To kick off our post of planet facts, we first take a look at what a planet is exactly. Then the different types of planets followed by all the planets in order from the sun outward.
Planet Facts – What Is a Planet?
The answer to this question is a highly controversial one. This has not always been the case, though. In fact, before 1978 the definition of a “planet” was not really necessary. Until that time a planet simply meant a body in orbit around the Sun, that reflected sunlight, and was not a planetary moon, asteroid, or comet.
When Pluto’s moon Charon was discovered in 1978, scientists were able to calculate Pluto’s mass much more accurately than ever before. They soon realized that it was much smaller than they had previously believed.
At a tiny fraction of the mass of Mercury, Pluto was clearly a body much smaller than any other planet. This discovery led some to question whether Pluto was actually a planet or some other type of object.
In the 1990s and early 2000s, some objects similar in size to Pluto were discovered in the outer solar system. Because of that, scientists gave a more detailed definition of a planet.
Such definition was needed to separate those types of objects like Pluto into a distinct class. Otherwise, all of the newly found objects would have to be called planets as well.
In response to this uncertainty, the International Astronomical Union (IAU), the official governing body for matters concerning naming astronomical objects, came to a definition of the term “planet.” According to the IAU, a planet is a celestial body that meets the following criteria:
- The object is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
Planet vs Dwarf Planet
Planets are dwarf planets are two different classifications of astronomical bodies. With the word “dwarf” we are already given the idea that dwarf planets are smaller objects. The other difference lies in their orbits.
We used to have nine planets but Pluto was reclassified as a dwarf planet in August 2006. It did not meet all the criteria because it has not “cleared the neighborhood” of its orbit over time. That means there are similar-sized objects as Pluto in its path around the Sun.
According to the IAU, a dwarf planet meets the following criteria:
- It is in orbit around the Sun.
- It has sufficient mass to assume a hydrostatic equilibrium (nearly round) shape.
- The object has not cleared the neighborhood around its orbit.
- It is not a satellite.
Together with Pluto in the dwarf planet category are Ceres, Haumea, Makemake, and Eris. Ceres lies in the inner solar system in the asteroid belt. The other dwarf planets are located beyond the orbit of Neptune, in the Kuiper belt.
Types Of Planets
Terrestrial Planets (Inner planets)
There are four terrestrial planets: Mercury, Venus, Earth, and Mars. These planets are those closest to the Sun. They are characterized by their dense, rocky composition with solid surfaces.
Gas Giants (Outer planets)
There are also four gas giant planets: Jupiter, Saturn, Uranus, Neptune. These planets are found in the outer solar system. They are characterized by their massiveness and gaseous composition. Specifically, Uranus and Neptune are most appropriately called “ice giants.”
Planets In Order From The Sun
The order of planets from closest to farthest from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. We could use mnemonics to easily remember the planets’ order such as: “My very eager mother just served us nine pizzas.” We can also make our own mnemonics and be creative about it.
The largest planet is Jupiter, followed by Saturn, Uranus, Neptune, Earth, Venus, Mars and, the smallest planet, Mercury.
If you include dwarf planets as well, the planets in order become Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris being the furthest from the Sun.
To compare distances, instead of kilometers or miles, we also use astronomical units (AU). This is the distance from the Sun to Earth. One AU equals 149,600,000 km.
Now, let’s take a look at the planets in order individually.
Mercury has been known for a long time because it is visible to the naked eye. Galileo Galilei made the first telescopic observation of this small planet in the 17th century. It was named after the swift Roman god of commerce.
Mercury is the closest planet to the Sun. It lies at an average distance of about 58 million kilometers (36 million miles). In terms of the Earth-Sun distance, it is 0.4 AU.
Measuring around 4,880 km (3,032 mi) across, Mercury is the smallest of all the eight planets. It is 2.6x smaller than our planet and is just slightly bigger than the Earth’s Moon.
