Scientists may have found the first signs that Pluto’s thin atmosphere is beginning to break down as the dwarf planet gradually moves farther from the sun. Recent observations indicate that atmospheric pressure on Pluto has started to decline, lending support to long-held predictions that its gases could eventually freeze onto its icy surface. These insights come from an international research team led by Amanda Sickafoose of the Planetary Science Institute and were published in The Planetary Science Journal.
Unlike the eight main planets, Pluto orbits the sun along a highly elongated, elliptical path. It reached its closest point to the sun in 1989 and has been steadily drifting outward ever since. It will continue journeying into the cold outer regions of the solar system until 2114 before reversing course and heading back inward. As Pluto cools, scientists have expected its atmosphere—primarily composed of nitrogen, with traces of methane and carbon monoxide—to gradually freeze and settle onto its surface as ice.
Confirming this process has been challenging because Pluto is nearly 6 billion kilometers away from Earth, and no spacecraft are currently nearby to study it directly. Instead, researchers use a technique called stellar occultation, where Pluto passes directly in front of a distant star from our perspective. During such events, the star’s light filters through Pluto’s atmosphere before vanishing behind the dwarf planet. By analyzing how the star’s light dims and reappears, scientists can infer details about the pressure and structure of Pluto’s atmosphere.
Since 2017, Sickafoose and her team have tracked ten of these rare occultations using telescopes around the world. Because Pluto’s shadow often crosses remote oceanic regions or sparsely populated areas, only four of these events were observable from multiple locations, enabling comparative analysis of different atmospheric regions.
Their analysis showed that from the time of NASA’s New Horizons mission in 2015 up through mid-2021, Pluto’s atmospheric pressure remained fairly stable. However, between mid-2021 and July 2023, they detected roughly a 16% drop in atmospheric pressure. The researchers incorporated computer models that factored in the hazy atmosphere observed by New Horizons, which contains minuscule haze particles formed when sunlight triggers chemical reactions involving methane and other carbon-based gases. These hazes scatter light and are crucial in interpreting occultation data.
If the atmosphere continues weakening, scientists expect the haze particles to become even thinner, settling closer to the surface as the gases freeze out. Although these new findings strongly suggest an atmospheric collapse may be underway, the researchers emphasize that more ongoing observations are necessary to confirm this trend. Since Pluto takes 248 Earth years to orbit the sun, its seasonal changes happen very slowly.
By consistently monitoring these rare occultations, scientists hope to deepen their understanding of how Pluto’s atmosphere interacts with its frozen surface and how its icy world evolves as it journeys through the distant reaches of our solar system.