Many people were disappointed a decade ago when the International Astronomical Union demoted Pluto from its status as a planet by defining, for the first time, what a planet is. There were at least two reasons that Pluto was removed from the group of objects that we call planets. One reason was that Pluto does not seem to fit with the other planets. Based upon common properties, we easily can group the planets into one of two categories, terrestrial (Earth-like) and Jovian (Jupiter-like) planets. The first four planets, Mercury, Venus, Earth, and Mars, are the terrestrial planets, while the next four, Jupiter, Saturn, Uranus, and Neptune, are the Jovian planets. Pluto's properties make it difficult to put it into either group. The other reason that Pluto was reclassified from planetary status is that it is very small compared to other planets. This was brought to the forefront in 2003 by the discovery of Eris, another body similar in size to Pluto and with a similar orbit. If Pluto is a planet, then shouldn't Eris be a planet, too? Eris was just the largest of many small bodies, discovered since 1992, orbiting the sun beyond the orbit of Neptune. More of these likely will be found, so shouldn't the larger ones of these be planets, too? The discovery of so many small objects beyond the orbit of Neptune has been motivated by the search for the Kuiper Belt, a hypothetical collection of bodies where short-period comets supposedly come from. Many astronomers call these distant denizens of the solar system Kuiper Belt Objects (KBOs), though some prefer (as I do) the name trans Neptunian objects (TNOs). Astronomers now think that Pluto was just the first TNO discovered, more than seventy years before discovery of the second one. But now many Pluto fans are sure to be encouraged by the January 20, 2016, announcement that two astronomers think that they have evidence that there is a ninth planet after all, a sort of replacement for Pluto.
Discovery of the Outer Planets
To understand this new ninth planet, we ought to review the discovery of planets in the outer solar system. There are five planets that appear as bright stars to the naked eye. In fact, all five are visible in the early morning sky in late January and early February 2016.
These planets have been known since ancient times, which is why our names for them are those of gods in the ancient Roman pantheon. The first modern planet discovery was in 1781, when the German-English astronomer William Herschel (1738–1822) stumbled upon an object while surveying the sky with his telescope. While stars are mere pin points when viewed through a telescope, Herschel could see that this object was a small disk. He thought at first that he was seeing a comet, but after he recorded the motion of the object over several nights, Herschel was able to compute an orbit. He was astonished to learn that it had a circular orbit about twice as far from the sun that Saturn is. This could not be a comet, but rather had to be a planet. It took years for astronomers to decide what to call this new planet, but eventually they decided to follow the custom of naming planets for ancient Roman deities and called it Uranus.