Reasons to Believe

Rare Moon Just Got Rarer

We owe our very existence to the Moon. Because we are living on a relatively small planet orbited by a single gigantic moon, our planet’s rotation axis tilt is stable, our planet possesses tidal pool ecosystems, and our planet’s rotation rate has been slowed down to a value that is optimal for advanced life. The stability of Earth’s rotation axis tilt is the most life-critical feature of the Moon. If it were not for the Moon, Earth’s rotation axis tilt would change so dramatically as to induce climate changes severe enough to wipe out nearly all forms of life.

So far, astronomers have detected moons only in our solar system. The list now stands at 431 confirmed moons. Of these, 173 orbit the eight planets and the other 258 orbit dwarf planets, asteroids, and comets.

Of the moons orbiting the solar system, our Moon sticks out like a sore thumb. Its mass relative to the mass of its host planet (Earth) is fifty times greater than that of any other moon orbiting a solar system planet. It is because of this enormous mass ratio that only our Earth has a stable rotation axis tilt.

The long term habitability of a planet requires a moon like our Moon orbiting a planet like our Earth. According to a recent study,1 however, moons of any kind orbiting rocky planets that are relatively close to their host stars may be extremely rare.

Author Stephen Kane explained that for a planet to retain a moon, the moon must orbit the planet between the Roche limit and Hill radius.The Roche limit is the minimum distance to which a large satellite can approach its primary body without being torn apart by tidal forces exerted by the primary body. The Hill radius is an astronomical body’s spherical region in which it dominates the attraction of satellites to a sufficient degree that such satellites do not escape. Satellites interior to the Hill radius remain in stable orbits about the astronomical body. 

For Earth, the Hill radius is a little less than 1 million miles and the Roche limit is about 12,000 miles. The Hill radius presumes no perturbative effects from the host star or from any other planets in the planetary system. Typically, these perturbative effects reduce the Hill radius to a factor of three times.

Kane pointed out that currently, astrobiologists in their quest to find an extrasolar habitable planet are especially enamored with planets orbiting stars that are smaller than the Sun. These stars are numerous, comprising 97% of all stars in our Milky Way Galaxy. Most of the discovered extrasolar planets orbit their stars closer than Earth orbits the Sun. Since small stars are much cooler than large stars, the zones where water could exist in liquid form will be closer to these stars than would be the case for larger stars.

Kane then showed that for planets orbiting their host stars significantly closer than Earth orbits the Sun, the Hill radius is dramatically closer to the planet. For a planet orbiting a small star at a distance of 5 million miles the theoretical Hill radius is only  56,000 miles and the practical Hill radius just 19,000 miles. Thus, the Hill radius is a mere 58% larger than the Roche limit. (For Earth, the Hill radius is about 7,000% larger than the Roche limit.)

A difference of just 58% effectively implies that planets orbiting their stars much closer to their hosts stars than Earth orbits the Sun will be, in Kane’s words, “worlds without moons.” Kane also pointed out another problem that makes moonless planets even more unlikely.

The reason why so many extrasolar planets orbit their host stars closer than Earth orbits the Sun is that in their infancy they migrated from their birthing locations to orbital positions much closer to their host stars. The migration process, Kane demonstrated, inevitably exposed the migrating planets to highly variable gravitational perturbations from the host star and especially from other planets and planetesimals in the same system. These perturbations will almost always strip the migrating planets of any large moons they may be hosting.

The bottom line of Kane’s research is that large moons orbiting planets a few times larger than Earth, or smaller moons that orbit their host stars close enough where water will not be permanently frozen, are likely to be exceedingly rare.

This rarity is affirmed by the statistics of moons orbiting the Sun’s planets. None orbit Mercury or Venus. The two orbiting Mars possess insignificant masses—about one ten-millionth the mass of our Moon. Of the 173 moons orbiting the Sun’s planets, 170 orbit the gas giant planets, all of which orbit the Sun far beyond the snow line. (The snow line is that distance from a star at which volatiles like water, carbon dioxide, and carbon monoxide are permanently frozen.)

If you have not thanked God for the Moon lately, today may be a good day to start. Paul writes in 1 Corinthians 15:41 that the Moon possesses one kind of glory. Indeed, the Moon stands as a testimony of how God designed our planet and every other body in our solar system so that we humans could thrive on Earth.


  1. Stephen R. Kane, “Worlds without Moons: Exomoon Constraints for Compact Planetary Systems,” Astrophysical Journal Letters 839 (April 13, 2017): id. L19, doi:10.3847/2041-8213/aa6bf2.
  2. Kane was one of 13 Christian astronomers who reviewed the debate between me and astronomer Danny Faulkner on the age of the universe. You can read their review at Hugh Ross, “An Evaluation of Evidence for the Age of the Universe,” Today’s New Reason to Believe (blog), Reasons to Believe, March 7, 2012,
  3. Kane, “Worlds without Moons.”

Subjects: Astronomy, Milky Way Galaxy, Moon & Its Formation, Extrasolar Planets, Life on Other Planets, Solar System Design