Pluto’s tenuous atmosphere appears to have begun contracting after years of holding steady, according to a new study published in the Planetary Science Journal.
Complex chemical compounds called tholins give Pluto a distinctive blue haze. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
Pluto’s atmosphere is composed mainly of nitrogen with traces of methane and carbon monoxide.
First detected via stellar occultations in the 1980s, it is sustained by a delicate balance with the ices on its surface.
Because Pluto has a highly eccentric orbit and an extreme axial tilt, the amount of sunlight reaching different parts of its surface changes dramatically over its 248-year ‘year,’ which in turn affects how much ice sublimates into gas.
Planetary scientists have long debated whether this seasonal cycle would eventually cause the atmosphere to collapse entirely as Pluto moves farther from the Sun, or whether reservoirs of nitrogen ice — particularly in the heart-shaped basin known as Sputnik Planitia — would keep enough gas in circulation to sustain it.
“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 to even the best space telescopes,” said lead author Dr. Amanda Sickafoose from the Planetary Science Institute and her colleagues.
“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, we can characterize Pluto’s atmosphere.”
The team’s new research draws on 10 stellar occultations observed between 2017 and 2023.
Their findings suggest that after decades of increasing pressure, Pluto’s atmosphere held in a stable plateau from around the time of NASA’s New Horizons flyby in 2015 through about 2021.
Since then, the data point to a decline: comparing observations from 2015-2021 to those from 2022, the researchers calculated a pressure drop of about 7% in the clear upper atmosphere and about 16% when accounting for haze lower in the atmosphere.
“We’re at a particularly interesting point for Pluto,” Dr. 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 next years to decades to specifically confirm or refute this trend.”
The authors also found that while the upper atmosphere’s basic structure has remained consistent since 2017, the lower atmosphere shows signs of change, with a shallower light-curve slope in the most recent data.
They attribute this to settling haze particles, a process that can occur on timescales of weeks to about a year.
One 2018 observation also captured brief spikes in the light curve, which the researchers interpret as signs of vertically propagating buoyancy waves in the atmosphere.
They caution that the apparent pressure decline needs confirmation from additional observations, noting that data quality varied widely across the events studied, with some measurements too noisy to draw firm conclusions.
“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 the Earth’s — on a world two-thirds the size of our Moon and 30 times farther from the Sun,” Dr. Sickafoose said.
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Amanda A. Sickafoose et al. 2026. Changes in Pluto’s Atmosphere Based on Stellar Occultation Data from 2017 to 2023. Planet. Sci. J 7, 180; doi: 10.3847/PSJ/ae6cdd
