Pluto’s Wink

Pluto (center, largest) and its moons Charon, Nix,
and Hydra captured by the Hubble Space Telescope.In the past year, the quiet and unassuming Pluto has been having its moments of fame. First was the attention brought by the launch of NASA’s New Horizons spacecraft, the robot that, when it arrives at Pluto nearly a decade from now, will give us our first up-close look at this cold, distant, and tiny world. Myself, I can hardly wait.

Then last August, the astronomers of the International Astronomical Union (IAU) got together to debate and vote on a scientific definition for the word planet (because that’s what the IAU does, among other things). Alas for Pluto, the ratified scientific definition of planet was a standard that Pluto didn’t quite measure up to, and our Solar System suddenly had eight official planets.

Personally, I've made my peace with this--and I like the way my daughter put it: "Dad, I’m sad because, guess what? Pluto isn't a planet anymore. But guess what? It's now a dwarf planet! Cool!"

However Pluto may have been treated by the IAU, an upcoming astronomical event serves as a reminder that this far-flung, icy dwarf planet has gone on with its life, serenely unaffected by human debate.

On March 18, close to 4:00 AM Pacific Daylight Time (yes, Daylight Savings Time starts early this year), Pluto will pass in front of a star—a star about as bright as Pluto itself—casting a faint shadow onto the Earth in a much less showy version of a solar eclipse, when the Moon passes in front of the Sun. This type of event is called an occultation.

It's an uncommon event for Pluto to occult a star bright enough to be of use to astronomers, but the March 18 event is even more rare because of where it will be visible from: the Western United States, a region densely populated with professional and amateur astronomers and telescopes both large and small—and you can bet that a lot of them will be up all night getting ready to observe.

Occultations are important events for astronomers. By measuring the brightness and the colors in the light of the star being occulted by a planet, astronomers can learn a number of things about the planet that would not be forthcoming through ordinary, direct observation. The object's size, speed, and even the structure and composition of its atmosphere, if it has one, can be gleaned by observing the effects on the occulted star's light.

In the past, astronomers have learned a lot about Pluto's thin, chilly methane atmosphere by observing occultations. The rings of Uranus were discovered accidentally by astronomers observing an occultation of a star by that planet.

At Chabot Space & Science Center, observers who normally spend their nights hunting for occultations of stars by their own planets-- extra-solar planets, or exoplanets-- will attempt to capture the Pluto occultation using the antique 20-inch refracting telescope named Rachel and a modern digital CCD camera. Now, if only we have clear skies on this dark March morning...

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.