Mercury is the closest planet to the sun. But although it boasts the most widely varying temperatures in the solar system, it is not the hottest planet. How is this possible?
Highs and lows
Orbiting between 28 and 43 million miles (46 and 70 million kilometers) from the sun, Mercury, also the smallest planet, feels the brunt of the solar rays. The planet has an average temperature of 332 degrees Fahrenheit (167 degrees Celsius) The day side of the planet reaches temperatures of up to 801 F (427 degrees C). In contrast, the chilly night side can get as cold as minus 279 F (minus 173 C).
These variations are relatively long-lived. Scientists once thought that Mercury kept a single side perpetually facing the sun, in a condition known as tidal locking. Because the planet lies so close to the sun, it could only be studied when it showed the same rocky, cratered face toward Earth, though at different points in its orbit. However, further studies revealed that the planet spun very slowly — only three times every two Mercury years, or once every 60 Earth days.
Mercury's low mass and close proximity to the sun keep it from having anything but the thinnest of atmospheres, and this is the reason it must pass on being the hottest planet. An atmosphere helps to cloak a planet, keeping heat from leaking into space and balancing it, to some degree. Without an atmosphere, Mercury loses a great deal of heat into space, rather than sharing with its night side.
Location, location, location
On Earth, seasonal temperature shifts are caused by the tilt of the planet's axis. If the Southern Hemisphere is closer to the sun than its northern counterpart, it experiences spring and summer instead of autumn and winter. But on Mercury, the planet has essentially no tilt, which means that the hemispheres experience no significant difference in temperature from one another.
However, Mercury has the least circular, most eccentric orbit of all the planets (Pluto's is more eccentric but the tiny rock is only considered a dwarf planet). The huge range in its distance from the sun means that the planet does feel some variation in temperature based on where it travels over the course of its 88 Earth-day year.
— Nola Taylor Redd, SPACE.com Contributor