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Much ado about Pluto

Recent discoveries of Pluto-like objects in the outer solar system have sparked debate about the nature of the tiniest "planet"

Feb 17, 1999: In 1979 the Solar System became a bit mixed up. That's when Pluto, which travels in a highly elliptical orbit, temporarily moved closer to the sun than Neptune. Every 248 years the two planets swap places and for about 20 years Pluto becomes the eighth planet and Neptune the ninth. This topsy-turvy situation was rectified last Thursday, Feb. 11, when Pluto crossed Neptune's orbit and became the ninth planet once again.

Right: Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface, shown in this image of one hemisphere. The brightness variations could be due to craters and basins, methane and nitrogen frosts, or even areas of primordial organic matter. No one knows, and the mystery may remain until some future spacecraft pays a visit to this distant planet. More information

One of these things is not like the others

But is Pluto really a planet? That's what astronomers have been discussing since late last year when some members of the International Astronomical Union suggested that Pluto be given a minor planet designation. Why? For one thing Pluto is very small. It's 6 times smaller than Earth, and even smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). Pluto's own moon, Charon, is larger in proportion to its planet than any other satellite in the solar system. Some astronomers consider the pair to be a double planet.

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Pluto's elliptical orbit is also unusual. It is the only planetary orbit which crosses that of another planet (Neptune), and it is tilted 17 degress with respect to the plane of the solar system. Astronomers once thought that Pluto may have been a satellite of Neptune's that was ejected to follow a tilted elliptical path around the sun. However, careful simulations of the orbits and dynamics of Pluto and Neptune indicate that this is an unlikely scenario.

Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of rock and ice. All the other rocky planets -- Mercury, Venus, Earth and Mars -- are located in the inner solar system, close to the Sun. Except for Pluto, all of the outer planets -- Jupiter, Saturn, Uranus and Neptune -- are gaseous giants. Once again, Pluto is a misfit.

Pluto behind the 8 ball

Despite its well-known peculiarities, Pluto's official status as a planet was never in jeopardy until 1992 when David Jewitt and J. Luu discovered a curious object called 1992 QB1. QB1 is a small icy body, similar in size to an asteroid, orbiting 1.5 times further from the sun than Neptune. QB1 was the first hint that there might be more than just Pluto in the distant reaches of the solar system.

Right: Kuiper Belt object 1995 WY2 observed UT 1995 Nov 18 using the Keck 10 meter telescope by D. Jewitt (University of Hawaii) and J. Luu (University of California, Berkeley). These are the discovery images. Each is a 900 second integration. The images are separated by about 18 minutes, during which time the object moves 1 arcsecond. Stationary background galaxies are also evident in this short movie. The apparent magnitude is V = 24.2. For more information see David Jewitt's Kuiper Belt Home Page.

Since then nearly 100 objects like QB1 have been found. They are thought to be similar to Pluto in composition and, like Pluto, many orbit the sun in a 3:2 resonance with Neptune. This swarm of Pluto-like objects beyond Neptune is known as the Kuiper Belt, after Gerard Kuiper, who first proposed that such a belt existed and served as a source of short period comets. Astronomers estimate that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number (and mass) of similar sized objects in the main asteroid belt.

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So, is Pluto really a planet or is it more like a dormant comet, simply the largest known member of the Kuiper Belt? That's the question that astronomers have recently been debating.

Other than its relatively large size, Pluto is practically indistinguishable from the Kuiper Belt objects (KBOs) and short period comets. The main difference is Pluto's reflectivity, which is much higher than that of known KBOs. "Pluto has a higher albedo (60%) than we suspect for the other KBOs," explains Dr. David Jewitt of the University of Hawaii. "But this is an artifact of size - Pluto has enough mass & gravity to retain a tenuous atmosphere from which bright surface frosts may be deposited on the surface."

David Jewitt and his colleagues at the Institute for Astronomy are leaders in the search for new members of the Kuiper Belt. They are presently scanning a 50 sq. degree area of the sky for KBO's using the University of Hawaii's 8192 x 8192 pixel CCD camera, and are experimenting with an even larger 12,000 x 8,000 pixel camera for faster searches. So far their efforts have met with considerable success. They have discovered over 40 KBO's in recent years, some of which are comparable in size to Pluto.

"We've already found objects 1/3rd the diameter of Pluto," says David Jewitt," even though we have examined only a tiny fraction of the sky. An example is 1996 TO66, which is 800 km diameter. It would be incredible in its own right if Pluto proved to be the only 2000 km object. I think we'll have Pluto II, Pluto III....within a few years."

artist's concept of
the Pluto-Kuiper Express mission. Credit JPLRight: Artist's conception of the Pluto-Kuiper Express, a spacecraft that NASA planners hope will visit Pluto and Charon around 2010. Pluto is the only planet in our Solar System not yet viewed close-up by spacecraft. As a result many basic questions about Pluto remain unanswered. For example, what are the mysterious dark patches revealed in Hubble images of Pluto? Could they be due to photochemical reactions caused by cosmic rays, or perhaps areas of primordial organic matter? Does Charon, like Pluto, have dark spots and an atmosphere? Or is it a very different kind of body? Mission planners hope to answer these questions and many more. Depending on the success of the Pluto-Charon encounter, the space probe could go on to visit objects in the Kuiper Belt.

Nine's a charm

Dr. Jewitt raises the interesting possibility that Kupier Belt objects might one day be discovered that are even larger than our ninth planet. If that happens, what does it mean for Pluto? Should it be stripped of planetary status and reclassified as a member of the Kuiper Belt? Or should newly discovered "Plutos" be classified as planets as well?

Left: A Hubble telescope image of Pluto and its satellite Charon.

These are difficult questions that await the astronomical community. For now, however, Pluto's status as a planet seems secure. In a press release dated Feb. 3, 1999 the International Astronomical Union stated that "No proposal to change the status of Pluto as the ninth planet in the solar system has been made by any Division, Commission or Working Group of the IAU responsible for solar system science. Lately, a substantial number of smaller objects have been discovered in the outer solar system, beyond Neptune, with orbits and possibly other properties similar to those of Pluto. It has been proposed to assign Pluto a number in a technical catalogue or list of such Trans-Neptunian Objects (TNOs) so that observations and computations concerning these objects can be conveniently collated. This process was explicitly designed to not change Pluto's status as a planet."

Mistaken reports that the IAU intended to strip Pluto of its planetary status caused an uproar among astronomers and in the popular press. It seems that Pluto is a sentimental favorite to remain a planet among both scientists and the public. However, if more trans-Neptunian objects are discovered that are even larger than Pluto, the debate could begin anew.

Web Links

IAU Press Release-- on the status of Pluto, Feb 3, 1999

A discussion of Pluto's status-- from Division III of the IAU. Includes "What is a planet?"

The Pluto-Kuiper Express-- a mission to explore Pluto/Charon and the fringes of our Solar System from JPL.

Facts about Pluto - from SEDS

Facts about the Kuiper Belt and the Oort Cloud - from SEDS

More Space Science Headlines - NASA research on the web

NASA's Office of Space Science press releases and other news related to NASA and astrophysics


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Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Linda Porter
NASA Official: Ron Koczor