A new study of the orbit of comets suggests that the plane of the ecliptic, where the Earth's orbit rests, may not be the only major alignment in the solar system. By tracking the point where long-period comets are farthest from the Sun, Arika Higuchi, an assistant professor at the University of Occupational and Environmental Health in Japan, has shown that there may be a second "empty ecliptic" alignment plane.
If you look at a chart of the solar system, one of the most obvious facts about it is that almost everything in it lies more or less in the plane of the ecliptic, which is the plane formed by the Earth's orbit. The other planets, asteroids, and everything else except comets have orbits that lie within a few degrees of this plane.
The reason for this is that as the solar system formed, it did so as a flat disc of gas and cosmic dust revolving around a bulge that eventually became the Sun. In this disc, the debris also drew together to form the planets and the rest. Even the comets formed in this way, although the interaction of the various planetary gravitational fields scattered their orbits at oblique angles. The aphelion, or the spot where they are farthest from the Sun, however, tends to remain in the ecliptic.
By studying comet orbits, Higuchi has now found that there was another alignment. The plane of the elliptic sits at an angle of about 60 degrees to the disc of the Milky Way, which also can influence the orbit of comets. When the aphelia of the comets were charted, they aligned not only with the ecliptic, but also with a so-called empty ecliptic that is inclined 60 degrees to the Milky Way in the opposite direction. "Empty ecliptic" refers to a plane that was originally empty, but was later populated by comets.
This hypothesis was later confirmed by Higuchi by cross-checking her findings with computations by the PC Cluster at the Center for Computational Astrophysics of NAOJ, and analysis of the comets in NASA's JPL Small Body Database, which showed peaks in the ecliptic and empty ecliptic. However, Higuchi does not see this as conclusive.
"The sharp peaks are not exactly at the ecliptic or empty ecliptic planes, but near them," she says. "An investigation of the distribution of observed small bodies has to include many factors. Detailed examination of the distribution of long-period comets will be our future work. The all-sky survey project known as the Legacy Survey of Space and Time (LSST) will provide valuable information for this study."
