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Space
Space
Science
Paul Sutter

Why is Mercury so weird? Blame the giant outer planets.

Mercury Earth Solar System NASA
The planet Mercury seen from NASA's Messenger spacecraft in December 2009.

Mercury is a planet that just doesn't make sense. It's incredibly small yet hosts a relatively massive core. Mercury is so strange that astronomers have not been able to explain its properties with simulations of the solar system's formation. But now, researchers have found an important clue, and Mercury's weirdness appears to be the fault of the giant planets.

Mercury is by far the smallest planet in the solar system, with only around 5.5% of both the mass and volume of Earth. Despite that diminutiveness, Mercury manages to be the second-densest planet in the solar system, with a whopping 98% the density of Earth.

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