Mercury | The Closest Planet to the Sun
Of the eight planets in the solar system, Mercury is the closest planet to the sun. It is also the smallest planet only slightly larger than our moon. Despite being the smallest planet with a mean diameter of 3,032 miles (4,879 km), Mercury is the second densest planet.
With the shortest orbit, thanks to its proximity to the sun, a year goes by fast on Mercury orbiting the sun in only 88 Earth days. However, the planet itself spins slowly, taking 59 Earth days to make a full rotation. This means that one day on Mercury lasts a long time.
Mercury is part of the terrestrial planets, along with Venus and Earth, and is visible to the unaided eye from our planet. It orbits the Sun within Earth’s orbit (like Venus), making it appear in Earth’s sky only as a ‘morning star’ or ‘evening star’ that’s fairly close to the Sun.
With an atmosphere so thin there is nothing to stop impacts, the planet is marked with craters similar to the surface of the moon.
Captured with MESSENGER’s Wide Angle Camera as the spacecraft receded after making its closest approach January 2008 | NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie
Mercury Facts:
- Distance from the sun: 36 million miles (58 million km)
- Length of 1 Day: 59 Earth days
- Length of 1 Year: 88 Earth days
- Diameter: 3,032 miles (4,879 km)
- Radius: 1,516 miles (2,439.7 km)
- Temperature: High of 800°F (430°C), low of -290°F (-180°C)
What do we know about Mercury?
Studying the planet Mercury is a challenge. With Mercury being closest to the sun, observations from Earth are only possible during dawn or twilight hours, when the Sun’s brightness is dim.
Spacecraft exploration has also been a challenge due to Mercury’s fast orbiting speeds. The first spacecraft to visit the planet was NASA’s Mariner 10, in the 1970s, which photographed nearly half of the surface of Mercury, providing evidence of a crater surface.
Eminescu crater on the planet Mercury from the MESSENGER mission | NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
NASA’s MESSENGER spacecraft, the first to orbit the planet, revealed more information than ever before, including discoveries of an unusually large core, magnetic field, and evidence of water ice on Mercury’s poles. It orbited the planet for four years before ending its mission.
Most recently, BepiColombo is a joint mission between the European Space Agency and JAXA that was launched in 2018. A total of nine flybys are planned to steer the spacecraft into orbit in late 2025 and begin its science mission early 2026.
One mystery that still remains about the planet is how and why it has such a large iron core. There are a few ideas but more data is needed to explain it and come to a conclusion.
Why is Mercury NOT the hottest planet?
Despite being the closest planet to the sun, Mercury is not the hottest planet in the solar system. Though it does receive the most intense solar radiation being the closest to the Sun, Venus is actually the hottest planet.
This is because Venus, the second planet from the sun, has a very thick atmosphere full of greenhouse gas carbon dioxide. This atmosphere traps heat via the greenhouse effect, creating the hottest surface despite being almost twice as far from the sun as Mercury.
The surface temperatures on Venus are approximately 900°F (475°C), which is hot enough to melt lead. Compare this to the temperature on Mercury, which gets as hot as roughly 800°F (430°C).
The greenhouse effect makes Venus roughly 700°F (390°C) hotter than it would be without a greenhouse effect.
Interesting Facts about Mercury
- It is the closest planet to the Sun
- It is the smallest planet in the solar system
- Though it is the closest planet to the sun, it is not the hottest planet in the solar system
- It orbits the fastest of any other planet in the solar system
- It has the shortest year of all the planets at 88 days
- It is unlikely that life could survive on Mercury due to solar radiation and extreme temperatures
- It is named after the Roman god Mercurius, the god of communication and the messenger of gods
- The planet is shrinking thanks to a single continental plate over a cooling iron core that solidifies, reducing the planet’s volume
