Pluto

Pluto has the most eccentric (non-circular) orbit of all the planets in our solar system. While the planet's mean distance from the Sun is 39.44 Astronomical Units (AU), it can be as far as 49.19 AU from the Sun and as close as 29.58 AU. The time required for Pluto to complete one orbit about the Sun (its sidereal period) is 248.03 years, and the time for the planet to repeat alignments with respect to the earth and the Sun (its synodic period) is 366.7 days.

While commonly referred to as the ninth and outermost planet of our solar system, the large eccentricity of Pluto's orbit can bring the planet closer to the Sun than Neptune. Pluto, in fact, last edged closer to the Sun than Neptune in January of 1979, and remained the eighth most distant planet from the Sun until March of 1999. On September 5, 1989, Pluto reached perihelion, its closest point to the Sun, when it was at its brightest when viewed from Earth. Pluto is not a conspicuous night-sky object, and can only be viewed with telescopic aid. Under good viewing conditions, Pluto can be seen as a star-like point in any telescope having an objective diameter greater than 7.9 in (20 cm). Pluto moves only slowly through the constellations; due to the fact that the planet is both small and very distant.

At its closest approach to Earth, Pluto's planetary disk is smaller than 0.25 arc seconds (that is, 0.00007°) across. Periodic variations in the planet's brightness, however, have revealed that Pluto rotates once every 6.3827 days. Pluto's spin axis is inclined at 123° to the plane of its orbit about the Sun and consequently its rotation is retrograde. The extreme tilt of Pluto's spin-axis Figure 1. The Pluto-Charon system. Illustration by Hans & Cassidy. Courtesy of Gale Group. results in the earth-based observer seeing different hemispheric projections as the planet moves around the Sun. In the early 1950s, for example, Pluto presented its south pole towards the earth, today, we see its equatorial regions. In the year 2050 Pluto will present its north pole towards the Earth.

Careful long-term monitoring of the variations in Pluto's brightness indicate that the planet is brightest when seen pole-on. This observation suggests that the poles are covered by reflective ices, and that the planet has a dark patch (lower albedo) on, or near its equator. It is highly likely that Pluto's brightness variations undergo seasonal changes, but as yet, astronomers have only been able to monitor the planet during about 1/6 of one orbit about the Sun.

At its mean distance of about 40 AU from the Sun, Pluto receives 1/1600 the amount of sunlight received at Earth. Consequently Pluto is a very cold world, with a typical daytime surface temperature of about -351°F (-213°C). Spectroscopic observations indicate the presence of methane, nitrogen and carbon monoxide ices on Pluto's surface. Most surprisingly, however, and in spite of its small size and low escape velocity (0.68 mi/sec (1.1 km/sec), Pluto is able to support a very tenuous atmosphere.

That Pluto might have a thin methane atmosphere was first suggested, on the basis of spectroscopic observations, in the early 1980s. Conclusive evidence for the existence of a Plutonian atmosphere was finally obtained, however, on June 9, 1988, when Pluto passed in front of a faint star producing what astronomers call a stellar occultation. As Pluto moved between the star and the earth, observers found that rather than simply vanishing from view, the star gradually dimmed. This observation indicates the presence of a Plutonian atmosphere. Indeed, Pluto's atmosphere appears to have a tenuous outer layer and a more opaque layer near its surface.

It has been suggested that Pluto only supports an atmosphere when it is near perihelion, and that as the planet moves further away from the Sun the atmosphere freezes out. This freezing and thawing of Pluto's atmosphere may explain why the planet has a relatively high surface albedo of about 40%. Essentially the periodic freezing and thawing of Pluto's atmosphere continually refreshes the methane ice at the planet's surface.