There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The sun is gigantic compared to the planets, containing about 99.8% of the entire mass of the solar system. On the other hand, the planets are much smaller; together, they make up less than 1% of the solar system’s mass.
Each of the planets in the solar system has unique characteristics that make it interesting to study. For example, Mercury is the closest planet to the sun and has a very small atmosphere. It’s also the smallest planet – only about a third as large as Earth. Venus is considered the hottest planet due to its thick carbon dioxide atmosphere. Finally, Earth is the only known planet with suitable conditions for life.
Mars has distinct red color due to its rusty iron-rich soil. Jupiter is the largest planet in the solar system, with a diameter of a staggering 11 times that of Earth. Saturn is famous for its rings made up of ice and dust particles. Uranus, unlike all the other planets, rotates on its side.
Neptune is the farthest planet from the sun and has a large, continuous storm called the “Great Dark Spot.”
The largest planet is Jupiter, which is more than 300 times the size of Earth.
The four inner planets (Mercury, Venus, Earth, and Mars) are all made mostly of rock and metal. They are sometimes called the terrestrial planets because they have solid surfaces like Earth’s. The four outer planets (Jupiter, Saturn, Uranus, and Neptune) are primarily made up of gas and ice. They are sometimes called the gas giants because they have substantial atmospheres of hydrogen and helium.
The solar system also contains numerous smaller bodies, including asteroids, comets, meteoroids, and moons. These bodies orbit around the sun or the larger planets.
Space enthusiasts are often surprised to learn that the solar system doesn’t end at Neptune. In fact, it extends far beyond the gas giants, through the asteroid belt and out to the Kuiper Belt (a larger asteroid belt). Beyond that lies a region known as the heliopause, where the sun’s influence ends and interstellar space begins.
According to scientists, this boundary lies roughly 9 billion miles from the sun. This may seem like a tremendous distance, but it’s just a drop in the bucket compared to the vast space.
What About Pluto?
Pluto is conspicuously missing from the solar system’s list of planets, which may be surprising to some. After all, it’s long been referred to as the ninth planet and has even been included in middle school science curricula.
The debate over Pluto’s status began in the late 1990s when astronomers started questioning whether it met the criteria for being classified as a planet. At the time, Pluto was considered the outermost planet in the solar system. Still, several unusual characteristics led some scientists to believe it might be more similar to a comet than a planet. But how exactly is Pluto different from other planets in the solar system?
Pluto is no longer considered a planet because it fails to meet the “planet” criteria necessary. The first and most important criterion is that a planet must orbit around the sun. Pluto, however, has an irregular orbit. It spends about 20 of its 248-year lap around the Sun inside Neptune’s orbit, technically making it a satellite of Neptune.
Another criterion for being classified as a planet is having enough mass to create a spherical shape due to gravity. Although Pluto has enough mass, its sphere is only about two-thirds the size of Mercury’s. Finally, planets are expected to clear their neighborhood of small objects. Since Pluto shares its orbital space with other dwarf planets and asteroids, it does not meet this criterion either.
Due to these reasons, in 2006, the International Astronomical Union (IAU) made the controversial decision to reclassify Pluto as a “dwarf planet.” Although this decision was widely unpopular, it ultimately helped to clarify the definition of a planet and resulted in a better understanding of our solar system.
The IAU’s decision does not mean Pluto is any less fascinating or important. On the contrary, it remains an intriguing celestial body that continues to capture our imaginations.
Let’s now look at the sun and then discuss each of the eight planets in the solar system in greater detail.
The sun is the largest object in our solar system and is the centerpiece of our planetary neighborhood. It’s the most critical enabler of life on Earth, thanks to its emission of heat and light. Its central position in our solar system enables the sun to profoundly influence all the planets that orbit it.
Light travels from the sun to Earth in eight minutes, which seems like a relatively short period given the vast distance between them. However, it takes millions of years for the sun’s energy to travel from its core to its surface. This goes to show just how powerful and bright the sun truly is. Its light can reach us in mere minutes, but it takes much longer for its energy to make its way to the surface. This highlights the incredible journey the sun’s light takes before reaching us here on Earth.
Every planet orbits the sun in an oval-shaped path known as an ellipse. The sun sits slightly off-center in each ellipse, with the planets closer to the sun experiencing shorter orbital periods.
Ellipses are often described using two measurements: the semi-major axis (which is the distance from the sun to the farthest point of an ellipse) and the eccentricity (the measure of how elongated an ellipse is).
