As seen from the Earth, the Sun, Moon, and planets all appear to move along the ecliptic. More precisely, the ecliptic is the Sun’s apparent path among the stars for a year. But there are exceptions such as comets.

The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic.

Models of solar formation propose that even long-period comets initially shaped near the ecliptic and were later dispersed into the orbits observed today through gravitational interactions, most prominently with the gas giant planets. However, even with planetary scattering, the comet’s aphelion, where it is farthest from the Sun, ought to remain near the ecliptic. Other external forces are expected to clarify the observed distribution.

The gravitational field of the Milky Way galaxy in which the solar system resides also exerts a small but non-negligible influence.

Arika Higuchi, an assistant professor at the University of Occupational and Environmental Health in Japan and previously a member of the NAOJ RISE Project, studied the effects of the galactic gravity on long-period comets analytical investigation of the equations governing orbital motion.

Her study has shown that when considering the galactic gravity, the aphelia of long-period comets collects around two planes: 1. Ecliptic, and 2. Empty ecliptic.

The ecliptic is inclined with respect to the disk of the Milky Way by about 60 degrees. The empty ecliptic is also inclined by 60 degrees but in the opposite direction. 

Higuchi calls this the “empty ecliptic” based on mathematical nomenclature and because initially, it contains no objects, only later being populated with scattered comets.

Higuchi carried out numerical computations in part on the PC Cluster at the Center for Computational Astrophysics of NAOJ. 

The study indicates that the solar system has a second alignment plane. Analytical investigation of the orbits of long-period comets shows that the comets’ aphelia, the point where they are farthest from the Sun, tend to fall close to either the well-known ecliptic plane where the planets reside or a newly discovered “empty ecliptic.”

Comparing the analytical and computational results to the data for long-period comets listed in NASA’s JPL Small-Body Database showed that the distribution has two peaks, near the ecliptic and empty ecliptic as predicted. This is a strong indication that the formation models are correct and long-period comets formed on the ecliptic.

Higuchi cautions, “The sharp peaks are not exactly at the ecliptic or empty ecliptic planes, but near them. 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.”

Journal Reference:

1. Arika Higuchi. Anisotropy of Long-period Comets Explained by Their Formation Process, The Astronomical Journal (2020). DOI: 10.3847/1538-3881/aba94d