How many planets are there in the solar system
How many planets are there in the solar system
How many planets are there in the solar system
There are more planets than stars in our galaxy. The current count orbiting our star: eight.
The inner, rocky planets are Mercury, Venus, Earth, and Mars. NASA’s newest rover — Perseverance — landed on Mars on Feb. 18, 2021. The outer planets are gas giants Jupiter and Saturn and ice giants Uranus and Neptune.
Beyond Neptune, a newer class of smaller worlds called dwarf planets reign, including longtime favorite Pluto. Thousands more planets have been discovered beyond our solar system. Scientists call them exoplanets (exo means «from outside»).
Planets of our Solar System
The key difference between a planet and a dwarf planet is the kinds of objects that share its orbit around the Sun. Pluto, for example, has not cleared its orbit of similar objects while Earth or Jupiter have no similarly-sized worlds on the same path around the Sun. Like planets, dwarf planets are generally round (Haumea looks like an overinflated football) and orbit the Sun.
There are likely thousands of dwarf planets waiting to be discovered beyond Neptune. The five best-known dwarf planets are Ceres, Pluto, Makemake, Haumea, and Eris. Except for Ceres, which lies in the main asteroid belt, these small worlds are located in the Kuiper Belt. They’re considered dwarfs because they are massive, round, and orbit the Sun, but haven’t cleared their orbital path.
Real-Time, Interactive Solar System
Real-Time, Real Data: Your Galactic Neighborhood
This simulated view of our solar system at the top of this page (and below) runs on real data. The position of the planets, moons, and spacecraft are shown where they are right now. This digital orrery (a model of the solar system) runs on a light, mobile-mobile friendly version of NASA’s Eyes on the Solar System software.
This snapshot focuses on active NASA missions and select ESA missions. Showcasing the entire operating international fleet is too much of a data wallop at the moment (but we are working on it!). Active international missions such as Japan’s Akatsuki Venus Orbiter and ESA and Japan’s Mercury-bound BepiColombo are not yet available.
Use the HD button to upload higher-resolution imagery of the planets. It may not work properly on older mobile devices. Have fun, and keep exploring.
Solar system planets, order and formation: A guide
Explore our solar system’s planets from the nearest to the sun to the furthest.
The order of the planets in the solar system, starting nearest the sun and working outward is the following: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and then the possible Planet Nine.
The solar system extends from the sun, called Sol by the ancient Romans, and goes past the four inner planets, through the asteroid belt to the four gas giants and on to the disk-shaped Kuiper Belt and far beyond to the teardrop-shaped heliopause.
Scientists estimate that the edge of the solar system is about 9 billion miles (15 billion kilometers) from the sun. Beyond the heliopause lies the giant, spherical Oort Cloud, which is thought to surround the solar system.
Ever since the discovery of Pluto in 1930, kids grew up learning that the solar system has nine planets. That all changed in the late 1990s when astronomers started arguing about whether Pluto was indeed a planet. In a highly controversial decision, the International Astronomical Union ultimately decided in 2006 to designate Pluto as a «dwarf planet,» reducing the list of the solar system’s true planets to just eight.
If you insist on including Pluto, it would come after Neptune on the list. Pluto is truly way out there and on a wildly tilted, elliptical orbit (two of the several reasons it was demoted).
Astronomers, however, are still hunting for another possible planet in our solar system, a true ninth planet, after mathematical evidence of its existence was revealed on Jan. 20, 2016. The alleged «Planet Nine,» also called «Planet X,» is believed to be about 10 times the mass of Earth and 5,000 times the mass of Pluto.
Types of planets in the solar system
The inner four planets closest to the sun — Mercury, Venus, Earth and Mars — are often called the «terrestrial planets» because their surfaces are rocky. Pluto also has a rocky, albeit frozen, surface but has never been grouped with the four terrestrials.
The four large outer worlds — Jupiter, Saturn, Uranus and Neptune — are sometimes called the Jovian or «Jupiter-like» planets because of their enormous size relative to the terrestrial planets. They’re also mostly made of gases like hydrogen, helium and ammonia rather than of rocky surfaces, although astronomers believe some or all of them may have solid cores.