For example, a perfectly circular orbit would have an eccentricity of 0, while a highly elongated orbit might have an eccentricity of 0.9. All orbits are elliptical to some degree, but those with small eccentricities are often referred to as being nearly circular.
This arrangement creates a fascinating dynamic whereby the planets are constantly drawn closer to and farther away from the sun. The result is a dance that has captivated observers for centuries and is a source of wonder today.
According to scientific estimates, the sun was formed approximately 4.59 billion years ago from a solar nebula. This is a cloud of hydrogen and dust from which a star and its planets form. Once the star ignites and begins to emit light, the solar nebula starts to dissipate slowly.
Currently, the sun is in the reactive stage, slowly burning through its stockpile of hydrogen fuel. However, the expectation is that after about 5 billion years, the sun will enter the self-destructive phase. During this phase, it will swell up and absorb Earth and all the other planets in the solar system. The sun will then discard its outer layers, creating a new planetary nebula.
Finally, the sun will cool and shrink to become a dwarf star. Although this eventual fate of the sun may seem bleak, it’s actually an essential part of the life cycle of stars. Without stars like our sun going through this process, our universe would have no new planets or life.
Thanks to recent technological advances, we can now study the sun in greater detail than ever before. NASA has already commissioned a fleet of spacecraft that are currently observing the sun.
One of them is the Parker Solar Probe, launched in 2018 with the top goal of observing the sun’s surface. Unfortunately, the spacecraft will have to swoop to within 4 million miles of the sun to achieve its goal. This will put the spacecraft under massive heat and radiation. Luckily, it’s equipped with hardware designed to protect it from that.
Quick Facts About the Sun
- Has a surface temperature of 5,000°-5,700° Celsius
- 93 million miles away from Earth
- Rotates in the same direction as Earth
- Create energy through nuclear fusion
- Can hold over 1 million Earths
Mercury is the planet closest to the sun and is one of the smaller planets in the solar system. It’s a rocky planet with an average diameter of about 4,878 kilometers.
It’s only a little larger than Earth’s moon and takes only 88 days to lap the sun. However, Mercury is so close to our star that it receives about twice as much sunlight as Earth. This makes Mercury’s surface very hot, enough to melt lead.
Mercury is small and barren, with a surface scarred by craters. And yet, it holds several surprises. One of these is that, due to its elliptical orbit and sluggish rotation, the sun appears briefly, sets, and then rises again at certain points on the planet’s surface. The reverse happens at sunset. As a result, a day on Mercury lasts 176 Earth days or slightly over two Mercurian years.
Even though it’s relatively closer to the Sun, Mercury is not the hottest planet. That honor goes to Venus, which has thick clouds that trap heat in its atmosphere. Mercury does not have an atmosphere, so there is nothing to trap heat and make the surface temperature more bearable. As a result, Mercury’s surface can reach temperatures of 430° Celsius during the day.
At night, however, the temperature plummets because there is no atmosphere to hold in any heat. Consequently, Mercury has the most significant temperature range of any planet in the solar system — from -170° Celsius at night to 430° Celsius during the day.
Mercury is slightly less than one-third the size of Earth and has a very thin atmosphere consisting mostly of sodium, oxygen, helium, hydrogen, and potassium. The planet’s thin atmosphere is incapable of deflecting incoming meteoroids, and as a result, it has a heavily cratered surface.
Mercury is not an airless world despite its small size and lack of atmosphere. Instead, the planet has a surprisingly large iron core that generates a magnetic field. This field protects it from the solar wind and makes it an important target for future exploration.
The first successful mission to Mercury—Mariner 10—happened in 1973. A more recent spacecraft, Messenger, was launched in 2004 and went on to orbit Mercury for four years before crashing onto the planet’s surface in 2015. From the mission, scientists discovered frozen organic compounds at the planet’s north pole and evidence that volcanic eruptions were rampant during the formation of Mercury’s surface.
Despite its small size and inhospitable environment, Mercury continues to fascinate astronomers and provide clues about the formation of our solar system.
Quick Facts About Mercury
- Discovery: First observed by Galileo Galilei and Thomas Harriot in the 17th century
- Namesake: The Roman messenger god
- Complete orbit: 88 Earth days
- Number of moons: 0
- Diameter: 4,879 km
Venus is the closest planet to Earth. It’s often called the “sister planet” to Earth because of its similar sizes.
