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When looking up into the night sky, we can only imagine the vast planets above us, but exactly how large are the planets? The smallest planet in our Solar System, Mercury only has a diameter of 4,878 km. As a comparison, Mercury is not much larger than our Moon, which has a diameter of 3,473 km.
One reason why Venus is nicknamed Earth’s twin is because it is almost the same size as our own planet. Venus has a diameter of 12,100 km, which is only 5 percent smaller than Earth’s diameter. The similarities do not extend much farther than that though. Venus is a toxic, barren planet almost the complete opposite of our own.
The Earth’s diameter is 12,742 km. You may have heard a different number listed though. This is because 12,742 km is actually an average of the distance between the north and south poles and the diameter at the equator. The equatorial diameter is about 41 km larger than the diameter of the poles because the Earth bulges outward slightly from spinning so quickly.
The diameter of Jupiter is 142,981 km at its equator. The planet’s diameter is actually different than the length of the planet from pole to pole because the planet rotates so quickly that it bulges out at the center, the same way Earth does. The diameter of the poles is only 133,708 km, so the planet is much more compressed than Earth is. How big is the largest planet in our Solar System? It would take 11.2 Earths to match Jupiter’s diameter.
Saturn has a diameter of 120,536 km, making it the second largest planet in the Solar System. Like Jupiter, Saturn flattens out because it rotates quickly. It is only 108,728 km between the poles. This is a difference of more than 10,000 km between the two diameters. To put it into focus, the difference between its two diameters is more than twice the diameter of the Moon.
Uranus’ diameter is 51,118 km. Like most of the other planets, Uranus’ diameter is different between the equator and the north and south poles. Although Uranus is the larger of the two planets, it is actually lighter than Neptune is because it has a very low density.
The diameters of Neptune’s poles and equator average out to around 49,500 km. The actual equatorial diameter of the planet is 49,528 km, while the diameter of its poles is 48,682 km. The fourth largest planet in the Solar System, Neptune could fit 3.9 Earths side by side.
Astronomy Cast has episodes on all the planets. Here is Mercury to start out with.