Pluto's atmosphere appears to be freezing out as the dwarf planet moves ever further away from the sun, according to careful observations of Pluto astronomers made as it was seen passing in front of stars in the sky.
Pluto is already cold. Its surface temperature is about –382 degrees Fahrenheit (–230 degrees Celsius), while its upper atmosphere is a little warmer at –333 degrees Fahrenheit (–203 degrees Celsius). As Pluto moves away from the sun it will grow even colder and its nitrogen-rich atmosphere should start to collapse and freeze out, condensing as frost on the frigid surface.
Now, a team led by Amanda Sickafoose of Arizona's Planetary Science Institute believe they have seen the onset of this process. "We're at a particularly interesting point for Pluto," said Sickafoose in a statement. "Our work suggests that the atmosphere has recently started decreasing in pressure."
Pluto's orbit is highly elliptical and very oblique to the ecliptic plane of the solar system. In fact, so eccentric is Pluto's orbit that for ten years either side of its perihelion in 1989 (when Pluto was closest to the sun), it was actually closer to the sun than Neptune was.
However, ever since 1989 Pluto has been moving away from the sun on its 248-year-long orbit – we've not even seen Pluto complete half of its orbit yet since its discovery in 1930. Pluto will reach its furthest point from the sun – called aphelion – in the year 2114 when it will be about 49 astronomical units (7.3 billion kilometers/4.6 billion miles) distant.
When NASA's New Horizons mission flew past Pluto in July 2015, it measured Pluto's atmospheric pressure to be 10 microbars. This is a tiny amount, millions of times lower than the atmospheric pressure on Earth, resulting from Pluto's thin nitrogen atmosphere that also contains hazes made from organic molecules including methane and carbon monoxide.
With New Horizons now having flown past Pluto and deeper into the Kuiper Belt, astronomers have to instead rely on more distant and less direct means of keeping tabs on Pluto's atmosphere. From our point of view on Earth, Pluto is sometimes seen to pass in front of a distant star. This is called a stellar occultation. When an airless body, such as an asteroid, occults (or hides) a background star the star's light winks out in an instant, before suddenly reappearing. When there is an atmosphere involved, however, the light of the star doesn't go out all at once when blocked by Pluto, but steadily fades as the atmosphere scatters and refracts the starlight before the star vanishes behind the dwarf planet's solid body.
Between 2017 and 2023, Sickafoose's team probed Pluto's atmosphere during ten occultations of stars ranging in brightness from about magnitude 12 to magnitude 18, far below naked-eye visibility.
Observing stellar occultations is not always a simple matter. They are not visible from everywhere on Earth, but only along narrow tracks, often in difficult to reach areas. The tracks, which are just a few kilometers wide, are effectively Pluto's shadow falling on the Earth. By spacing observers across the width of the track, it is possible to measure the extent of Pluto's atmosphere (there usually also has to be an unlucky observer just outside the track to confirm that is indeed the edge of the shadow track).
"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.
Of the ten occultations, four could be seen from observatories in multiple locations, which meant that the different observatories could confirm each other's results.
"We often prefer to observe predicted occultations that have shadow paths over large observatories because that equipment and data have proven successful," said Sickafoose. "Then, we try to improve those datasets by collaborating with local observers in the shadow paths."
Previous work had suggested that Pluto's atmosphere began freezing out in 2018, but Sickafoose's team found that actually, Pluto's atmosphere remained constant from 2015 to midway through 2021. However, between mid-2021 and the last occultation that Sickafoose's team observed in 2023, Pluto's atmospheric pressure dropped by 16 per cent.
The pressure drop was deduced from the light curves, which show how the star's brightness changed with time during an occultation. The light curves revealed that while the structure of Pluto's upper atmosphere remained fixed, the lower atmosphere changed.
Sickafoose's team interprets this as being the haze particles settling towards lower latitudes. This is what is expected to happen when the atmospheric pressure drops.
"I am hopeful that we'll be able to get more data in the next years to decades to specifically confirm or refute this trend," said Sickafoose.
It is thought that Pluto's atmosphere won't have completely frozen onto the dwarf planet's surface by the time it reaches aphelion, and then as Pluto begins moving in towards the sun once more the feeble solar heating that Pluto receives will slowly begin to increase, causing nitrogen frost to sublimate from the surface and thicken the atmosphere once more.
The findings were published on July 31 in the Planetary Science Journal.
