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QUESTIONS ABOUT THE PLANETS OF THE SOLAR SYSTEM

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Planet Song- teach the order of the planets!

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Oh My Genius - Nursery Rhymes And Kids Songs

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Facts About the Solar System And Its Formation

Before we dive right into the detail, here's some bite sized Solar System facts to get us started!

It is believed that the solar system formed 4.6 billion years ago from a cloud of gas and dust called the solar nebula.

The Sun and the planets were born from the solar nebula. The leftovers from this formation became comets, asteroids, and other small bodies. There are millions of these leftovers in our solar system!

We can think of the solar system in terms of three different main areas. From the Sun, we have the inner and then outer solar system. The third is the unexplored area beyond Neptune called the trans-Neptunian region.

Located beyond Neptune, the Kuiper belt is a disk-like region with solar system leftovers. Even further from this belt is the Oort Cloud. It is a spherical space that is said to be the end of the solar system.

There are five major dwarf planets in the solar system. Only one, Ceres, is located in the asteroid belt. The others, Pluto, Makemake, Haumea, and Eris, are in the Kuiper belt. Some of these dwarf planets have their own moons!

The eight planets lie on the same plane around the Sun called the ecliptic. They also orbit in the same direction. However, as they move on their axis, Venus and Uranus rotate differently.

More than 219 moons have been discovered in our solar system. While the Earth only has one, Saturn has the most known moons which is 82. All planets have moons except the closest planets to the Sun, Mercury and Venus.

Our solar system is one of the many star systems in the Milky Way galaxy. It is located in the Orion Arm, roughly 26,000 light-years away from the galactic center.

Proxima Centauri is our nearest star neighbor, located roughly 4.25 light-years from us. It is part of the multiple star system of Alpha Centaurus.

Solar System Profile

What's in a Name?

Our Sun is called Sol, from the Latin word "solis." Sol is the Sun god in Roman mythology, equivalent to Helios in Greek mythology. From the name Sol, we call everything related to our Sun "solar."

The Formation of the Solar System

There is some debate as to how the solar system came to be. Currently, the best-known explanation is that it formed about 4.6 billion years ago.

The Sun was born out of the solar nebula, a cloud of gas and dust. The planets and other objects in the solar system also formed from the same cloud. Our solar system took millions of years to develop.

The Sun formed first, then the planets, and eventually moons, asteroids, meteoroids, and other minor planets.

The Sun Was Born

Approximately 4.6 billion years ago a large cloud of gas and dust was disturbed by some force. Scientists have theorized that this force was a nearby supernova.

As a result of this disturbance and the energy introduced to the cloud, the cloud began to move.

Once the movement began, the cloud started to collapse in on itself due to its own gravity.

During the process of collapsing, the cloud began to rotate and heat up.

As the cloud continued to collapse, the cloud's temperature continued to rise and its rotation became faster and faster. As a result, the cloud eventually began to flatten out into a disk shape with most of the mass located at its center.

At some point, the pressure and temperature became so great at the cloud's center that nuclear fusion began to take place. It was then that the Sun was born.

The Planets Formed

After the Sun was born, the gases and dust further out from the disk's center began to cool and condense into tiny particles.

As more and more particles formed they began to collide with one another and stick together, thus creating particles as large as rocks and boulders.

Like the smaller particles that collided, the boulder-sized particles began to impact and join together. These larger bodies are known as planetesimals.

Eventually, enough planetesimals joined together to form planetary embryos.

Unlike the small particles, boulders, and planetesimals, planetary embryos were massive enough to exert significant gravitational force on surrounding objects. Hence, instead of random collisions between objects, planetary embryos pulled objects in the surrounding area to themselves.

Once all of the material in the area of each planetary embryo was pulled in, the planets were born.

Other Solar System Objects Followed

All other significant material in the solar system that did not join to form the Sun or the planets condensed to form the moons, asteroids, or comets.

Over time, the orbits of the planets and other bodies stabilized into the solar system that we know today.

The Objects in Our Solar System

There are many different types of objects found in the solar system: a star, planets, moons, dwarf planets, comets, asteroids, gas, and dust. In terms of the numbers of each of these objects, our current knowledge is as follows:

The Sun comprises over 99.86% of the Solar System's mass. The remaining mass is mostly accounted for by the mass of the giant planets, especially Jupiter. The inner planets, asteroids, and other smaller objects account for only about 0.002% of the total mass.

As shown in the graphic above, we can divide the structure of the solar system into three regions: the inner solar system, the outer solar system, and the trans-Neptunian region.

Visit the planets or dwarf planets pages for more information. You can also browse the various objects and phenomena in the solar system by clicking the images below.