Venus holds a unique history in the annals of astronomy because it’s the first planet to be explored by a spacecraft. On December 14, 1962, NASA’s Mariner 2 flew past and scanned the cloud-covered world. This event opened a new era of exploration, and since then, numerous spacecraft from various space agencies have gone on to explore Venus in greater detail.
One such mission was NASA’s Magellan, which mapped the planet’s surface using radar. More recent missions have continued to build on this body of knowledge, revealing new insights into the nature of Venus and its unique environment.
The main component of Venus’s dense atmosphere is carbon dioxide. This creates a “greenhouse effect,” which makes the surface of Venus very hot—about 465° Celsius. The atmospheric pressure on Venus is also much higher than on Earth. As a result, any water on Venus would be in the form of vapor, not liquid. The atmosphere of Venus is also very acidic, which has led to the planet being nicknamed “the lemon” by astronomers.
The surface of Venus is covered in volcanoes and is crisscrossed by cracks called “graben.” The daytime sky on Venus is bright yellow, while the nighttime sky is violet. Venus takes 225 Earth days to orbit the sun and rotates once every 243 Earth days.
It’s no secret that Venus is an inhospitable place. The average temperature on its surface is a toasty 900 degrees Fahrenheit (465° Celsius), and the atmospheric pressure at the surface is 92 bars—roughly the same as the pressure at a depth of 1 kilometer underwater on Earth.
So, what’s it like to stand on the surface of Venus? For starters, you’d be immediately crushed by the atmospheric pressure. And within seconds, you’d be cooked alive by the searing surface temperature. In short, it’s not a place you want to spend any time.
Venus has been observed since the beginning of astronomy and has been a source of fascination for cultures worldwide. For example, the Ancient Greeks believed that Venus was two different objects: one that appeared in the morning sky (Phosphorus) and another in the evening sky (Hesperus). In Chinese astronomy, Venus is associated with the legendary goddess Xi Wang Mu.
Due to its brightness, Venus has often been associated with UFOs. A famous instance of this occurred in 1965 when a retired police officer reported seeing a UFO near Brighton, England. The UFO turned out to be Venus, which was exceptionally bright that night due to its position in relation to the Earth.
As our understanding of astronomy has improved, instances of “UFO sightings” caused by Venus have become less common. However, the planet still holds a place in human mythology and continues to fascinate astronomers.
Venus also spins slowly from east to west, in the opposite direction of most of the other planets in the solar system.
- Discovery: First observed by ancient Greek astronomers. Also visible to the naked eye
- Namesake: Roman goddess of love and beauty
- Complete orbit: 225 Earth days
- Number of moons: 0
- Diameter: 12,104 km
Earth is the third planet from the sun and the only planet known to support life. According to estimates, Earth is 4.5 billion years old. Since its formation, it has undergone numerous changes in its physical appearance and the life forms that inhabit it.
Today, Earth is home to a diverse range of species, from tiny bacteria to large mammals, and its landscapes range from sprawling deserts to lush rainforests.
The name Earth is thought to be at least 1,000 years old. Unlike the names of the other planets in our solar system, Earth is a Germanic word that simply means “the ground.” Possibly, the name was chosen because it is so descriptive of our planet. After all, Earth is the only planet on which we find solid ground. The other planets are mainly gas and dust.
Earth is primarily covered in water and has a nitrogen-rich atmosphere capable of supporting life. It’s the only planet known to have all these conditions. Oceans supply 97% of Earth’s water. The remaining 3% is fresh water vital for human and animal life.
Although Earth’s atmosphere mainly comprises nitrogen, it also contains 21% oxygen. This combination is necessary for sustaining human and animal life. Earth’s atmosphere also helps regulate the planet’s temperature, protecting it from the extreme temperatures of space. Without this atmosphere, Earth would be a cold, barren world.
As anyone who has ever been on a merry-go-round can attest, things move faster the further you are from the center. The same is true for our planet. Earth rotates fastest at the equator, where its surface moves at about 1,000 miles (1,600 kilometers) per hour. That may not seem very fast, but it’s fast enough to make watching the sun rise twice in one day possible.
