The Origin of Comets

Proplyds in Orion

Beta Pictoris from MKO

The comets formed in the outer regions of the protoplanetary disk. The high content of volatiles like water and carbon monoxide in the cometary nucleus provides relatively direct evidence for formation at low temperatures. Dynamical considerations are compatible with the physical evidence: the majority of the comets in the Oort Cloud were formed near and ejected by Uranus and Neptune. Comets now in the Kuiper belt were formed near where we see them today.

If comets are icy planetesimals from the sun's protoplanetary disk, how did they come to be where we see them today? Planetesimals in the vicinity of the growing cores of the gas-giant planets were either absorbed by those planets, or hurled away by close encounters with them. Planetesimals ejected from the solar system by a kind of gravitational sling-shot were, for the most part, lost to the interstellar medium of our galaxy. A small fraction (maybe 10%) of those ejected were perturbed by passing stars into the Oort Cloud. Planetesimals substantially beyond the orbit of Neptune were immune to gravitational disturbances and so were not ejected. These bodies are seen today in the Kuiper Belt.

So, you can see that the comets in the Oort Cloud originated closer to the sun than the objects of the Kuiper Belt. Since Long-Period Comets are thought to be associated with the Oort Cloud and Short-Period Comets with the Kuiper Belt, there is reason to expect that the LPCs formed at higher temperatures than the SPCs. Chemical evidence for (or against) this idea is presently being sought.

Evidence for disks around young stars is now compelling: they are observed via their thermal emission and are seen in projection against background gas. When observed in projection, they have been (rather oddly) named "proplyds".

More surprisingly, some relatively old, main-sequence stars like Beta Pictoris also exhibit circumstellar disks. These older disks are unlikely to be accretion disks of the kind present around T-Tauri stars. Instead, they must be continuously regenerated, perhaps by sublimation of unseen comets or by collisions in an unseen Kuiper Belt. Some nice images of regenerated disks have been compiled by Paul Kalas.

Comet Jewitt