If you were to order the planets by size from smallest to largest they would be Mercury, Mars, Venus, Earth, Neptune, Uranus, Saturn and Jupiter.
What is (and isn’t) a planet?
The IAU defines (opens in new tab) a true planet as a body that circles the sun without being some other object’s satellite; is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star); and has «cleared its neighborhood» of most other orbiting bodies.
But that restrictive definition helped isolate what should and should not be considered a planet — a problem that arose as astronomers discovered more and more planet-like objects in the solar system. Pluto was among the bodies that didn’t make the cut and was re-classified as a dwarf planet.
The problem with Pluto, aside from its small size and offbeat orbit, is that it doesn’t clear its neighborhood of debris — it shares its space with lots of other objects in the Kuiper Belt. Still, the demotion of Pluto remains controversial.
The IAU planet definition also put other small, round worlds into the dwarf planet category, including the Kuiper Belt objects Eris, Haumea and Makemake.
Ceres, a round object in the Asteroid Belt between Mars and Jupiter, also got the boot. Ceres was considered a planet when it was discovered in 1801, but it was later deemed to be an asteroid. That still didn’t quite fit because it was so much larger (and rounder) than the other asteroids. Astronomers instead deemed it a dwarf planet in 2006, although some astronomers like to consider Ceres as a 10th planet (not to be confused with Nibiru or Planet X).
Below is a brief overview of the eight true planets in our solar system, moving from that closest to the sun to the farthest from the sun:
The sun
The sun is by far the largest object in our solar system, containing 99.8% of the solar system’s mass. It sheds most of the heat and light that makes life possible on Earth and possibly elsewhere. Planets orbit the sun in oval-shaped paths called ellipses, with the sun slightly off-center of each ellipse.
NASA has a fleet of spacecraft observing the sun, such as the Parker Solar Probe, to learn more about its composition, and to make better predictions about space weather and its effect on Earth.
Mercury: The closest planet to the sun
Mercury is the closest planet to the sun and the smallest planet in the solar system — it is only a little larger than Earth’s moon. Mercury zips around the sun in only 88 days and because it is so close to our star (about two-fifths the distance between Earth and the sun).
Mercury experiences dramatic changes in its day and night temperatures. Mercury temperatures can reach a scorching 840 F (450 C) in the day, which is hot enough to melt lead. Meanwhile, on the night side, temperatures drop to minus 290 F (minus 180 C).
— Discovery: Known to the ancient Greeks and visible to the naked eye
— Named for the messenger of the Roman gods
— Diameter: 3,031 miles (4,878 km)
— Orbit: 88 Earth days
— Day: 58.6 Earth days
— Number of moons: 0
Mercury’s atmosphere is very thin and primarily composed of oxygen, sodium, hydrogen, helium and potassium. Because the atmosphere is so thin it cannot incoming meteors, its surface is therefore pockmarked with craters, just like our moon.
Over its four-year mission, NASA’s MESSENGER spacecraft revealed incredible discoveries that challenged astronomers’ expectations. Among those findings was the discovery of water ice and frozen organic compounds at Mercury’s north pole and that volcanism played a major role in shaping the planet’s surface.
Venus: Earth’s solar system twin
Venus is the second planet from the sun and is the hottest planet in the solar system. Its thick atmosphere is extremely toxic and composed of sulfuric acid clouds, the planet is an extreme example of the greenhouse effect.
— Discovery: Known to the ancient Greeks and visible to the naked eye
— Named for the Roman goddess of love and beauty
— Diameter: 7,521 miles (12,104 km)
— Orbit: 225 Earth days
— Day: 241 Earth days
— Number of moons: 0
The average temperature on Venus’ surface is 900 F (465 C). At 92 bar, the pressure at the surface would crush and kill you. And oddly, Venus spins slowly from east to west, the opposite direction of most of the other planets.