Of course, Earth isn’t just rotating on its axis; it’s also orbiting the sun. Our planet completes one orbit every 365.24 days, which we call a year. As it circles the Sun, Earth moves at an average speed of more than 18 miles (29 km) per second or nearly 67,000 miles (108,000 km) per hour. This means that Earth is constantly on the move—zipping around the sun while simultaneously spinning on its axis. And though it might not always feel like it, we’re all along for the ride.
Unlike most of the other planets in the solar system, Earth has one moon. Scientists believe the moon formed approximately 4.51 billion years ago, 800 million years after the solar system’s formation. A widely accepted theory is that the moon formed after a tremendous impact between Earth and a planet as big as Mars called Theia.
Although the moon has no atmosphere, it has an extremely weak gravitational field responsible for the high and low ocean tides observed on Earth. The moon’s traditional English name is “moon,” derived from its Old English name, mona. Its orbit around Earth causes it to regularly pass into Earth’s shadow, resulting in a lunar eclipse on average once every 6 months.
Scientists contend that human activity has profoundly impacted Earth, and the planet’s future remains uncertain. In the face of climate change and other environmental challenges, it is clear that Planet Earth is at a crossroads. Whether it will continue to be a hospitable home for life or become an uninhabitable wasteland remains to be seen.
Quick Facts About Earth
- Namesake: German word meaning “the ground.”
- Complete orbit: 365.24 days
- Number of moons: 1
- Diameter: 12,760 km
- Day: 23 hours, 56 minutes
At 141 million miles from the sun, Mars is the fourth planet in our solar system. It’s a small world with a diameter of just over 4,000 miles. But despite its size, Mars has always been of great interest to astronomers and scientists.
This is because Mars shares many similarities with our own planet Earth. Like Earth, Mars is a rocky world with mountains, ridges, valleys, escarpments, and canyons. There are also dust storms that can cover the entire planet and smaller dust devils that can engulf an entire region. And of course, Mars is also famous for its red hue, which is caused by iron oxide dust covering the planet’s surface.
There is strong evidence that water once flowed across the Martian surface. Ancient riverbeds, valleys, and deltas have been discovered, and minerals that can only form in the presence of water have been found. In addition, there is evidence Mars once had a much thicker atmosphere. Over time, the planet has lost almost all its atmospheric gases to space. These discoveries suggest that Mars was once a habitable world, with conditions similar to those on Earth.
Despite these similarities, there are also some significant differences between Earth and Mars. For starters, Mars is a much colder world than Earth, with an average temperature of only about 55° Fahrenheit. Additionally, Mars is a desert-like planet with very little water. In fact, 97% of the Martian surface is covered in dry, dusty plains.
For centuries, Mars has been the subject of human fascination. Early astronomers believed that the planet was home to intelligent life, and fictional works like H.G. Wells’ “The War of the Worlds” only added to the public’s interest. Finally, in 1965, NASA’s Mariner 4 spacecraft completed a flyby of Mars, providing the first close-up images of the planet.
These images showed a barren, cratered surface, leading many to believe that their hopes for finding life on Mars had been dashed. However, recent discoveries have reignited the search for life on Earth’s neighbor.
In 2018, NASA’s InSight lander detected small vibrations on Mars likely caused by seismic activity. Although there is no direct evidence of life on Mars, these vibrations suggest that the planet is more geologically active than previously thought, making it a potentially hospitable place for microbial life.
Quick Facts About Mars
- Namesake: Gravidus (Originally Mars, Roman god of war)
- Complete orbit: 687 days
- Number of moons: 2
- Diameter: 6,787 km
- Day: 24 hours and 37 minutes
The Asteroid Belt
The asteroid belt is a torus-shaped region that lies between the orbits of Mars and Jupiter. It’s home to a large number of small, minor planets. It’s estimated that more than 1.5 million asteroids are in the belt. Most of these asteroids are made of rock and metal and are thought to be the remains of a once-massive planet destroyed by a powerful collision billions of years ago.
One of the largest objects in the region is the dwarf planet Ceres, which has a diameter of about 950 km. Other notable asteroids include Vesta, Pallas, and Hygiea. Many asteroids have orbits that intersect Earth’s orbit. As a result, they occasionally collide with Earth or the other inner planets. These collisions can cause significant damage, as was evident during the Chelyabinsk meteor event in 2013.
Although the asteroid belt is often portrayed as a dangerous place for spacecraft to venture, it’s relatively safe. Most asteroids are spaced far apart, and there have only been a handful of collisions with belts in recorded history. Nevertheless, the asteroid belt remains a fascinating place for astronomers to explore, and many believe it could hold clues about how our planets formed and evolved.
