Astronomical distances in the Solar System are often measured in specialized units, such as the Astronomical Unit (AU), which represents the average distance between Earth and the Sun. The inner region of the system is home to four dense, rocky worlds whose characteristics are shaped by their proximity to the Sun.
The Factual Answer: Mercury
The planet occupying the orbit closest to the Sun is Mercury. This small terrestrial world travels along an eccentric, or highly elliptical, path that brings it as close as 46 million kilometers (29 million miles) and as far as 70 million kilometers (43 million miles) from the stellar center. Its average distance from the Sun is approximately 58 million kilometers, which is equivalent to 0.39 Astronomical Units.
The short distance from the Sun means Mercury is the fastest planet, zipping around its orbit at an average speed of about 47 kilometers per second. Due to this incredible velocity, Mercury completes a full revolution around the Sun in just 88 Earth days, giving it the shortest year in the entire Solar System. This rapid orbital period is why the ancient Romans named the planet after their swift messenger god.
Life on the Edge: Mercury’s Extreme Environment
Mercury’s close orbit creates the most extreme thermal variations experienced by any planet in our system. During the long Mercurian day, the surface facing the Sun can be scorched by temperatures reaching up to 430 degrees Celsius (800 degrees Fahrenheit). This temperature is hot enough to melt lead and is a direct consequence of the intense solar radiation received at such a short range.
Despite its proximity, Mercury is not the hottest planet because it lacks a substantial atmosphere to trap heat, unlike Venus. Instead of a true atmosphere, Mercury possesses an extremely tenuous exosphere, made up of atoms like oxygen, sodium, and hydrogen that have been blasted off the surface by the solar wind. Without an insulating layer, the side of the planet facing away from the Sun quickly radiates its heat into space.
When the long night falls on Mercury, temperatures plummet dramatically, dropping to frigid lows of approximately -180 degrees Celsius (-290 degrees Fahrenheit). This massive swing of over 600 degrees Celsius between day and night is the largest equatorial temperature fluctuation in the Solar System. The planet’s surface is heavily pocked with craters, a feature it shares with Earth’s Moon, which is a result of the lack of atmospheric protection from impacting space debris.
Scientific observations have also revealed a surprising phenomenon at Mercury’s poles. Due to the planet’s virtually non-existent axial tilt, certain deep craters near the poles are permanently shadowed from solar radiation. Radar data indicates that these shadowed regions are cold enough to harbor deposits of water ice, existing in a frozen state despite the intense heat elsewhere on the planet.
The Inner Solar System Lineup
Mercury is the first of the four terrestrial planets in the inner Solar System. Following Mercury, the planets are arranged sequentially by distance from the Sun as Venus, Earth, and finally Mars. Mercury is the only planet that is always closer to the Sun than Venus.
Venus holds the title of the hottest planet, demonstrating that distance alone does not determine a planet’s environment. Venus’s thick, carbon dioxide-rich atmosphere creates a runaway greenhouse effect, trapping heat far more efficiently than Mercury’s thin exosphere. The four inner planets—Mercury, Venus, Earth, and Mars—are dense, rocky bodies that formed within the warmer region of the protoplanetary disk.