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Would happen if two gas planets collided? © Getty Images

Here’s what would happen if two gas planets like Jupiter and Saturn collided

Published: 03rd October, 2021 at 11:00
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Astrophysicist Dr Alastair Gunn explains the outcome of celestial collisions – and what this tells us about our own Solar System.

Although orbital dynamics make it highly unlikely that two gas giants would collide, there’s a small chance that such an impact could happen during the formation of a planetary system. The result of a collision mainly depends on the speed and angle of impact.


Head-on collisions would generally lead to a complete merger of the gas giants without any loss of material, either in their solid cores or in their gaseous envelopes. However, a higher speed head-on collision would likely lead to the loss of most of the envelope gas as the two cores merge. Very high speeds would completely fragment and destroy both planets.

Collisions that are not head-on (known as oblique collisions) can give different results. For instance, if the two cores avoid each other completely, both gas giants will lose a substantial fraction of their gaseous envelopes to space, but will not merge. If the cores collide at an angle then the planets may or may not merge, but in all cases a large amount of the gaseous envelope will be lost.

Very oblique collisions do not disrupt the planets at all and both would continue on almost the same orbits without losing any mass. Oblique impacts between gas giants can lead to changes in their shape, or ‘oblateness’. This has been proposed as a means of studying the early impact history of gas giants in other star systems.

Although there is no concrete evidence that Jupiter or Saturn formed from the merger of smaller gas giants, it is a possibility. Jupiter and Saturn have a higher fraction of heavy elements than the Sun, suggesting that one or both of them may have been formed by such a collision.

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Asked by: Mark Buckmaster


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