Mercury Fast Facts
- Distance: 57,910,000 km (35,983,605 mi)
- Radius: 2,440 km (1,516 mi)
- Temperature: -180 to 430 °C (-290 to 800 °F)
- Day length: 59 Earth days
- Year length: 87.97 Earth days
- Number of moons: 0
Since it is close to the Sun, the temperature on Mercury is extreme. At night, it can be as cold as -180°C (-290°F) but during the daytime, the temperature can soar up to 800°F (430°C). It does not have an atmosphere, but only a thin exosphere.
Mercury is tidally-locked to the Sun. It follows an egg-shaped orbit, the most eccentric of all planets. However, its axis has the least tilt. This small planet rotates slowly on its axis but orbits quickly around the Sun. A day there takes 59 days and a year is only 88 Earth days. It is in a 3:2 spin-orbit resonance. That means it makes three rotations for every two revolutions it completes around the Sun.
Mercury is a dense planet with a large metallic core. It has a lot of impact craters, some of them with distinct rays. It does not have any moons or rings.
Fun Facts About Mercury
- If we were to stand on Mercury, the Sun would look three times as large as we see it on Earth. It is seven times brighter too!
- While it takes about eight minutes for sunlight to travel to Earth, it reaches Mercury in about three minutes.
- Man-made spacecraft have visited Mercury. The first was NASA’s Mariner 10 which performed three flybys on the planet. MESSENGER orbited Mercury for about four years between 2011 and 2015. Joining the two is BepiColombo which launched in October 2018.
Venus is only the second-closest planet to the Sun but it is the hottest. It has a very thick atmosphere which traps the heat from the Sun, thus causing its temperature to rise. It got its name from the Roman goddess of love and beauty, whose Greek counterpart is Aphrodite.
This planet lies around 108 million km (67 million mi) from the Sun. It is 12,104 km (7,521 mi) across, about the same size as Earth. Because of that, it is often called “Earth’s twin.” However, other than size, the two are very different. Venus is scorching hot, with temperatures around 475 °C (900 °F).
Venus is one of the brightest objects in our night sky, second only to the Moon. It has been a common sky fixture ever since antiquity, so one is credited with its discovery. With an apparent magnitude of -4.14, it is much brighter than the -1.46 magnitude Sirius. Because it is close to the Earth, Venus appears exceptionally bright to us. Even during the day, it is bright enough to be seen.
Venus Fast Facts
- Distance: 108,200,000 km (67,232,363 mi)
- Radius: 6,052 km (3,761 mi)
- Temperature: 438 to 482°C (820 to 900 °F)
- Day length: 243 Earth days
- Year length: 225 Earth days
- Number of moons: 0
While Earth rotates in an anticlockwise direction, Venus spins backward. Because of that, the Sun rises in the west in Venus and sets in the east. Another thing that makes it different is that it spins on its axis at a very slow pace. It orbits the Sun in 225 Earth days but it takes 243 days for it to rotate on its axis. That means a day on Venus is 18 days longer than a year!
Venus is a volcanic planet with lots of craters on the surface. Its thick atmosphere is dominated by carbon dioxide and the yellow clouds are made up of sulfuric acid. Just like Mercury, it has no rings or moons.
Venus was first the first planet to be visited by a spacecraft through Mariner 2 in 1962. Many landers made their way to the planet’s surface. However, they were short-lived because of the blazing heat.
Fun Facts About Venus
- Among all planets, Venus is the only one named after a female god. Its surface features are also named after women, both real and mythical.
- Venus rotates on its axis very slowly. It is so slow that a day that there is longer than a year!
- Ancient people thought that Venus was two different objects. Because of that, it has two nicknames: “the evening star” and “the morning star.” It rises just before sunrise and fades away when the much brighter Sun dominates the daytime sky.
The Earth is our home planet and the only one known where life exists. It has many names such as Gaia, Tellus, and even “the world.” Whatever we call it, it sure is an interesting one.
The Earth is the largest terrestrial planet and the fifth-largest overall. It is the third planet from the Sun, with an average distance of 150 million km (93 million mi). This is just the perfect distance, not too close but also not too far from the Sun. It lies in the habitable zone, the area where the temperature is just enough to allow liquid water on its surface.
