The Evolution of Long-Period Comets

TL;DR

This study uses numerical simulations to analyze the evolution and fading of long-period comets, matching observed distributions and exploring effects of gravitational and non-gravitational forces in the solar system.

Contribution

It introduces a detailed model incorporating fading laws and external forces, providing new insights into comet lifetimes and orbital distributions.

Findings

Observed comet distribution requires fading or depletion processes.

A fading law proportional to m^-0.6 fits observations well.

Most comets last only a few returns before fading or destruction.

Abstract

We study the evolution of long-period comets by numerical integration of their orbits, following comets from their origin in the Oort cloud until their final escape or destruction, in a model solar system consisting of the Sun, the four giant planets and the Galactic tide. We also examine the effects of non-gravitational forces and the gravitational forces from a hypothetical solar companion or circumsolar disk. We confirm the conclusion of Oort and other investigators that the observed distribution of long-period comet orbits does not match the expected steady-state distribution unless there is fading or some similar process that depletes the population of older comets. We investigate several simple fading laws. We can match the observed orbit distribution if the fraction of comets remaining observable after m apparitions is proportional to m to the power -0.6 +/- 0.1 (close to the fading law originally proposed by Whipple 1962); or if approximately 95% of comets live for only a few (~6) returns and the remainder last indefinitely. Our results also yield statistics such as the expected perihelion distribution, distribution of aphelion directions, frequency of encounters with the giant planets and the rate of production of Halley-type comets.