There are 8 official planets in our Solar System. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
According the hierarchy of space, the universe, being massive beyond imagination, is placed at the top. The second place belongs to the galaxy, which is also a massive unit. The observable universe contains about 200 billions galaxies as estimated by space researchers, yet according pictures taken by NASA’s Hubble Space Telescope and other observatories, ideas have been proposed that there are actually at least 2 trillion galaxies scattered throughout the universe, 10 times more than previously thought.
The third place is the solar system. The galaxy that we happen to be in is called the Milky Way, yet, in this enormous galaxy, we are not alone. So far, more than 500 solar systems have been found in the Milky Way and new ones are being discovered by space experts every year.
What is a planet?
In simple terms, a planet is defined as a celestial body moving in an orbit around a star, which is in our case, the Sun. When you were a little kid, you have probably learned in school that there are 9 different planets in our solar system and it was true. But it isn’t anymore.
In 2006, a new definition of planet was established among the astronomy community. The International Astronomical Union decided that, in order for an object to be classified as a planet, it must pass all of the following criterion.
- It must be in orbit around the Sun.
- It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- It must have cleared the neighborhood around its orbit.
8 of the 9 bodies successfully fulfilled all of these criterion and were able to keep their title. The only planet that was unable to do so was Pluto. It does orbit the Sun, it does have enough gravity to be spherical, yet, it has not cleared out its orbit path. In fact, astronomers have discovered that Pluto belongs to the Kuiper Belt, a ring of icy objects locating beyond the 8 official planets, therefore, it shares its orbit with many other objects settling in this region.
Due to this, Pluto was demoted to being a dwarf planet.
8 planets of our Solar System
Mercury is the nearest planet to the Sun and it is also the smallest in our solar system. In fact, its radius is only about 2,440 kilometers long, making it only slightly larger than Earth’s moon. Despite being the tiniest, this planet is the second densest planetary body, with a density of 5.43 grams per cubic centimeter. A day on Mercury is very long whereas a year up there is rather short. Mercury spins on its axis once every 176 Earth days while the time needed for it to fully orbit the Sun is only 88 Earth days, so if you live there, you can celebrate your birthday every three months. How exciting!
Fittingly to its fast orbital speed, the planet was named after the Roman messenger god. Additionally, Mercury spins around the Sun so quickly that people in the ancient world actually believed that it was 2 different stars, one rose up in the morning and the other in the evening. Mercury
One interesting fact about Mercury is that it has the most dramatic day-night temperature difference out of all planets due to its proximity to the Sun and lack of atmosphere. The temperature on Mercury can drop from 427°C during the day to -173°C at night.
Being the only planet named after a goddess, the goddess of love and beauty, Venus is known as Earth’s planetary sister. This is largely due to the fact that they are very much alike in terms of size, mass and volume. Venus is roughly 12071 kilometers across, weighs 4,867×10^24 kilograms and has a volume of approximately 92,843×10^10 cubic kilometers, making it only slightly smaller than Earth. Moreover, they also have similar composition and structure as they are both primarily composed of rocky material, making them terrestrial planets.
Venus is the second closest planet to the Sun, yet, with an average surface temperature of 462°C, it is the hottest one in our solar system. This extreme weather condition is the result of the severe greenhouse effect Venus experiences. The planet’s thick atmosphere consists of up to 96.5% carbon dioxide with clouds of sulphuric acid which traps solar radiation, causing the greenhouse effect.
One interesting fact about Venus is that since it is the brightest planetary body in our solar system, its glow is sometimes mistaken for a UFO.
Our beautiful home is the third planet from the Sun and and the only celestial body which we know so far to be inhabitable. Earth is the only planet which was not named after a mythological god or goddess; in fact, its name comes from the English and German words, “ertha” and “erde”, both of which mean “ground”. Having a mass of 6×10^24 kilograms and a radius of 6,371 kilometers, Earth is the largest terrestrial planet and the fifth largest in our solar system. Additionally, it is the densest out of all planets with a density of 5,513 kilograms per cubic meter.
We are living in a breathable atmosphere of which 78% is nitrogen, 21% is oxygen and 1% argon, carbon dioxide and some other gases. Around 71% of Earth’s surface is covered with water, yet, only about 3% of this amount is fresh. Of that 3% freshwater, over 2% is trapped in glaciers and ice sheets, leaving us with just under 1% of fresh and accessible water. So be mindful of your water usage!
One interesting fact about Earth is that Route 66, also known as America’s Mother Route is longer than the planet’s core, which is roughly 3000 kilometers long.
The forth planet from the Sun, Mars is the last of the four terrestrial planets, named after the Roman god of war. It is often known as the Red Planet because of the brownish-red rusty iron layer blending in its soil and atmosphere. Mars has an orbital radius of approximately 228 million kilometers, thus, it needs 687 Earth days to complete 1 orbit around the Sun, almost twice as long as a year on Earth. Given that Mars’s axial tilt is 25°, just a fraction greater than that of our planet, it also experiences 4 distinct seasons with noticeable temperature variations.
