Thanks to a passing star, scientists have been able to measure the thin atmosphere of the dwarf planet. And that shows that it is slowly but surely freezing again.

On the night of August 15, 2018, dwarf planet Pluto passed in front of a star. Astronomers took advantage of this event – also known as occultation – to observe the thin atmosphere of the dwarf planet through a variety of telescopes. It leads to an interesting discovery. Because Pluto’s atmosphere appears to be slowly disappearing.

More about Pluto’s atmosphere
Pluto has a thin, tenuous atmosphere that expands as it gets closer to the sun and collapses as it moves farther from the sun — similar to a comet. The main component is molecular nitrogen, although molecules of methane and carbon monoxide have also been detected. When Pluto is close to the sun, its surface sublimes (evaporates) and then rises, temporarily forming a thin atmosphere. Due to Pluto’s low gravity, the atmosphere is much more expansive than our planet’s atmosphere. When Pluto moves away from the sun, the temperature on the dwarf planet drops sharply. During this time, most of Pluto’s atmosphere freezes and falls to the surface as snow.

Occultation is an astronomical event in which an object comes between the Earth and a distant star, causing it to completely cover the star. The star is therefore hidden from our view. Scientists can then demystify properties of the object in question thanks to such an event. In Pluto’s case, the occultation lasted about two minutes, meaning the star disappeared from view during this time. And that gives us more insight into the so-called ‘density profile’ of Pluto’s atmosphere.

Nitrogen

As you could already read in the box, Pluto’s atmosphere consists mainly of nitrogen. Unlike Earth, Pluto’s atmosphere is supported by the vapor pressure of surface ice. This means that small changes in the temperature of the ice can lead to large changes in atmospheric density. The temperature is then again determined by the distance from the sun. Pluto takes 248 Earth years to complete a full orbit around the sun. The distance ranges from the nearest point of about 30 AU (astronomical units; 1AU is the distance from Earth to the sun) to 50 AU.

Sunlight

Over the past quarter of a century, Pluto has been receiving less and less sunlight the farther it gets from the sun. Still, as you might expect, the dwarf planet’s atmospheric density did not immediately decrease, but increased until 2018. How is that possible? “Compare it to the way the sun heats up the sand on a beach,” explains researcher Leslie Young. “Sunlight is most intense around noon. But the sand continues to absorb the heat during the afternoon, making the grains hottest in the late afternoon. That Pluto’s atmosphere was preserved for so long suggests that nitrogen reservoirs on Pluto’s surface were kept warm by stored heat below the surface.”

But that time seems to be over now. “The new data suggests it’s starting to cool down,” Young said. And as mentioned, when the temperature drops, most of Pluto’s atmosphere will freeze and fall to the surface as snow. Pluto’s atmosphere was previously thought to be collapsing, although that was later questioned. But now researchers expect that Pluto’s atmosphere will continue to get thinner and thinner over time, until it eventually disappears completely.