More than 2 billion kilometers farther from the sun than Pluto, a frigid world named Makemake sports the most distant gas ever seen in our solar system, new observations reveal.
“By surprise, we found evidence of gas” on Makemake, even though it currently resides 53 times as far from the sun as Earth does, says Silvia Protopapa, a planetary scientist at the Southwest Research Institute in Boulder, Colo. She and her colleagues submitted the discovery September 8 to arXiv.org.
Makemake is so remote that it takes 306 years to orbit the sun, compared with 248 years for Pluto, the previous record-holder. Pluto’s atmosphere revealed itself in 1988 while passing in front of a star and blocking its light.
Protopapa and her colleagues did not expect to find any gas around Makemake. Previous observations had shown no trace of gas when the world passed in front of a background star. But that’s only because the gas is barely there: If it constitutes an atmosphere, the surface pressure is roughly 100 billionth Earth’s atmospheric pressure, or one millionth Pluto’s. Yet observations from the James Webb Space Telescope, or JWST, in 2023 managed to discern the gas. The telescope’s large size gives it great sensitivity, and it focuses on infrared light, the spectral range where these frigid, far-off worlds reveal their compositions.
“The thing that [the detection] says most of all is the extreme, fabulous power of the Webb telescope to make discoveries,” says William McKinnon, a planetary scientist at Washington University in St. Louis who was not part of the discovery team. “It’s blown the doors off the outer solar system in terms of figuring out what’s on the surfaces of all these mysterious worlds.”
Makemake is so cold that methane freezes to its surface and blankets it with ice reflecting about 80 percent of sunlight. The methane gas may come from this ice vaporizing in the weak sunlight, producing a tenuous atmosphere.
But a more exciting possibility exists, Protopapa says. Plumes may erupt the gas from Makemake’s interior, similar to the geysers that shoot water into space from Saturn’s moon Enceladus.
At only a third the diameter of Makemake, Enceladus owes its drama to gravitationally-driven heating from the ringed planet. While Makemake doesn’t have a planet nearby to trigger geysers, its relatively large size could make it active. At about 1,430 kilometers across — 60 percent the width of Pluto — it is probably the fourth largest known object revolving around the sun beyond Neptune’s orbit.
Like Pluto, Makemake is orange — probably, Protopapa says, because sunlight and cosmic rays alter its methane into more complex compounds. But unlike Pluto, which is rich in nitrogen ice and gas, JWST saw no sign of nitrogen anywhere on Makemake. The researchers suspect the smaller body may have lost its nitrogen over time: The gas is more volatile than methane at Makemake’s surface temperature, and the world’s weak gravity may have been unable to hold on to the ensuing nitrogen gas. It’s possible, though, that nitrogen ice still lurks underneath Makemake’s methane ice.
If Makemake can have gas, might even more remote worlds in our solar system harbor it too? Eris is nearly twice as far from the sun as Makemake and almost as large as Pluto; furthermore, Eris’ surface has both methane and nitrogen ice. A 2010 passage of Eris in front of a star revealed no gas, nor yet have any JWST observations. For now, Protopapa says, Makemake is unique in showing methane emission at such a large distance from the sun. But who knows what JWST’s sharp infrared eye might see on Eris in the future?