Artist conception of New Horizons spacecraft at the Solar System's ninth planet Pluto and its moon Charon (JHUAPL/SwRI)
New Horizons over Mysterious Pluto
Faraway History Planet Moons Spacecraft Go Now

It's a Warm Winter on Pluto


Hubble Space Telescope image of the Solar System's ninth planet Pluto and its moon Charon
Hubble sees Pluto and its Moon Charon

The European Space Agency's Faint Object Camera aboard the Hubble Space Telescope spacecraft snapped this clear view of the distant planet Pluto and its moon Charon on February 21, 1994, when they were 2.6 billion miles (4.4 billion kilometers) from Earth. That's nearly 30 times the separation between Earth and the Sun. Pluto and Charon are 12,200 miles apart.

This image allowed astronomers to measure directly Pluto's diameter of 1,440 miles and Charon's diameter of 790 miles. The observation showed that Charon is bluer than Pluto, indicating that they have different surface composition and structure. A bright highlight seen on Pluto suggests it has a smoothly reflecting surface layer.

Pluto was discovered back in 1930, but Charon wasn't detected until 1978 because the moon is so close to Pluto that the two blur together when seen through telescopes on Earth. Hubble is in space above earth's atmosphere. Pluto sometimes is referred to as a double planet because Charon is half the diameter of Pluto. BY comparison, Earth's Moon is one-quarter the diameter of planet Earth.

Credit: NASA; Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility   [click image to enlarge]
Wintertime is making Pluto warm and expansive!

One might have imagined that a Winter's chill would have brought some shrinkage to Pluto's atmosphere. Not so, according to researchers at the Massachusetts Institute of Technology and others.

The surprised astronomers, looking through the Hubble Space Telescope (HST) at the little planet out at the edge of the Solar System, saw Pluto's atmosphere expanding even as it was moving away from the Sun.

Scientists had supposed the atmosphere – mostly nitrogen – would contract as the planet moved away from the Sun and into its winter. Instead, the planet's temperature appeared to have gone up by slightly more than 1.8° Fahrenheit (1° Celsius) since its closest approach to the Sun in 1989. Of course, it's still quite cold at -390° to -275° F.

The team suggested the temperature increase may be something like a heat-lag they have experienced here on Earth. For example, even though our Sun is highest at noon on a hot day, the warmest temperatures come a few hours later.

Out there on Pluto, such a lag could last for decades. Winter lasts for more than six decades as Pluto travels its 248-year orbit around the Sun.

Who can say what will happen across the entire season?

How do they measure that? Pluto's atmosphere also seemed to have become larger.

The researchers discovered the changing size of the atmosphere by comparing what they saw when a bright star passed behind the planet in 1988 and then again in 2002.

To measure a distant planet like Pluto, astronomers look for "stellar occultations," when starlight passes through a planet's atmosphere.

The atmosphere surrounding Pluto has been known only since astronomers observed an occultation in 1985. They then did some measuring during an occultation in 1988 and again in 2002. They found that atmospheric pressure had doubled over 14 years.

To get a closer look at what's going on out near the edge of the Solar System, NASA would like to send a probe called New Horizons to Pluto and Charon in January 2006. If successful, it would arrive in 2015. Pluto is the only planet in our Solar System that never has been visited by spacecraft from Earth.

Not your typical planet? Pluto is an unusual planet. For instance: Some naysayers go as far as to suggest that Pluto really can't be a planet. They look at it as part of a collection of small icy bodies lying beyond planet Neptune in an area known as the Kuiper Belt.

SOURCE: Nature 424, 165-168 (2003)

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