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Space : Our Solar System

  • NASA
  • Nov 1, 2017
  • 6 min read

10 Need-to-Know Things

This artist's illustration shows the Earth and our solar system's place in the Universe. Image Credit: Andrew Z. Colvin

  1. What is it? Our solar system is made up of the sun and everything that travels around it. This includes eight planets and their natural satellites such as Earth's moon, dwarf planets such as Pluto and Ceres, asteroids, comets and meteoroids.

  2. Sun-Centered The sun is the center of our solar system. It contains almost all of the mass in our solar system and exerts a tremendous gravitational pull on planets and other bodies.

  3. Age Our solar system formed about 4.6 billion years ago.

  4. Can Stand on Them The four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are called the terrestrial planets because they have solid, rocky surfaces.

  5. Can't Stand on Them Two of the outer planets beyond the orbit of Mars - Jupiter and Saturn - are known as gas giants; the more distant Uranus and Neptune are called ice giants.

  6. Beyond Neptune Most of the known dwarf planets exist in an icy zone beyond Neptune called the Kuiper Belt, which is also the point of origin for many comets. Ceres is the exception. It is in the main asteroid belt.

  7. Hard to Breathe Many objects in our solar system have atmospheres, including planets, some dwarf planets and even a couple moons. But none of them are suitable for humans.

  8. Spiral Galaxy Our solar system is located in the Orion Arm of the Milky Way Galaxy. There are most likely billions of other solar systems in our galaxy. And there are billions of galaxies in the Universe.

  9. Taking Measure We measure distances in our solar system by Astronomical Units (AU). One AU is equal to the distance between the sun and the Earth, which is about 93 million miles (150 million km).

  10. Going the Distance NASA's twin Voyager 1 and Voyager 2spacecraft are the first spacecraft to explore the outer reaches of our solar system.

Our Solar System: Overview

A solar system is a star and all of the objects that travel around it — planets, moons, asteroids, comets and meteoroids. Most stars host their own planets, so there are likely tens of billions of other solar systems in the Milky Way galaxy alone. Solar systems can also have more than one star. These are called binary star systems if there are two stars, or multi-star systems if there are three or more stars. The solar system we call home is located in an outer spiral arm of the vast Milky Way galaxy. It consists of the sun (our star) and everything that orbits around it. This includes the eight planets and their natural satellites (such as our moon), dwarf planets and their satellites, as well as asteroids, comets and countless particles of smaller debris.

Our Solar System: In Depth

The planets and dwarf planet Pluto are shown in their correct order of distance from the sun, their correct relative sizes and their correct relative orbital distances. The sizes of the bodies are greatly exaggerated relative to the orbital distances.

The planets and dwarf planet Pluto are shown in their correct order of distance from the sun, their correct relative sizes and their correct relative orbital distances. The sizes of the bodies are greatly exaggerated relative to the orbital distances.

Quick Facts

  • Planets: 8

  • Dwarf Planets: 5

  • Moons: Known = 149 | Provisional = 24 | Total = 173

  • Comets: More than 3,400

  • Asteroids: More than 715,000

A solar system is a star and all of the objects that travel around it — planets, moons, asteroids, comets and meteoroids. Most stars host their own planets, so there are likely tens of billions of other solar systems in the Milky Way galaxy alone. Solar systems can also have more than one star. These are called binary star systems if there are two stars, or multi-star systems if there are three or more stars.

The solar system we call home is located in an outer spiral arm of the vast Milky Way galaxy. It consists of the sun (our star) and everything that orbits around it. This includes the eight planets and their natural satellites (such as our moon), dwarf planets and their satellites, as well as asteroids, comets and countless particles of smaller debris.

Size and Distance

The 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.

And 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 the sizes of mountains and sometimes larger, 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. But it will be many thousands of years before the two Voyagers exit the Oort Cloud.

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 percent 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 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're small with solid, rocky surfaces.

Voyager 1 was the first spacecraft to leave our solar system.

Voyager 1 was the first spacecraft to leave our solar system.

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 ice giants Uranus and Neptune.

Exploration

In the early 17th century, Galileo Galilei's discoveries using the recently invented telescope strongly supported the concept of a solar system in which all the planets, including Earth, revolve around a central star — the sun. At the time this was called Copernican heliocentric theory, and it was a revolutionary idea, as most people thought Earth was the center of the universe.

Since then, we have learned much about our solar system and what lies beyond it using ground-based telescopes, spacecraft and mathematical models.

Significant Dates

  • 1543: Nicolaus Copernicus publishes his theory of heliocentrism, a model that places the sun at the center of the known universe with the planets orbiting in circles around it.

  • 1609, 1619: Johannes Kepler publishes three laws of planetary motion.

  • 1610: Galileo Galilei publishes "The Starry Messenger," a pamphlet about his telescopic observations of Earth's moon, the phases of Venus, and four of Jupiter's moons.

  • 1705: Edmond Halley mathematically demonstrates orbital similarities for comets observed in 1456, 1531, 1607 and 1682 and successfully predicts the return of the same comet in 1758.

  • 1781: William Herschel extends the solar system by telescopically discovering a new planet, later named Uranus.

  • 1801-1808: The first four asteroids were discovered (though the first and largest, Ceres, was reclassified as a dwarf planet 200 years later).

  • 1846: Urbain Le Verrier, with confirming observations by Johann Galle, further extends the solar system by predicting (Le Verrier) and observing (Galle) a new planet, Neptune.

  • 1930: Clyde Tombaugh's photographic search discovers a member of the solar system beyond Neptune, named Pluto, initially classified as a planet but now considered a dwarf planet.

  • 1977: Voyagers 1 and 2 launch from Earth, beginning their mission to explore the far reaches of our solar system.

  • 2004: Voyager 1 crosses the termination shock of our solar system.

  • 2007: Voyager 2 crosses the termination shock of our solar system.

  • 2012: Voyager 1 enters interstellar space.

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