Our Solar System is such an incredible place. Although many objects are still waiting to be found, planets have been known to humans for thousands of years. We’ve discovered a few more since then and even sent spacecrafts to view them up close. What is the order of planets from the Sun, and what do we know about them?
So What is a Planet?
Before we take a look at the planets that make up our Solar System, we first need to know what a planet is. This seemingly simple question does not have a simple answer. The definition has already changed a few times in history, and chances are it will change again.
The word “planet” is derived from the Greek word “planetes”, which means wanderer. In ancient times, planets were thought to be deities. A lot has changed since then, but our planets are still wanderers of the night sky.
In 2006, the International Astronomical Union (IAU) adopted the latest definition of a planet. In order to be classified as a planet, a celestial object needs to meet three different criteria:
- It must orbit a star.
- It must be big enough to have enough gravity to force it into a spherical shape.
- It must be big enough that its gravity cleared away any objects of a similar size near its orbit.
While the first two criteria are simple to understand, the third has left a few people wondering. Most astronomers interpret the phrase to mean that a planet has way more mass than anything else around it.
Planets of Our Solar System
There are currently eight objects in our Solar System that meet the criteria listed above. Let’s take a brief look at each one in their order from the Sun.
Mercury
Mercury is the closest planet to our star, the Sun, and moves incredibly quickly around it. The planet flies around the Sun in only 88 days, which is why it was named Mercury after the swift-footed messenger of the gods.
Its close proximity to the Sun has slowed Mercury’s rotation over time, and it now takes 58.65 Earth days to rotate a single time – that’s close to two-thirds of a Mercurian year!
Because of these two factors, if you were standing on one spot on Mercury at sunrise, it would take 176 days (two Mercurian years) to see another one. One year is entirely light, and the next is completely dark. During daylight, the Sun would appear to move backward at times in the sky and even perform a loop.
Mercury isn’t a whole lot bigger than Earth’s Moon but is the second densest planet in the Solar System because of its composition of mainly heavy metals and rock. Its small size and proximity to the Sun means it has very little atmosphere. The air it does have is quickly taken and then replenished by solar winds.
Due largely to its size and proximity to the Sun, Mercury has no moons or rings to speak of. It sits an average of 36 million miles (58 million km) from the Sun, and can see temperatures of 800 °F (430 °C) during the day.
Without much of an atmosphere to retain heat, those temperatures can drop to -290 °F (-180 °C) at night! Even with such highs, Mercury still has some ice on its surface in permanently shadowed craters.
Even with such high temperatures, Mercury is not the hottest planet in our Solar System. That title goes to the next planet on our list.
Venus
Venus, the second planet from the Sun, is our closest neighbor in many ways. Besides our Moon, it is physically the nearest object to us in the sky. After the Moon, it shines brightest in our nighttime sky. This brightness may be why Venus was named after the Roman goddess of beauty.
Venus is also the closest planet to Earth in terms of size, just 373 miles (600 km) smaller. Like Earth, it is a rocky planet and is even sometimes referred to as our twin. That being said, there are some radical differences between Venus and Earth.
We have our bad weather days here on Earth, but Venus is perpetually shrouded in a thick layer of sulfuric acid clouds. Those clouds, mixed with a toxic atmosphere of carbon dioxide, create an out-of-control greenhouse effect. As a result, temperatures can exceed 850 °F (454 °C), making it the hottest planet in the Solar System.
Venus didn’t get the memo about which way to rotate and rotates opposite all the other planets except one. The Sun rises in the west and sets in the east! Some astronomers believe this happened due to a massive impact, but there’s no evidence of such an event.
Whatever the reason, Venus rotates so slowly that it takes 243 Earth days to rotate once. This is by far the slowest rotation of any planet in our Solar System. Considering the planet only takes 225 days to revolve around the Sun, a day on Venus is actually longer than a year!
Besides Mercury, Venus is the only other planet in our Solar System without a moon. Given its size, no one is quite sure why it couldn’t grab one at some point.
Earth
As the planet we’re very likely all from, the Earth is the third rock from the Sun and the only planet in the Solar System with known life. We sit among over 1.5 million other species that all flourish around us.
When it comes to naming conventions, the Earth is the only planet not named after a Roman god or goddess. The moniker comes from old English and German words like ‘ertha’ that simply mean ‘ground’. The word Earth is believed to be over 1000 years old, but no one knows precisely why it was used.