The Sun

The Sun is the source of light and energy in the solar system. This yellow dwarf star is a big ball of glowing gases made up of hydrogen and helium. The Sun's gravity holds the solar system together. It generates energy through nuclear fusion, and without it, life and everything that we know will not exist. [ Learn more about the Sun]

Inner Solar System

The inner solar system is made up of the terrestrial planets (Mercury, Venus, Earth, and Mars) and the asteroid belt. The inner planets have more rocky compositions because the strong solar wind blew away the lighter elements into the outer zones.

Mercury

Mercury is the closest planet to the Sun. It is the smallest of the eight planets but the second densest after Earth. It is nearly tidally locked to the Sun. Because of that, it makes three rotations for every two orbits that it completes around the Sun.

Mercury's close distance from the Sun does not allow it to hold a moon. The Sun's gravity is much stronger and it will just snatch the moon from this small planet. [ Learn more about Mercury]

Venus

Venus is the second-closest planet to the Sun. The thick atmosphere is largely made up of carbon dioxide which makes it the hottest planet in the solar system. This scorching hot world has an average temperature of about 468 degrees Celsius (875 degrees Fahrenheit).

At night, Venus is the second-brightest natural object after the Moon. It is only slightly smaller than Earth, which is why it is nicknamed Earth's sister planet. It spins on its axis clockwise, opposite to the majority of planets which rotate counterclockwise. Like Mercury, it also has no moons. [ Learn more about Venus]

Earth

The Earth is our home planet and the only one known to hold life. It is just at the right distance from the Sun where water can exist on its surface. Also, the Earth's atmosphere protects us from solar radiation.

Overall, the Earth is only the fifth largest planet but it is the densest of them all. For a long time, it was believed to be the center of the universe. We only have one natural satellite-the Moon. This moon is responsible for the tides in our oceans. Interestingly, a day on Earth is getting longer, but at a very slow rate. [ Learn more about Earth]

Mars

Mars is the farthest terrestrial planet from the Sun. It has a distinct color which earned it the nickname the Red Planet. On this planet, a day is roughly the same length as our day here on Earth.

Mars is the second smallest planet in the solar system. It has two moons, Phobos and Deimos. Many orbiters, landers, and rovers are in the Martian System right now which makes it a favorite mission destination. [ Learn more about Mars]

The Asteroid Belt

As the name suggests, the asteroid belt is a donut-shaped region composed of many asteroids. These are rocky leftovers from the formation of the solar system.

In a way, the asteroid belt marks the boundary of the inner solar system. It is located between the orbits of Mars and Jupiter. The closest dwarf planet to the Sun, Ceres, is located in the asteroid belt. Together with it are the largest asteroids in the region, Vesta, Pallas, and Hygiea. [ Learn more about the asteroid belt]

Outer Solar System

The outer solar system is composed of giant planets (Jupiter, Saturn, Uranus, and Neptune), centaurs, and other small bodies.

Jupiter

Jupiter is the largest of all the planets. As a gas giant, it does not have a solid surface like the terrestrial planets. It is the third brightest object in our night sky after the Moon and Venus.

With a rotation period of about 10 hours, it has the shortest day among the eight planets. It has 79 moons in total. Its grandest moon, Ganymede, is the largest moon in the solar system. Jupiter also has a faint ring system around it. [ Learn more about Jupiter]

Saturn

Saturn is very famous for its magnificent rings. Known as the "ringed planet," it is surrounded by icy debris and dust which forms this ring system around. Also, it is the second-largest planet in the solar system but the least dense of all.

Saturn's moons come in all sizes and compositions. Some of them, like Titan and Enceladus, can possibly host life. Not only is this planet grand in its rings, but it also has the most number of moons known-82 moons and counting. [ Learn more about Saturn]

Uranus

Uranus is known as the "sideways planet" because its orbit is tilted by 98 degrees. A collision with a large object in the past may have caused this unusual tilt. Aside from Venus, it is the only planet in the solar system that follows a retrograde rotation.

This ice giant is a low-density planet. It has 27 known moons and 17 faint rings. [ Learn more about Uranus]

Neptune

Neptune is the farthest planet from the Sun. Because of its deep blue color, it was named after the Roman god of the sea. A day on Neptune lasts only 16 hours but it takes 165 Earth years to complete one orbit.

Neptune has 14 known moons. The largest of them is Triton, a unique satellite that has a retrograde orbit. Like the other outer planets, it also has a ring system around it. [ Learn more about Neptune]

Trans-Neptunian Region

Often dubbed the "third zone," the trans-Neptunian region is where the Kuiper belt and the Oort Cloud lie. Much about these regions is still a mystery since they are located very far from us.

The Kuiper Belt

The Kuiper belt is a donut-shaped region like the asteroid belt. However, it is located past the orbit of Neptune, about 30 to 50 astronomical units (AU) from the Sun. Since it is very far away, it is a very dark and cold region.

