While there are many planetary systems and even innumerable galaxies in the universe, there is only one which we call home and it is located on an outer spiral arm of the Milky Way Galaxy. Our solar system consists of all the planets, moons, asteroids, comets and any other space debris which orbit the sun. Our system, known as the Solar System, is named after our sun, or star, Sol. Everything else, including our planet, are bound to the sun in a gravitational pull called an orbit. Our solar system consists of eight planets, subdivided into the inner and outer planets, and out lying dwarf planets. Temperatures on the planets depend on the sun and other factors. Those closer to the sun are generally hotter.
The eight planets in our solar system are, in order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four, known as the inner planets, are terrestrial bodies. This means they are rock and metal based planets. When the universe was young, these would have been the only planetary bodies that would have been able to withstand the heat of the young star. Because of this, the inner planets are solid, either rock based, metal or a combination of both, and all have higher average temperatures than the outer planets.
The outer planets, Jupiter, Saturn, Uranus and Neptune, are separated from the inner by a large asteroid belt. These planets are significantly cooler in temperature than their inner counterparts, and instead of being rock based, they are actually composed primarily of hydrogen, helium; dense liquids, other gases and ice. These plants are also much larger than the inner planets, and have faster orbits and spins. Their gaseous composition is what causes the outer planets to often be referred to as the gas giants.
Is Pluto A Planet?
Pluto, at one point considered the smallest and farthest planet in our solar system, was officially demoted from planetary status in 2006. This was because it did not, in fact, meet the three criteria necessary for distinguishing a planet. These criteria are: the planet must be in orbit around the sun; it must have sufficient mass for its own gravity to overcome rigid body forces so that is assumes a nearly round shape, and it must have cleared the neighbourhood around its orbit of other objects. As Pluto did not meet the third rule, it was deemed a ‘dwarf planet’. There are five dwarf planets in our solar system, including Pluto, and they are Ceres, Makemake, Haumea and Eris. The outer reaches of the solar system also include the Kuiper Belt, home to four out of five of these dwarves, and the Oort Cloud, thought to surround the entire solar system in a cloud-like shell.
Planetary temperatures are effected by the sun, and the biggest contributing factor to a planet's average temperature is its proximity to the sun. However, there are other important factors such as the planet’s atmosphere, rotation speed, and density of the planet itself. Generally, the temperature decreases for planets the further away they are from the sun, meaning closer inner planets such as Mercury, Venus and Earth, have warmer temperatures than those in the outer reaches of the solar system such as Neptune or Uranus.
Just as we have seasons and temperature fluctuations on Earth, other planets experience large changes in temperature between their day side and night side. For Earth, the rotation period is one day, or 24 hours, meaning the amount of time in ‘night’ or with the planet facing away from the sun, isn’t overly long. On a planet such as Mercury, however, which rotates more slowly (58.6 days per rotation), the night side faces away from the sun for much longer periods. Because of this, temperatures can be more than 1,000 degrees Fahrenheit lower than those on the day side. This is also why both temperatures are listed in the diagram, as the difference between day and night is so great. Temperatures for the gas planets don’t tend to fluctuate as much, as the sun does not warm and cool the surface to the same extent that it would with rock and metal. Additionally, because there is no fixed surface from which to take the temperature readings, average temperatures are taken from an atmosphere level that is equal in pressure to that of sea level on Earth.
The Hottest Planet
Venus is the hottest planet in our solar system with an average temperature of 880 degrees Fahrenheit. Though planetary temperatures tend to rise with the proximity to the sun, Venus is actually warmer than its neighbour Mercury, for a few reasons. As mentioned, Mercury has a slow rotation period, meaning it differs drastically in temperature between its daylight and nighttime sides. Additionally, it has a thin atmosphere which allows heat to escape. In contrast, Venus has a very dense atmosphere, which means it traps heat much like a greenhouse. This means heat is contained within the planet’s atmosphere, causing a distinctly higher temperature than on any other planet. The average temperature of 880 degrees Fahrenheit is so hot it can melt lead. At these temperatures, it is hypothesized that no life forms could exist.
The Coldest Planet
The coldest planet in our solar system is Neptune. As the farthest planet from the sun, Neptune receives little to no warmth from solar rays. As such, it has an extremely low average temperature, resting at negative 331 degrees Fahrenheit, and is composed largely of ice and frozen matter. Neptune is so far from the sun, at a distance of roughly 2.8 billion miles, that it is the only planet which cannot be seen with the human eye alone. It is a volatile planet, with a rapid rotation of 16 hrs to one Earth day, and has intense gusting winds which whip through its gaseous atmosphere. Because of this, and its frigid temperatures, Neptune cannot sustain life as we know it.
Life Beyond Earth
As the graphic shows, the range of temperatures within our own solar system is very large. When compared to the average temperature on Earth, the other planets can clearly be seen to be far outside our understanding of life supporting temperature ranges. However, exploration continues to determine whether or not life exists, could exist, or even once existed on these planets. Mars has long been thought to be the most viable option for sustained lifeforms. At an average temperature of minus 20 degrees Fahrenheit, it is the closest to the Earth’s temperature. Indeed, certain parts of the Earth routinely reach these temperatures either in winter months, or year round in polar areas. For this reason, Mars remains the most viable option for future human life, or for discovery of existing out-planetary life forms. At the present, however, definitive proof of life has yet to be discovered, and the current knowledge indicates that most planets could not sustain life as we know it due first and foremost to hostile temperatures and atmospheres.