Jupiter is the fifth planet from the sun and the largest in the solar system. It’s a gas giant with a mass one thousand times that of the sun.
Jupiter has a diameter of about 143,000 kilometers at its equator, making it more than 11 times the size of Earth. Research indicates that Jupiter is mainly composed of hydrogen and helium. The planet has been known since ancient times. It’s named after the Roman god Jupiter, the god of the sky.
Jupiter rotates very rapidly, with a day lasting only 9 hours and 56 minutes. Its rapid rotation creates extreme weather conditions, including strong winds and storms. The planet also has a series of faint rings consisting of dust and debris particles. These are thought to be left over from collisions between asteroids or comets and the small moons orbiting Jupiter.
Jupiter has four large moons—Io, Europa, Ganymede, and Callisto – which were discovered by Galileo Galilei in 1610. The planet also has dozens of smaller moons. Jupiter’s most notable feature is its Great Red Spot, a massive storm that has been raging for centuries.
Jupiter has a higher mass than all the other planets in the solar system combined. This makes Jupiter very important in shaping the solar system. In addition, its strong gravity affects both asteroids and comets as they orbit around it. As a result, Jupiter acts as a “shield” for Earth, protecting us from dangerous impacts by comets and asteroids.
Jupiter is also known for intense radio bursts. These bursts were first detected in 1955 by Bernard F. Burke and Kenneth W. Franklin at Ohio State University. They are now known to be caused by Jupiter’s intense magnetic field interacting with particles in the solar wind.
The interaction creates a giant plasma wave oscillating at frequencies ranging from 10 MHz to 10 GHz. The plasma wave can be hundreds of kilometers long and millions of watts in power. When this wave hits Earth, it can be detected as a burst of radio noise.
Jupiter has colorful clouds created by different trace gases in the atmosphere, including ammonia ice, ammonium hydrosulfide crystals, water vapor, and ice. The different colors are created by the sun’s light reflecting off these particles at different wavelengths.
Ammonia ice is thought to be responsible for the blue hues, while ammonium hydrosulfide crystals give Jupiter its characteristic orange and brown stripes. Water vapor and ice likely contribute to the white and gray colors seen in the clouds. Jupiter’s atmospheric conditions constantly change, so the gas composition and cloud colors can also change over time.
Scientists believe these changes may be related to the dynamics of Jupiter’s interior, which is still largely unknown.
Quick Facts About Jupiter
- Namesake: Roman ruler of the gods and heavens (Jove)
- Complete orbit: 12 Earth years
- Number of moons: 79
- Diameter: 139,822 km
- Day: 9 hours, 56 minutes
Named after the Roman god of sowing or seed, Saturn is a gas giant with an atmosphere of hydrogen and helium. Its most notable feature is its ring system, which consists of millions of small icy objects orbiting the planet.
Galileo Galilei wasn’t just the first to observe Saturn, but he was also the first to document it. In 1610, he used a small telescope to study the planet and thought he saw three objects: a planet and two large moons on either side.
He was baffled by what he saw and made a small drawing of one large circle and two smaller ones in his notebook. This was his way of describing his discovery. However, it wasn’t until 1659, when Dutch astronomer Christiaan Huygens used a larger telescope, that the rings of Saturn were discovered. Huygens also gave the planet its current name, the Roman god of agriculture and harvest.
Saturn’s rings are thought to be the result of the planet’s intense gravitational pull tearing apart a large moon that got too close. However, it has also been suggested that the rings are material left behind after Saturn was formed.
However, evidence suggests the rings are gradually dissipating into Saturn. After a few million years, they are expected to disappear completely. The rings are made up of ice and dust particles, colliding with each other and breaking into smaller pieces. Over time, these small pieces will fall into Saturn’s atmosphere and be vaporized
Saturn is also home to several conspicuous moons, including Titan (the second largest moon in the solar system), Dione, Rhea, Hyperion, Iapetus, and Pan. Dione and Rhea, for example, have heavily cratered surfaces and extremely rocky cores. Hyperion, however, looks more like a giant sponge or coral reef.
On September 1, 1979, NASA’s Pioneer 11 became the first spacecraft to visit Saturn, flying past the planet at a distance of 21,000 km. The spacecraft’s onboard camera took some spectacular photos of Saturn’s rings and discovered a new outer ring. One year later, on November 12, 1980, Voyager 1 had a similar flyby of Saturn, but this time the spacecraft also made a close encounter with Titan.
