Although billions of kilometers from the sun, frigid Pluto has an Earthly air: an atmosphere made mostly of nitrogen, the same gas that constitutes 78 percent of the air we breathe. But Pluto pursues such an elliptical orbit around the sun that all of that gas might freeze onto its surface when farthest and coldest. On May 4, however, Pluto passed in front of a star in the constellation Sagittarius, allowing observers to watch the atmosphere block some of the star's light and deduce that the air is so substantial it never disappears.
That passage was key to understanding the atmosphere's future, says Catherine Olkin, a planetary scientist at the Southwest Research Institute in Boulder, Colo., whose team tracked the so-called occultation. In work submitted to Icarus she and her colleagues report that Pluto's atmosphere is now thicker than ever before seen.
Astronomers discovered the atmosphere in 1988, when Pluto occulted another star. An airless Pluto would have cut off the star's light abruptly, but instead the starlight faded gradually, revealing air with roughly one one-hundred-thousandth the surface pressure of our own—equivalent to the terrestrial atmosphere 80 kilometers high.
Pluto is so distant that completing a single orbit takes it 248 years. Pluto came closest to the sun in 1989 and has been receding from the star ever since. When Pluto ventures out to its most distant point, in 2113, it will be 3 billion kilometers farther, and sunlight on its surface will be 36 percent weaker, than in 1989. "Many scientists have predicted that Pluto's atmosphere would collapse as it traveled away from the sun," Olkin says. "Receiving less sunlight, the gas would condense onto the surface." Mars, whose orbit is also rather elliptical, temporarily loses a quarter of its air every time its southern hemisphere experiences winter, when Martian gas freezes onto the south polar cap.
Pluto is mostly rock, but its crust consists of water ice. At Pluto's temperature of approximately 40 kelvins (–233 Celsius), water is as hard as rock, constituting a stage on which nitrogen and also methane dance back and forth between ice and gas.
The new observations indicate that Pluto's air is now three times denser than in 1988, contradicting models that predicted the atmosphere would someday vanish. Instead, Olkin says, the higher pressure accords with a model indicating that the region around a hundred meters below the surface retains heat during Pluto's close encounters with the sun and releases that heat only slowly, thereby keeping the surface warm enough so that some of the nitrogen always stays gaseous. "As Pluto goes around the sun, its atmosphere does not completely condense," Olkin says. Her work implies that Pluto's water-ice layer is compact, because a porous subsurface would quickly lose its warmth.
"It's a nice piece of work," says John Stansberry, a planetary scientist at the Space Telescope Science Institute. "These kinds of observations are critical for studying seasonal evolution on Pluto." Stansberry worries, however, that Pluto is more complex than the model assumes, which means the atmosphere's behavior is less clear than Olkin asserts. "Based on these results, it's certainly fair to say that Pluto's atmosphere is not going to collapse any time soon, but to say it's going to be there in 2140 is maybe stretching it a bit," Stansberry says.
Both Olkin and Stansberry do agree on a far more famous controversy: Pluto is a planet. In 2005 astronomers discovered Eris, a distant world proclaimed to be larger than Pluto, adding to arguments that Pluto should lose its planetary status and prompting predictions that a plethora of worlds surpassing Pluto in size awaited discovery.
But things didn't work out that way. In 2010 Eris passed in front of a star and failed to live up to the hype. The short duration of the occultation revealed Eris to be just 2,326 kilometers across—versus about 2,350 kilometers for Pluto. And no one has ever found anything else orbiting the sun beyond Neptune's path exceeding Pluto’s size.
Pluto's diameter, however, is uncertain: It could be as small as 2,300 kilometers or as large as 2,400 kilometers. Ironically, the villain is the atmosphere, which bends starlight during occultations and complicates measurements of its diameter.
Fortunately, help is on the way. In July 2015 NASA's New Horizons spacecraft will sail past Pluto and its five known moons. "I'm not sure what we'll see, but I can't wait to get there," Olkin says. "It's going to revolutionize our view."