When the International Astronomical Union (IAU) decided in 2006 on a modified definition of the concept “Planet” and consequently Pluto was demoted to the class of “Dwarf Planet”, there was big international outcry by many who seemed to take this kind of personal. However this icy body itself, in its elliptical and tilted orbit mostly beyond planet Neptune, remained exactly the same as it has always been during the last 4.5 billion years.
We will hear a lot more about Pluto in the near future when the New Horizons spacecraft will finally reach this Solar system body in July 2015.
But what do we presently know about Pluto and its companions?
Pluto was discovered in 1930 by Clyde Tombaugh who systematically researched photographic images in search of “Planet X”. The name “Pluto” (the mythological god of the underworld) was suggested by an eleven year old girl to her grandfather, who passed this on to the astronomer community.
In 1951 astronomer Gerard Kuiper predicted that some comet-like debris from the formation of the solar system should be just beyond the orbit of Neptune. In the early 1980’s computer simulations of the solar system's formation confirmed that a disk of debris could have formed around the edge of the solar system. This region is called the Kuiper Belt or Edgeworth-Kuiper Belt. According to this scenario, planets had clumped together rather quickly in the inner region around the Sun, and gravitationally swept up remaining debris. However, beyond Neptune, the last of the gas planets, there should be a debris-field of icy objects that formed into smaller bodies. A few of these actually became big enough to take on a spherical shape.
In 1992 a 240 km wide body, called 1992QB1 was detected, at the distance of the suspected belt. Soon, several similar-sized objects were discovered, confirming that the Kuiper belt was real. Since 1992 almost a thousand Kuiper Belt Objects (KBO’s) have been discovered but only the largest have been named. One object called Eris is actually larger than Pluto.
Pluto’s largest companion Charon was discovered in 1978, and is about half the size of Pluto. In 2005, two small additional moons were discovered and were named Nix and Hydra. Recently two more moons were discovered by the Hubble Space telescope, Kerberos in 2011 and Styx in 2012. These names all refer to the Greek mythology of Pluton or Haides, the god of the underworld.
Is Charon a Moon of Pluto? When the IAU redefined the concept “Planet” in 2006, they might as well have done something about the term “Moon”. Currently the idea of a “Moon” is of an object that orbits about another body that itself orbits the Sun. But where does that lead us? Yes, planets have moons (we have discovered almost 200 moons in the Solar system, mainly around the gas planets), but asteroids also have moons. Only 18 of the moons around planets in the Solar system are big enough to have become spherical in shape, all the others are irregular objects.
So the IAU has told us that Pluto is a Dwarf planet. But Charon (1200 km) is about half the size of Pluto (2300 km) and that is very odd for a planet and moon pair. Earth’s Moon Luna, is about a quarter the size of planet Earth and that is already a big exception in the Solar System. Most moons are very much smaller than their host planet.
Pluto and Charon are in an unusual orbit around each other. The animation shows the “dance” of Pluto and Charon in the correct scale as to size and distance. The red dot indicates the common centre of gravity of the two bodies, which lies far outside Pluto. Because of this “intimate dance”, Pluto and Charon are in mutual tidal locking that causes both objects to face each other with the same side all the time. If you are on the “back” side of Pluto, you will never see Charon and similarly for an observer on Charon who never can see Pluto when he is on the "wrong" side. They orbit each other in about 6.4 Earth days at a distance of 19,570 km, about a common centre of gravity like a giant dumbbell.
Could that be called a (dwarf) planet and its moon? My suggestion is to call Pluto – Charon a Binary (or double) Dwarf Planet system, similar to the many double stars that we have in the Universe. It comes down to the vague definition we have of a “Moon”. Relatively big objects like Charon should be linked up with their bigger companion as a “Binary” system. On the other end of the spectrum where moons are very small, such as objects in the rings of the gas planets, we don’t call them moons, but technically they are. I think it is time for the IAU to give us some sharper definitions of the concept of a moon. What do you think?
The Pluto-Charon system is special in other ways too. Similar to Uranus, Pluto is "rotating on its side" with an axial tilt of 120 degrees. Charon orbits its companion in the equatorial plane as shown. In the period 1985 - 1990 Charon's orbital plane was seen edge-on from Earth, showing that Charon has an orbital period of 6.4 days.
Pluto’s (and Charon’s) orbit around the Sun is quite elliptical, more so than any of the eight planets. Its distance to the Sun varies between 30 and 49 AU. During part of its orbit (of a total of 248 Earth years), Pluto is actually closer to the Sun than Neptune.
The orbit is also inclined with respect to the ecliptic by about 17 degrees, much more than that of any of the planets.
Anyway, we are going to learn a lot more about Pluto, Charon and its other companions in July 2015 when the New Horizons spacecraft, after a voyage of nine and a half years, will fly-by this remote system. Recently New Horizons was woken up out of hibernation for the last time, in preparation for its historic encounter.
Many discoveries are going to be made about this interesting “Binary Dwarf Planet” system.