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Pomona College Astronomers Take Part in Rare Viewing of Pluto

Group Project Finds Distant Planet’s Atmosphere Has Unexpectedly Expanded

On a cloudy summer night high in the mountains above Wrightwood, Pomona College astronomers Bryan Penprase and Alper Ates aimed the school’s one-meter telescope at Pluto and collected data that has helped lead to the surprising observation that Pluto’s atmosphere has unexpectedly expanded rather than contracted over the past 14 years. The findings were published this month in the esteemed scientific journal, Nature.

Penprase, associate professor of physics and astronomy and coordinator of Pomona’s Astronomy Program, and Ates, staff astronomer, were part of a group of astronomers stationed at powerful telescopes around the world that same night for a rare opportunity to view Pluto. In addition to Pomona, the astronomers hailed from MIT, Boston University, Williams College, Lowell Observatory and Cornell University.

“The exciting thing about the event was that Pluto was crossing right in front of a pretty bright star. The star could then probe through the atmosphere of Pluto and give hints about what Pluto is made of,” said Penprase. “Alper and I put the one-meter telescope on ‘cruise control’ so it would track the star and automatically take pictures of Pluto, which appeared on the screen as a small dot with the star behind it. Our one-meter telescope was able to gather information that, when combined with the observations of the others and carefully studied, revealed the presence of an atmosphere on Pluto that had expanded since a similar study was done in 1988.”

Despite the fact that Pluto was discovered more than 80 years ago, little is known about our solar system’s most remote planet – the only one still unvisited by spacecraft. We know its basic size and that it has a moon named Charon – and from the orbits of Pluto and Charon we can guess at its mass and density. Anything else is pure conjecture.

But when a bright star passes behind Pluto, astronomers can make some Earth-bound observations – in particular viewing Pluto's thin atmosphere, which exerts a surface pressure roughly a million times lower than that at the Earth's surface. This alignment of planet and star is an uncommon occurrence. Following the initial detection of Pluto's atmosphere in the 1980s, there was a 14-year period during which astronomers tried but failed to get a good view of the planet – an unproductive time that ended with the recent group viewing of Pluto.

Penprase said he and Ates were thrilled to participate in the collective viewing of Pluto and they consider themselves lucky to have such ready access to a powerful telescope.

“We were able to participate in this project easily since we can flexibly schedule observing time on our telescope,” Penprase said. “Many of the other observers waited for as long as a year to get the telescope time and had to travel thousands of miles. Alper and I were able to have dinner with our families and then drive up to our telescope.”

Students in the Astronomy Program are making new discoveries with the telescope, as well. For example, this summer, juniors Millie Meier and Alex Thoreen are monitoring protostars with the college’s new infrared camera on the one-meter telescope. They are hoping to detect variability in the protostars that would provide a better understanding of how stars are formed.

To view the piece in Nature to which Penprase and Ates contributed, go to http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v424/n6945/full/nature01762_fs.html.