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Mercury is the smallest planet in the Solar System ever since Pluto lost its status and was reclassified as a dwarf planet. Mercury is not that much larger than the Moon and its volume is only about 5.4% of Earth’s volume. It is hard to believe that Pluto was even smaller than Mercury.
Although Mars is often thought of a twin of Earth because it has so often been the subject of searches for life, it is actually much smaller than our own planet. Mars is the second smallest planet in our Solar System with a volume only 15% of Earth’s volume.
Venus is so bright that it can cast shadows on a dark night, but it is actually a little smaller than Earth. Venus is roughly 90% the size of the Earth. Although the planet is the closest one in size to Earth, the similarities seem to end there. Venus is a hostile place with burning temperatures and a toxic atmosphere.
Earth is the fourth smallest planet in our Solar System and the biggest of the four inner planets. Our planet is also the densest planet. While the Earth is large, its size is a fraction of the four outer planets, which are all gas giants.
The planets mentioned above are the 4 inner planets.
Neptune is the fifth smallest planet and the smallest gas giant in our Solar System, but it is much larger than any of the inner planets. It is also almost sixty times larger than the Earth is.
Uranus is only slightly bigger than Neptune is, but the planet is more than sixty times the size of the Earth. Although it is the seventh planet from the Sun, you can actually see the planet without the help of a telescope, but it will take some searching.
Saturn is the 2nd largest planet in our Solar System and is famous for its incredible planetary ring system. Saturn’s volume is about 764 times Earth’s volume and about 12 times larger than Uranus’ volume.
Jupiter dwarfs all the other planets by comparison. By far the largest planet in our Solar System, you could fit 1321 Earths inside the planet. Jupiter actually cannot get any larger because it would begin to fold in on itself. While the planets are not exactly lined up in increasing size, the larger planets do tend to be farther toward the edge of the Solar System, especially since Pluto was demoted to a dwarf planet.
Astronomy Cast has episodes on all of the planets including Mars.