The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud.
The Solar System is the gravitational bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets with the remainder being significantly smaller objects such as dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury.
The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with the majority of the remaining mass contained in Jupiter.
- The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal.
- The four outer planets are giant planets, being substantially more massive than the terrestrials.
- The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium.
- The two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane.
All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
- Planets 8
- Dwarf planets: 5
- Moons: Known = 149 | Provisional = 26 | Total = 175
- Comets: More than 3,400
- Asteroids: More than 715,000
- Known to the ancients: Mercury, Venus, Mars, Jupiter, and Saturn. These planets are visible to the naked eye and have been known since prehistoric times.
- Modern: Uranus (1781), Neptune (1846) and dwarf planet, Pluto (1930) were discovered only after the invention of the telescope. It was in 2006 when Pluto was reassigned as a dwarf planet.
- Solar system: Planets, small bodies, moons, regions and stars
The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.
- Sun: 99.85%
- Planets: 0.135% (our Earth is part of this share while Jupiter contains more than twice the matter of all the other planets combined)
- Comets: 0.01% ?
- Satellites: 0.00005%
- Minor Planets: 0.0000002% ?
- Meteoroids: 0.0000001% ?
- Interplanetary Medium: 0.0000001% ?
Consider the movement of the earth’s surface with respect to the planet’s center. The earth rotates once every 23 hours, 56 minutes and 4.09053 seconds, called the orbital period and its circumference is roughly 40,075 kilometers. Thus, the surface of the earth at the equator moves at a speed of 460 meters per second (1,656 kilometer per hour).
The earth is moving around our sun in a very nearly circular orbit. It covers this route at a speed of nearly 30 kilometers per second (110,000 kilometers per hour). In addition, our solar system – Sun, Earth and all that – moves through space at 20 kilometers per second (72,000 kilometers per hour) and whirls around the center of our galaxy at some 220 kilometers per second (792,000 kilometers per hour).
The galactic year, also known as a cosmic year (roughly 225 million earth years) is the duration of time required for the Sun to orbit once around the center of our Milky Way Galaxy. → Timeline of the universe and Earth’s history in galactic years.
The galaxies in our neighborhood are also rushing at a speed of nearly 1,000 kilometers per second towards a structure called the Great Attractor, a region of space roughly 150 million light-years (one light year is about 9,5 trillion kilometers) away from us. This Great Attractor, having a mass 100 quadrillion times greater than our sun and span of 500 million light-years, is made of both the visible matter (15%) that we can see along with the so-called dark matter (85%) that we cannot see.
Our whole galaxy is falling into something at freaking warp speed! But the Great Attractor itself is also moving in reference to other things…