Our Earth is the largest of the rocky planets, but that rocky surface is two-thirds of the way covered in water. That’s far more water than any other planet in our Solar System. Our metallic core also makes Earth the most dense. You’re probably aware that Earth’s atmosphere has the perfect balance of nitrogen and oxygen for us to breathe.
Speaking of our planet being livable, the Earth has an extremely powerful magnetic field caused by a combination of its nickel-iron core and fast rotation. This magnetic field blocks the harmful effects of solar wind.
Our planet revolves around the Sun once every 365.24 days taking us through four seasons that can vary greatly depending where on the planet you are. The extra 0.24 days that it takes to revolve around the Sun is made up for once every four years by adding a day to the month of February.
Seasons are actually a result of the fact that the Earth is tilted at a 23.45° angle instead of being perfectly straight up and down. This also attributes to longer daylight hours during certain times of the year and longer nighttime hours during others – unless you live on the equator.
The Earth is not without extreme temperatures, but nothing like our neighbors closer to the Sun. The highest recorded temperature was 56.7 °C (134.1 °F) in Death Valley, California. The coldest temperature was recorded at Vostok Station in Antarctica at −89.2 °C (−128.6 °F).
The Earth currently rotates once every 23 hours, 56 minutes, and 4 seconds and this rotation is slowing by 17 milliseconds every hundred years. No worries though, it will take 140 million years before a day on Earth is 25 hours long.
Our one Moon plays with our tides and casts reflected light down from the Sun to illuminate us at night.
Mars
Mars is the fourth planet away from the Sun. It is the last of the planets in our Solar System made of rock. It’s known as the Red Planet because of countless iron minerals in the surface that have rusted, leaving the soil and atmosphere looking red. Mars is named after the Roman god of war, likely due to this red color.
While scientists have dreamed about living on Mars someday, the planet can not support life in its current state. The atmosphere is quite thin for its size, and the air is mostly carbon dioxide. Theories abound that Mars once had a much thicker atmosphere, as evidenced by gullies that were probably once filled with water.
Mars has ice, but scientists are still looking to see if there is liquid water somewhere on the planet. This water would have to be salty to prevent it from freezing.
If that wasn’t bad enough for the concept of life, Mars is a cold, dusty, desert world. The planet has the largest dust storms in the Solar System, often lasting for months on end and at times covering the entire planet.
It takes Mars just over 24 hours to rotate on its axis, and due to its distance from the Sun, the length of a Martian year is 687 Earth days. It sees similar cold temperatures during its winter, but even the hottest day on Mars never quite gets above the freezing point of water.
When Mars does have a clear day, the rust in the atmosphere leaves the sky looking pinkish-red during the day. At sunset, this changes to a sharp blue color, which is the exact opposite of Earth.
Mars is host to the highest mountain in the entire Solar System, Olympus Mons. At 13 miles high (21 km), it stands over twice as tall as Mount Everest, which sits as a measly 5.5 miles (8.9 km).
Our red neighbor has two moons, likely permanently borrowed from the nearby asteroid belt. They are some of the smallest moons in the Solar System.
Jupiter
Jumping over the asteroid belt, we reach Jupiter, the fifth planet away from the Sun. Named after the Roman king of the gods, Jupiter is by far the biggest planet in our Solar System – it’s more than twice the mass of all the other planets combined. It has a diameter roughly 11 times the size of our planet.
With a massive atmosphere of mainly hydrogen and helium, Jupiter is a gas giant. The swirls and stripes we see on the surface are frigid clouds of water and ammonia that float above those other elements. It’s clear that Jupiter’s atmosphere runs deep, but no one knows precisely how far. If Jupiter has a solid or liquid core, it’s likely only the size of Earth.
Jupiter’s Great Red Spot is one big storm that has been raging across the planet for more than three centuries. It’s so large that you could fit three Earths inside of it.
The intense gravity given off by this gas giant has attracted over 75 moons to date, and Jupiter has been known to grab objects like comets. Some of these moons are quite large, with Ganymede being the largest in the Solar System.
The four small rings that circle the planet are made of dust resulting from meteor collisions on the planet’s moons. The particles are just small enough to stay suspended in orbit.
Jupiter has the fastest rotation of any planet in the Solar System, and a day on the surface only takes 9 hours and 55 minutes. Being three times further away from the Sun than Mars is, this gas giant takes nearly 12 years to orbit the Sun.
Saturn
It’s hard to think about Saturn, the sixth planet from the Sun, without thinking about the magnificent rings that adorn it. The seen distinct rings are made up of rock and ice particles usually no more than 10 feet (3 meters) wide. No one knows for sure, but some speculate these rings were once a moon ripped to pieces by the planet’s gravity.
