As always, where we’re figuring out the size of things, we’re going to compare it to the Earth. There are also many ways to deal with the question “big”. So let’s tackle it every way.
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First, let’s start with mass. This is a measure of the amount of stuff in Pluto. Thanks to the fact that Pluto its moon Charon, astronomers were able to calculate its mass very accurately. They came up with a number of 1.31 x 1022 kg. For comparison, that’s 0.24% the mass of Earth. Pluto isn’t very massive.
Next, let’s look at Pluto’s size. Once again, astronomers have Pluto’s moon Charon to thank. Because the moon passes in front of Pluto, astronomers were able to calculate its size very accurately. The current estimate puts Pluto’s size at 2,390 km.
Obviously, this makes Pluto smaller than every other planet in the Solar System, but it also makes it even smaller than seven of the Moons in the Solar System: Ganymede, Titan, Callisto, Io, Earth’s Moon, Europa, and Triton. Pluto is much larger than the asteroid Ceres, but smaller and less massive than Eris.
Because of its small size and mass, it failed to clear out its orbit, and so Pluto was demoted from planet to dwarf planet status by the International Astronomical Union.