicking.
New Horizons didn’t see any active eruptions of alchemical slurries of water, nitrogen and methane lava, as some theorists had hoped. But the landscape nonetheless looked to be smothered in youthful flows, snows and frosts. “After Pluto, everything changed. All of a sudden it made sense to think about cryovolcanoes, which had been kind of a myth,” says David Rabinowitz, an astrophysicist at Yale University and co-discoverer of Eris and Makemake, who wasn’t involved with the new work.
Perhaps, then, Eris and Makemake weren’t dissimilar to Pluto, scientists thought. But without a robotic envoy heading their way anytime soon, how would anyone know?
JWST came in clutch. Using its Near Infrared Spectrograph to watch the glints of reflected sunlight from Eris and Makemake alike, researchers ascertained the ratio of heavier-to-lighter methane upon each world’s surface. Primordial methane would have a higher proportion of deuterium—something like that of comet 67P/Churyumov-Gerasimenko, an astronomically antediluvian object born not long after the sun began to shine. But as reported in their first study, published in December 2023, both worlds instead had mid-to-low ratios, indicating their methane veneers were far younger.
In its second study, published this month, the team speculates on how this fresh methane may have formed. One “hotspot” model features a scorching interior, with scattered patches of each world’s rocky core roasting at circa 400 degrees Celsius; fluids from overlying layers of liquid water would filter down to mingle with any carbon dioxide or carbon monoxide within these natural ovens, brewing into methane that then rushes upward, punching through a thin ice shell to erupt onto the surface.
Another possibility is that “the core is just slowly baking,” Glein says. In this scenario, carbon-rich matter in and around the core gently simmers into methane until an eruptive process similar to that of the hotspot model unfolds, albeit through a thicker ice shell and a thinner liquid-water layer. Alternatively, if the worlds have drier, more tepid innards, then methane might simply seep up into a churning icy layer before eventually trickling out at the surface. These latter two scenarios are thought to best fit the smaller Makemake, while the larger core of bulkier Eris may be a better candidate for hosting many hotspots.
All three models require a source of heat. Tidal heating—which is known to power eruptions on Jupiter’s Io and Saturn’s Enceladus—cannot be invoked here because it would demand orbital dynamics that neither Eris nor Makemake possess.
That may not be a problem, however. “We’ve learned more recently that bodies lacking tidal heating can support potential oceans and active geology,” Glein says. “The poster child for that was Pluto.” Heat leftover from their formation as well as that generated by decaying radioactive compounds in their rocky core, plus some thermal insulation provided by their icy shell, could keep both worlds somewhat toasty.
Their hypothetical cryovolcanism is also probably not that dissimilar from the (equally hypothetical) Plutonian variety. Plumes of twinkling methane ice gushing out of crevasses seem plausible. The methane could spray out, then rain down to condense and freeze on the surface, Radebaugh speculates. Perhaps, in some cases, it even oozes out like a strange, toothpastelike icy lava.
That these worlds are so flamboyantly active isn’t a certainty. The deuterium-to-hydrogen ratio is a potent tool for gauging worldly ages, but scientists do not definitively know what a pristine primordial ratio looks like. “That is the question,” says Stefanie Milam, JWST deputy project scientist for planetary science at NASA’s Goddard Space Flight Center, who was a co-author of the December 2023 study. Comets, which are often thought of as ancient, are not immutable: their surface chemistry is altered in various and uncertain ways by solar and cosmic radiation, making any deuterium-based dating a bit of a guesstimate. But the fundamental notion of inner heat on these two icy worlds, as presented by both studies, is reasonable.
The findings about the two dwarf planets are just preludes of what’s to come. “We’ve also got JWST data on Pluto and Triton,” says Will Grundy, a planetary scientist at Lowell Observatory in Flagstaff, Ariz., and an author on both studies. And eventually, researchers will try to get even smaller objects in JWST’s crosshairs.
“Each world may have its own story,” Glein says. They may not be visited by spacecraft for many decades, but thanks to JWST, these once unfathomable celestial objects are becoming tangible, knowable realms—and it already seems like many of them won’t be iced-up planetary husks but dominions humming with geologic life.