Mercury (planet)
Smallest planet and closest to the Sun.
Mercury is the first planet from the Sun and the smallest in the Solar System. It is a rocky planet with a trace atmosphere and a surface gravity slightly lower than that of Mars. As the most inferior orbiting planet, it always appears close to the Sun in Earth's sky, either as a morning star or an evening star.
- type
- Terrestrial planet
- orbital_period
- 88.0 Earth days
- rotation_period
- 58.65 Earth days (sidereal)
- surface_temperature_range
- −170 °C to 420 °C
- known_for
- Smallest planet, highest delta-v requirement for travel from Earth
Lore & Background
Mercury is a classical planet observed throughout history. The ancient Greeks realized the evening and morning stars were the same, calling it Stilbōn and Hermēs. The Romans named it after the messenger god Mercury. The planet has a very tenuous exosphere and a faint magnetic field.
Reader's Guide
Mercury formed approximately 4.5 billion years ago. Its high density suggests a large iron-rich core occupying about 57% of its volume. Competing hypotheses explain its composition: a giant impact stripping the crust, vaporization by the early Sun, or drag from the solar nebula. Data from the MESSENGER orbiter (ended 2015) found higher-than-expected potassium and sulfur, favoring the third hypothesis, but further analysis is needed. Mercury is expected to be destroyed when the Sun becomes a red giant in about seven or eight billion years. Its significance lies in being the innermost planet, with extreme temperature variations and a unique spin–orbit resonance.
Did You Know?
- Despite being closest to the Sun, Mercury is the second hottest planet after Venus due to lacking an atmosphere to trap heat.
- Mercury's core occupies about 57% of its volume, compared to 17% for Earth.
Frequently Asked Questions
What is Mercury?
Mercury is the innermost and smallest planet in our Solar System, orbiting closest to the Sun. It is a rocky terrestrial world with only a trace atmosphere and a surface gravity just slightly lower than that of Mars.
Why is Mercury so hard to spot from Earth?
As the most inferior planet, Mercury never strays far from the Sun in our sky, so it is only visible as a faint morning or evening star low on the horizon. Its proximity to the Sun's glare makes it one of the trickiest planets to catch with the naked eye.
How long is a day and a year on Mercury?
A full orbit around the Sun takes just 88 Earth days, the shortest of any planet. Its sidereal rotation period is about 58.65 Earth days, meaning a single Mercury day stretches longer than half of its year.
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Elsewhere in the Astronomy & Space universe
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