A dwarf planet far beyond Pluto might have a paper-thin atmosphere

Scientists have detected methane gas glowing faintly above Makemake, an icy dwarf planet smaller and farther than Pluto in space. 

Until now, Pluto was the only object that far out in the solar system known to have gas. The study, led by the Southwest Research Institute, made the discovery using the James Webb Space Telescope, a joint observatory of NASA and its European and Canadian counterparts. 

The new results could mean Makemake has an ultra-thin atmosphere, or that methane is bursting out of its frozen surface in plumes, similar to the geysers observed on Saturn’s moon Enceladus. Either way, they show the small distant body isn't just a frozen leftover, but a world that's still active and evolving. 

The findings will soon be published in The Astrophysical Journal Letters.

"Makemake is one of the largest and brightest icy worlds beyond Neptune, and its surface is dominated by frozen methane," said Silvia Protopapa, lead author, in a statement. "The Webb telescope has now revealed that methane is also present in the gas phase above the surface, a finding that makes Makemake even more fascinating."

Makemake, discovered 20 years ago, is named after the Rapanui god of fertility. It is only 890 miles wide — just two-thirds the size of Pluto. 

The dwarf planet is so far out in the solar system, about 4.25 billion miles from the sun, that it takes about 305 Earth-years for it to make a complete trip around it. In 2016, astronomers figured out that it doesn't make this journey alone: NASA's Hubble Space Telescope saw a moon orbiting it. 

NASA's Hubble Space Telescope spotted a moon orbiting dwarf planet Makemake a decade ago. Credit: NASA / ESA / Southwest Research Institute /A. Parker / M. Buie

The new Webb data offer the best look yet at Makemake's surface chemistry. The observations have revealed that some of the tiny world's frozen surface is turning into gas. Webb was able to detect this because when sunlight hits the gas molecules, they give off a faint glow. 

Where Makemake’s supply of methane comes from is unclear. Some scientists think it may have formed inside the dwarf planet through water–rock chemistry and later escaped. Others think it could be leftover methane from the birth of the solar system.

Over time, sunlight and radiation break down methane into more complex molecules. Scientists already found some of these on Makemake, such as ethane, ethylene, and acetylene, which match this process. 

But the results can't yet conclude whether the gas is from an atmosphere or a plume-like cloud. Neither explanation is a perfect match for the data, so follow-up Webb observations are necessary. The atmosphere, if one exists, is extremely faint — billions of times thinner than Earth’s and much weaker than Pluto’s. 

Scientists wonder if methane is bursting out of Makemake in plumes, similar to the geysers seen here on Saturn’s moon Enceladus. Credit: NASA / JPL-Caltech / SSI

"If this scenario is confirmed," said co-author Emmanuel Lellouch in a statement, "Makemake would join the small handful of outer solar system bodies where surface–atmosphere exchanges are still active today."

If the methane isn't forming an atmosphere at all, Makemake could be releasing "a few hundred kilograms" of methane bursts every second, Protopapa said, comparable to what's been seen on Enceladus, which blasts water into space.

Webb and other infrared telescopes also showed that spots on the dwarf planet are unexpectedly "warm" for the outer realm — about -190 degrees Fahrenheit. But so far the researchers haven't seen any direct evidence of leaking gas from the warm spots.

If someone connected all the dots, they might assume Makemake has a methane-based weather cycle, but Ian Wong, one of the coauthors, cautions it's premature to draw that conclusion. 

"While the temptation to link Makemake’s various spectral and thermal anomalies is strong," Wong said in a statement, "establishing the mechanism driving the volatile activity remains a necessary step toward interpreting these observations."