Saturn is another gas giant like its neighbor Jupiter and also has an atmosphere consisting of mostly hydrogen and helium. Different bands of clouds make up layers of the atmosphere, and it’s uncertain whether Saturn has a solid core somewhere below. The pale-yellow color comes from ammonia crystals in the upper atmosphere.
Even the ancients likened this planet to Jupiter, as Saturn is the father of Jupiter in Roman mythology. It is the furthest planet that one can see with the naked eye.
Saturn spins nearly as fast as Jupiter, and one day on the world is 10.5 Earth hours. Sitting twice as far from the Sun as Jupiter, it takes Saturn 29.5 years to make one complete orbit.
Scientists have lost count of just how many moons Saturn has. The average number is somewhere in the ’80s, but many speculate that the planet has upwards of 150 or more.
While the surface temperature is a freezing -220 °F (-140 °C), chemical reactions in the interior of the gas giant cause temperatures there to reach over 11,700°C (21,000 °F).
Uranus
Uranus is located beyond the realm of visibility with the naked eye and is the seventh planet from the Sun. The planet was only discovered in 1781, and astronomers weren’t even sure what they had located until they observed the object had a circular orbit around the Sun.
Although a different name was proposed at the time of discovery, astronomers chose to name the planet Uranus, after the Greek god of the sky.
Another gas giant consisting primarily of hydrogen and helium, Uranus is sometimes called the ice giant because of the methane ice crystals that give the planet its pale blue color. It is clearly a giant, four times wider than our Earth.
It’s almost unfathomable that Uranus sits 1.8 billion miles (2.9 billion km) away from the Sun. This distance means it takes the planet 84 Earth years to travel in a full circle around it.
What’s most unusual about this world is that its axis is nearly horizontal. Theories suggest that Uranus was once upon a time hit by an Earth-sized object, tipping it onto its side. Although the planet rotates once every 17 hours 14 minutes, this horizontal axis leaves half the world exposed to the Sun for 42 years. That’s one long day.
In terms of satellites, Uranus has 27 known moons, all named after characters from William Shakespeare and Alexander Pope. 13 distinct rings surround the planet at its equator. The inner rings are narrow and dark, and the outer rings have a bright color.
Neptune
More than 30 times further from the Sun than Earth, Neptune is the most distant planet in the Solar System. It’s so far away that it takes the planet an incredible 165 Earth years to make an entire orbit. A scientist calculated the planet’s existence in 1846 before it was observed by any telescope.
Neptune is another gas giant of hydrogen and helium, with red-light absorbing methane that gives the planet a deep blue hue. Neptune’s name comes from the Roman god of the sea due to this color and due to its location in the far into the deep “sea” of space. The blue planet is the smallest of the gas giants, but not by much.
Being so far out, Neptune probably has many secrets left to discover. Astronomers have confirmed the existence of 14 moons and at least five main rings made up of debris from a nearby moon. We also know it takes the planet 16 hours to perform one complete rotation.
The upper atmosphere of Neptune is very active. Winds rage around the planet at speeds of up to 1,100 miles per hour (1,770 kilometers per hour), which is faster than the speed of sound on Earth. One such storm, known as the Great Dark Spot, lasted five years before finally fading away.
What About Pluto?
Many of us grew up with a Solar System of nine planets. Unfortunately for Pluto, it was bumped to a dwarf planet in 2006 when the IAU agreed upon the latest definition of a planet. Pluto has not been able to clear the neighborhood around its orbit.
Pluto was discovered in 1930 and classified as a planet for 76 years. The name Pluto comes from the Roman god of the underworld, fitting into the heavenly naming pattern.
In terms of size, Pluto is smaller than many moons in our Solar System, including our own. Even so, it has five moons of its own. Charon, the largest moon, is so big compared to Pluto that the two actually orbit each other like a double planet.
Pluto is over 40 times further from the Sun than the Earth is and is one of many objects located in what’s now known as the Kuiper Belt. This region represents the region of the Solar System that’s believed to contain many comets, asteroids, and other objects. This distance also means it takes the dwarf planet 248 years to orbit the Sun.
Pluto’s strange elliptical orbit takes it anywhere from 2.7 to 4.5 billion miles (4.4 to 7.3 billion km) away from the Sun. There are times during its orbit where it is actually closer to the Sun than Neptune, although the two objects will never collide.
