Here's an interesting comparison of the terrestrial planets in our Solar System—left to right Mercury, Venus, Earth and Mars to scale. Looking at the numbers, their radii are: Mercury 2439 km, Venus 6051 km, Earth 6371 km, Mars 3389 km.
I grabbed this from the first video lecture of Dr Bruce Betts' Introduction to Astronomy course. Bruce is from California State University and he's also the guy who does the "What's Up?" sections of the Planetary Society's Planetary Radio podcasts (also available via iTunes).
You can watch all his lectures as they become available each week, or if you can't wait, there's a link on that page and you can dive right into last year's lectures...
Back to the planets with a few more numbers: check out the overall density of each planet.
- Mercury mass ....3.3 × 10^23 kg
- Venus mass .......4.8 × 10^24 kg
- Earth mass .........6.0 × 10^24 kg
- Mars mass .........6.4 × 10^23 kg
And why is Mercury so small? Well it seems something huge hit Mercury, knocking most of it into space and leaving just the dense core. No big deal—that happened to most of the planets at some stage. Except the difference was that Mercury orbits so close to the Sun that the solar winds blasted most of this lighter material away before the gravity of the core could draw it back down to reform the planet. Very small, but very dense, but still no atmosphere, thanks again to those intense solar winds. As another comparison, Mercury isn't a lot bigger than our Moon, which has a radius of 1737 km, but it is way denser than the Moon, which has a mass of 7.3 × 10^22 kg.
Of course they all pale into insignificance when compared to our Sun, with a radius of 695,508 km and a mass of around 2.0 × 10^30 kg.
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