Friday Night Science: Why We Have Only One Moon?

Have you looked at the Moon and wondered why one? 

It’s not fair that Earth has only one natural satellite, which we call The Moon, while Jupiter is swarmed by ninety-five (orbits confirmed so far) and perhaps a dozen more hiding in the outbacks. With Saturn, it’s even worse — one hundred and forty-six. Uranus and Neptune, individually, with twenty-seven and fourteen moons, bring the score to a splendid forty-one. And how about Pluto? That not-much-of-a-planet dare pick up five. Mars with Deimos and Phobos, those hijacked asteroids, finish second last, while our beloved Earth loses the game. 

Ask yourself what factor controls the number of natural satellites for a particular planet. If it’s all about mass (or size), Jupiter, no doubt, should have the most satellites. Yet, Saturn places the highest bidder. Among the ice giants, the lighter one, Uranus, weighing about 15 Earths, has more moons than Neptune (heavy at 17 Earths). On the opposite end, Pluto, at only 1/6th the width of Earth, dwells in a crazy neighborhood. With all that in mind, Earth, moderately large by terrestrial standards, has only the Moon. 

That's the Moon coming up . . . or hanging down.
Shot from the International Space Station flying over the Pacific, New Zealand on December 27, 2021. 
Credit: NASA

Shift your focus to Mercury and Venus*, two moonless worlds, and you begin to think differently. Perhaps the closer a planet is to the Sun, the fewer moons it can have or none; the Sun’s gravity is too strong. Good. You’re doing great! Planets have a Hill Sphere — the immediate neighborhood where its own gravity dominates over the Sun’s. A planet can hold its satellites as long as they’re within its Hill sphere. If a satellite migrates beyond the Hill sphere (for some reason) the Sun will capture it for itself. Astronomers believe that eons ago, Mercury may have had a moon. But being the smallest planet in the solar system, Mercury’s gravity isn’t strong enough to hold a moon, and all likely, the moon got ‘eaten’ (literally) by the Sun. 

Why not address something more fundamental — how do planets get their moons? 

As far as the consensus goes, planets acquire their moons in one of three ways:

• Accretion: Proponents of the accretion theory believe that as planets cumulatively grow over time by accreting gas, dirt, dust, and minor bits of rock, they can have their own — so-called — circumplanetary discs. On the nature of orbits, natural satellites fall under two broad categories: regular and irregular. Regular satellites don’t steer far away from their primaries; they maintain nearly circular orbits along the planet’s equatorial plane, and above all, they have prograde motion, i.e., they orbit (and also rotate) in the same direction as their primary’s spin. Io, Europa, Ganymede, and Callisto, Jupiter's four major moons, and Titan of Saturn likely formed alongside their parent planets in their younger days.

• Impact: Earth and moons could have been twins — the accretion model allows that. However, compelling evidence guides astronomers towards a violent scenario. When Earth was almost complete as a planet, it was struck by a Mars-sized object dubbed Theia. The cataclysmic collision tore them both apart down to their very cores. Astronomers suspect that while Earth reacquired most of its material from the debris field and grew in size, some material reached orbit and coalesced into the Moon. Pluto and Uranus are also believed to have met a similar fate.   

• Capture: Out of 264 natural satellites, unevenly distributed across six planetary systems, excluding Pluto, a bulkload of them are actually asteroids, comets, and leftover planetesimals that never grew into proper planets. Although irregular satellites can have prograde or retrograde orbits — opposite to the direction of their planet’s rotation, they are usually found at the farthest distances from their primary in highly inclined orbits. Most of Jupiter, Saturn, Uranus, and Neptune are captured satellites. The red planet, more than twenty magnitudes smaller than the gas giants, is no exception. The four giant planets have such (gravitational) influence in their immediate neighborhood that if any stray object passes too close, they fall in orbit. Triton — Neptune’s largest moon is an oddity. Though Triton stays close and maintains a neat circular orbit, its large size and a retrograde orbit tilting over 156 degrees away from Neptune’s equatorial plane make Triton a possible suspect for a captured Kuiper Belt object.   

These are only probable hypotheses — educated guesses and reasonable conclusions drawn from the best data. Scientists are yet to decipher the first few lines of the obscure script that writes the solar system's Genesis story. 

In summary, Earth has the Moon because it got it by pure chance — a freak collision. If not for the Theia impact, Earth would have come third on the list of moonless worlds. Or like Mars, Earth would've captured an asteroid if one passed under its Hill sphere. Without that fancy silvery orb up there, Earth could've spun faster, might've tilted on its side like Uranus, or had a sloth rotation like Venus; you can't know. Did the Moon play a part . . . in life's origin? 

With all that, every once in a while Earth does have more than one moon. It so happens that when a stray asteroid body veers within our planet's gravitational influence, Earth captures it into a temporary orbit before the same gravity once again puts the asteroid into an escape orbit. But by all rights, these objects shouldn't be considered as Earth's second moons. 

*Venus’s story is a bit complicated. At first blush, its proximity to the Sun may explain why it has no moons. However, astronomers speculate that Venus, at a tender age, got hit by a planetary body. Venus might have gained a moon in the aftermath of the collision. But due to its proximity to the Sun, that moon eventually must have fallen into the Sun. That’ll be our topic for the future.