Kuiper belt objects are icy leftovers like comets. This region is also the home of the dwarf planets Pluto, Haumea, and Makemake. In the outer stretches of the Kuiper belt lies the scattered disk where the dwarf planet Eris is located. [ Learn more about the Kuiper belt]

The Oort Cloud

The Oort Cloud is a hypothetical region that is believed to be where the gravitational influence of the Sun ends. In simple terms, it is the edge of the solar system. Scientists believe that it is the origin of long-period comets.

Unlike the asteroid belt and the Kuiper belt, the Oort Cloud is said to be spherical in shape. It surrounds the solar system and everything in it. About 2 trillion objects are estimated to be in the cloud. We have not observed this region directly yet so it remains hypothetical. [ Learn more about the Oort Cloud]

How the objects in the Solar System interact

All objects in the Solar System orbit the Sun; that is, they move around the Sun in elliptical paths. Moreover, the orbits of these objects lie roughly in the same plane, called the ecliptic plane. They also orbit in the same direction.

The mechanism that causes the orbit of objects in the Solar System is one of the fundamental forces in nature: gravity. While the natural tendency for objects in the Solar System is to continue in a straight line of motion, the Sun exerts a force (gravity) on each object and therefore "bends" the straight path into a curved one.

Additionally, other objects in the Solar System are massive enough to exert gravitational forces significant enough to alter the orbit of smaller objects. For example, the Earth's gravity is strong enough to keep the Moon in orbit around the Earth.

The Size of the Solar System

Though it is common for most people to believe that the edge of the Solar System is that of Pluto's orbit, this is far from the truth.

Over the course of the 20th century, scientists hypothesized that the size of the Solar System extends to around 100,000 times the distance from the Sun to the Earth. They are also right in believing that there are still so many undiscovered objects beyond Pluto.

Scientists now believe that there are two major regions beyond Pluto. The first is the Kuiper Belt, a region of leftover objects similar to the asteroid belt between Mars and Jupiter. The second one is the Oort Cloud, a spherical region that contains numerous comets.

Our Views of the Solar System Over Time

Our understanding of the universe has evolved over time. For more than thousands of years, astronomers had a geocentric view. They thought that the Earth was the center of the universe. This idea also held that the Earth was stationary and that it had no relations to the objects in our sky.

The invention of telescopes provided the first exploration of the solar system. This allowed Galileo Galilei to observe the physical details of celestial bodies.

He was able to observe sunspots and discover the moons of Jupiter in 1610. These moons-Ganymede, Callisto, Io, and Europa-became known as the Galilean moons. It was the first time that other natural satellites from another planet were discovered. Because of his discoveries, he became known as the father of observational astronomy.

As science progressed, our understanding of the world also expanded. We have learned that the other planets are very much like the Earth. They rotate on their axis and orbit around the Sun. Though they have different compositions, they follow the same natural laws.

These discoveries shifted our view from being geocentric to heliocentric. Now, we know that the Sun is the center of the solar system and the Earth is just one of the many planets out there. Our solar system is also just one of the trillions of possible planetary systems too.

In the early 1700s, Edmond Halley discovered that comets can make repeated appearances. The Great Debate of 1920 even broadened our understanding of the universe. After this, through Cepheid variable stars, Edwin Hubble proved that the then known Andromeda Nebula is actually a separate galaxy.

Models of the Universe: Geocentric vs Heliocentric

In ancient Greek astronomy, the belief that Earth was the center of the universe had been around even before the 4th century BC. Plato and Aristotle adopted this concept and wrote works based on it. This view became even more recognized in the 2nd century AD by the works of Claudius Ptolemaeus.

The Almagest became Ptolemy's most important work. According to Ptolemy's cosmos, the Earth is a sphere and it is the center of the cosmos. It does not move but instead, the Sun and other objects move around it. This was the established view of the universe for centuries until the 1500s.

Nicolaus Copernicus proposed the heliocentric model in the book De Revolutionibus Orbium Coelestium. The ancient Greek astronomer, Aristarchus, also had this idea before. However, this was dismissed in favor of the geocentric view of Aristotle and Ptolemy.

Copernicus's heliocentric theory was also faced with the same skepticism when it came out in 1543, just before his death. This work marked the beginning of the scientific revolution. But still, the geocentric model remained the generally-accepted view.

Around the 1600s, Galileo Galilei championed the heliocentric theory. He used his telescopic observations to support it but, still, there were so many oppositions. It conflicted with the belief of many, especially that of the church. He was imprisoned because of this.

Heliocentrism later became widely accepted when it aligned with other scientific concepts. This includes Johannes Kepler's elliptical orbits and Isaac Newton's law of motion and the law of universal gravitation.

Discovery of Planet-Like Objects

Now, it is already established that the Sun is indeed the center of the solar system. But as technology advanced, so did our knowledge of the solar system.

