Our solar system consists of a star of average size and luminosity we call the Sun, the planets (in order of their distance from the Sun) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Kuiper Belt objects (e.g. Pluto), the satellites or moons of the planets, numerous comets, asteroids, meteoroids and the interplanetary medium.

The planets, most of the satellites of the planets and the asteroids revolve around the Sun in the same direction (counterclockwise), in nearly circular orbits (ellipses, but close to circles). When looking down from above the Sun's north pole, the planets orbit in a counter-clockwise direction. A new class of dwarf planets was added in 2006, these objects are found mostly in the Asteroid Belt (like Ceres) and the Kuiper Belt (like Pluto, Haumea, Makemake and Eris).

The planets orbit the Sun in or near the same plane, called the ecliptic. Pluto is a special dwarf planet in that its orbit is the most highly inclined (18 degrees) and the most highly elliptical of all the planets.

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.

Planet Categories:

Terrestrial Planets:

The four primary terrestrial worlds are the innermost planets in the solar system, Mercury, Venus, Earth and Mars. There are an additional 8 other terrestrial worlds; the Moon, Io, Europa, Ganymede, Callisto (the four Galilean moons), Titan (a moon of Saturn), Triton (a moon of Neptune) and Pluto.

They are called terrestrial because they have a compact, rocky surface like the Earth's and are spherical in shape. The other moons are not spherical and are more asteroid-like (i.e. irregular). Venus, Earth, Mars and Titan have significant atmospheres, the rest have little to zero.

This category has the following 2 subcategories:

Gas giant planets:

Also called Jovian (Jupiter-like) Planets, Jupiter, Saturn, Uranus, and Neptune, because they are all similar in size and structure, i.e. gigantic compared with Earth and having a non-solid, gaseous nature.

The Jovian planets are also referred to as the gas giants, although all of them might have small solid cores below their thick atmospheres.

This category has the following 3 subcategories:

Circumbinary planets:

A circumbinary planet is a planet that orbits two stars instead of one.

The Scale diagram above shows the Kepler-16 system, from Doyle et al. (2011). Stars marked with A and B, planet marked with b. System centre of mass marked with +.

Pulsar planets:

Pulsar planets are planets that are found orbiting pulsars, or rapidly rotating neutron stars.

The Schematic view above shows the Pulsar-Planet system PSR J1719-1438 showing the pulsar with 5.7 ms rotation period in the centre, and the orbit of the planet in comparison to the size of the sun (marked in yellow).

Hypothetical planets:

Any hypothetical astronomical object is an astronomical object (such as a star, planet or moon) that is believed or speculated to exist but whose existence has not been scientifically proven.

The Illustration above shows Tyche, a Hypothetical Planet That Could Exist In the Oort Cloud in our Solar System

This category has the following 4 subcategories:

Planet vs. Moon:

A planet (this definition applies only to the Solar System) revolves around the Sun. A moon or satellite revolves around a planet. The term planet or moon is not selected by mass or size of the body.


The period of revolution of a planet is determined by timing and astrometry (the science of measuring stellar and planetary positions). Periods of rotation are determined by either

  1. timing surface features
  2. timing clouds and atmospheric features
  3. reflected sunlight (light curves)
  4. Doppler radar measurements of planet limb

Note that timing atmospheric features usually reveals that Jovian planets have differential rotation (meaning their equators rotate faster than the pole regions, i.e. the planet is not solid).

Information about the planets is obtained by:


There are several moons in the Solar System which are larger than the planet Mercury for instance. Neither are the moons less exciting than the planets. Some of them have volcanos, atmospheres, and even quite possibly liquid water oceans.

Example: The Saturn's moon called Titan, which happens to be the second largest moon in the entire solar system, is the only other planetary body in the solar system apart from Earth on which clear evidence of liquid bodies on the surface has been found. In the image above, Titan is compared in scale with Earth and it's Moon.

In the Solar System, neither Mercury nor Venus have any moons.

Moons by Planet:

Solar System Formation:

Any model of Solar System formation must explain the following facts:

  1. All the orbits of the planets are prograde (i.e. if seen from above the North pole of the Sun they all revolve in a counter-clockwise direction).
  2. All the planets have orbital planes that are inclined by less than 6 degrees with respect to each other (i.e. all in the same plane).
  3. All the planets have low eccentricities.
  4. All planets have prograde rotation except Venus and Uranus.

