The distant dwarf planet Pluto was long considered a cold, dead world. The images and data that NASA’s New Horizons space probe sent to Earth in July 2015 were all the more surprising: Despite its extreme cold, the dwarf planet turned out to be a dynamic world with a thin but amazingly complex atmosphere: the gas shell of the dwarf planet consists of several layers, changes over the course of the seasons and even generates winds.
Does the gas envelope collapse or is it preserved?
What remains unclear, however, is what will happen to Pluto’s gas shell in the coming years. The orbit of the dwarf planet is tilted and extremely eccentric: at the point of its orbit closest to the sun, Pluto is only about 30 astronomical units away from the sun, but at its furthest point it is almost 50. As a result, the already sparse solar radiation varies greatly over the course of Pluto’s year, which lasts around 248 years. Pluto passed its closest point to the sun in 1989 and has been moving increasingly further away since then.
But as the distance from the sun increases, it gets colder on Pluto. “This changes the distribution of volatile substances on its surface and also the atmospheric pressure,” explain Amanda Sickafoose from the Planetary Science Institute in Tucson and her colleagues. However, it is unclear what this means for the dwarf planet’s already thin atmosphere: “The model-based predictions range from a complete collapse of the gas envelope to continued existence as before,” say the researchers.
Star occultations as measuring aids
In order to find out more without visiting Pluto, Sickafoose and her team used an astronomical approach: They analyzed the light curves of stars that passed directly behind Pluto between 2017 and 2023. During such occultations, starlight penetrates the atmosphere at the beginning and end of the occultation and is only partially dimmed. The length, intensity and structure of this transition phase provide information about the nature of the gas envelope.
The light curve when Pluto occults a star also provides information about its gas envelope. — © A. Sickafoose
“It never ceases to amaze me that using such a simple method – dimming and re-lighting starlight – we can observe the thin atmosphere of such a distant, small celestial body,” says Sickafoose. By tracking Pluto’s ten occultations, sometimes using several observatories in parallel, the astronomers were able to follow the development of its gas envelope in detail.
First a plateau, then decreasing pressure
This showed that from 2015 to 2021, Pluto’s gas envelope initially barely changed. The density remained largely the same and the obscuration of the lower layers of the atmosphere by ice crystals did not show any significant changes. These observations match measurements from 2020, as the astronomers explain. At that time, researchers had also found evidence of such a stable plateau in Pluto’s atmosphere.
But since 2021, this has changed: While Pluto’s upper atmosphere remained relatively stable, the density of the lower atmosphere decreased rapidly from mid-2021 to mid-2023. “The pressure fell from 6.17 microbars to 5.2 microbars, which corresponds to a drop of around 16 percent,” report Sickafoose and her colleagues. The lower layers of Pluto’s gas shell are therefore becoming thinner.
This suggests that the increasing distance from the sun is gradually becoming noticeable: Because it is getting colder on Pluto, less nitrogen and methane are being released from its ice surfaces. This causes the atmosphere to run out of supplies. At the same time, gases from the lower atmosphere freeze out and fall to the ground as ice crystals and snow made from frozen nitrogen or methane. “According to models, these particles need between 100 and 400 days,” explain the researchers.
Will the trend continue or not?
It is still unclear whether and how this trend will continue. “Pluto is at a particularly exciting point right now,” says Sickafoose. If the thinning of Pluto’s atmosphere continues, it could actually collapse completely in the next few years or decades. Then it would be a temporary phenomenon, similar to the gas shell of comets. They also only form their shell of dust and outgassing particles near the sun.
It would also be conceivable that Pluto’s atmosphere stabilizes in a new, thinner state. “I hope that in the next few years we can collect more data to confirm or refute the current trend,” said the astronomer.
Source: Amanda Sickafoose (Planetary Science Institute, Tucson, Arizona) et al., The Planetary Science Journal, 2026; doi: 10.3847/PSJ/ae6cdd