Pluto, the dwarf planet located in the Kuiper Belt, may be undergoing a change in its atmosphere, which manifests itself in a gradual condensation towards its surface. Recent observations, published in 2026 in The Planetary Science Journal, indicate that the celestial body’s atmospheric pressure began to decrease after a period of stability that extended from 2015 to 2021. According to the model that considers atmospheric haze, the post-2021 pressure was 16% lower.
The nitrogen that makes up much of this atmosphere is probably not dispersing into deep space. Most of it appears to be turning into nitrogen ice and depositing on Pluto’s surface as the dwarf planet continues its slow move away from the Sun.
This phenomenon does not represent an “end” to Pluto’s atmosphere, but rather a natural phase in a planetary system that has been exchanging nitrogen between the surface and atmosphere for millions of years. The current understanding is of a change of state rather than an irreversible loss.
Measurement of atmospheric pressure by stellar occultations
The absence of an orbital probe on Pluto causes astronomers to monitor its atmosphere through stellar occultations. This technique involves watching the dwarf planet pass in front of a distant star, as seen from Earth.
If there were no atmosphere, the star would disappear abruptly. However, Pluto’s atmosphere refracts and absorbs starlight, causing a gradual drop in brightness that provides data on pressure, temperature and atmospheric structure.
The new research analyzed ten occultations recorded between August 2017 and July 17, 2023. Four of these events were observed on multiple trajectories through Pluto’s shadow, while six came from unique locations, which complicates distinguishing their geometry from the atmosphere itself.
Although indirect, the occultation technique has a relevant history, having revealed the existence of Pluto’s atmosphere in 1988, long before the New Horizons probe carried out its detailed observations in 2015.
The 16% drop in Pluto’s atmospheric pressure
The most significant reduction was observed in models that incorporate the presence of haze in Pluto’s lower atmosphere. At a reference altitude of 1,215 kilometers from the center of the dwarf planet, the pressure fell from 6.17 microbars in the period from 2015 to 2021 to 5.20 microbars after 2021. This variation represents a decrease of 16%, with a margin of uncertainty of 2 percentage points.
Another method of analysis, focusing only on the clearest upper atmosphere and calculating the pressure at a radius of 1,275 kilometers, indicated a more conservative reduction of 7%, with an uncertainty of 6 percentage points.
It is important to note that these reference rays are crucial, as most light curves from occultations do not reach Pluto’s surface. Scientists adjust the atmosphere at altitudes where starlight actually interacts, using atmospheric profiles to infer conditions at greater depths. Changes in atmospheric structure can therefore modify the conversion to surface pressure.
The observation campaign that detected the change covered the period between August 2021 and July 2023. The formal estimate of 16% is a weighted comparison between the stability period from 2015 to 2021 and the post-2021 adjusted data, influenced mainly by observations from 2022. The July 2023 event, being of low intensity and single trajectory, presented greater uncertainties. The drop became visible in this interval, but cannot be interpreted as two exact pressure readings on these specific dates.
The evolution of Pluto’s atmosphere and its relationship with the distance from the sun
Pluto reached perihelion, its closest point to the Sun, in 1989. The logical expectation would be an immediate contraction of its atmosphere as it moved away.
However, the occultations revealed surprising behavior: atmospheric pressure practically doubled between 1988 and 2002, remained stable in 2006 and 2007, and continued to rise until approximately 2015, remaining stable until 2021.
This delay in response is indicative of the complexity of planetary seasons. Pluto has an axial tilt of almost 122 degrees, an elliptical orbit and a subsoil with the capacity to retain heat. Its climate not only reacts to the distance from the Sun, but also to the incidence of sunlight, the availability of volatile ice and the slowness with which stored heat dissipates.
The orbital distance establishes the general trend, while the dwarf planet’s thermal inertia and geography determine the exact timing and intensity of the changes.
