Is Pluto a planet? No — but it’s a world, and one whose largely nitrogen-based atmosphere may finally be starting to thin as it travels farther from the sun, with scientists finding evidence that its atmospheric pressure has begun to fall.

Researchers at the Planetary Science Institute in Arizona analyzed observations of the dwarf planet passing in front of distant stars between 2017 and 2023. Combined with earlier observations, the results indicate Pluto’s atmospheric pressure increased from 1988 until around 2015, remained roughly stable through 2021 and then began declining.

When the researchers included Pluto’s haze in their atmospheric models, they calculated a pressure drop of about 16%.

The findings, published in The Planetary Science Journal , could mark an important turning point in Pluto’s atmosphere, although the researchers say more observations are needed to confirm the trend.

Pluto Is Getting Colder As It Moves Away

Pluto takes about 248 years to orbit the sun and follows a highly elliptical path. It made its closest approach to the sun in 1989 and is now moving towards colder parts of the outer solar system. It will reach its farthest from the sun in 2114, before gradually traveling back towards it.

Pluto’s atmosphere is mostly nitrogen, with smaller amounts of methane, carbon monoxide and other compounds. It is closely connected to ice on the surface, which can release gas when warmed and freeze atmospheric gases back onto the surface as temperatures fall.

Scientists therefore expect Pluto’s atmospheric pressure to decline as the dwarf planet moves away from the sun.

Starlight Reveals Pluto’s Thinning Atmosphere

With no spacecraft currently near Pluto, researchers monitor its atmosphere using occultations, which occur when Pluto passes in front of a distant star. By measuring how the starlight fades and reappears, astronomers can study the dwarf planet’s extremely thin atmosphere.

The researchers analyzed 10 occultations between 2017 and 2023 and found Pluto’s upper atmosphere remained largely unchanged, but detected changes lower down that could be caused by haze particles settling towards the surface.

“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,” said Sickafoose.

Will Pluto’s Atmosphere Collapse?

Some previous models predicted Pluto’s atmosphere could eventually collapse as nitrogen and other gases freeze onto its surface. However, that does not necessarily mean its atmosphere will completely disappear. More recent models suggest a reservoir of nitrogen ice in Pluto’s vast Sputnik Planitia basin — once thought to hide an ocean — combined with the ability of its subsurface to retain heat, could sustain an atmosphere throughout its orbit.

The researchers therefore caution that the recent pressure decline still needs confirmation. “We’re at a particularly interesting point for Pluto,” said Sickafoose. “Our work suggests that the atmosphere has recently started decreasing in pressure.”

Wishing you clear skies and wide eyes.