Planets in our Solar System orbit the Sun because it

Question: The planets in our solar system are orbiting the Sun counter clockwise, why? Do the laws of physics dictate that all planet orbit their respective stars counter clockwise or is it possible to have a solar system where the planets are in a clockwise motion around their star? — David

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Size and Distance

Answer: Most of the objects in our solar system, including the Sun, planets, and asteroids, all rotate counter-clockwise. This is due to the initial conditions in the cloud of gas and dust from which our solar system formed. As this gas and dust cloud began to collapse it also began to rotate. That rotation just happened to be in a counter-clockwise direction. There is nothing special about a counter-clockwise rotation, though. We could easily have found ourselves living in a solar system which was rotating clockwise about our Sun, if that was the initial state of rotation of the gas and dust cloud from which our solar system formed. Note, though, that there are two oddballs in our solar system that do not rotate in the same way as the rest of the planets. Uranus rotates about an axis that is nearly parallel with its orbital plane (i.e. on its side), while Venus rotates about its axis in a clockwise direction. These oddities are thought to be caused by events, such as collisions, which occurred during the formation of the solar system.

Jeff Mangum

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In ancient times, astronomers thought that all celestial objects – the Sun, Moon, planets and stars – orbited around the Earth in a series of crystal spheres. But as modern science developed, astronomers were better able to understand our place in the cosmos. They discovered that all the planets, including the Earth, actually orbit around the Sun.

Not only did scientists discover that the simple fact that the planets orbit the Sun, they uncovered the underlying reasons for why. What chain of events led us to our current Solar System, with planets orbiting the Sun?

Astronomers Used to Think the Earth was the Center of the Solar System

Because we live on Earth, and we see objects passing across our view of the skies, it’s natural to assume that the Earth is the center of the Universe. In fact, this perspective – known as geocentrism – was the default for all ancient civilizations. The Sun, the Moon, the planets and the stars appeared to move around the Earth each day. And because the Earth itself didn’t seem to be moving, astronomers like Ptolemy assumed that Earth was the center of the Universe. In fact, they went so far as to create very detailed models for predicting the motions of objects with a high degree of accuracy, using this completely inaccurate model of the Solar System. The predictions made by Ptolemy were used to make astrological predictions for more than 1500 years, until a much better model came along.

Actually, the Sun is the Center of the Solar System

A new, more accurate model of the Solar System didn’t come around until the 16th century, when the Polish astronomer Nicolai Copernicus published his Universe-changing book: On the Revolutions of the Heavenly Bodies. Copernicus accurately reorganized the Solar System, putting the Sun at the center in a heliocentric model. And the Earth took its proper place, as just another planet orbiting the Sun – one of the 6 known to astronomers at the time.

Copernicus’ model helped answer two questions which had troubled astronomers for centuries: why the planets brighten and dim over the course of several months (because they’re getting closer and further away), and why the planets seem to reverse and move in a retrograde direction. Easily explained because of the changing positions of the Earth, planets and the background stars.

But Why Do They Orbit the Sun?

Once they could accurately describe the nature of the planetary motion in the Solar System, they were left with a more fundimental question: Why do the planets orbit the Sun? What sequence of events led to the current motions of the planets around the Sun?

To explain this, we need to look back 4.6 billion years ago, before there was even a Solar System. In our place instead, there was a massive cloud of hydrogen gas left over from the Big Bang. Some event, like a nearby supernova explosion triggered a gravitational collapse of the cloud, causing the hydrogen atoms to attach to one another through mutual gravity.

Each individual hydrogen atom had its own momentum, and so when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning. Imagine two spinning skydivers colliding with one another in mid-air; after their collision, they’ll have a new rotation speed and direction based on the addition of their original directions.

Eventually all of this hydrogen gas was collected together into a massive spinning ball of gas that continued to collapse under its own gravity. As it collapsed, it began to spin faster and faster, just like a figure skater pulling in her arms increases her rotation speed.

The spinning cloud of gas and dust flattened out because of the rotational force, with the Sun at the center, and then a pancake-shaped disc of material surrounding it. The planets formed out of this disk of material, collecting together particles of dust into larger and larger rocks until planet-sized objects had accumulated together.

The Planets are in Perfect Balance

The planets orbit the Sun because they’re left over from the formation of the Solar System. Their current motion depends on the gravitational attraction of the Sun at the center of the Solar System. In fact, they’re in perfect balance.

There are two opposing forces acting on the planets: gravity pulling them inward, and the inertia of their orbit driving them outwards. If gravity was dominant, the planets would spiral inward. If their inertia was dominant, the planets would spiral outward into deep space.

The planets are trying to fly out into deep space, but the gravity of the Sun is pulling them into a curved orbit.

Research further: Cornell Astronomy

The Universe of Aristotle and Ptolemy

Copernical Model: A Sun-Centered Solar System
The Solar Nebula
On the Revolution of the Heavenly Bodies
The Copernican Revolution

Introduction

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

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.

NASA’s New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. Credit: NASA/JHUAPL/SwRI | Full caption and image

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 Oort Cloud is made of icy pieces of space debris - some bigger than mountains – orbiting our Sun as far as 1.6 light-years away. This shell of material is thick, extending from 5,000 astronomical units to 100,000 astronomical units. One astronomical unit (or AU) is the distance from the Sun to Earth, or about 93 million miles (150 million kilometers). The Oort Cloud is the boundary of the Sun's gravitational influence, where orbiting objects can turn around and return closer to our Sun.

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.

These six narrow-angle color images were made from the first-ever 'portrait' of the solar system taken by Voyager 1, which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. Credit: NASA Planetary Photojournal

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.