The temperatures across the eight planets in our Solar System range profoundly, moving from scorching inner worlds to frigid, distant giants. This thermal distribution is largely governed by the Sun’s diminishing energy output as distance increases, though individual planetary characteristics introduce complex variables. While the four inner, rocky planets experience intense solar heating, the four outer, gaseous planets exist in a realm of extreme cold. This discussion identifies the third coldest official planet and explores the physical principles defining its deep chill.
The Ranking of Coldest Planets
The coldest planet in the Solar System is Neptune, the most distant of the eight official planets. Its average effective temperature is \(-201^\circ\text{C}\) (\(-331^\circ\text{F}\)). Uranus, the seventh planet from the Sun, claims the second-coldest spot, maintaining an average temperature of approximately \(-195^\circ\text{C}\) (\(-320^\circ\text{F}\)).
The third coldest planet is Saturn, the ringed gas giant. It registers a mean temperature of about \(-140^\circ\text{C}\) (\(-220^\circ\text{F}\)) at the 1 bar pressure level in its atmosphere. This places Saturn behind the two ice giants, Neptune and Uranus, but significantly colder than the fourth coldest planet, Jupiter, which has a mean atmospheric temperature of about \(-110^\circ\text{C}\) (\(-166^\circ\text{F}\)).
Factors Determining Planetary Temperature
The main determinant of a planet’s temperature is its average distance from the Sun, as solar radiation intensity drops exponentially with distance. Planets farther out receive less solar energy to heat their atmospheres and surfaces, explaining the trend of colder temperatures beyond Mars. For the outer planets, this low level of incoming sunlight means their thermal profiles are influenced by other factors.
A second variable is a planet’s albedo, which measures how reflective its surface or atmosphere is. A higher albedo means more incoming sunlight is scattered back into space, preventing heat absorption. The bright, reflective cloud tops of the gas and ice giants contribute to their chilliness by minimizing solar energy absorption.
A third factor influencing the temperatures of the gas giants is internal heat generation. Both Jupiter and Saturn radiate more heat than they absorb from the Sun, indicating an internal heat source, likely from gravitational contraction. Uranus, in contrast, appears to have lost much of its primordial heat, which is why it is colder than its more distant neighbor, Neptune, and why its temperature is similar to Saturn’s despite its greater distance.
Profile of the Third Coldest Planet
Saturn orbits the Sun at an average distance of approximately 1.4 billion kilometers. Its atmosphere is a deep, turbulent layer composed predominantly of hydrogen (about 96% by volume), with helium constituting most of the remainder. Since this vast gaseous envelope lacks a solid surface, temperatures are measured at a standardized pressure level within the atmosphere.
The frigid average temperature results from the extreme distance from the Sun combined with the composition of its cloud layers. Saturn’s upper atmosphere is structured into distinct cloud decks made of various frozen compounds that condense at different temperatures and pressures. The highest visible clouds are composed of ammonia ice crystals, which give the planet its pale yellow hue.
Deeper within the atmosphere, layers of ammonium hydrosulfide and water ice clouds exist where temperatures are warmer. The planet’s internal heat source causes temperature and pressure to increase toward the core. However, the upper atmospheric layers that define the planet’s average temperature remain intensely cold due to the lack of solar heating and the reflective nature of the high-altitude ice crystals.