Pluto’s hazy atmosphere may be collapsing as it moves farther from the sun

Pluto is on an icy march away from the sun. In 2114, it will turn back, but until then, its elliptical orbit will carry it to colder and darker parts of the outer solar system. This is Pluto’s first time entering the outer solar system since its 1930 discovery, and along this journey, its atmosphere could slowly freeze out.

Details of atmospheric changes during this journey have been observed by a team of researchers led by Planetary Science Institute senior scientist Amanda Sickafoose, and they may have even detected the first signs of Pluto’s atmospheric collapse.

In a new paper published in the Planetary Science Journal, Sickafoose and her team describe how Pluto’s atmosphere changed over the 2017–2023 period. They found that between the 2015 New Horizons flyby and partway through 2021, Pluto’s atmospheric pressure remained constant. Then, between mid-2021 and July 2023, the bulk pressure decreased by 16%. Their models assumed a hazy atmosphere, as was seen during the Pluto flyby.

Reading Pluto by starlight

There are no spacecraft near enough to Pluto to see its atmosphere, and from Earth, Pluto appears to be nothing more than a tiny blob, even to the best space telescopes. Luckily, as Pluto, Earth and even the stars slowly move through the sky, chance alignments, called occultations, cause Pluto to pass in front of some stars. By observing how starlight changes as a star passes behind Pluto during such occultations, scientists can characterize Pluto’s atmosphere.

But capturing data from these events can be tricky. As a star passes behind Pluto, Pluto’s shadow is cast on Earth. Such occultation shadows can stretch across oceans or uninhabited land, so scientists create maps of these shadow paths to coordinate observations. Since 2017, Sickafoose and her collaborators have observed 10 different occultations; during only four were they able to witness the same event from multiple locations on Earth. These different observers could, when close enough, confirm one another’s results, and when more spread out, they were able to peer through different parts of Pluto’s atmosphere.

“We often prefer to observe predicted occultations that have shadow paths over large observatories because that equipment and data have proven successful,” Sickafoose said. “Then, we try to improve those data sets by collaborating with local observers in the shadow paths.”

While Pluto is faint—and will only get fainter as it moves away—what matters most in these observations is the brightness of the star. Telescopes carefully measure how much light is detected before, during and after the occultation to see how the star’s light fades and returns to view.

Gradual fading occurs during an occultation as Pluto’s atmosphere bends the star’s light (and any haze particles can scatter light like fog), and then the light drops off completely as the star passes behind Pluto’s icy body.

Pressure drop, haze questions

“I’m constantly amazed at how the simple technique of watching starlight dim and reappear allows us to study a thin atmosphere—a few millionths of Earth’s—on a world two-thirds the size of our moon and 30 times farther from the sun,” Sickafoose said.

By observing multiple occultations over several years, Sickafoose and her team measured the atmosphere’s changing pressure.

Pluto’s atmosphere is primarily nitrogen, along with other carbon-based compounds, like methane and carbon monoxide, that can combine to form hazes. As the atmospheric pressure drops, any hazes should start to thin out and settle to lower altitudes. The exact physics of the atmosphere isn’t well understood, but it is tightly coupled with the properties of Pluto’s surface ices, according to Sickafoose. Studies like this one, over Pluto’s multigenerational orbit, should untangle the truth.

“We’re at a particularly interesting point for Pluto,” Sickafoose said. “Our work suggests that the atmosphere has recently started decreasing in pressure. I am hopeful that we’ll be able to get more data in the coming years to decades to specifically confirm or refute this trend.”

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Gaby Clark

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Robert Egan

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