Other details for any formation theory:

T=rocky and small <-> J=gaseous and large

Any theory of Solar System formation must explain the following properties and why there is a separation of the planets into two types

Current Theory:

Scientists currently believe that the solar system was formed when a cloud of gas and dust in space was disturbed, maybe by the explosion of a nearby star (called a supernova). This explosion made waves in space which squeezed the cloud of gas and dust. Squeezing made the cloud start to collapse, as gravity pulled the gas and dust together, forming a solar nebula. Just like a dancer that spins faster as she pulls in her arms, the cloud began to spin as it collapsed.

Eventually, the cloud grew hotter and denser in the center, with a disk of gas and dust surrounding it that was hot in the center but cool at the edges. As the disk got thinner and thinner, particles began to stick together and form clumps. Some clumps got bigger, as particles and small clumps stuck to them, eventually forming planets or moons .

Near the center of the cloud, where planets like Earth formed, only rocky material could stand the great heat. Icy matter settled in the outer regions of the disk along with rocky material, where the giant planets like Jupiter formed. As the cloud continued to fall in, the center eventually got so hot that it became a star, the Sun, and blew most of the gas and dust of the new solar system with a strong stellar wind.

By studying meteorites, which are thought to be left over from this early phase of the solar system, scientists have found that the solar system is about 4600 million years old!

Common Elements/Compounds in the Solar System:

The most common elements on each planet tell us something about its formation process and later evolution. For example, small planets have gravitational fields that are too weak to maintain light elements such as H and He. Jovian worlds are cold and have many compounds as ice rather than gas or liquids.

And about Pluto?

Pluto may be the current most intriguing object in the Solar System. It is Tiny, it is dense, it is very very far way from us. So, is Pluto a Planet or not?


The first notice from Pluto was made in the 1840s. Using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus.

There are many unknowingly photographs of Pluto from 1909 to 1916, with the oldest being made by the Yerkes Observatory.

The official Pluto's discovery was announced on March 13, 1930, by by Clyde Tombaugh at the Lowell Observatory.

The object was officially named on March 24, 1930. It was an unanimous decision from the members of the Lowell Observatory. The name Pluto was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England.

For a long period, Pluto was known as the smallest planet in the solar system and the ninth planet from the sun.

Why Is Pluto Not Called a Planet Anymore?

In 2003, an astronomer saw a new object beyond Pluto. The astronomer thought he had found a new planet. The object he saw was larger than Pluto. He named the object Eris (EER-is).

Finding Eris caused other astronomers to talk about what makes a planet a "planet". There is a group of astronomers that names objects in space. This group decided that Pluto was not really a planet because of its size and location in space. So Pluto and objects like it are now called dwarf planets.

Pluto is also called a plutoid. A plutoid is a dwarf planet that is farther out in space than the planet Neptune. The four known plutoids are Pluto, Haumea, Makemake (MAH-kee-MAH-kee) and Eris. Astronomers use telescopes to discover new objects like plutoids.

Scientists are learning more about the universe and Earth's place in it. What they learn may cause them to think about how objects like planets are grouped. Scientists group objects that are like each other to better understand them. Learning more about faraway objects in the solar system is helping astronomers learn more about what it means to be a planet.

What Is Pluto Like?

Pluto is very, very cold. The temperature on Pluto is 375 to 400 degrees below zero. Pluto is so far away from Earth that scientists know very little about what it is like. Pluto is probably covered with ice.

Pluto has about one-fifteenth the gravity of Earth. A person who weighs 100 pounds on Earth would weigh only 7 pounds on Pluto.

Most planets orbit the sun in a near-circle. The sun is in the center of the circle. But Pluto does not orbit in a circle! The orbit of Pluto is shaped like an oval. And the sun is not in the center. Pluto's orbit is also tilted.

Pluto's orbit and the ecliptic.

Orbit of Pluto—ecliptic view. This 'side view' of Pluto's orbit (in red) shows its large inclination to Earth's ecliptic orbital plane.


The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of rocky asteroids, and four gas giants surrounded by the Kuiper belt of icy objects.

Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four gas giants.

Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. And more Recently The Asteroid Belt is considered the line that separates the inner and outer solar system.