Additionally, Mars takes 24 hours and 40 minutes to finish a single sidereal rotation, which is very close to an Earth’s day. A majority of the astronomy community has a strong belief in life on Mars, hence, a number of missions have been launched to visit this planet, making it the most explored celestial body.
One interesting fact about Mars is that it is where our solar system’s tallest known mountain stands, the Olympus Mons. This giant, being 21 kilometers tall, is a volcano formed billions of years ago.
Jupiter, one of the four gas giants of our solar system, is the fifth planet from the Sun and the first of the outer planet group. It is the most gigantic planetary bodies in our solar system with a diameter of approximately 140,000 kilometers, 11 times larger than that of Earth. So imagine if Earth is shrunken to the size of a grape, Jupiter would still be as big as a basketball. When the Romans first observed the planet, they named it after the supreme god in Roman mythology, for it appeared larger than any other planets.
Jupiter has very short days whereas a year up there goes by very slowly. The planet rotates on its axis once every 10 hours, making it the planet that has the shortest day in our Solar System. Its fast spinning speed however, results in a flattening at its polar regions and a bulging at its equatorial region which causes the planet to be oblate. Despite having a very fast axial rotation, Jupiter needs 11.86 years to fully orbit the Sun. Jupiter, being a gas giant, does not have a solid surface. Instead, it is mainly composed of hydrogen, which accounts for 90% of its atmosphere.
On interesting fact about Jupiter is that the planet’s iconic red spot, namely The Great Red Spot, is in fact, an enormous storm which has been spinning for more than 300 years.
Being known as The Ringed Planet, Saturn is the sixth planet from the Sun, of which name belonged to the Roman god of agriculture. It is the second largest planet in our solar system, with a radius of 58.232 kilometers, about nine times bigger than that of Earth and a volume of 8.2713 x 10^14 cubic kilometers, heavier than 760 Earths. Just like Jupiter, Saturn has short days but very long years. It takes this planet only 10.6 hours to complete a rotation around its own axis, yet, 10,756 Earth days to fully orbit the Sun.
Saturn is best known for its unique ring system, which consist of billions of particles made out of icy, dusty and rocky remnants left over by passing comets, asteroids and nearby moons. You can read more about the planet’s rings here: https://susitv.net/saturn-ring-system-how-many-rings-does-saturn-have/. In addition to the ring system, Saturn is also very famous for its moons. Having a total of 150 natural satellites, it holds the record for the number of moons orbiting a planet in our Solar System.
One interesting fact about Saturn is that despite its massive size, it is the least dense planet in our solar system. It has a density of only 0.687 grams per cubic centimeter, meaning that when being placed in water, it would actually float!
The first planet discovered by astronomers with the aid of a telescope, Uranus is the seventh planet from the Sun. It is also the only planet, of which name was directly derived from a Greek mythological figure, the Greek God of the Sky. Although this planet belongs to the gas giant group, astronomers often refer to it as “the ice giant”. This is due to the fact that it has a thick and strong icy layer surrounding its rocky core. But other than that Uranus has an atmosphere made up of hydrogen and helium just like its fellow gas giants.
Although, Uranus is not the most distant planet from the Sun, it has the coldest atmosphere out of all planets. It can reach a minimum temperature of -224°C, which is about 135°C colder than the lowest temperature ever recorded on Earth. An Uranian day is only 17 hours long while the average length of an Uranian year is 84 Earth days.
One interesting fact about Uranus is that it orbits the Sun in a rather unusual way. With a tilt of approximately 98°, the planet’s equator is nearly perpendicular to its orbit path, making it the only planet that rotates on the side. This unique feature also causes extreme seasonal conditions on this planet. For 21 years, Uranus’s north pole is directly pointed at the Sun, meaning that it experiences 21 years of daytime. That is summer. And when winter comes, it will experience another 21 years of nighttime while the south pole is directed at the Sun.
Neptune, the other ice giant, is the eighth planet in line from the Sun, the most distant world. The average distance from it to the Sun is about 4.5 billion kilometers, more than 30 times longer than that from Earth. As Neptune is so far away, it is not visible to the naked eyes and thus, becomes the first planet to be discovered using mathematics. When being observed through a telescope, the planet appears blue; therefore, it was named after the Roman God of the Sea.
The atmosphere of Neptune is composed of mostly hydrogen and helium with a trace of methane, yet this tiny amount is what causes the planet’s blue appearance. When light reaches Neptune, the methane gas particles will absorb light of a wavelength corresponding to the color red, thus will turn out blue.
One interesting fact about Neptune is that it is the windiest world in our solar system. Up there, winds strike at a speed of more than 2,000 kilometers per hour. If you are unable to imagine that, Earth’s most powerful wind in the history blows at 400 kilometers per hour.
More about the planets in our Solar System
Besides the 8 main planets introduced above, there are 5 more dwarf planets in our Solar System. They are Ceres, Pluto, Haumea, Makemake and Eris. Similar to Pluto, Ceres, Haumea, Makemake and Eris all failed to satisfy the IAU criterion for a planet and as a result were classified as dwarf planets. So Pluto, don’t be so sad, you’re not the only one!