When worlds collide

13 December, 2009

Massive Smash-up at Vega.
Massive Smash-up at Vega.
Source: NASA/JPL-Caltech

Question: What would happen if two planets collided and could this ever happen to Earth?

Answer: The 1932 novel When Worlds Collide, by Edwin Balmer and Philip Wylie, tells the story of Earth’s destruction when it collides with another planet. While this sounds like the stuff of science fiction, collisions have already happened to several planets in our own solar system, including Earth.

About 4.5 billion years ago, just after the formation of the Solar System, the Earth was believed to have collided with another large planet-like body, probably about the same size as Mars. This collision would have had a catastrophic effect on both bodies, with the molten debris from their rocky mantles ejected into space.

Today, this is the most commonly accepted theory of how the Moon was formed. The debris that resulted from the collision was captured by Earth’s gravity and coalesced to form the Moon.

When it was proposed in the 1970s, the idea of this giant impact was not readily accepted, but today there is a great deal of evidence that such impacts are common during the formation of solar systems. Not only Earth, but other planets like Uranus and Venus, as well as smaller bodies like Pluto, appear to have had major impacts when the Solar System was forming.

Evidence has also been found in other solar systems that suggest the catastrophic collision of planets. Recently NASA reported that it had located debris from two rocky planets, about the size of the Moon and Mars, that collided at the speed of about 36,000 km per hour in a solar system about 100 light years from Earth. It gives us a some idea of what happened to Earth 4.5 billion years ago: ‘The impact … destroyed the smaller planet, causing vast amounts of rock to vaporise and flinging huge plumes of hot molten lava into space’ (Scientific American, 11 August, 2009).

The path of the planets in our Solar System has been mapped for about the next billion years and it seems unlikely that Earth will ever collide with any of its planetary neighbours. But there is a remote chance. Astronomers in France have recently conducted computer simulations suggesting that there is a 1-in-2500 chance of Earth colliding with Mars or Venus in 3.5 to 5 billion years if the orbits of the planets were to destabilise.

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