There are now only eight official planets in the solar system. Where did Pluto go? And what about those new planets they keep announcing?
For thousands of years there were just five planets, Mercury, Venus, Mars, Jupiter, and Saturn. Nobody knew they were planets, they were just wandering stars obviously different from the fixed stars. Something of the true complexity of the solar system started to be seen with the invention of the telescope, when smaller objects were discovered orbiting around some of the planets. The Earth eventually became classified as just another planet orbiting the sun. But not until the late 18th century was Uranus discovered, and soon after Neptune, bringing the total to eight. These planets were all fairly big, four of them huge gas giants far larger than the Earth, and all of them orbited the sun in fairly circular paths, the dominant body in their part of space. Everything was clearcut, even though some of the largest gas giant moons were the size of Mercury.
A variety of smaller wandering planets were discovered with the aid of telescopes, and they all turned out to be orbiting in the gap between Mars and Jupiter. Eventually thousands of asteroids would be found in a belt, all of them much smaller than Pluto, many irregularly shaped, and very rocky or even metallic. Further asteroids were to be found in harmonic orbits with Jupiter, and eventually near Saturn and even in eccentric orbits passing near the Earth.
Neptune had been discovered by measuring small gravitational tugs on the orbit of Uranus. Astronomers worked out where to look and there was the big blue gas giant. Using the same technique, they looked long and hard for yet another large planet further out in the solar system. After many years of nothing, Clyde Tombaugh discovered Pluto orbiting about where it should be, but it quickly became apparent that Pluto was not what they had been looking for. It was a small rocky or icy body, smaller than any of the other planets and nowhere near large enough to tug anything about. It also had a highly eccentric orbit, tilted at an angle to the sun and other planets, and even passing inside the orbit of Neptune at times. Then and now there was speculation that Pluto was an ejected gas giant moon, a deflected dead comet, or some other object that hadn't originally formed in a planetary orbit. Still, there was little opposition to classifying it as the ninth planet, it was comfortably larger than anything else orbiting directly about the sun and it fitted nicely in the outward progression of planets. Astronomers kept looking for a tenth planet which they were fairly sure would be out there beyond Pluto, but nothing appeared.
A handful of very large moons had been discovered orbiting around the various gas giants, comparable to the size of Mercury and all larger than Pluto, but there was no confusion. These were clearly satellites in orbit about other planets, while Mercury and Pluto were not.
New Discoveries Cloud the Picture
Towards the end of the 20th century, space probes began to reach the gas giants, the Hubble space telescope was brought into service, ground-based telescopes made leaps forward in resolution, and automated electronic imaging vastly increased the capacity to spot faint objects orbiting the sun Several discoveries began to muddy the waters. Pluto was shown to be much smaller and lighter than thought, and with a large companion object. Pluto was now much smaller than many gas giant satellites, smaller even than our own Moon. There were mutterings that this insignificant object was hardly a true planet.
In the same period, other solar system objects started to be discovered. Between the orbits of Jupiter and Neptune, a number of substantial objects were discovered, now referred to as Centaurs, all smaller than Pluto. Outside the orbit of Neptune, the Trans Neptunian Objects (TNOs) started to show up. In time, at least two were shown to be even larger than Pluto
and the situation had finally become untenable. Pluto was no longer a well-defined planet but simply a largish example of hundreds, probably thousands, of similar bodies. Either there were dozens, possibly hundreds, of planets, or Pluto could not realistically be classed as a planet at all.
Defining a Planet
In 2006, the International Astronomical Union (IAU) decided to address the issue at its General Assembly meeting. It produced a formal definition of a planet within the solar system, something which had originally been fairly obvious, recently very confused, but never formally defined. The headline news was that there were now only eight planets and Pluto wasn't one of them.
In detail, a planet must first be large enough for gravity to pull it together into a roughly spherical shape. This included all the nine planets at that time, plus a number of asteroids, a number of satellites of the planets, and many of the TNOs and other newly discovered objects in the outer solar system. A second criteria was that the object should be in direct orbit about the sun, not about another planet, so all the satellites were excluded. The last criteria for being a planet was that the object should "have cleared the neighbourhood around its orbit", meaning that the object should not be part of a group of objects in similar orbits and should not be locked in resonance with a larger object. This excluded all the asteroids and all the TNOs including Pluto. Notice that there is no strict size limit on planets, but it is extremely unlikely that any further planets will be discovered under this definition since there just isn't any space for them to have a cleared orbit.
The IAU also collectively defined all other objects as Small Solar System Bodies, but the terms asteroid, minor planet, comet, satellite, etc. remain to describe them more specifically.
The Dwarf Planets
A completely new category of solar system objects was defined, the Dwarf Planets. These were defined as for planets except that they need not have cleared the neighbourhood around its orbit. Asteroids large enough to be spherical under their own gravity, and equivalently large TNOs, would now be classed as Dwarf Planets. Again, there are no strict size limits on how large or small a Dwarf Planet can be, but in practice an object must be several hundred kilometers across to have the mass to pull itself into a sphere.
Pluto is the obvious first candidate as a Dwarf Planet, clearly massive enough and round enough, as well as being the reason for the new definition in the first place.
Charon, the satellite of Pluto, is another obvious candidate. It is well enough studied that its size, shape, and mass are known and it is a spherical body pulled together by its own gravity. However, it is unclear whether it is in orbit directly about the sun as a double object with Pluto or should be considered simply as a sateliite of Pluto. Currently Charon is not officially defined as a Dwarf Planet. One definition of a double system is that the centre of gravity of the system lies outside the larger body and this is the case with the Pluto Charon system. Pluto also has two other tiny objects in orbit about it, called Nix and Hydra.
Ceres is the largest of the asteroids orbiting in the belt between Mars and Jupiter, although it is not the brightest as seen from Earth. Ceres has been studied well enough to show conclusively that it is a differentiated body with a rocky core and icy overlay all pulled together by gravity. It has been formally classified as a Dwarf Planet.
Eris is the largest known TNO, about 5% larger than Pluto. Orbiting about 50% further from the sun than Pluto, and without a large companion, it is much fainter than Pluto and was only discovered in 2003. Eris has been formally classified as the third Dwarf Planet.
Vesta is the brightest of the asteroids as seen from Earth and almost certainly meets the criteria as a Dwarf Planet but is not yet formally accepted as the fourth. Around 50 other objects are likely to also meet the Dwarf Planet criteria but are not yet well enough studied to be certain. For example, there are about a dozen objects thought to be around 1000km or larger and all will likely be classified as Dwarf Planets when they are studied well enough. Possibly many more will be discovered further out towards the Oort cloud.