The Earth has four layers: the inner core, outer core, mantle, and crust. We live on the crust, the thinnest layer of the planet. The surface is 71% water and land only occupies only 29% of it.
Earth Fast Facts
- Distance: 149,600,000 km (92,957,130 mi)
- Radius: 6,371 km (3,959 mi)
- Average Temperature: 14°C ( 57.2 °F)
- Day length: 23.9 hours
- Year length: 365.25 days
- Number of moons: 1
Our atmosphere has the perfect ingredient for us to breathe. It is 78% nitrogen and 21 oxygen, and the remaining 1% is other gases. Aside from making life possible, this atmosphere also acts as a shield from incoming bodies such as meteoroids. These objects burn up once they enter our atmosphere, which results in meteor showers or “falling stars.”
The Earth follows a slightly elliptical orbit around the Sun. This journey around the star takes 365.25 days. Additionally, it takes 24 hours to spin on its axis. The tilt of this rotational axis (23°) is the reason why we experience different seasons in a year. In comparison, Venus is only tilted by around 3° so it has a uniform season throughout. We have one Moon, which causes the tides in our oceans.
Fun Facts About Earth
- The name “Earth” has a Germanic origin which means “the ground.” Of all the eight planets, it is the only one that was not named after any gods or goddesses.
- The exact length of a year on Earth is 365.25 days. To make things easier for our calendar system, we usually have 365 days in a year. The leap year (366 days) comes every four years. During that time, an extra day is added to the month of February.
- The Earth’s crust is divided into different tectonic plates. These plates cause earthquakes, they also give rise to volcanoes and rift valleys. Most of all, the tectonic plates are important to the carbon cycle of our planet.
Mars is often dubbed the “Red Planet” because of the iron on its surface. It is the fourth planet from the Sun, located around 228 million km (142 million mi) away. This is a dry desert world that is also very cold.
Mars has been known by the ancients, looking like a reddish point in the night sky. With an apparent magnitude of around 1.50, we can readily spot it under a dark sky.
The red planet got its name from the Roman god of war whose Greek equivalent is Ares. It has two satellites—the most moons of any terrestrial planet. Phobos and Deimos have irregular shapes. They are named after the sons of Ares (Mars) in Greek mythology.
Mars Fast Facts
- Distance: 227,940,000 km (141,635,349 mi)
- Radius: 3,400 km (2,113 mi)
- Temperature: -153 to 20 °C ( -225 to 70 °F)
- Day length: 24.6 Earth days
- Year length: 687 Earth days
- Number of moons: 2
Mars is about 1.9 times smaller than Earth. Its diameter is around 6,805 km (4,228 mi). Though it is smaller, it shares some similarities with our planet. For one, a day there is roughly the same length as a day here on Earth. Martian days are called sols and one sol takes 24.6 Earth hours. Mars is tilted by 25° so it experiences different seasons as well.
Mars also has volcanoes, canyons, and similar surface structures. In fact, the Olympus Mons is the largest volcano in our solar system. Since Mars is farther from the Sun than us, it takes longer to complete one revolution. It orbits the Sun in 687 Earth days.
Mars has an atmosphere but it is very thin. Inhaling it would be toxic to us since it is largely composed of carbon dioxide, argon, and nitrogen. This thin atmosphere is also the reason for the planet’s extreme temperature.
Fun Facts About Mars
- Phobos is predicted to crash into Mars around 50 million years from now. The potato-shaped moon will shatter, which will likely create a ring around the planet.
- As of 2021, three rovers are exploring its surface: Curiosity, Perseverance, and Zhurong. There are also eight orbiters studying it from above.
- The sky on Mars looks reddish during the day and turns blue during sunsets. The total opposite of what our sky looks like here on Earth!
Jupiter is the fifth planet from the Sun and the largest of all the solar system planets. It was named after the king of the gods in Roman mythology. With an apparent magnitude of about -2, it is easily visible to the naked eye. In fact, it is the third brightest object in our night sky.
