Is the Earth Young?

A Response to the Short Period Comets Argument

9. Short period comets "boil off" some of their mass each time they pass the sun. Nothing should remain of these comets after about 10,000 years. There are no known sources for replenishing comets. If comets came into existence at the same time as the solar system, the solar system must be less than 10,000 years old.
The argument is presented without a reference source. I looked in the Impact index, but could not find an Impact leaflet on comets either.

It is correct that the shortest of short period comets, the ones with periods less than 10 years or so, will probably not last longer than 10,000 years, maybe even 5,000 years. But the lifetime of a typical comet is best measured in orbits, rather than years, since that's the unit of time that controls the ablation process - each time the comet swings close to the sun, it loses some stuff. Those ablation events happen once per orbit. A typical comet should last about 400 orbits.

But "short period" in comet jargon is less than 200 years, while greater than 200 is "long period". Comet Halley has an orbital period about 76 years. 400 times 76 is 30,000, and that's bad if 10,000 years is a hard limit on the age of a young solar system. So the argument is in trouble already.

But the big mistake is the statement "There are no known sources for replenishing comets." This is now known to be wrong, and it kills the entire argument at once. The long predicted Kuiper Belt has now been directly observed, and it serves quite well as a source for comets with anticipated lifetimes well in excess of the short lifetimes required to make the comet argument sing. The existence of the Kuiper Belt is by it self sufficient to put the short period comet argument out of the rest.

For much longer period comets, the putative source is the Oort cloud, which models place from 10,000 to 100,000 AU from the sun [1 AU = astronomical unit = the average earth-sun distance]. Although not directly seen, its presence is reasonably inferred from the known properties of comet orbits (see, for instance, Zheng et al., 1996 or Weissman, 1998). Furthermore, even if we can't see our own Oort cloud, we do see many similar features around other stars (Backman et al., 1997)


Backman, D.E. et al.
"Exploring Planetary Debris Disks Around Solar Type Stars"
A chapter in "From Stardust to Planetesimals"
ASP Conference Series, vol 122, 1997; Pages 49-66

Weissman, P.R.
"The Oort Cloud"
Scientific American 279(3): 84-89 (1998 Sep)

Zheng, J.Q. et al.
" Orbits of Short Period Comets Captured from the Oort Cloud"
Earth, Moon and Planets 72(1-3): 45-50 (1996 Feb)
[This issue deals heavily with the Oort cloud and comet aging]

  • The SEDS Comet Page
  • The JPL Comet Observations Page
  • Kuiper Belt Home Page
  • The Oort Cloud