The planet Pluto

Pluto, which was discovered in 1930, is but a dot of light in even the largest Earth-based telescopes. Pluto is 2/3 the size of Earth's moon but 1,200 times farther away, which makes viewing surface detail as difficult as trying to read the printing on a golf ball located thirty-three miles away (more info). The adjacent movie made fromrecent Hubble Space Telescope computer enhanced images (Ref) indicates that we are finally beginning to resolve some detail on the surface of this distant planet.

General Features

Pluto is on a highly elliptical orbit at an average separation of almost 40 A. U. from the Sun, with an orbital period of 248 years. Since the planet was only discovered in 1930, we have observed only a portion of its orbit so far. Further, the orbit is tilted by about 17 degrees relative to the plane of the ecliptic, much more than for any other planet. Its equatorial radius of 1150 km is only 20% of that of the Earth, and its mass is only 0.0025 that of the Earth. Thus, it is by far the smallest planet, either in mass or diameter. Its period of rotation appears to be almost 6 1/2 days.

The 9th Planet or the 8th Planet?

Because of the eccentricity of Pluto's orbit, its orbit lies inside that of Neptune for the period from 1979 to 1999. Thus, strictly, Pluto isn't even the most distant planet at present, as illustrated in the following diagram. Of course, we term it the 9th planet because its average separation from the Sun (length of semimajor axis) is greater than that for Neptune.

What is Pluto?

This, and other peculiar aspects of Pluto's orbit, have led to some speculation that Pluto is not really a planet but instead an escaped moon of one of the gas giant planets, most likely Neptune. Its composition, as inferred from its density of 2.1 g/cc, is largely ices. Thus, Pluto is more similar in structure to moons of the gas giant planets than it is to the terrestrial planets. However, other details may favor an origin of Pluto independent of Neptune, so this is an open question at present.

Are There Planets Beyond Pluto?

From time to time there has been speculation of another planet beyond Pluto. As of this time, there is little conclusive evidence of such a planet. Here is an interesting summary of historical proposals and speculation on additional planets in the Solar System.

The Surface and the Interior

First images of Pluto's surface

Hubble Space Telescope Images of Pluto's Surface

The surface of Pluto is resolved for the first time in these NASA Hubble Space Telescope pictures, taken with the European Space Agency's (ESA) Faint Object Camera (FOC) in 1994 (Ref). These images, which were made in blue light, show that Pluto has large-scale contrast than any planet except Earth.

The two smaller inset pictures at the top are actual images from Hubble of opposite hemispheres of Pluto. Each square pixel (picture element) is more than 100 miles across. At this resolution, Hubble discerns roughly 12 major "regions" where the surface is either bright or dark. The larger images (bottom) are from a global map constructed through computer image processing performed on the Hubble data. The tilepattern is an artifact of the image enhancement technique (more info). Here is a Hubble Space Telescope movie of Pluto rotating.

Surface map of Pluto (Hubble Space Telescope-Faint Object Camera)

A Map of Pluto's Surface

The preceding figure (Ref) is the first image-based surface map of Pluto. This map was assembled by computer image processing software from four separate images of Pluto's disk, as described above. The map covers 85% of the planet's surface and indicates that Pluto has a dark equatorial belt and bright polar caps.

The brightness variations in this map may be due to topographic features such as basins and fresh impact craters. However, most of the surface features are likely produced by frosts that migrate across Pluto's surface with its orbital and seasonal cycles (more info).

Atmosphere and Interior

Pluto isn't large enough to retain much of an atmosphere, but it has a thin one that appears to be mostly nitrogen with some methane. We know essentially nothing about Pluto's interior at this point.

The Moon Charon

In 1978, careful Earth-based observation indicated that the image of Pluto had a slight bulge. This was interpreted as evidence for a previously unknown moon, named Charon. The adjacent image (Ref) shows subsequent higher quality ground-based observation, and early Hubble Space Telescope observations that show conclusively the existence of Charon. The orbit is show in the inset.

The figure shown below shows a more recent Hubble Space Telescope image of Pluto and Charon.

Pluto (left) and Charon (right)

With the presence of a moon, it was now possible to determine the mass of Pluto to much better precision than before because of the gravitational interaction between the moon and planet. This caused a drastic decrease in the previously assumed value for the mass of Pluto (previously the mass had been assumed to be as large as 10-100% of that of the Earth).