Jupiter is roughly 142,984 km (88,846 mi) across. With that size, we can put 11 Earths side by side along its equator. It is also the most massive planet. If we combine all the seven other planets, Jupiter would still be twice as massive. A day on Jupiter is only 10 hours long—the shortest in the solar system. A year on this giant planet is much longer, taking about 12 Earth years.
Jupiter Fast Facts
- Distance: 778,330,000 km (483,631,840 mi)
- Radius: 71,492 km ( 44,423 mi)
- Temperature: -108 °C ( -162 °F)
- Day length: 9.93 Earth hours
- Year length: 11.86 Earth years
- Number of moons: 79
This enormous planet is a gas giant made up of hydrogen and helium. It has extreme temperatures and pressures. The swirling bands and stripes that we often see on this planet are clouds of water and ammonia. Giant storms on Jupiter are common and they often last for a very long time. The famous Great Red Spot, for example, has raged for 300 years or more.
People have observed Jupiter for a long time. Astronomers became even more excited about it in 1610 when Galileo Galilei discovered its moons. It was the first time that satellites from another planet were discovered. The Galilean moons are Ganymede, Io, Europa, and Callisto. From then on, the number of Jovian moons has grown to 79. It has a ring system too!
Fun Facts About Jupiter
- The Great Red Spot is a very large storm that could fit two to three Earths. It was first observed in 1665 by astronomer Giovanni Cassini. Though it might disappear in the next hundred years, a new one will likely replace it.
- Auroras on Jupiter are the brightest in the solar system. They are much more intense than the light shows we have on Earth. Other than solar storms, auroras on Jupiter are also caused by its moon Io.
- Since it has the same composition as the Sun, Jupiter is sometimes considered a failed star. It was not massive enough to fuse hydrogen and helium to produce energy. But if it were 75 times more massive, it would have become another star!
We know Saturn as the “ringed planet.” It is the sixth planet from the Sun, located about 1.5 billion kilometers away. It was named after the Roman god of agriculture, whose Greek counterpart is Cronus.
Saturn is the second-largest planet, and like Jupiter, it is a gas giant with no solid surface. It measures roughly 120,536 km (74,897 mi) across, more than 9 times larger than Earth. The temperature on this planet is very cold. It also has extreme pressure and storms like those in Jupiter.
Saturn Fast Facts
- Distance: 1,424,600,000 km (885,205,400 mi)
- Radius: 60,268 km (37,448 mi)
- Temperature: -138 °C (-218 °F)
- Day length: 10.7 Earth hours
- Year length: 29 Earth years
- Number of moons: 82
A day on Saturn is only 10.7 hours long but a year there takes about 29 Earth years. Its fast rotation causes it to be oblate in shape. It is flat at the poles while its equator has an unmistakable bulge. Its low density also contributes to this.
The other outer planets also have rings but none as breathtaking as Saturn. This complex system, made up of ice and rock, has seven main rings. It extends more than 280,000 km (173, 983 mi) from the giant planet.
What makes Saturn more interesting is that it has the most moons in the solar system. The 82 known moons range in different sizes and compositions. Some of them have effects on the rings too. Life on this planet is impossible. However, some of its moons have the potential for life, especially the ocean worlds of Enceladus and Titan.
Fun Facts About Saturn
- Saturn is the least dense planet in the entire solar system. With a density of 0.687g/cm3, it is even less dense than water (1 g/cm3). As a result, it would float if put in a very large bathtub!
- Much of what we know now about the Saturni was because of the Cassini mission. In its grand finale in 2017, the spacecraft deliberately crashed into Saturn. It was done to make sure that the potentially habitable moons are unharmed.
- Aside from the main rings around the planet, the Saturnian system has another set of rings. In 2009, a sparse but very large ring was discovered around the irregular moon, Phoebe. It is said to be much larger than Saturn and its main rings!
