Why doesn’t Earth have rings? © Science Photo Library

Why doesn’t Earth have rings?

Although Earth doesn't have a ring system today, it may have done in the past.

All gas giant planets (Jupiter, Saturn, Uranus and Neptune) in the Solar System have rings, while the terrestrial ones (Mercury, Venus, Earth and Mars) do not. There are two theories about how ring systems develop. They may have formed from leftover material from the formation of the planet, or be the remains of a moon that was destroyed by an impact, or was simply broken apart by the gravitational tugging of its parent.

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It is unclear why only the gas giants have rings. However, the fact that gas giants formed in the outer Solar System, while rocky planets formed only in the inner Solar System may be a clue.

Perhaps the inner planets were shielded from the collisions that could have formed rings, or perhaps the higher incidence of moons in the outer Solar System means rings also became more common there. Another factor may be that larger planets simply have a larger volume in which a ring system can form and remain stable.

Scientists believe the Earth did have a ring system in the past. Very early in its history a Mars-sized object collided with the Earth, probably resulting in a dense ring of debris. However, unlike the outer planets, Earth’s ring system soon coalesced to form the Moon.

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Asked by: Moe Mbago, London

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