Although it is not currently known to support life, Saturn’s unique features make it an interesting target for future exploration.
Quick Facts About Saturn
- Namesake: Roman god of sowing or seed
- Complete orbit: 29 Earth years
- Number of moons: 82
- Diameter: 120,500 km
- Day: 10 hours, 34 minutes
In 1781, Uranus made history as the first planet to be discovered by telescope by Sir William Herschel, who initially believed it was a star. He named it Georgium Sidus, after King George III. In 1850, John Russell Hind realized it was a planet, giving it the name Uranus.
Uranus has a diameter of 50,724 kilometers and is mainly made up of rock and ice. The atmosphere of Uranus is made up of hydrogen, helium, and methane, and it has a very faint blue color.
The planet takes 84 years to orbit the sun and rotates on its axis every 17 hours. Uranus’ distance from the sun makes it very cold, with an average temperature of -216° Celsius. Despite its frigid temperatures, Uranus is home to interesting weather phenomena like methane rainstorms.
Among the planets in the solar system, Uranus has several features that make it stand out. First, unlike any other planet, its equator is nearly at a right angle to its orbit. As a result, Uranus rotates from east to west rather than from west to east like most planets.
Additionally, its axis of rotation is tilted nearly horizontally to its orbit. Scientists believe this strange orientation is the result of a massive collision early in the history of our solar system. When Uranus was first forming, it was hit by a massive object, which caused it to reorient itself.
The planet’s unusual tilt means that for part of its 84-year orbit around the sun, one of its poles points directly toward the sun while the other pole is in complete darkness. As a result, Uranus has extreme seasons, with 20 years of daylight followed by 20 years of darkness.
Uranus has 27 moons, most of which were discovered in the 20th century. The largest and most well-known of these is Titania, discovered in 1787 by Herschel. Titania is 1,578 kilometers in diameter and is covered in glaciers of water ice. Uranus also has two tiny inner moons, Ariel and Umbriel, discovered in 1851 by William Lassell. These two moons are just 584 and 772 kilometers in diameter, respectively. In 1986, Uranus’ largest moon, Miranda, was discovered by the Voyager 2 spacecraft. Miranda is just 470 kilometers in diameter but is one of the most fascinating moons in the solar system due to its strange, heavily cratered surface.
All of Uranus’ moons are named after characters from William Shakespeare’s plays.
In a decadal survey conducted by planetary scientists in 2022, a new flagship mission to Uranus was selected as a top science goal for the next 10 years. The planet has only been visited by one spacecraft, Voyager 2, which flew by in 1986. Since then, our understanding of Uranus has advanced significantly, and there is a growing body of evidence suggesting the planet may be hiding a liquid ocean beneath its icy surface.
Quick Facts About Uranus
- Namesake: Ancient Greek god of the sky
- Complete orbit: 84 Earth years
- Number of moons: 27
- Diameter: 50,724 km
- Day: 18 Earth hours
Neptune is the eighth and farthest known planet from the sun. Neptune was discovered through mathematical predictions.
In 1846, two mathematicians independently predicted a new planet’s existence beyond Uranus’s orbit. John Couch Adams, a student at Cambridge University, began working on the problem after hearing about the irregularities exhibited by Uranus in its orbital path from his astronomy professor. Meanwhile, Urbain Le Verrier, a French mathematician, became interested in the issue after reading a paper on the subject by Adams.
After long hours of calculations, both men finally arrived at very similar predictions for the position of the new planet. They submitted their findings to astronomers for verification, and Neptune was discovered exactly where they had predicted it would be within months. The success of Adams and Le Verrier’s calculations proved that the solar system is governed by mathematical laws and opened a new era of astronomical discovery.
Neptune orbits the sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km). It takes the planet 60,215 Earth days to complete one revolution around the sun. On the other hand, one rotation takes 16 Earth hours. The interior of Neptune consists of rock and ice, including water ice, methane ice, and ammonia ice. Silicates are thought to comprise most of Neptune’s rocky material.
The planet is also known for its supersonic winds, reaching speeds of up to 2,000 kilometers per hour. But why?
Neptune is a net energy generator because it generates more energy than it receives from the sun. This excess energy is thought to come from the planet’s core, where radioactive elements like uranium and thorium decay. As the elements decay, they release heat and energy, which radiates outwards towards the planet’s surface. This heat drives convection currents in the atmosphere, creating strong winds.
