☿️ Mercury: The Solar System's Innermost Planet

Mercury is the smallest planet in the solar system and the closest to the Sun, completing an orbit in just 88 Earth days. Despite its proximity to the Sun, it is not the hottest planet -- that title belongs to Venus -- because Mercury has almost no atmosphere to trap heat.

A World of Extremes

With virtually no atmosphere to insulate it, Mercury's surface temperature swings dramatically: daytime highs reach about 430°C (800°F), while nighttime lows plunge to around -180°C (-290°F) -- the largest temperature range of any planet in the solar system. Its surface is heavily cratered, resembling the Moon, since there's no weather or plate tectonics to erode old impact scars.

Orbit and Rotation

Mercury has the most eccentric (elliptical) orbit of any planet, and a strange rotational relationship with the Sun: it rotates on its axis three times for every two orbits it completes, a 3:2 spin-orbit resonance caused by the Sun's strong tidal forces. A single Mercury solar day -- sunrise to sunrise -- actually lasts about 176 Earth days, longer than its own year.

Structure and Composition

Mercury is a dense, rocky planet with an unusually large iron core that makes up roughly 85% of its radius -- proportionally the largest core of any planet. One leading theory is that a massive ancient impact stripped away much of Mercury's outer rocky layers, leaving the core dominant.

Exploration

Mercury has been visited by only two NASA missions: Mariner 10, which flew by three times in 1974-75 and returned the first close-up images, and MESSENGER, which orbited Mercury from 2011 to 2015 and mapped its surface and composition in detail. The European Space Agency and JAXA's joint BepiColombo mission is currently en route, having launched in 2018.

Frequently Asked Questions

Is Mercury the hottest planet?

No -- despite being closest to the Sun, Mercury is not the hottest planet. Venus is hotter due to its thick, heat-trapping carbon dioxide atmosphere, while Mercury's near-total lack of atmosphere lets heat escape rapidly at night.

Does Mercury have moons?

No. Mercury and Venus are the only two planets in the solar system with no moons at all.

How long is a year on Mercury?

Mercury orbits the Sun in about 88 Earth days, the shortest year of any planet.