Ten Years of Stellar Occultations Show Pluto’s Atmosphere Thinning

Pluto’s atmosphere has begun to thin, according to a decade of stellar occultation observations led by the Planetary Science Institute. Between 2015 and 2022, the dwarf planet’s atmospheric pressure dropped by as much as 16%, the clearest sign yet that the long-predicted seasonal collapse of Pluto’s nitrogen atmosphere may be starting.

A Decade of Watching Pluto’s Shadow

Planetary Science Institute Senior Scientist Amanda Sickafoose led a team that analyzed ten stellar occultations by Pluto recorded between August 2017 and July 2023. In a stellar occultation, Pluto passes directly between Earth and a distant star, and the star’s light briefly dims and vanishes as it filters through Pluto’s atmosphere before reappearing on the other side.

Four of the ten events were multi-chord observations, meaning multiple telescopes at different locations recorded the same occultation from different vantage points, while six were captured from single sites. Instruments involved included Gemini South’s Zorro camera, the Las Cumbres Observatory network, the Lowell Discovery Telescope, the Magellan Clay telescope, and the South African Astronomical Observatory’s 74-inch telescope. The results are published in The Planetary Science Journal.

Why Pluto’s Atmosphere Rises and Falls

Pluto’s atmosphere is composed primarily of nitrogen, with traces of methane and carbon monoxide that combine to form haze. Because Pluto’s surface ices exist in vapor-pressure equilibrium with the atmosphere above them, any change in how much sunlight reaches the surface directly affects atmospheric pressure.

That relationship is complicated by Pluto’s orbit. The dwarf planet has an eccentricity of roughly 0.25 and an axial tilt near 122 degrees, producing large swings in solar heating over its 248-year orbit. Pluto passed perihelion, its closest approach to the Sun, in 1989 and will not reach aphelion until 2114. It is now moving into colder, darker parts of the outer solar system for the first time since its discovery in 1930.

What the New Data Show

The team’s measurements indicate that Pluto’s atmospheric pressure held roughly steady in a plateau from the 2015 New Horizons flyby through about 2021. Between that period and 2022, clear-atmosphere pressure at an altitude of 1,275 kilometers decreased by 7% ± 6%. When haze was included, pressure at 1,215 kilometers dropped by 16% ± 2%.

The upper atmosphere’s structure remained consistent across the full 2017–2023 span, but the team detected a change in light-curve slope in the lower atmosphere, consistent with haze particles settling to lower altitudes over the course of a year or less. One light curve also showed spikes indicating intermittent buoyancy waves moving through the atmosphere.

How You Measure an Atmosphere You Cannot See

No spacecraft is currently near enough to image Pluto’s atmosphere directly, and from Earth-based telescopes the dwarf planet appears as little more than a point of light. Occultations remain the only way to probe its atmospheric structure. Because occultation shadows can sweep across oceans or uninhabited terrain, Sickafoose’s team maps predicted shadow paths in advance and coordinates observers along the track.

“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,” Sickafoose said in a statement released by the Planetary Science Institute.

What This Means for Planetary Science

The finding adds a new data point to a long and sometimes contradictory observational record. Earlier occultation studies found that Pluto’s atmospheric pressure doubled between 1988 and 2002, held steady through 2006–2007, and then increased monotonically through 2016, according to a 2019 analysis by Eliot Meza and colleagues. Other researchers proposed a pressure drop based on 2018–2019 observations, and a 2021 study reported the atmosphere had entered a “freezing out” phase, while a separate 2021 paper described a plateau persisting since 2015.

Sickafoose’s team cautions that more observations are needed before the recent pressure decrease can be confirmed as the start of a sustained decline rather than a temporary fluctuation. The research was supported by NASA’s Solar System Observations program.

Pluto is the only trans-Neptunian object confirmed to host a global atmosphere, making it a benchmark for understanding volatile transport on icy worlds throughout the outer solar system, including bodies like Triton and Eris. Continued monitoring over the coming years, as Pluto moves farther from the Sun, should determine whether this measured pressure drop marks the beginning of the seasonal atmospheric collapse that volatile-transport models have anticipated for decades, or another temporary shift in a still poorly understood system.

Source: Planetary Science Institute — Amanda Sickafoose et al. Published in The Planetary Science Journal. Primary source: arxiv.org/abs/2606.25675 (preprint); psi.edu press release.