On June 11, 2007, David Jewitt, Nuno Peixinho, and myself observed that comet 133P/Elst-Pizarro has become active after a roughly 5 year period of inactivity (PDF). Figure 1 shows 133P indicated by an arrow. The fuzzy appearance and the tail are also clearly visible. The remaining objects are background stars — they appear elongated due to our following the movement of the comet.
Meet the MBCs.
133P is the prototype of a new class of objects, the main-belt comets (abbreviated MBCs). The MBCs orbit the Sun together with the main-belt asteroids, in almost circular, low inclination trajectories. This is easily seen in Figure 2, where the MBCs (red lines) and asteroids (yellow lines) lie in the same region, whereas comets (blue lines) Halley, a Halley-family comet, and Tempel 1, a Jupiter-family comet, have more elongated orbits. These cometary orbits are also inclined (into or out of the screen) but this is not visible in Figure 2. In conclusion: MBCs have asteroid-like orbits. However, unlike asteroids which have mostly rocky and inert surfaces, the MBCs show cometary activity, i.e., they are surrounded by a fuzzy envelope (known as coma) and have tails extending away from their nucleus. The coma and tail are mostly made up of small dust particles which reflect sunlight and give the MBC its nebulous appearance. The dust particles were ejected from the solid nucleus of the MBC by sublimation of (probably water) ice on the surface. Two other MBCs are known to date. Their uninspiring names are P/2005 U1 (Read) and 118401 (1999 RE70).
Why are the MBCs interesting?
The orbits of MBCs are very stable, which means they must have been there since formation. Because they formed so close to the Sun, closer than other comets such as the ones in the Jupiter-family and the Hubble-family, their ice is likely different in (isotopic) composition. This is important because it may explain the origin of the Earth's oceans. It has long been suggested that our oceans were brought by icy comets which impacted the Earth after its very hot formation. The problem is that Jupiter-family comets and Hubble-family comets have ice which is isotopically different from our ocean water. The isotopical composition of the MBCs ice is still not known, but if it turns out to match that of our water we may have found the reservoir where oceans came from.
If you would like to know more...
To find out more about main-belt comets refer to Henry Hsieh's page on the subject. Henry Hsieh and David Jewitt are the astronomers who have identified this new kind of comet. They also wrote a paper about this, which you can find here.
Figure 1. Shifted and added composite of 133P.
Figure 2. Top view of MBC orbits (in red). The orbits of comets Halley and Tempel 1 are shown in blue and those of main-belt asteroids in yellow. Black lines indicate the orbits of the planets from Mercury, closest to the Sun, to Jupiter.