Ceres

Pluto was discovered on February 18, 1930, and was considered the ninth planet. However, more and more objects like it were getting discovered. In January 1801, Ceres was discovered in the asteroid belt. It was thought to be a planet at first, but then later on classified as an asteroid. It was the first asteroid to be classified.

When it was discovered on January 5, 2005, some considered Eris the tenth planet. That means the number of planets is going to increase every time an object like it is discovered. Because of that, a new category of objects called "dwarf planet" was established.

Pluto Reclassified

We used to think that there were nine planets. After the discovery of Eris, experts came together during the International Astronomical Union (IAU) general assembly in August 2006. They refined the criteria of what makes an object a planet. The definition for the new dwarf planet category was also finalized.

According to the IAU, a planet:

Is in orbit around the Sun

Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)

Has "cleared the neighborhood" around its orbit

Pluto's size was calculated when its moon Charon was discovered. Astronomers found out that it is actually much smaller than Mercury. It did not fulfill the three requirements for planets because it had not cleared its path of similar objects. Because of that, it was reclassified as a dwarf planet and we only have eight planets now.

According to the IAU, a dwarf planet:

Is in orbit around the Sun

Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape

Has not cleared the neighborhood around its orbit

Is not a satellite

The first dwarf planets to be considered were Pluto, Ceres, and Eris. Makemake and the egg-shaped Haumea were later added to this category. These five are the officially recognized dwarf planets. However, there are still many objects like these that are awaiting confirmation and discovery.

More Interesting Solar System Facts

Over 300 unmanned spacecraft have left Earth to explore the solar system. As of 2021, NASA's Curiosity, InSight, Perseverance, and Ingenuity on Mars are still operational. The Zhurong rover of the China National Space Administration (CNSA) became the new addition to these robotic spacecraft on the red planet.

Voyager 1 and 2 launched in 1977. Now, they are still operating and have become the first man-made spacecraft to reach interstellar space. Pioneer 10 and 11, as well as New Horizons, will follow them in the coming years.

Leading the group is Voyager 1 which entered interstellar space in 2012. Still, it will take around 300 years for it to enter the Oort Cloud and 30,000 years to exit it. In that case, and if it doesn't run out of fuel, it will officially leave our solar system.

Our planets orbit the Sun and the Sun, in turn, also orbits around the galactic center. While the longest-orbiting planet, Neptune, takes 165 to go around the Sun, our solar system takes roughly 230 million years to orbit!

Other star systems also have planets which we call exoplanets. These exoplanets are just so far from us that we cannot see them in detail. Still, we can get a sense of their sizes and compositions by studying the planets in our solar system.

Solar System Sources:

https://solarsystem.nasa.gov/solar-system/our-solar-system/overview/
https://en.wikipedia.org/wiki/Solar_System
https://en.wikipedia.org/wiki/Nicolaus_Copernicus#Galileo
https://en.wikipedia.org/wiki/Heliocentrism

Solar System Image Sources:

Solar system: https://upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Solar_sys8.jpg/1280px-Solar_sys8.jpg

The Sun was born: https://upload.wikimedia.org/wikipedia/commons/thumb/0/0b/Artist%E2%80%99s_Impression_of_a_Baby_Star_Still_Surrounded_by_a_Protoplanetary_Disc.jpg/1280px-Artist%E2%80%99s_Impression_of_a_Baby_Star_Still_Surrounded_by_a_Protoplanetary_Disc.jpg

The planets formed: https://rocketstem.b-cdn.net/wp-content/uploads/2020/12/NASA-ExocometsAroundBetaPictoris-ArtistView-2.jpg

Other Solar System Objects Followed: https://res.cloudinary.com/jerrick/image/upload/c_fill,f_jpg,fl_progressive,h_375,q_auto,w_625/wfstnflfhmmydslhf0fu.jpg

Earth and Moon: https://upload.wikimedia.org/wikipedia/commons/thumb/5/50/Dscovrepicmoontransitfull.gif/800px-Dscovrepicmoontransitfull.gif

Sun to Alpha Centauri: https://upload.wikimedia.org/wikipedia/commons/9/9f/Solarmap.png

Galileo Galilei using a telescope:
https://upload.wikimedia.org/wikipedia/commons/e/e7/Bertini_fresco_of_Galileo_Galilei_and_Doge_of_Venice.jpg

Halley's comet: https://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Lspn_comet_halley.jpg/1024px-Lspn_comet_halley.jpg

Geocentrism: https://upload.wikimedia.org/wikipedia/commons/thumb/8/8f/Ptolemaic_system_%28PSF%29.png/800px-Ptolemaic_system_%28PSF%29.png

Heliocentric model: https://upload.wikimedia.org/wikipedia/en/c/cb/De_Revolutionibus_ms_p9v.jpg

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Solar System Facts: Interesting Facts about Our Solar System

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