Pluto's atmosphere is starting to disappear

Pluto's atmosphere is going through a strange transformation, scientists are finding. 

The icy dwarf planet, which lies over 3 billion miles (4.8 billion kilometers) away from Earth in the Kuiper Belt, caught astronomers' attention as it passed in front of a star back in 2018. 

With the star backlighting Pluto, the team of researchers was able to make observations of the dwarf planet and its atmosphere. With this unique view, they came to a surprising conclusion, which they describe in a new study. 

The astronomers found evidence that Pluto's atmosphere is starting to disappear. 

Related: Why Isn't Pluto a Planet Anymore?

Using telescopes at multiple sites in both the U.S. and Mexico, the team observed Pluto and its thin atmosphere, which is primarily made of nitrogen, like that of Earth. Pluto's atmosphere is supported by the vapor pressure of ices on the dwarf planet's surface. So, if ice warms up on Pluto, it can dramatically alter the density of its atmosphere, according to a statement from Southwest Research Institute (SwRI), the home institution of multiple members of the research team.

For about 25 years, Pluto has been moving farther and farther away from the sun, so its surface temperature has been going down. And with these recent observations, the researchers found evidence showing that Pluto's atmosphere is actually refreezing back onto its surface as the dwarf planet gets colder and colder. Pluto is so far from the sun that, as time goes on, it will get distinctly farther away (and colder) before getting closer to the sun in other regions of its immense orbit.

Thanks to a phenomenon known as thermal inertia, Pluto'ssurface pressure and atmospheric density continued to rise until 2018. Essentially, Pluto had residual heat from when it was closer to the sun. However, the inertia starting to wear off and, as Pluto gets colder, more and more of its atmosphere will freeze back onto its surface and "disappear."

"An analogy to this is the way the sun heats up sand on a beach," SwRI staff scientist Leslie Young, who studies the interaction between icy solar system bodies and their surfaces and atmospheres, said in the same statement. 

"Sunlight is most intense at high noon, but the sand then continues soaking up the heat over the course of the afternoon, so it is hottest in the late afternoon. The continued persistence of Pluto’s atmosphere suggests that nitrogen ice reservoirs on Pluto’s surface were kept warm by stored heat under the surface. The new data suggests they are starting to cool," Young said.

So how did they figure this out just by watching Pluto move in front of a star?

The researchers watched the star fade as Pluto moved in front of it and then come back into view once the dwarf planet had passed. Using the rate that the star came in and out of view, a transition that lasted about 2 minutes, they were able to determine the density of the dwarf planet's atmosphere, according to the statement.

This method relies upon what is known as an "occultation," an event that happens when one cosmic object is hidden by another passing in front of it. Studying occultations is an old and well-worn technique in the world of astronomy, and researchers have used it to study Pluto's atmosphere since 1988, Eliot Young, a SwRI senior program manager, said in the statement. 

"The New Horizons mission obtained an excellent density profile from its 2015 flyby, consistent with Pluto’s bulk atmosphere doubling every decade, but our 2018 observations do not show that trend continuing from 2015," Young added, referring to the pioneering NASA mission that gave the world its first up-close look at Pluto..

In observing Pluto as it passed in front of the star, the team noticed a "central flash" in the middle of the path of the dwarf planet's shadow. The flash, caused by Pluto's atmosphere refracting light into the center of the shadow, changed the light curve that usually happens during occultation from a "u-shape" into a "w-shape."

"The central flash seen in 2018 was by far the strongest that anyone has ever seen in a Pluto occultation," Young said. "The central flash gives us very accurate knowledge of Pluto’s shadow path on the Earth."

Young discussed the results of this study Oct. 4 at the 53rd American Astronomical Society Division for Planetary Sciences Annual Meeting.

Email Chelsea Gohd at cgohd@space.com or follow her on Twitter @chelsea_gohd. Follow us on Twitter @Spacedotcom and on Facebook.