It takes Pluto 248 years to complete one journey around the Sun. Humanity has known of its existence for less than half of that time, and we are still discovering the myriad ways that Pluto is unlike other celestial objects. Among other oddities: Pluto is the only object beyond Neptune’s orbit to have an atmosphere.
But that might not always be the case.
Because Pluto experiences such a drastic change in the amount of sunlight it receives throughout its 2-century year, planetary scientists have theorized that Pluto’s nitrogen-dominated atmosphere might come and go with its seasons. Now, new research suggests that Pluto’s atmospheric pressure is beginning to drop, potentially heralding atmospheric collapse ahead of Plutonian winter.
Power of the Occult
Pluto was discovered in 1930, but astronomers didn’t detect its atmosphere until 1988. They carefully observed as Pluto briefly blocked the light from a distant star and noticed that the starlight traveled through a thin gaseous layer of mostly nitrogen, with a little bit of methane, carbon monoxide, and hydrocarbons. This method of detection, called stellar occultation, allows scientists to monitor the bulk properties of Pluto’s atmosphere, like the height and pressure, as well as some finer details like atmospheric waves and the presence of haze.
Then in 2015, NASA’s New Horizons spacecraft flew through the Pluto system. It provided the clearest pictures to date of Pluto’s atmosphere and revealed surprising complexity. Since then, planetary scientists have continued to monitor the dwarf planet’s atmosphere via occultations.
“Stellar occultations provide a snapshot of the physical properties of Pluto’s atmosphere [at] a given time. The path that the starlight takes through a thin atmosphere is a function of its composition as well as the temperature and density profiles,” Amanda Sickafoose, a planetary scientist at the Planetary Science Institute in Tucson, Ariz., told Eos via email. Sickafoose is the lead author on the new discovery.
But occultations happen only when Pluto and a background star line up just right, which is maybe once or twice a year. What’s more, given how subtle the signal from Pluto’s atmosphere is, the answers inferred from one telescope and analysis method might differ from those of another method. These challenges make it difficult for astronomers to put together a uniform and cohesive dataset that shows the subtle changes in Pluto’s atmosphere over time.
An Almost Airless Winter
Sickafoose and her colleagues sought to overcome these challenges by compiling Pluto occultation data spanning 1988–2023, including 10 new occultations from 2017 to 2023, and analyzing them in a consistent way. The observations were made at more than 15 different telescope sites around the world. But by reanalyzing the data in a uniform way, the team minimized telescope-specific differences and was able to tease out details of how Pluto’s atmosphere has evolved.
The team’s analysis showed that Pluto’s upper and lower atmospheres have responded differently to weakening sunlight.
“In terms of size and pressure, we find that Pluto’s atmosphere remained roughly stable from the New Horizons flyby in 2015 through 2021,” Sickafoose said. “Our most recent datasets in 2022 and 2023 suggest that the pressure has dropped, at a level of 16%, in the lower atmosphere.”
The data also indicate that while Pluto’s upper atmosphere remains haze free, its lower atmosphere haze has started to clear up. Astronomers have noted the change in haze before and suspect that haze particles are snowing down as Pluto’s atmosphere cools. The team published these results in The Planetary Science Journal in July.
“This is great work!” said Perianne Johnson, a Pluto climate scientist at Purdue University in West Lafayette, Ind., who was not involved with this research. “We are hoping to identify slight changes in Pluto’s atmosphere, and it can be difficult to ascertain if reported differences are truly changes to the atmosphere or just differences between the way two scientists processed the data and different assumptions they made about unknown atmospheric properties. In this work, we can trust that the reported changes to the atmospheric pressure are real.”
Nitrogen Snow
Like many trans-Neptunian objects (TNOs), Pluto’s orbit is very noncircular and angled steeply from the ecliptic plane. Its temperature changes drastically throughout its year, depending on how far it is from the Sun. Since the discovery of Pluto’s atmosphere, astronomers have theorized that the atmosphere may freeze out, or collapse, as Pluto’s temperature drops.
Astronomers “anticipated a drop in pressure sometime in the upcoming decades, based on atmospheric models that consider things like Pluto’s orbit, the amount of Sun received at different locations on its surface, and the surface ice properties,” Sickafoose said.
“Pluto is the best studied TNO, and what we learn about Pluto helps put other TNOs in context.”
These data suggest that that collapse might be starting. And while a 16% drop in atmospheric pressure may sound small, Johnson said, “it is important to note that that change occurred over a period of just a few years, which is only a few percent of Pluto’s orbital timescale. So this is rapid, exciting change and is indicative of ice condensation occurring somewhere on Pluto.”
Most theories predict that once the collapse starts, Pluto’s atmospheric pressure will continue to drop for more than 150 years before eventually bouncing back in the 23rd century. Pinning down when and how fast the collapse happens holds clues for where atmospheric nitrogen is condensing and snowing down onto Pluto’s southern hemisphere ice deposits.
The next opportunity for high-quality occultation observations of Pluto is in 2027 and could verify whether atmospheric collapse is imminent.
“Occultations are an incredibly powerful tool for observing the distant solar system from Earth,” Johnson said. “Pluto is the best studied TNO, and what we learn about Pluto helps put other TNOs in context.”
—Kimberly M. S. Cartier (@astrokimcartier.bsky.social), Staff Writer