Solar system

Solar system, assemblage consisting of the Sun—an average star in the Milky Way Galaxy—and those bodies orbiting around it: 8 (formerly 9) planets with about 210 known planetary satellites (moons); countless asteroids, some with their own satellites; comets and other icy bodies; and vast reaches of highly tenuous gas and dust known as the interplanetary medium

When did people discover that the Sun is a star?

the first person to come up with the idea that stars and the Sun are the same thing, just at different distances, was Anaxagoras, in about 450 B.C. Later, Aristarchus, around 220 B.C., thought similarly. In 1600 A.D., Giordano Bruno was burnt at the stake for heresy, for asserting that the Sun is a star, among other things. It wasn't until the mid-1800s, after the work of Galileo, Kepler, Huygens, Newton, and finally, Friedrich Bessel, that it could be proven. The distance to other stars was calculated, and it was found that stars were about as bright as the Sun, when you account for the difference in distance. Also, chemical composition and surface temperature could be determined, and this added further evidence.


Sun, star around which Earth and the other components of the solar system revolve. It is the dominant body of the system, constituting more than 99 percent of its entire mass. The Sun is the source of an enormous amount of energy, a portion of which provides Earth with the light and heat necessary to support life.


The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things:
1. It must orbit a star (in our cosmic neighborhood, the Sun).
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun.


Moons — also called natural satellites — come in many shapes, sizes and types. They are generally solid bodies, and few have atmospheres. Most planetary moons probably formed from the discs of gas and dust circulating around planets in the early solar system.

Asteroid Belt

The asteroid belt is a region between the orbits of Mars and Jupiter that contains many asteroids—small, rocky bodies. Our solar system has about a million asteroids. When Jupiter formed 4.5 billion years ago, its massive gravity disturbed the orbits of nearby bodies, causing collisions. Most of the resulting fragments fell toward Jupiter, but many remain m the belt.

Asteroid Belt

Sometimes asteroids are knocked out of the belt, causing meteoroids (small pieces of asteroids in space). When meteoroids fall into Earth's atmosphere, they create bright streaks of light, which are often referred to as "shooting stars" (a misnomer). The pieces of meteoroids that survive the fall to the ground are called meteorites.

Interplanetary Medium

The space between the planets is called the Interplanetary medium. Its density is extremely low, and temperatures vary widely depending on proximity to the Sun.

Interplanetary Medium

The interplanetary medium contains dust, cosmic rays (high-energy radiation), and fast-Traveling charged particles. Earth's atmosphere and magnetic field protect us not only from pressure and temperature extremes, but also from harmful radiation.

Kuiper Belt

The Kuiper belt extends from the orbit of Neptune outward to approximately 8 billion kilometers away from the Sum It is a doughnut-shaped region that contains about a trillion short-period comets: which complete their orbit around the Sun relatively quickly (in less than 200 years).

Kuiper Belt

The Kuiper belt also contains hundreds of thousands of icy objects more than 100 kilometers wide, known as Kuiper belt objects (KBOs)_ KBOs include Pluto, Eris, Makemake, Haumea, and others and are likely remnants from the solar system's formation .

The Oort Cloud

The Oort cloud is a collection of trillions of comets and icy objects orbiting the Sun at the outermost reaches of our solar system. It is so far away that if the image above were to scale, its inner edge would be 27 meters away from the Sun.

The Oort Cloud

Unlike the Kuiper belt, the Oort cloud is spherical. Its so-called long-period comets travel outside of the ecliptic plane, with orbits so long that no such comet has ever been seen twice.


These are the rocky and airless leftovers from the formation of planets in our solar system. They mostly orbit our sun in the asteroid belt between Mars and Jupiter and range from the size of cars to dwarf planets.


Comets are dirty space snowballs of mostly ice and dust that formed during the birth of the solar system 4.6 billion years ago. Most comets have stable orbits in the outer reaches of the solar system past the planet Neptune.

Meteoroids, Meteors, Meteorites

Meteoroids are tiny asteroids or the broken-off crumbs of comets and sometimes planets. They range in size from a grain of sand to boulders 3 feet (1 meter) wide. When meteoroids collide with a planet's atmosphere, they become meteors. If those meteors survive the atmosphere and hit the planet's surface, their remains are called meteorites.

Space Prob

A probe is a spacecraft that travels through space to collect science information. Probes do not have astronauts. Probes send data back to Earth for scientists to study. Many probes study Earth or measure properties of space. Other probes use telescopes or other instruments to study planets, stars, and galaxies that are far away.

Space Prob

Probes that travel to other planets have changed from simple machines that could study a few features of a planet to sophisticated probes that travel great distances to study a wide range of features on planets, moons, asteroids and comets. We tend to call these more sophisticated probes spacecraft, orbiters, landers and rovers.

Voyager 1

One of the most famous probes is Voyager 1. It has traveled further in space than any human-made object. It launched into space in 1977. Voyager 1 flew past Jupiter and Saturn and then headed for the edge of our solar system. As of Feb. 1, 2010, the spacecraft was 16.8 billion kilometers (about 10.4 billion miles) from Earth.

Space Debris

Space debris is defunct human-made objects in space—principally in Earth orbit—which no longer serve a useful function.