Scientists may have detected the first signs that Pluto’s thin atmosphere is beginning to collapse as the dwarf planet slowly travels farther from the sun.
New observations suggest the atmosphere’s pressure has started to fall, supporting long-standing predictions that Pluto’s gases could eventually freeze onto its icy surface.
The findings come from an international team led by Amanda Sickafoose of the Planetary Science Institute and were published in The Planetary Science Journal.
Unlike the eight major planets, Pluto follows a highly stretched, elliptical orbit around the sun. It reached its closest point to the sun in 1989 and has been moving steadily outward ever since.
It will continue traveling deeper into the cold outer solar system until 2114 before beginning its long journey back toward the sun.
As Pluto becomes colder, scientists have expected that its atmosphere, which is mostly made of nitrogen with small amounts of methane and carbon monoxide, would gradually freeze and settle onto the surface as ice.
However, confirming this process has been difficult because Pluto is nearly 6 billion kilometers from Earth, and no spacecraft are currently close enough to study it directly.
Instead, researchers rely on a clever observing method known as a stellar occultation. Occasionally, Pluto passes directly in front of a distant star as seen from Earth.
When this happens, the star’s light passes through Pluto’s atmosphere before disappearing behind the dwarf planet. By carefully measuring how the starlight fades and then reappears, scientists can determine the structure and pressure of Pluto’s atmosphere.
Since 2017, Sickafoose and her colleagues have observed 10 of these rare events from telescopes around the world. Because Pluto’s shadow often crosses remote regions or oceans, only four of the occultations were successfully observed from multiple locations, allowing researchers to compare measurements from different parts of the atmosphere.
The team’s analysis found that Pluto’s atmospheric pressure remained largely unchanged between NASA’s New Horizons flyby in 2015 and about the middle of 2021. After that, however, the picture changed. Between mid-2021 and July 2023, the atmosphere’s overall pressure dropped by about 16%.
The researchers used computer models that included the hazy atmosphere observed by the New Horizons spacecraft. Pluto’s atmosphere contains tiny haze particles formed when sunlight triggers chemical reactions involving methane and other carbon-containing gases. These hazes help scatter light, making them important for interpreting occultation measurements.
If Pluto’s atmosphere continues losing pressure, scientists expect these hazes to become thinner and gradually settle closer to the surface as the gases freeze out.
Although the new results strongly suggest that atmospheric collapse may have begun, the researchers say additional observations over the coming years and decades will be needed to confirm the trend. Because Pluto takes 248 Earth years to complete one orbit around the sun, its seasonal changes happen extremely slowly.
By continuing to monitor these rare occultations, scientists hope to better understand how Pluto’s atmosphere interacts with its frozen surface and how distant icy worlds change as they move through the outer reaches of the solar system.
