By Robert Hazen, Ph.D., George Mason University
According to studies and scientific findings, the planets of our solar system were formed in a solar nebula 4.5 billion years ago. The four closest to the Sun are called terrestrial planets, while the outer four are the gas giants. However, our solar system doesn’t end there; several other enigmatic celestial objects cavort in space beyond.
Creation of Solar System
Mercury, Venus, Earth, and Mars are the four inner terrestrial planets. These are rocky planets; small, dense objects, each with a distinct characteristic. Jupiter, Saturn, Uranus, and Neptune are the Jovian planets, the gas giants; planets composed primarily of hydrogen and helium, with swirling atmospheres, much farther out from the Sun. The differences between these inner and outer planets are the result of the Sun’s radiant energy.
When the Sun ignited 4.5 billion years ago, the solar wind and the heat energy pushed the hydrogen and helium away, sweeping it out to the outer regions where it could condense as the gas giant planets.
However, there are a number of additional objects that litter space beyond the gas giants. The first of those is Pluto.
Discovery of Pluto
American astronomer Percival Lowell suspected the existence of Pluto from slight perturbations in Neptune’s orbit. He called the missing object Planet X, and he searched futilely for it until his death in 1916.
The search was resumed by American astronomer Clyde W. Tombaugh, who painstakingly compared photographic plates of the night sky taken several months apart. He was looking for an object that moved relative to the fixed backdrop of the stars.
Pluto was discovered by Tombaugh on February 18, 1930, from this photographic survey.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Pluto’s Characteristics
Pluto is unlike any of the other planets. First of all, it’s pretty small: it has a radius of only 1,140 kilometers; that’s only two-thirds the size of the Moon. Though small, like a terrestrial planet, it seems to have a mixed composition, that’s about half rock and half ice. So, in that case also, it’s different from the Jovian planets and the terrestrial planets.
Pluto has a highly elliptical orbit. It averages about 40 times the Earth-Sun distance during a 249-year orbit around the Sun. Yet, at some times, for nearly 20 years out of that orbit, it’s actually inside the orbit of Neptune. Pluto’s orbit is significantly tilted to the plane of other planets. This is another thing that makes it odd.
These characteristics have led some astronomers to suspect that Pluto is, in fact, an escaped moon of Neptune.
Think about how this might have happened; if there had been collision, for example, with a comet, or a large asteroid, or some other object, it might have pulled that object right away from Neptune, swept it into its orbit, which then is the current orbit of Pluto.
Learn more about the solar system.
The Kuiper Belt
Beyond Pluto lies the Kuiper Belt. The Kuiper Belt contains hundreds of objects, up to a few hundred kilometers in diameter. From the darkness of the surfaces of these objects, they appear to be at least partly rocky.
Looking at the light coming to us that’s reflected off the surface, it appears that there are two different populations of these objects. Some objects the light is reflecting off have more red wavelengths, the reddish Kuiper Belt objects, while others are slightly bluish. This suggests that there are at least two different surfaces, perhaps two different compositions of objects.
It’s hard to know what these different compositions are exactly. Maybe some are icier, and maybe some are rockier; we don’t know. Some scientists have suggested that Pluto is, in fact, the largest of these Kuiper Belt objects. Again, we just don’t know; they’re too far away, and we need more studies of these distant objects.
Learn more about the life cycle of stars.
The Oort Cloud
Even farther away are the comets, the most distant objects known that are gravitationally tied to the Sun. These have extremely elliptical orbits, with an average distance about 50,000 times the Earth-Sun separation— that’s three-quarters of a light-year. Dutch astronomer Jan Oort, who lived from 1900 to 1992, suggested that a large number of comet-like objects occupy a distant band called the Oort Cloud.
These would be primarily frozen liquids and gasses—clumps of this material, far out in space. Once in a while, some of this material is jostled and starts its long journey inward toward the Sun. These comets that are occasionally jostled free start an orbit that sometimes takes millions of years to complete from deep space.
Of course, when a comet gets close to the Sun, the tail forms. That’s when the volatiles—the methane, and the water ice, and other materials— start vaporizing, and the long tail forms. That’s when we see the comet in its spectacular glory as it approaches the Sun.
If the orbit’s further perturbed— for example, by Jupiter or another large planet—the comet can adopt a much shorter orbit, closer to the Sun. Comet Halley, for example, has a period of about 75 years, so it’s a much shorter orbit.
Common Questions about the Search for Pluto and Other Celestial Objects in the Solar System
The gas giants are primarily made up of hydrogen and helium. In contrast, the inner terrestrial planets have mostly rocky surfaces. The differences between these inner and outer planets are the result of the Sun’s radiant energy.
Pluto is pretty small, only two-thirds the size of the Moon. Though small, it seems to have a mixed composition, that’s about half rock and half ice. It has a highly elliptical orbit. Moreover, its orbit is significantly tilted to the plane of other planets.
Dutch astronomer Jan Oort suggested that a large number of comet-like objects occupy a distant band called the Oort Cloud, lying beyond the Kuiper Belt.
