How Big Is Pluto?
QUICK ANSWER
Pluto has a diameter of about 1,477 miles (2,377 km), smaller than Earth's Moon (2,160 miles) and roughly two-thirds the size of Mercury. Pluto's small size was part of why astronomers were uncomfortable classifying it as a full planet, leading to its 2006 reclassification as a dwarf planet.
Pluto is small. Not just smaller than the giant planets, but smaller than Earth's Moon, Mercury, and several other moons in the solar system. The small size was always one of the awkward facts about Pluto's planet status, since it was dramatically smaller than even the smallest accepted planets. Eventually that awkwardness became too much to ignore.
How big is Pluto exactly?
Diameter of about 1,477 miles (2,377 km). According to NASA, Pluto is the largest dwarf planet in our solar system, but it's still smaller than Earth's Moon (2,160 miles) and significantly smaller than Mercury (3,030 miles). Pluto's mass is just 0.2 percent of Earth's and 17 percent of Earth's Moon's mass. Pluto is also smaller than several major moons in our solar system, including Jupiter's Ganymede, Saturn's Titan, and Neptune's Triton.
How does Pluto compare to Earth?
Tiny. Pluto's diameter is about 18 percent of Earth's. By volume, you could fit about 200 Plutos inside Earth. The mass ratio is even more extreme: Pluto's mass is about 0.002 of Earth's, or 1/500th. If Earth were the size of a basketball, Pluto would be about the size of a small kiwi fruit. The size comparison is part of why the IAU's planet definition specifically excluded objects in the Pluto range; the bottom end of accepted planets (Mercury) is dramatically larger than Pluto.
Why does Pluto's size matter for classification?
Because it's not just small, it's small relative to its neighborhood. Pluto sits in the Kuiper Belt, surrounded by many similar icy objects. Eris, discovered in 2005, is roughly the same size as Pluto, and there may be dozens of other similar-sized objects waiting to be discovered. If Pluto counted as a planet, all the similar-sized Kuiper Belt objects would have to count too, potentially adding dozens of planets to the solar system. The IAU's solution was to tighten the planet definition to exclude them.
How does Pluto compare to other dwarf planets?
Pluto is the largest of the recognized dwarf planets, though just barely. Pluto and Eris are similar in size, with recent measurements suggesting Pluto is slightly larger but Eris is slightly more massive. Haumea (about 1,000 miles wide) and Makemake (about 890 miles) are noticeably smaller. Ceres (in the asteroid belt) is about 590 miles, the smallest. Pluto is also probably bigger than several other candidate dwarf planets that haven't yet been formally classified, like Sedna and Quaoar.
Pluto has a diameter of about 1,477 miles, smaller than Earth's Moon and significantly smaller than Mercury. The small size was one of the main reasons Pluto's planet status was always somewhat awkward, and it's part of why the IAU created the dwarf planet category in 2006. Pluto is still the largest dwarf planet we know of, but only by a thin margin over Eris.
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