Comets are scraps of the formation of the planets, more than 4 billion years ago. We believe comets formed between the current orbits of Jupiter and Neptune (5 to 30 AU¹) and were pushed – or in some cases hurled – out to great distances by the gravity of the planets as these migrated through the disc. Many ended up in the Kuiper belt (~40 AU) but probably more were sent to the Oort cloud (~10,000 AU) or even to interstellar space.
In the Kuiper belt and Oort cloud these comets were kept very cold and were able to retain ices of all the ingredients that were present in the disc when the planets were forming. That is why comets are important, because they are frozen relics from a time we cannot directly observe.
The comets we currently see have left their cold reservoirs possibly due to perturbations by Neptune (if they come from the Kuiper belt) or a passing star (if they come from the Oort cloud) and are returning to the inner solar system on elliptical orbits. As they approach the Sun, their ices are heated and sublimate into jets. The jets carry some dust that was embedded in the ice and give rise to the coma: an unstable atmosphere of gas and dust that surrounds the nucleus. The coma is what gives comets their fuzzy appearance. The solar radiation then pushes part of the coma into the elongated tail structure that gives a comet its famous appearance.
In the past, I have led a Max Planck Research group in Cometary Science to analyse the results of the Rosetta mission in the context of the formation and evolution of the solar system.
The images and data we are getting from comet 67P never cease to surprise us.
I have also worked on a paper about the main-belt comet 133P/Elst-Pizarro.
- Lacerda (2013), MNRAS 428, 1818
- Lacerda & Jewitt (2012), ApJL 760, 2
- Hsieh, Ishiguro, Lacerda & Jewitt (2011), AJ 142, 29
- Hsieh, Jewitt, Lacerda et al. (2010), MNRAS 403, 363
¹ Astronomical Unit, the distance between the Sun and the Earth, convenient to measure distances in the solar system.