Venus is sometimes referred to as Earth’s twin as they are similar in size and radar images beneath its atmosphere reveal numerous mountains and volcanoes. But beyond that, the planets could not be more different.
The Greeks believed Venus was two different objects — one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky, Venus has generated many UFO reports.
Earth: Our home planet, filled with life
Earth, our home planet, is the third planet from the sun. It is a water world with two-thirds of the planet covered by water. Earth’s atmosphere is rich in nitrogen and oxygen and it is the only world known to harbor life.
— Name originates from «Die Erde,» the German word for «the ground.»
— Diameter: 7,926 miles (12,760 km)
— Orbit: 365.24 days
— Day: 23 hours, 56 minutes
— Number of moons: 1
Earth rotates on its axis at 1,532 feet per second (467 meters per second) — slightly more than 1,000 mph (1,600 kph) — at the equator. The planet zips around the sun at more than 18 miles per second (29 km per second).
Mars: The solar system’s Red Planet
Mars is the fourth planet from the sun. It is a cold, desert-like planet covered in iron oxide dust that gives the planet its signature red hue. Mars shares similarities with Earth: It is rocky, has mountains, valleys and canyons, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms.
Substantial scientific evidence suggests that Mars at one point billions of years ago was a much warmer, wetter world, rivers and maybe even oceans existed. Although Mars’ atmosphere is too thin for liquid water to exist on the surface for any length of time, remnants of that wetter Mars still exist today. Sheets of water ice the size of California lie beneath Mars’ surface, and at both poles are ice caps made in part of frozen water.
— Discovery: Known to the ancient Greeks and visible to the naked eye
— Named for the Roman god of war
— Diameter: 4,217 miles (6,787 km)
— Day: Just more than one Earth day (24 hours, 37 minutes)
— Number of moons: 2
Scientists also think ancient Mars would have had the conditions to support life like bacteria and other microbes. Hope that signs of this past life — and the possibility of even current lifeforms — may exist on the Red Planet has driven numerous Mars missions and the Red Planet is now one of the most explored planets in the solar system.
The asteroid belt
Between Mars and Jupiter lies the asteroid belt. Asteroids are minor planets, and according to NASA there are approximately between 1.1 and 1.9 million asteroids (opens in new tab) in the main asteroid belt larger than 0.6 miles (1 km) in diameter and millions more smaller asteroids.
The dwarf planet Ceres, about 590 miles (950 km) in diameter, resides here. A number of asteroids have orbits that take them closer into the solar system that sometimes lead them to collide with Earth or the other inner planets.
Jupiter: The largest planet in our solar system
— Discovery: Known to the ancient Greeks and visible to the naked eye
— Named for the ruler of the Roman gods
— Diameter: 86,881 miles (139,822 km)
— Orbit: 11.9 Earth years
— Day: 9.8 Earth hours
— Number of moons: 79 (53 confirmed, 26 provisional)
Its swirling clouds are colorful due to different types of trace gases including ammonia ice, ammonium hydrosulfide crystals as well as water ice and vapor.
A famous feature in its swirling clouds is Jupiter’s Great Red Spot, a giant storm more than 10,000 miles wide, first observed in 1831 by amateur astronomer Samuel Heinrich Schwabe. It has raged at more than 400 mph for the last 150 years, at least.
Jupiter has a strong magnetic field, and with 75 moons, including the largest moon in the solar system, Ganymede.
Saturn: The ringed jewel of the solar system
Saturn is the sixth planet from the sun and is famous for its large and distinct ring system. Though Saturn is not the only planet in the solar system with rings.
— Discovery: Known to the ancient Greeks and visible to the naked eye
— Named for Roman god of agriculture
— Diameter: 74,900 miles (120,500 km)
— Orbit: 29.5 Earth years
— Day: About 10.5 Earth hours
— Number of moons: 82 (53 confirmed, 29 provisional)
If you put Saturn in a bathtub it would float as Saturn has an average density that is less than water. You’d just need to find a bathtub big enough…
When polymath Galileo Galilei first studied Saturn in the early 1600s, he thought it was an object with three parts: a planet and two large moons on either side. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing — a symbol with one large circle and two smaller ones — in his notebook, as a noun in a sentence describing his discovery. More than 40 years later, Christiaan Huygens proposed that they were rings.
