- A planet's Hill sphere — where its gravity overpowers the Sun's — determines its moon-holding capacity.
- Saturn leads the solar system with 146 moons; Mercury's tiny Hill sphere can retain none.
- Earth's Moon formed from debris after a Mars-sized body struck the young Earth 4.5 billion years ago.
Saturn holds 146 moons in its gravitational embrace. Mercury, the smallest planet, keeps not a single one.
The difference has nothing to do with luck, and everything to do with physics that astronomers have understood since 1878.
Nicole Granucci, a physics instructor at Quinnipiac University, wrote about it for The Conversation to answer a six-year-old's question. We think it is equally interesting for adults.
The key concept is something called the Hill sphere, named for American astronomer George Hill. It marks the zone where a planet's gravitational pull overpowers the Sun's.
What is the Hill sphere?
The Hill sphere is the region around a planet where its own gravity is stronger than the Sun's pull. Any moon orbiting within this zone stays bound to the planet; anything beyond it gets pulled away by the Sun. The larger and more distant from the Sun a planet is, the bigger its Hill sphere – and the more moons it can collect and keep.
Every planet has one. The larger the planet and the farther from the Sun, the bigger its Hill sphere becomes. And the bigger the Hill sphere, the more moons a planet can hold.
Comparison of the Hill spheres and Roche limits of the Sun-Earth-Moon system (not to scale) with shaded regions denoting stable orbits of satellites of each body Credits: CMG Lee/WikiMedia. CC BY-SA 4.0.
The Hill sphere is the outer boundary - how far a planet's gravity can hold onto a moon before the Sun steals it. The Roche limit is the inner boundary - how close a moon can orbit before the planet's tidal forces rip it apart.
So moons must orbit in the zone between the two: far enough out to survive, close enough to stay.
Key figure
146
Moons orbiting Saturn – the most of any planet in the solar system
Mercury's Gravitational Handicap
Mercury faces a double disadvantage. It is small, and it orbits closer to the Sun than any other planet. Its Hill sphere extends only about 175,000 kilometers. That is roughly the distance from Earth to the Moon. But here, the Sun's gravity constantly threatens to steal anything that wanders close.
Any moon Mercury might capture would likely get yanked away before completing a single orbit.
Venus presents the strangest case of all. A 2006 computational study by Caltech researchers modeled a scenario where Venus had a moon billions of years ago. It formed by a massive impact, much like Earth's Moon.
Then, in the researchers' simulation, fate intervened. A second impact reversed Venus's spin direction entirely. The resulting tidal forces pulled the Venusian moon inward. It crashed back into the very planet that once held it.
The Outer Planets Collect Moons Like Gravity Wells
Jupiter and Saturn play by different rules entirely.
Their enormous masses create Hill spheres spanning tens of millions of kilometers. Earth's Hill sphere extends about 1.5 million kilometers. Jupiter's stretches more than 50 million kilometers. At such distances from the Sun, solar gravity weakens considerably. Passing asteroids that drift too close become permanently trapped.
Most of Jupiter's and Saturn's outer moons arrived this way. They are cosmic hitchhikers that wandered into gravitational territory they could never escape.
Their inner moons likely formed differently. When the solar system was young, gas and dust swirled around these giant planets. These disks were miniature versions of the one that surrounded the Sun. Some of that material condensed into moons that have orbited ever since.
Earth's Moon Tells a Stranger Story
Our Moon fits neither pattern.
Around 4.5 billion years ago, a Mars-sized body called Theia slammed into the young Earth. The collision sprayed material into orbit. That debris eventually coalesced into the Moon we see today.
The evidence came from rock chemistry. Lunar basalt carries an isotopic signature nearly identical to rocks from Earth's interior. If Theia had contributed most of the Moon's material, the chemistry would tell a different story. It would look alien. Instead, it looks like home.
Recent research has refined this picture. A 2024 study in Science traced iron isotopes in lunar samples. The analysis concluded that Theia probably formed in the inner solar system, close to where Earth was taking shape.
The giant impact hypothesis remains the leading explanation, though scientists continue debating the precise mechanics.
What the Cosmic Lottery Really Measures
As Granucci notes in The Conversation, the question of planetary moons "is still widely debated." Each moon in our solar system carries its own origin story.
Mars's two tiny moons, Phobos and Deimos, might be captured asteroids - Mars sits conveniently close to the asteroid belt. Or they might have formed alongside the planet. The evidence supports capture, but certainty remains elusive.
What we do know: a planet's moon count measures its gravitational reach, its distance from the Sun, and billions of years of cosmic collisions.
Saturn wins the competition with 146. Mercury never had a fair chance to compete.
(Story updated on January 2, 2026.)
Fact Check: Claim-by-Claim Verification Verified
The recap accurately summarizes the source article by Nicole Granucci, correctly explaining Hill sphere concepts, moon counts, and planetary examples with appropriate hedging on debated origins.
Commentary
- Source notes scientists "still don't fully understand," and recap appropriately hedges on moon origins as "widely debated."
- Hill sphere size for Mercury (~175,000 km) and Earth (~1.5 million km) aligns with physics; Jupiter's is indeed much larger (~50 million km).
- 2024 Science study on iron isotopes refining Theia from inner solar system is cited accurately, though full paper unavailable.
Sources used for verification
Academic/Peer-reviewed:
- Moon Formation - nasa.gov
Other reliable sources:
- Why do some planets have moons? A physics expert explains - theconversation.com
- Scientists still don't fully understand why some planets have hundreds of moons - livescience.com
Fact-checked by Perplexity Sonar Pro on 2026-01-02