Being so far away, only one spacecraft has ever made the journey. The New Horizons spacecraft discovered that Pluto is 33% water, with the rest of the world being rocky, mountainous, and cratered. Its size makes it very difficult for the dwarf planet to hold an atmosphere.
Dwarf Planets
While the jury is still out on just how many dwarf planets our Solar System currently has, there are four other well-known ones. Three exist past Pluto, with the other being located in the Asteroid Belt between Mars and Jupiter.
Ceres
Ceres is the largest object in the Asteroid Belt and was classified as an asteroid until 2006 when it was bumped up to dwarf planet status. It comprises upwards of one-third of the total mass in the Asteroid Belt but is still 14 times smaller than Pluto.
The name Ceres comes from the Roman goddess of harvests, but is a barren, rocky world without an atmosphere. It does, however, have water, which has piqued the interest of astronomers as few objects in the Asteroid Belt do. The surface has a mysterious white spot, and no one knows what it is.
With a diameter of just 592 miles (952 km), Ceres is an incredibly small world. It travels around the Sun once every 4.6 years but manages to complete a full rotation in just nine Earth hours.
Despite all the asteroids around it, it is the only dwarf planet without at least a single moon.
Haumea
Existing in the Kuiper Belt even further out than Pluto, we don’t yet know a lot about Haumea. The dwarf planet has a unique, elongated shape that somewhat resembles an overinflated American football. It’s believed to have this shape because of its rapid rotation – the entire system rotates completely in just 3.9 Earth hours.
Haumea is the Hawaiian goddess of childbirth, and the dwarf planet has two small moons. These moons are thought to have been birthed from Haumea following a collision with a large object.
At only 770 miles (1240 km) in diameter, Haumea is half the size of Pluto. It takes 285 years for the dwarf planet to revolve around the Sun.
Little is yet known about the surface. We do know the world is made of rock with a thick coating of ice around it. There is a dark red spot of unknown origin that could be a concentrated area of a particular type of mineral.
Makemake
Makemake, named for the Rapa Nui god of fertility, this dwarf planet sits beyond Haumea in the Kuiper Belt and is a bit smaller than Pluto. Makemake has one known moon and takes 305 years to orbit the Sun. It completes one rotation every 22.5 hours.
We don’t know much about Makemake’s surface, but it appears to consist of frozen methane and ethane, giving it a reddish-brown color. It does not have a discernible atmosphere, which puzzles scientists, given its similar size to Pluto.
Eris
Eris, roughly the same size as Pluto, was the spark that started the 2006 debate about what a planet actually is. It was in line for status as the tenth planet in the Solar System before new planetary criteria were set. The dwarf planet has one moon.
Named for the Greek goddess of discord, Eris exists so far out into space that it takes 557 years to orbit the Sun in an area known as the scattered disc beyond the Kuiper Belt. A day on Eris is 25.9 Earth hours, similar to our own.
The dwarf planet has a rocky surface with an atmosphere that only appears when it is close enough to the Sun. When it drifts away, the atmosphere freezes and falls as snow to the surface.
Tips To Remember the Order of the Planets
There are a few fun tools that you can use to remember this order of planets.
1. Mnemonic Device
The most common is a mnemonic device, which uses the first letter of each planet as the first letter of the phrase. Here’s one of the most common:
My Very Educated Mother Just Served Us Noodles
If you feel like adding the dwarf planets into the mix, try something like this:
My Very Educated Mother Cannot Just Serve Us Nine Pizzas—Hundreds May Eat!
2. Create a Song
It may also be helpful to create a short song with the names of the planets that you can recall whenever you get stuck. Here’s an example:
Amazing Mercury is closest to the Sun,
Hot, hot Venus is the second one,
Earth comes third: it’s not too hot,
Freezing Mars awaits an astronaut,
Jupiter is bigger than all the rest,
Sixth comes Saturn, its rings look best,
Uranus sideways falls, and along with Neptune, they are big gas balls.
3. Use a Video
If a fun video is more your style, there’s an awesome video on YouTube that’s perfect for learning the order of planets in our Solar System.
4. Flash Cards
Creating flashcards with names and pictures involves seeing and speaking as a method of learning the order of the planets. Either draw the pictures or use some on a site like this one. Memory experts say the more senses you involve in learning, the better the result will be.
Conclusion
With eight unique planets and five classified dwarf planets, our Solar System is host to an incredible display of heavenly bodies. Unless the definition of a planet changes significantly in the future, it’s unlikely we’ll be adding any new planets to our current eight. That being said, only time will tell what awaits us far into the Kuiper Belt and beyond.