Uranus is the second-farthest planet from the Sun. As the seventh planet, it lies roughly 2.9 billion km (1.7 mi) away from the star. It is famously known as the “sideways planet” because it is tilted on its axis by about 98°. As a result, it looks like it is rotating on its side!
The astronomer William Herschel discovered it in 1781 through a telescope. It had been observed many times before discovery but it was thought to be a star, just like Herschel initially did. It got its name from Greek mythology’s primal god of the sky.
Uranus Fast Facts
- Distance: 2,873,550,000 km (1,785,541,189 mi)
- Radius: 25,559 km (15,881 mi)
- Temperature: -195 °C (-320 °F)
- Day length: 17 Earth hours
- Year length: 84 Earth years
- Number of moons: 27
Uranus is a big ball of gas just like Jupiter and Saturn. However, aside from hydrogen and helium, it largely contains water, ammonia, and methane. These materials are called “ices” so we call Uranus an “ice giant.” It has 27 moons and a faint ring system.
Uranus has a diameter of about 51,118 km (31,763 km). At that size, we can fit four Earths along its equator. Also, it is the third-largest planet in the solar system. It is a cold world with temperatures around -195 °C (-320 °F). With its distance, sunlight travels nearly 3 hours to reach it.
A day on Uranus is shorter than a day here on Earth. A year there takes 84 years—even longer than a human lifetime. Aside from spinning on its side, Uranus rotates from east to west (retrograde) just like Venus. This tilt causes extreme seasons on this planet. The methane gas in its atmosphere gives Uranus its blue-green appearance.
Fun Facts About Uranus
- The moons of Uranus are named after literary characters. Specifically, they are inspired by the works of William Shakespeare and Alexander Pope. Its largest moon, Titania, got its name from Shakespeare’s A Midsummer Night’s Dream.
- NASA’s Voyager 2 was the only spacecraft that visited Uranus. Its closest approach was around 81,500 km (50,600 mi) from the planet. This visit took place a long time ago in January 1986.
- Uranus is colder than Neptune in some areas, particularly in the atmosphere. With a minimum temperature of -224°C (-321°F), it has the coldest planetary atmosphere in the solar system.
Neptune is the eighth planet and the farthest from the Sun. At 4.5 billion km (2.8 billion miles), it is 30 times as distant as the Earth is from the Sun. It was named after the god of the sea in Roman mythology.
The astronomer Urbain Le Verrier made mathematical predictions about Neptune’s existence. Using his calculations, Johann Galle discovered the planet through a telescope in September 1846. Just days later, its largest moon, Triton, was also discovered. So far, 14 moons are known to orbit this planet. It also has a ring system.
Neptune Fast Facts
- Distance: 4,501,000,000 km (2,796,791,736 mi)
- Radius: 24,764 km (15,387 mi)
- Temperature: -201 °C (-331 °F)
- Day length: 16 Earth hours
- Year length: 165 Earth years
- Number of moons: 14
Neptune is about four times as large as Earth. It is an ice giant and is mostly composed of water, methane, and ammonia. Though it is a bit smaller than Uranus, it is much denser. At 1.638 g/cm3, it is the densest giant planet. It also has the strongest winds in the entire solar system.
Sunlight reaches Neptune’s dark and cold world in four hours. Also, the light that our planet receives is nearly 1,000 times brighter than that on Neptune. A day there is only 16 hours long but a year takes 165 Earth years. It is in a 3:2 orbital resonance with Pluto. That means every time Neptune makes three orbits around the Sun, Pluto makes two. This relationship makes sure they never collide.
Scientists are still figuring out why Neptune has a darker blue color than Uranus. It was visited only once by a spacecraft. Voyager 2 made a flyby on this dark world in 1989.
Fun Facts About Neptune
- Neptune follows a very elongated orbit. It is so elongated that sometimes it becomes farther than Pluto.
- Neptune’s tilt on its axis of rotation is 28 degrees, which is closely similar to the Earth’s. This will give the planet a change in seasons like us. But since a year on Neptune is 165 Earth years long, it will take about 40 years for the seasons to change!