The rings are made of ice and rock and scientists are not yet sure how they formed. The gaseous planet is mostly hydrogen and helium and has numerous moons.
Uranus: The tilted, sideways planet in our solar system
Uranus is the seventh planet from the sun and is a bit of an oddball.
It has clouds made of hydrogen sulfide, the same chemical that makes rotten eggs smell so foul. It rotates from east to west like Venus. But unlike Venus or any other planet, its equator is nearly at right angles to its orbit — it basically orbits on its side.
— Discovery: 1781 by William Herschel (was originally thought to be a star)
— Named for the personification of heaven in ancient myth
— Diameter: 31,763 miles (51,120 km)
— Orbit: 84 Earth years
— Day: 18 Earth hours
— Number of moons: 27
Astronomers believe an object twice the size of Earth collided with Uranus roughly 4 billion years ago, causing Uranus to tilt. That tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years at a time.
The collision is also thought to have knocked rock and ice into Uranus’ orbit. These later became some of the planet’s 27 moons. Methane in Uranus’ atmosphere gives the planet its blue-green tint. It also has 13 sets of faint rings.
Uranus holds the record for the coldest temperature ever measured in the solar system — minus 371.56 degrees F (minus 224.2 degrees C). The average temperature of Uranus is minus 320 degrees Fahrenheit (-195 degrees Celsius).
Neptune: A giant, stormy blue planet
Neptune is the eighth planet from the sun and is on average the coldest planet in the solar system. The average temperature of Neptune at the top of the clouds is minus 346 degrees Fahrenheit (minus 210 degrees Celsius).
— Discovery: 1846
— Named for the Roman god of water
— Diameter: 30,775 miles (49,530 km)
— Orbit: 165 Earth years
— Day: 19 Earth hours
— Number of moons: 14
Neptune is approximately the same size as Uranus and is known for its supersonic strong winds. The planet is more than 30 times as far from the sun as Earth.
Neptune was the first planet predicted to exist by using math, rather than being visually detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other planet might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth and has a rocky core.
Trans-Neptunian region
Astronomers had long suspected that a band of icy material known as the Kuiper Belt existed past the orbit of Neptune extending from about 30 to 55 times the distance of Earth to the sun, and from the last decade of the 20th century up to now, they have found more than a thousand of such objects. Scientists estimate the Kuiper Belt is likely home to hundreds of thousands of icy bodies larger than 60 miles (100 km) wide, as well as an estimated trillion or more comets.
Pluto, now considered a dwarf planet, dwells in the Kuiper Belt. It is not alone — recent additions include Makemake, Haumea and Eris. Another Kuiper Belt object dubbed Quaoar is probably massive enough to be considered a dwarf planet, but it has not been classified as such yet. Sedna, which is about three-fourths the size of Pluto, is the first dwarf planet discovered in the Oort Cloud. NASA’s New Horizons mission performed history’s first flyby of the Pluto system on July 14, 2015.
Pluto: Once a planet, now a dwarf planet
Pluto was once the ninth planet from the sun and is unlike any other planet in the solar system.
— Discovery: 1930 by Clyde Tombaugh
— Named for the Roman god of the underworld, Hades
— Diameter: 1,430 miles (2,301 km)
— Orbit: 248 Earth years
— Day: 6.4 Earth days
— Number of moons: 5
It is smaller than Earth’s moon; its orbit is highly elliptical, falling inside Neptune’s orbit at some points and far beyond it at others; and Pluto’s orbit doesn’t fall on the same plane as all the other planets — instead, it orbits 17.1 degrees above or below.