The planet is 4.5 billion kilometers from the sun. By comparison, Earth is 152 million kilometers away from the sun. It has six known moons: Triton, Nereid, Halimede, Sao, Laomedeia, and Neso. The largest is Triton, discovered just 17 days after the planet was discovered in 1846. Measuring 2,700 kilometers in diameter, Triton is significantly smaller than our own moon but slightly larger than Pluto.
Triton is also unique because it’s the only large satellite in the solar system with a retrograde orbit (orbiting in the opposite direction to its planet’s rotation). Neptune’s other moons are much smaller, with the majority less than 100 kilometers in diameter.
Quick Facts About Neptune
- Namesake: Roman god of water
- Complete orbit: 165 Earth years
- Number of moons: 14
- Diameter: 49,244 km
- Day: 16 Earth hours
The Kuiper Belt
Beyond Neptune lies the Kuiper Belt, a large region shaped like a doughnut. It’s thought to be home to thousands of icy bodies, including comets, asteroids, and dwarf planets. Many of these objects are believed to be leftovers from the formation of our solar system. It’s made up of small icy bodies, most of which are less than 200 miles across. The largest and perhaps most famous resident of the Kuiper Belt is Pluto.
The Kuiper Belt is thought to be the source of comets that enter our solar system. These comets are called short-period comets because their orbital periods are less than 200 years. The Kuiper Belt was named after Gerard Kuiper, a Dutch-American astronomer who predicted its existence in 1951. However, it wasn’t until 1992 that the first body in the Kuiper Belt was discovered.
Since then, astronomers have found hundreds of these small icy worlds. Many of them are probably similar to Pluto’s size and composition. In 2006, a new class of objects was discovered in the Kuiper Belt. These objects are much larger than anything previously found in the belt and are thought to be similar to the giant planets in our solar system. One of them, Eris, is almost as large as Pluto.
A Possible Planet Nine
For a long time, astronomers have been hunting for a ninth planet in our solar system.
The possibility of a ninth planet in our solar system was first proposed in 2014 by California Institute of Technology researchers. Their work was based on the unusual orbits of six small planets beyond Neptune, which they suggested could be explained by the gravitational pull of a larger, as-yet-undiscovered planet.
In 2016, another team of astronomers used mathematical models and computer simulations to further explore this hypothesis. Their work, published in the Astronomical Journal, provides new evidence that Planet Nine indeed exists.
According to their simulations, Planet Nine would need to be about four times the size of Earth and orbit about 20 times farther from the sun than Neptune does. As a result, it would take Planet Nine between 10,000 and 20,000 years to make one complete orbit around the sun.
In September 2019, Jaku Scholtz of Durham University and James Unwin of the University of Illinois at Chicago proposed a new hypothesis on the preprint server arXiv. They suggested that Planet Nine might not be a planet at all but a primordial black hole. This type of black hole is thought to have formed shortly after the Big Bang, and it is possible that our solar system later captured it.
If this hypothesis is correct, it could explain why Planet Nine has not been directly observed. Unfortunately, primordial black holes are extremely difficult to detect because they are small and emit very little light. However, if more data is collected on Planet Nine, it may be possible to confirm or refute this hypothesis. Either way, this new research provides an interesting perspective on one of the most mysterious objects in our solar system.
The Edge of the Solar System
Peering past the Kuiper Belt leads us to the heliosphere, the very edge of the solar system. The heliosphere is a vast, teardrop-shaped space region containing electrically charged particles released by the sun. It’s divided into two regions: the inner heliosphere, which is dominated by the sun’s radiation and magnetic field, and the outer heliosphere, which is influenced by the interstellar medium.
Many astronomers estimate the heliosphere limit, known as the heliopause, at 9 billion miles (15 billion km) from the sun. Beyond the heliopause is a region of space that is filled with extremely low-density interstellar matter and radiation. This region is thought to extend to the edge of the Milky Way galaxy, about 100,000 light-years from Earth.
Beyond the heliopause, the solar wind can no longer push back against interstellar gas, and a shock front separates the two regions. In addition, high-energy particles from outside are constantly buffeting the heliosphere, so its size and shape are continually changing.
Many astronomers believe that Voyager 1, launched in 1977, has already reached the heliopause and is now journeying through interstellar space.