- Neptune’s Triton is the only large natural satellite that moves in the opposite direction as the planet. It most likely formed somewhere else and was just captured later by Neptune.
Order Of The Planets In The Solar System: By the Numbers
Distance Of The Planets From The Sun:
|Planet||Distance from the Sun||Diameter||Mass||Important Notes|
|Mercury||57,910,000 km (0.387 AU)||4,879 km||3.3022 x 1023 kg||The closest planet to the Sun
|Venus||108,200,000 km (0.723 AU)||12,104 km||4.8685 x 1024 kg||The hottest
The first planet visited by a spacecraft
Has the longest rotation period (243 days)
|Earth||149,600,000 km (1.000 AU)||12,756 km||5.9736 x 1024 kg||The only planet known to harbor life|
|Mars||227,940,000 km (1.524 AU)||6,805 km||6.4185 x 1023 kg||The “Red Planet”
The terrestrial planet with the most moons
Has the largest volcano (Olympus Mons)
|Jupiter||778,330,000 km (5.203 AU)||142,984 km||1.8986 x 1027 kg||The grandest planet
The third-brightest object seen at night
Has the shortest rotation period (10 hours)
|Saturn||1,424,600,000 km (9.523 AU)||120,536 km||5.6846 x 1026 kg||The "ringed planet"
Has the most moons
The least dense
|Uranus||2,873,550,000 km (19.208 AU)||51,118 km||8.6810 x 1025 kg||The "sideways planet"
The second-least dense
The third-largest planet
|Neptune||4,501,000,000 km (30.087 AU)||49,528 km||10.243 x 1025 kg||Discovered through mathematical calculations
The densest giant planet
The windiest world
The Hypothetical Planet X
There have been several additional planets hypothesized throughout history. However, none of these planets has ever been found.
The most recent of these theorized planets was Planet X, also nicknamed Planet Nine. This is supposedly a giant planet with a very elongated orbit. And because of that, it takes thousands of years to orbit the Sun.
Scientists have not made any direct observation of Planet X. The existence of an undiscovered planet was just hypothesized to explain the unusual orbits of Kuiper belt objects (KBOs). However, scientists are also open to other theories that may explain these strange orbits.
How many planets are in the Solar System?
According to the IAU’s definition of planets, there are 8 known planets in the Solar System. These are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is no longer considered a planet under the IAU definition.
Does Mars have oxygen?
Yes, but only a very small amount. While the Earth’s atmosphere is 21% oxygen, it is only 0.13% on Mars. Carbon dioxide makes up 96% of the atmosphere on the red planet. With that said, we will need spacesuits on Mars because its air is toxic to us.
Can you walk on Saturn’s rings?
Unfortunately, no. They look may look like solid structures but Saturn’s rings are actually made up of millions of debris floating in space. Some of these materials are as small as dust while some are as large as houses. Saturn’s gravity holds them together. But not only that, the planet has “shepherd moons” that keep these icy particles in place.
What body is most like Earth?
Saturn’s biggest moon, Titan, is comparable to Earth. It has an atmosphere that protects it from radiation. This icy world possibly has a water ocean below the surface. It also has the elements needed for life. Though it is a unique world of its own, it is one of the most possible places for human colonization.
Why is Mars red?
Mars looks red because of iron on its surface. Its rocks and dust are rich in iron. Since they are just exposed in the open, the iron reacts with oxygen which results in a red rust color. These swirls of rusty dust are swept up by large storms into the atmosphere making its sky reddish. Read our post here if you want to learn more about why Mars is red.
Planet Facts – Sources:
Planet Facts – Image Sources:
The eight planets: https://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Size_planets_comparison.jpg/1920px-Size_planets_comparison.jpg
Planet vs Dwarf Planet: https://www.jpl.nasa.gov/edu/images/videos/spaceshorts_dwarfplanet.jpg
Planet X: https://solarsystem.nasa.gov/system/content_pages/main_images/383_planet_9_art_1_1400.jpg