It is smaller than Earth’s moon; its orbit is highly elliptical, falling inside Neptune’s orbit at some points and far beyond it at others; and Pluto’s orbit doesn’t fall on the same plane as all the other planets — instead, it orbits 17.1 degrees above or below, taking 288 years to complete a single orbit according to ESA.
From 1979 until early 1999, Pluto had been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune’s path and once again became the solar system’s most distant planet — until it was redefined as a dwarf planet. It’s a cold, rocky world with a tenuous atmosphere.
Scientists thought it might be nothing more than a hunk of rock on the outskirts of the solar system. But when NASA’s New Horizons mission performed history’s first flyby of the Pluto system on July 14, 2015, it transformed scientists’ view of Pluto.
Pluto is a very active ice world that’s covered in glaciers, mountains of ice water, icy dunes and possibly even cryovolcanoes that erupt icy lava made of water, methane or ammonia.
Planet Nine: A planet search at solar system’s edge
In 2016, researchers proposed the possible existence of a ninth planet, for now, dubbed «Planet Nine» or Planet X. The planet is estimated to be about 10 times the mass of Earth and to orbit the sun between 300 and 1,000 times farther than the orbit of the Earth.
Scientists have not seen Planet Nine. They inferred its existence by its gravitational effects on other objects in the Kuiper Belt, a region at the fringe of the solar system that is home to icy rocks left over from the birth of the solar system. Also called trans-Neptunian objects, these Kuiper Belt objects have highly elliptical or oval orbits that align in the same direction.
A hypothesis proposed in September 2019 on the pre-print server arXiv suggests Planet Nine might not be a planet at all. Instead, Jaku Scholtz of Durham University and James Unwin of the University of Illinois at Chicago speculate it could be a primordial black hole that formed soon after the Big Bang and that our solar system later captured, according to Newsweek. Unlike black holes that form from the collapse of giant stars, primordial black holes are thought to have formed from gravitational perturbations less than a second after the Big Bang, and this one would be so small (5 centimeters in diameter) that it would be challenging to detect.
Astronomers continue to come up empty in their search for Planet 9. A recent 2022 sky survey using the 6-meter Atacama Cosmology Telescope (ACT) in Chile found thousands of tentative candidate sources but none could be confirmed.
The edge of the solar system
Past the Kuiper Belt is the very edge of the solar system, the heliosphere, a vast, teardrop-shaped region of space containing electrically charged particles given off by the sun. Many astronomers think that the limit of the heliosphere, known as the heliopause, is about 9 billion miles (15 billion km) from the sun.
Solar system formation and discovery
The heat and pressure eventually became so high that hydrogen atoms began to combine to form helium. The nuclear reactions released vast amounts of energy and our sun was formed.
The sun accumulated about 99% of the available matter and the remaining material further from the sun formed smaller clumps inside the spinning disk. Some of these clumps gained enough mass that their gravity shaped them into spheres, becoming planets, dwarf planets and moons. Other leftover pieces became asteroids, comets and smaller moons that make up our solar system.
For millennia, astronomers have followed points of light that seemed to move among the stars. The ancient Greeks named them planets, meaning «wanderers.» Mercury, Venus, Mars, Jupiter and Saturn were known in antiquity, and the invention of the telescope added the Asteroid Belt, Uranus, Neptune, Pluto and many of these worlds’ moons. The dawn of the space age saw dozens of probes launched to explore our system, an adventure that continues today.
There have been five human-made objects so far, Voyager 1, Voyager 2, New Horizons, Pioneer 10 and Pioneer 11, that have crossed the threshold into interstellar space.
Additional resources
Explore the solar system in greater detail with these interactive resources (opens in new tab) from NASA. Discover the wonders of the solar system with this educational material (opens in new tab) from ESA. See where the planets are in their current orbit of the sun with this interactive orrery (opens in new tab) from NASA.
Bibliography
Prialnik, Dina K., Antonella Barucci, and Leslie Young, eds. The Trans-Neptunian Solar System. (opens in new tab) Elsevier, 2019.
Pirani, Simona, et al. «Consequences of planetary migration on the minor bodies of the early solar system. (opens in new tab) » Astronomy & Astrophysics 623 (2019): A169.
