NASA scientists announced this week (20 July 2011) that they had discovered a new moon around the dwarf planet Pluto. The discovery was made by scientists using the Hubble Space Telescope in a set of five long exposure pictures taken over a period of two months and is referred to as 'P4' or 'S/2011 p1'. The moon has since been identified in earlier Hubble images, from 2006 and 2010.
The Pluto system as we currently understand it, incorporating the new moon P4.
Pluto was discovered in 1930 by Clyde Tombaugh, a young astronomer working at the Lowell Observatory in Arizona; its existence had been predicted as early as 1909, due to anomalies in the orbit of Neptune. At the time it was assumed that Pluto was a planet of some size, capable of disturbing the orbit of Neptune. Pluto spends part of its 248 earth year orbit inside the orbit of Neptune; this is not the same on every orbit, but alternates between a 20 and a 14 year stay.
In 1979 the first moon of Pluto was discovered by James Christy at the United States Naval Observatory. Dubbed 'Charon' it enabled scientists to make an estimate of the size of Pluto; to their surprise they found it has only 2% of the mass of Earth or 18% of that of the moon, making it far to small to affect the orbit of Neptune, re-starting the hunt for new planets in the outer solar-system. In fact it is now understood that (due to orbital effects) Pluto passes closer to Uranus than it ever does to Neptune. Charon is so large (relative to Pluto) that their mutual centre of gravity, the point about which they both rotate, is 2040 km above the surface of Pluto; they are the only known planet/moon system for which this is true (671 Patroclus is a pair of asteroids orbiting a mutual centre of gravity at Jupiter's trailing Lagrangian Point, but these are nothing like planets), and for this reason some scientists suggest they should be referred to as a pair of binary planets rather than a planet and its moon. Charon has a mass 2% that of the moon (1520 × 10¹⁸ kg) and a diameter of 1205 km. It orbits the systems centre of gravity at a distance of 17 530 km.
The Pluto/Charon system.
In 2004 another Pluto-like object was located in the outer solar system by a team led by Mike Brown at the California Institute of Technology (this is disputed; José Luis Ortiz Moreno of the Instituto de Astrofísica de Andelucía and his team at the Sierra Nevada Observatory in Grenada, Spain). Haumea has only a third the mass of Pluto, and orbits slightly further out, but it confirmed what scientists were beginning to suspect, that the outer solar system might contain a number of such objects.
An artists impression of Haumea. It is ellipsoid in shape and has a distinctive red patch. These cannot be directly imaged, but are the best interpretation of the available data on Haumea.
In 2005 Mike Brown's team working at Caltech's Palomar Observatory discovered two further objects in the outer solar system.
Makemake is similar to Haumea, roughly third the mass of Pluto, and further out, but the other new object, Eris, was more interesting. Eris is half again as far from the sun as Pluto - and a third again as big. Clearly if Pluto is a planet, then Eris is too.
Hubble images of Eris and it's moon Dysnomia.
Since it was likely that there are many more objects of this size in the outer solar system many astronomers were becoming uncomfortable with the term 'planet' to describe them. Thus in 2006 the International Astronomical Union settled on the term 'Dwarf Planet', to designate objects large enough to form a roughly spherical shape under their own gravity, but no so massive as to have cleared the area around their orbit of all other objects. Pluto, Haumea, Makemake, and Eris were placed in this category, as was Ceres in the asteroid belt. Ceres had also been classified as a planet at the time of its discovery in 1801, as were a number of other asteroids until the mid-nineteenth century, when it became clear that asteroids were too abundant to be classed as planets.
Also in 2005 two more moons of Pluto were discovered by the Hubble Space Telescope Pluto Companion Search Team. Nix and Hydra are further out and smaller than Charon, but orbit the same centre of gravity, so logically if Charon should be considered a planet, then so should Nix and Hydra. This is rather more problematic, as Nix is only 91 km in diameter and Hydra 114 km. Nix orbits the system's centre of gravity at 48 708 km and Hydra at 64 749 km.
The new moon, S/2011 p1, orbits the same centre of gravity as the rest of the system and therefore logically could also potentially be considered a planet, despite having a radius of between 14 and 34 km. It orbits between Nix and Hydra, at a distance of about 59 000 km.
Clearly the Pluto system is something very different to anything in the inner solar system, a cloud of objects more than a planet with satellites. Some scientists theorize that this may be the result of a collision early in the solar system's history, in the same way that Earth's moon (larger in comparison to its parent body than any other moon in the solar system except Charon) is thought to be the result of a collision between the early Earth and a Mars-sized object.
The images we have at the moment, even those taken with our best telescopes, are still pretty faint. In July 2015 NASA's New Horizon spacecraft is due to reach Pluto and will hopefully bring us far more information, and probably a good few more surprises.