The dynamics of nitrogen between the surface and the atmosphere
Pluto’s atmosphere is predominantly composed of molecular nitrogen, with smaller amounts of methane, carbon monoxide and traces of hydrocarbons. Its surface is also home to large reserves of nitrogen ice, especially evident in Sputnik Planitia, the heart-shaped region.
As already explored, this plain is not a static frozen area. Nitrogen ice moves by convection, flowing and renewing the surface, as well as fueling the atmosphere above.
Under Pluto’s temperature conditions, the boundary between ground and air is dynamic. Nitrogen ice sublimes directly to the gaseous state when heated, and nitrogen gas condenses directly back to the solid state (frost) when cooled. Consequently, atmospheric pressure is directly linked to the vapor pressure of surface ice and can vary drastically with small temperature fluctuations.
The New Horizons probe complemented this data with images that showed Pluto surrounded by several atmospheric layers. That bluish twilight is the result of the scattering of sunlight by organic particles, even though the gas itself is extremely rarefied.
The new study found that the basic shape of the upper atmosphere remained consistent between 2017 and 2023, within the measurement’s margins of error. The light curves of the lower atmosphere, however, showed changes. The authors suggest this is compatible with variations in thermal gradients, fog, or both.
For particles with a diameter between 0.2 micrometers and 10 nanometers, the estimated sedimentation time varies from about 10 to 400 days. A gentler slope in the recent light curve may therefore indicate the descent of haze as the atmosphere contracts.
Atmospheric collapse: a phase change, not annihilation
Planetary scientists use the term “atmospheric collapse” to describe a large loss of pressure caused by condensation. While it is a technically useful term, it can be emotionally misleading, suggesting a total disaster.
In the case of Pluto, much of the nitrogen just changes phase, joining the ice reservoir. It is not necessarily lost by the dwarf planet permanently. In warmer conditions, in another part of its seasonal cycle, some of this ice may sublime and return to the atmosphere. This means that the dwarf planet has an intricate system of mass exchange between its surface reservoirs and its thin gas layer.
Existing models present divergences regarding the intensity of the pressure drop. Older scenarios predicted the freezing of most of the atmosphere in the coldest regions of Pluto’s orbit. More recent models, which consider the transport of volatile substances, generally predict the maintenance of a global atmosphere, albeit tenuous, throughout its 248-year orbital cycle. This is because Sputnik Planitia holds a lot of nitrogen and the subsoil has considerable thermal inertia.
The distinction is significant. A thinner global atmosphere, a temporarily localized atmosphere, and a nearly complete seasonal collapse are not identical outcomes. Current observations are not yet precise enough to conclusively determine which model is most accurate.
The persistence of Pluto’s atmosphere over millions of years
There are no direct observations that prove the uninterrupted persistence of Pluto’s atmosphere over geological time. Detection by humans only occurred in 1988. The idea that it “persisted in some form” is an inference based on the inventory of volatile substances, the surface record, estimates of atmospheric escape, and climate models.
A long-term model for the nitrogen cycle has tracked the transport of volatile substances on Pluto for millions of years, including variations in obliquity and orbital geometry. Their simulations describe the movement of nitrogen between Sputnik Planitia and other regions, the formation and erosion of deposits, and surface pressures over the last 30 million years.
It is in this deeper sense that Pluto’s atmosphere can endure. The individual nitrogen molecule does not need to remain in a gaseous state. The atmosphere can contract, ice can shift, and global pressure can vary dramatically while the interconnected reservoir continues to function.
It would be an exaggeration to claim that Pluto has maintained current atmospheric pressure or a continuous global haze for millions of years. It is, however, reasonable to conclude that the mechanism capable of rebuilding a nitrogen atmosphere operated over many extensive climate cycles.
What the 2114 Forecast Means for Pluto
According to NASA information about Pluto, the dwarf planet orbits the Sun at a distance ranging between 30 and 49.3 astronomical units. Pluto is predicted to reach aphelion, its furthest point from the Sun in its orbit, around the year 2114.