Brown, Michael E., and Konstantin Batygin. «Observational constraints on the orbit and location of planet nine in the outer solar system. (opens in new tab) » The Astrophysical Journal Letters 824.2 (2016): L23.
Planets of the Solar System
This page provides a brief description of each of the planets (and links to dwarf planets) of our solar system. You can also find out about the difference between planets, dwarf planets and small solar system bodies (SSSBs) here.
Planets
Dwarf Planets
The Planets of the Solar System
Planet Formation
The planets and the solar system were formed from a huge cloud of gases and dust particles left over when a massive star exploded as a supernova.
Gravity was also working within the disk of rotating gas and dust pulling matter together to form primitive planets within the gas disk.
The heat of the Sun and the solar wind immediately began to have an effect on the huge cloud of gas and particles in the disk. Volatile substances such as water ice near the Sun would heat and sublimate into gas, and these and other gases such as hydrogen would be gently accelerated away from the Sun by the solar wind.
At the distance of Jupiter, the temperature the Sun was not high enough to cause water ice to evaporate and so this meant that large quantities of solid material were available to build larger planets. These planets could therefore attract and keep hold of more of gas from the gas disk. This is one theory as to why the gas giants became so large, and why there is a divide in planet size between the small inner rocky planets and the outer gas giants.
Planets and Dwarf Planets in Order from the Sun
The planets and dwarf planets are listed here in the order they are from the Sun. Click for more information on each.
How Many Planets are in our Solar System?
Conclusion For Pluto
This happened after astronomers settled upon what would define a planet, and Pluto seemed to lack criteria. Many, even from high positions at NASA, still consider Pluto, the ninth planet of our Solar System. As such, it is quite a matter of preference for how many planets are in our Solar System.
A New Planet?
Many believe a mysterious tenth (if considering Pluto) or ninth planet is orbiting in our Solar System, commonly referred to as Planet X. This hypothetical planet might be the size of Neptune, and it would have a highly elongated orbit, even more so than Pluto.
Planet X would complete one orbit around the Sun once every 10.000 or 20.000 years. Some mathematical evidence leads many to believe that this elusive planet indeed exists.
The Solar System
But let us get back to the known planets of our Solar System. The closest planet to the Sun is Mercury, followed by Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and the dwarf planet Pluto. This is the order of the planets.
The smallest planet in our Solar System is Mercury, which is only one third the size of Earth, while the biggest planet in the Solar System is Jupiter, which is 11 times bigger than our Earth, and more than 1,300 could fit inside it if it were hollow.
Mercury is only 0.4 AU or 58 million km / 36 million mi away from our Sun, while our Earth is at 1.00 AU or 150 million km / 93 million mi away. The farthest planet, Neptune, is 30 AU or 4.5 billion km / 2.8 billion mi away from the Sun, and if you want to know how far Pluto is, its 39.5 AU or 5.9 billion km / 3.7 billion mi away from the Sun.
Here is a table about the planets in our Solar System, their mass, size, and distance from the Sun.
The Solar System
Distance From the Sun
One interesting fact about our Solar System is that it lacks a specific type of planet called a Super-Earth. Super-Earth planets are terrestrial planets that are more massive and much bigger than our Earth, by several times.
These giant planets are usually smaller than the gas giants; however, they should harbor life, and it would be interesting to think about how life would evolve on such a planet. Just imagine our Earth being four times bigger, at least, how many species could there have been in such a world.
Are There More Planets in Our Solar System?
You could say that there are 13 planets in our Solar System, maybe even more. Pluto isn’t the only dwarf planet orbiting the Sun; there are others as well.
Some consider that the biggets moon of Pluto, named Charon, might also be a dwarf planet. Regardless of its classification, Charon is even bigger than Ceres.
Ceres is an attractive dwarf planet since it is also classified as an asteroid, the biggest yet discovered. Some believe that life may have come from Ceres through the process of panspermia.
