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Planetary News: Observing from Earth (2005)

Pluto's Partner Charon Dims a Star

By Emily Lakdawalla
July 20, 2005

For only the second time, astronomers on Earth have caught Pluto's moon Charon in the act of hiding a star. Charon is so tiny and so distant that such stellar occultation events are exceedingly rare; the last (and only other) one to have been observed was in 1980. By measuring the "light curve" (change of light intensity with time) of the star, the astronomers hope to determine whether Charon has an atmosphere, and to address the uncertainty that still exists as to the moon's size.

On the night of July 10-11, the 6.5-meter Clay Telescope at Las Campanas Observatory in Chile spotted Pluto, Charon, and a faint star. At left, Pluto (top) and the star (bottom) are visible; the star overwhelms the dim light from Charon. At center, Charon has crossed in front of the star, casting a shadow across the Clay telescope; only the light of Pluto (top) and Charon (bottom) are visible. At right, after less than a minute, Charon has completed its transit, and the star is again visible. Images: James Elliot, Jay Pasachoff, and others

The star was a faint one, "a million times fainter than the brightest stars in the sky," explained Williams College astronomer Jay Pasachoff, one of the leaders the observational effort. "It’s a very obscure star that just happened to be one that Pluto’s moon Charon lined up with." But at a visual magnitude of 14 -- approximately the same as Pluto -- it was quite bright enough to be useful as a probe of Charon's disk.

Whether Charon has an atmosphere, like the ones that have been observed at Neptune's moon Triton and theorized at Pluto itself, is still unknown. "The observation from 1980 indicated that there might be an atmosphere, and that’s one of the things that we’re really trying to check. At first glance there’s no obvious atmosphere, but we are now looking at it in more detail," Pasachoff said.

Charon and Pluto are very far from Earth, almost 4.5 billion kilometers (2.8 billion miles). But the star that Charon crossed is millions of times farther away than that. As a result, the shadow that Charon cast on the Earth when it crossed in front of the star was the same size as Charon itself, only 1200 kilometers (750 miles) across, or only a tenth the Earth's diameter. As a result, only a very few observatories on the Earth were in Charon's shadow and consequently in a position to observe the occultation.

Pasachoff and a large team of coworkers watched the event -- which lasted less than a minute -- from four telescopes in Chile and one in Brazil. Cloudy skies prevented the Brazilian telescope from making observations, but all four Chilean telescopes spotted the occultation. That provides four slightly different points of view on the transit. By studying the length of time that the star was obscured at the four observatories, the astronomers should get measurements of the width of Charon at four different positions across its disk.

Those measurements may help to resolve the uncertainty about the size and shape of Charon, Pasachoff said. "There’s a little debate as to the exact diameter of Charon, because with the two other ways of observing there’s a disagreement. Charon and Pluto mutually occulted each other over a period of years 10 years or so ago, and they gave a diameter that’s a few kilometers smaller than the one from the one previous occultation in 1980. So there’s a discrepancy in the scientific literature that we hope to resolve. At the end of this we should have some better idea of the shape."

Pluto and Charon's geometry for the July 10-11, 2005 Charon stellar occultation

Click to enlarge >

Pluto and Charon are often regarded as a double-planet system because Charon is so large relative to Pluto and it orbits so close to its primary. This diagram, showing the relative positions of Pluto and Charon on the night of July 10-11, 2005, shows the two bodies to scale with Charon's orbit around Pluto. Credit: JPL Solar System Simulator (Dave Seal)

Until spacecraft visit Pluto and Charon, occultations like the one on July 11 provide the best data available to elucidate the shapes and atmospheres of the two bodies. At long last, the New Horizons mission to Pluto is poised for launch in January of next year, but the encounter will not take place for a decade, in July of 2015.

The astronomers hope to have the occultation analyzed before the annual meeting of the Division of Planetary Sciences of the American Astronomical Society in Cambridge, England in early September. The Planetary Society will be there for the latest news on this and other developing stories in planetary science.