Diane M. Huot
Pluto lies about 40 Astronomical Units and is the ninth planet from the Sun. It is the last planet in the Solar System. It is the smallest planet, smaller even than our Moon. Pluto has a diameter of about 1,466 miles (2,360 km). This strange, icy planet is much more like the rocky planets that are found near our Sun. Tiny Pluto takes 6 days and 9 hours to make one complete rotation on its axis and 248 years to orbit the Sun. Pluto is the smallest of all the planets and has its own moon named Charon. Charon is half the size of Pluto at 746 miles (1,200 km). The two bodies rotate, facing one another.
Pluto's density suggests a rock and ice core covered with layers of ices. Its surface probably resembles Neptune's moon Triton, with nitrogen and methane frosts. The temperature is about –220 degrees Celsius. Relative to the orbits of all the other planets, Pluto is more tilted and eccentric. At times, this brings the planet closer that Neptune to the Sun. Pluto is the only planet in the Solar System not yet visited by spacecraft.
Is Pluto Really a Planet?
Pluto fits the classical criteria for a planet: it orbits the Sun, it is large enough to become round, and it even has a moon. But planetary scientists are now starting to think of Pluto as an object from the Kuiper Belt. This is a region within the solar system that extends from the zone of the planets out to a thousand astronomical units further. In addition to Pluto's tilted and eccentric orbit, Pluto's orbit is strange in another way. It is tilted away from the evenly arranged orbits of the other planets and crosses the plane of another major planet, Neptune. Pluto is a trans-Neptunian object (TNOs) of which there are many (Pluto being the largest) that orbit the sun twice for every three times that Neptune does. These appear to be stable orbits over millions of years. The objects in this group are now called "plutinos".
In recent years, there has been much discussion as to whether Pluto is a planet or not. If discovered today, Pluto would not be considered a major planet but a minor planet like Ceres. Due to the politics and social pressures of things assumed to be one way and then shown to be another it takes time for the public to accept that even major concepts and ideas must be abandoned to further progress. Sometimes things need to be reclassified or discussed in different ways in the light of new knowledge.
