Therefore, when Pluto passes in front of another star -- as it did in 1988 and again July 19 -- researchers look for subtle changes in the star's light just as Pluto's atmosphere crosses in front of it. Such events are called occultations.
Researchers at Lowell Observatory and the Massachusetts Institute of Technology (MIT) led the new observations.
"In the last 14 years, one or more changes have occurred," said Lowell's Marc Buie. "Pluto's atmosphere is undergoing global cooling, while other data indicates that the surface seems to be getting slightly warmer. Some change is inevitable as Pluto moves away from the Sun, but what we're seeing is more complex than expected."
Scrambling for a view
The new observations very nearly were not made.
A handful of research teams chased an uncertain occultation path, flying to various locations in South America. In some cases, they changed their minds at the last minute as refinements were made to the projections of exactly where the event would be visible.
Pluto occulted star called P126A. Buie made the observations with the help of Oscar Saa of the Cerro Tololo Inter-American Observatory in northern Chile using portable 14-inch (0.35-meter) telescope.
The data analyzed primarily by Buie and James Elliot of MIT, while a host of other astronomers contributed to the project.
From the 1988 occultation and other data, astronomers knew that Pluto's tenuous atmosphere is made mostly of nitrogen, with some methane and carbon monoxide. In that earlier event, the star's light dimmed gradually and then suddenly dropped off, indicating a possible smog layer or abrupt temperature drop-off near Pluto's surface.
In this year's occultation, the starlight dimmed gradually. Astronomers conclude that Pluto's atmosphere has cooled by 20 to 55 degrees Fahrenheit (11 to 31 degrees Celsius).
Let's go there
Buie hopes the new observations will get the so-called Pluto-Kuiper Belt mission on NASA's front burner. The probe, planned to launch in 2006 and reach Pluto a decade later, would represent humanity's first visit to Pluto, the only planet in the solar system that has not been studied up close.
"These drastic changes to its atmosphere, coupled with the possibility that Pluto's surface is getting warmer, make exploring the planet that much more compelling," Buie said in a statement released last night. "The Pluto-Kuiper Belt mission is our best hope for putting all the puzzle pieces together."
In late July, a Senate appropriations subcommittee voted to restore funding for the Pluto-Kuiper Belt mission, after a National Research Council report recommended it as a top priority. NASA had previously axed the mission, however, and did not request the funding. For now, the whole idea is in limbo, awaiting House action that won't come until after the August recess.
Meanwhile, in a remarkable coincidence of timing, astronomers will get a second chance this summer to watch Pluto occult a star. It will pass in front of the star P131.1 on Tuesday, Aug. 20.
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