Observers have known about Mercury's existence since antiquity. It's one of the five planets visible to the naked eye, along with Venus, Mars, Jupiter, and Saturn. Named after the swift Roman god of trade, Mercury takes just 88 Earth days to orbit the Sun. However, its day (from one sunrise to the next) is double that — it's 176 Earth days. So Mercury's day is two of its years.
This innermost planet is also the smallest. It's about 38 percent as wide as Earth and contains about 5.5 percent of our planet's mass. Mercury is the second densest — right behind Earth.Learn more about the innermost planet by registering with Astronomy.com and gaining access to the video, "Tour the solar system: Mercury."