Regardless, some other big-sized objects were discovered in our Solar System, such as Sedna, Quaoar, Orcus, Gonggong, Chiron, or Interamnia.
Some of them have been classified as asteroids, comets, but many are possible dwarf plantes. If this is true, then our Solar System really seems like a crowded place. Who knows what we will discover in the future and if the hypothetical Planet X will also be debunked one day.
Our Solar System
The planetary system we call home is located in an outer spiral arm of the Milky Way galaxy.
Our solar system consists of our star, the Sun, and everything bound to it by gravity – the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; dwarf planets such as Pluto; dozens of moons; and millions of asteroids, comets, and meteoroids.
Beyond our own solar system, there are more planets than stars in the night sky. So far, we have discovered thousands of planetary systems orbiting other stars in the Milky Way, with more planets being found. Most of the hundreds of billions of stars in our galaxy are thought to have planets of their own, and the Milky Way is but one of perhaps 100 billion galaxies in the universe.
While our planet is in some ways a mere speck in the vast cosmos, we have a lot of company out there. It seems that we live in a universe packed with planets – a web of countless stars accompanied by families of objects, perhaps some with life of their own.
Namesake
There are many planetary systems like ours in the universe, with planets orbiting a host star. Our planetary system is named the «solar system» because our Sun is named Sol, after the Latin word for Sun, «solis,» and anything related to the Sun we call «solar.»
Size and Distance
Size and Distance
Our solar system extends much farther than the eight planets that orbit the Sun. The solar system also includes the Kuiper Belt that lies past Neptune’s orbit. This is a sparsely occupied ring of icy bodies, almost all smaller than the most popular Kuiper Belt Object – dwarf planet Pluto.
Beyond the fringes of the Kuiper Belt is the Oort Cloud. This giant spherical shell surrounds our solar system. It has never been directly observed, but its existence is predicted based on mathematical models and observations of comets that likely originate there.
The Sun’s heliosphere doesn’t extend quite as far. The heliosphere is the bubble created by the solar wind – a stream of electrically charged gas blowing outward from the Sun in all directions. The boundary where the solar wind is abruptly slowed by pressure from interstellar gases is called the termination shock. This edge occurs between 80-100 astronomical units.
Two NASA spacecraft launched in 1977 have crossed the termination shock: Voyager 1 in 2004 and Voyager 2 in 2007. Voyager 1 went interstellar in 2012 and Voyager 2 joined it in 2018. But it will be many thousands of years before the two Voyagers exit the Oort Cloud.
Moons
There are more than 200 known moons in our solar system and several more awaiting confirmation of discovery. Of the eight planets, Mercury and Venus are the only ones with no moons. The giant planets Jupiter and Saturn lead our solar system’s moon counts. In some ways, the swarms of moons around these worlds resemble mini versions of our solar system. Pluto, smaller than our own moon, has five moons in its orbit, including the Charon, a moon so large it makes Pluto wobble. Even tiny asteroids can have moons. In 2017, scientists found asteroid 3122 Florence had two tiny moons.
Formation
Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula – a spinning, swirling disk of material.
At the center, gravity pulled more and more material in. Eventually, the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. With that, our Sun was born, and it eventually amassed more than 99% of the available matter.
Matter farther out in the disk was also clumping together. These clumps smashed into one another, forming larger and larger objects. Some of them grew big enough for their gravity to shape them into spheres, becoming planets, dwarf planets, and large moons. In other cases, planets did not form: the asteroid belt is made of bits and pieces of the early solar system that could never quite come together into a planet. Other smaller leftover pieces became asteroids, comets, meteoroids, and small, irregular moons.
Structure
The order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed. Nearest to the Sun, only rocky material could withstand the heat when the solar system was young. For this reason, the first four planets – Mercury, Venus, Earth, and Mars – are terrestrial planets. They are all small with solid, rocky surfaces.
Meanwhile, materials we are used to seeing as ice, liquid, or gas settled in the outer regions of the young solar system. Gravity pulled these materials together, and that is where we find gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune.