The planets in our solar system can be divided into three different, very distinct categories: terrestrial planets, Jovian planets and dwarf planets. Terrestrial planets are those planets in our solar system that are similar to Earth, hence the Latin name ‘terra’, meaning Earth. These planets are made either of rocks or metals and have a hard surface, differentiating them from the other planets which lack a solid surface. Those terrestrial planets are the inner 4 planets of our solar system – Mercury, Venus, Earth and Mars, while Pluto used to be regarded as a terrestrial planet because of its solid and frozen surface.
The Jovian planets are the 4 outer planets – Jupiter, Saturn, Uranus and Neptune. The term ‘Jovian’ denotes that they are similar to Jupiter, which means they are gaseous instead of solid. Jupiter and Saturn are known as gas giants, while Uranus and Neptune are regarded as ice giants. All 4 of these planets are made of mostly hydrogen and helium.
And then there are the dwarf planets. The International Astronomers Union defines planets as bodies that circle the Sun without being another planet’s satellite, and are large enough to be rounded by its gravity and also have cleared their neighborhood of almost all other bodies that could orbit it. Pluto just happened to be the smallest of all the planets and has an offbeat orbit, and in addition to this, it also shares its space with other objects that are part of the Kuiper Belt.
Mercury is the closest planet to the Sun, and it is smaller than Earth's moon. During the ‘day’, Mercury can heat up to 840°Fahrenheit, or 450°Celsius, while night temperatures usually drop to hundreds of degrees below freezing point. Mercury has almost no atmosphere, making it impossible to absorb impacts of meteors and thus its surface is marked by craters, similar to the Moon. The planet is always visible to the naked eye. It was named after the messenger of the Roman gods. Its days lasts the equivalent of 58.6 Earth days, and its year last for 88 Earth days.
Venus is the second-closest planet to the Sun and is even hotter than Mercury. Its atmosphere is toxic and the pressure on the surface will literally crush anything. The size and structure of Venus are similar to that of Earth, but the toxic and thick atmosphere that does not support life as we know it. Because it is brighter than most objects in the night sky, it is visible to the naked eye and has often been mistaken for UFOs. It was named after the Roman goddess Venus and is only a bit smaller than Earth, but its year lasts for 225 Earth days, while its day lasts for 241 Earth days. Unlike most planets in our solar system, it is important to note that Venus orbits in the opposite direction.
Earth is the 3rd planet from the Sun and has ideal conditions to support life on its surface due to 2/3 of it being covered by water. So far it is the only planet in our solar system and the universe known to support living beings. Its atmosphere is full of nitrogen and oxygen, which can sustain life. At the equator it spins with 1,600 km/h, while it revolves around the Sun with 29 km per second. It has a diameter of 12,760 km, its year last for 265.24 days and its day lasts for 23 hours and 56 minutes.
Mars is the 4th planet from the Sun; a cold and dusty planet that has a reddish color due to the iron oxide in the dust. Mars is often called Earth Two because of its many similarities to Earth; it has mountains and valleys, and even storms that can range from tornadoes that whirl dust around the planet all the way to dust storms that engulf the entire planet. Mars is known to have snow and water ice, which led scientists to believe it used to be warm and wet, like the Earth, but then turned into a cold, desert-like planet.
If you want to imagine Mars’ atmosphere, imagine it to be thinner than Earth’s, which makes it impossible for water to exist on the planet’s surface, but scientists are convinced there is water underneath the ice. Likewise, they believe that Mars used to have conditions similar to Earth's and supported life. It is visible with the naked eye and was named after the Roman god of war. Its diameter is almost half of Earth's, with its year lasting 687 Earth days and its day lasting a little more than Earth's days, 24 hours and 37 minutes.
Jupiter is the fifth planet from the Sun and is by far the largest and most massive planet in the entire solar system. It's made almost entirely out of gas, hydrogen and helium. Its atmosphere is full of colorful whirling clouds of trace gases that give it its multicolored look. The planet is known for its Great Red Spot, which consists of giant storms that have been going on for over hundreds of years. Moreover, Jupiter has a very strong magnetic field, and with its numerous moons, it even looks like a small solar systems in itself. It was named after the supreme ruler of the Roman gods. It has a fascinating diameter of 139,822 km, making it almost 11,000 times larger than Earth. Its year lasts for 11.9 Earth years, while its days only last for 9.8 Earth hours.
Saturn is the 6th planet from the Sun and is probably best known for its most distinct feature – its rings. Although Galileo Galilei was the first one to study Saturn, Dutch astronomer Christiaan Huygens was the first to suggest that it was a planet with rings. The rings around Saturn are made of rock and ice, but scientists are uncertain how those were formed. The planet is a gaseous giant consisting of hydrogen and helium, like Jupiter, and has 62 moons. Saturn was named after the Roman god of agriculture. It has a diameter of 120,500 km, and its year lasts for 29.5 Earth years and its day last for more than 10.5 Earth hours.
Uranus is the 7th planet from the Sun and, oddly enough, its equator is at a right angle to its orbit, making the planet look like its orbiting on its side. Scientists believe this is due to the planet colliding with another planet-sized object. This tilt has caused the planet to have extreme seasons, lasting for more than 20 years, with the Sun shining down on one side for 84 years. Uranus is almost the same size as Neptune. The methane in its atmosphere gives it a blueish-greenish color. Similar to Saturn, it has 27 moons and faint rings. Before it was discovered in 1781, it was thought to be a star, especially because it orbits so far from Sun. It has a diameter of 51,120 km, its year lasts for 84 Earth years, while its day lasts for 18 Earth hours.
Neptune is the 8th planet from the Sun and was discovered in 1846. It was named after the Roman god of water. Its diameter is almost 50,000 km, its year lasts 165 Earth years, while its day last for 19 Earth hours. Neptune is known for having strong winds that can blow faster than the speed of sound. Its distance from the Sun measures 30 times the distance from the Earth to the Sun. Neptune was also the first planet predicted to exist using mathematics, long before it was even discovered.
Pluto used to be the 9th planet from the Sun. It is smaller than the Moon, with its orbit carrying it inside the orbit of Neptune and then beyond that orbit. In 1999 it was declared that Pluto would no longer be considered a planet due to its orbit crossing paths with Neptune’s orbit. Pluto is a cold and rocky celestial body with a fleeting atmosphere. It was discovered in 1930 and was named after the Roman god Hades. It has a diameter of 2,301 km, and its year lasts for 248 Earth years, while its day lasts for 6.4 Earth years.
Like we mentioned before, it is believed that Mars used to have an atmosphere similar to that of Earth, but with time it changed to what is now considered to be harsh conditions which do not support human life. As of yet there still is no proof of life on Mars and scientists have still to find evidence that life existed on the red planet.
The similarities between Earth and Mars have led scientists to believe that Mars used to be just like Earth, but that changed long before humanity discovered Mars and found out that the planet is the closest to Earth and supporting living beings. It is strongly believed that Earth could one day look like Mars due to changes in atmosphere and temperature.
According to scientists, there are possibly 33 exoplanets – planets outside of our solar system that orbit a star – which could support some type of life. 11 of these are supposedly situated within their star’s habitable zone. The closest of these planets is Proxima Centauri b and is 4.22 light years away from Earth, while the most distant planet with conditions similar to Earth is almost 50 light years away from Earth, making it impossible to reach it by any spacecraft available to man.
In 2013, it was reported that there could be more than 40 billion planets in the Milky Way the size of Earth that orbit stars similar to our Sun and which could support life. 11 billion of these are supposed to orbit stars like our Sun, but the nearest of these is ‘only’ 12 light years away from Earth.
Proxima Centauri b was discovered in August 2014, and is 1.3 times Earth’s mass and its orbital period around its sun lasts for 11.2 Earth days. Proxima Centauri is believed to be a red dwarf, which ultimately casts doubt on whether any planet in its orbit could support life as we know it.
Alpha Centauri is another Sun-like star system that could support life on the planets orbiting it. It is 4.37 light years away, and the chances of it having habitable planets are 85%. There already have been efforts to determine whether planets in Alpha Centauri support life, Like Mission Centaur and Breakthrough Starshot, missions that are supposed to find exoplanets.