Planetary system

A planetary system is a group of objects that are pulled together by gravity and move around a star. These objects can include planets, small planets called dwarf planets, rocks called asteroids, small natural moons, pieces of rock and ice called comets, and other tiny pieces called meteoroids. Our own system, called the Solar System, is one example. In it, Earth and seven other planets, along with many smaller objects, move around the Sun.

Scientists have found many other planetary systems beyond our own. As of 23 April 2026, they have confirmed 6,416 planets in 4,809 different systems. Some of these systems have more than one planet. These discoveries help us learn about how planets form and what other worlds might be like.

One exciting area of study is the habitable zone in planetary systems. This is a region where planets might have liquid water on their surfaces, which could mean they might be able to support life similar to what we find on Earth. Scientists are especially interested in finding and studying planets in these zones.

Sun Earth exoplanets planets gravitationally orbit star system dwarf planets asteroids natural satellites meteoroids comets planetesimals circumstellar disks Solar System Debris disks astrobiology habitable zone

Definition

The International Astronomical Union says a planetary system is made up of planets that go around one or more stars, brown dwarfs, or stellar remnants. Both the International Astronomical Union and NASA call the Solar System a planetary system.

Some other definitions say a planetary system includes every object that is pulled by the gravity of one or more stars.

History

Heliocentrism

Heliocentrism is a way of thinking about space where the Sun is at the center of everything, instead of Earth being at the center. This idea was first suggested a long time ago in Western philosophy and Greek astronomy by a person named Aristarchus of Samos, but most others did not agree with it.

Discovery of the Solar System

Main article: Discovery and exploration of the Solar System

A book called De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, was the first to show a way to predict how planets move around the Sun. Later, scientists like Galileo Galilei, Johannes Kepler, and Sir Isaac Newton studied physics and helped people understand that Earth moves around the Sun, just like the other planets.

Speculation on extrasolar planetary systems

A long time ago, a thinker named Giordano Bruno believed that stars, like the Sun, might also have planets around them. Later, Sir Isaac Newton also thought about this idea. Even though no one could prove it at the time, this idea stayed popular and was often used in stories, especially science fiction.

Detection of exoplanets

In 1992, the first planets outside our Solar System were found around a type of star called a pulsar named PSR B1257+12. Then, in 1995, a big planet was discovered orbiting a normal star called 51 Pegasi. Since then, more and more of these distant planets have been found, thanks to better ways to look for them and special space missions like the Kepler mission.

Origin and evolution

See also: Nebular hypothesis, Planetary migration, and Formation and evolution of the Solar System

Planetary systems start from protoplanetary disks that form around stars when they are born.

During this time, a lot of material gets pushed far away, and some planets might get thrown out completely, becoming rogue planets.

As stars change and grow bigger, they can sometimes pull in the closest planets. If a star is part of a system with another star, the changing mass can create new disks and new planets.

Free-floating planets can sometimes be caught by stars and placed into wide orbits. This is less likely around certain types of stars and can be affected by the environment around them.

System architectures

The Solar System has small rocky planets close to the Sun and large giant planets farther out. But other planetary systems can look very different. Right now, we know of few systems that look like ours, with small planets close in and a big gas planet with a steady orbit. More often, we find systems with several super-Earths — planets a bit bigger than Earth — very close to their star. Some systems have a hot Jupiter, which is a big gas planet very close to its star. Scientists think these big planets might move closer to their stars over time.

Planetary systems can be grouped by how their planets are arranged. In "Similar" systems, the planets are about the same size. In "Mixed" systems, the planets vary more in size. In "Anti-Ordered" systems, the big planets are close to the star and smaller ones are farther out — but we haven’t found any examples yet. In "Ordered" systems, small planets are close to the star and bigger ones are farther out, like in our Solar System.

Most known planets orbit stars like our Sun. These stars are called main-sequence stars. Some smaller stars, called red dwarfs, might have fewer big planets, but we have found some planets around them too. There are many kinds of planets, including hot Jupiters, which are big gas planets very close to their stars, and super-Earths, which are bigger than Earth but smaller than planets like Neptune. Super-Earths can be made of rock, gas, or even water.

Young stars often have rings of dust and gas around them, called circumstellar disks. Our Solar System has several of these, like the asteroid belt. Other stars have similar disks too. We also know of comets — icy objects that orbit stars — in our Solar System and have found evidence of them around other stars as well.

Planets can have different ways of moving around their stars. Some systems have planets that move in very stretched-out paths, unlike the nearly circular orbits in our Solar System. Sometimes, planets line up in ways that their orbits affect each other, which can change how they move over time.

Zones

Habitable zone

Main article: Habitable zone

The habitable zone is the area around a star where a planet could have the right temperature for liquid water to exist. It’s not too close to the star, where water would evaporate, and not too far, where water would freeze. The exact location of this zone changes based on the star’s size and age, as well as the planet’s atmosphere.

Scientists have found that about 22% of stars similar to our Sun might have planets just the right size in their habitable zones.

Venus zone

The Venus zone is the area around a star where a planet might have very hot conditions, like Venus, but not so close that the atmosphere disappears completely. Like the habitable zone, its position depends on the star’s type and the planet’s properties. Studies suggest that many stars could have planets in this zone, but more observations are needed to confirm their conditions.

Galactic distribution of planets

See also: Galactic habitable zone, Extragalactic planet, and Globular cluster § Planets

The Milky Way is very large, spanning 100,000 light-years. However, as of July 2014, most planets we know about are within about 2000 light years of Earth. Scientists use a method called microlensing to find planets much farther away, and a new telescope, the Nancy Grace Roman Space Telescope, might help us learn more.

Young stars, which have more metals, are more likely to have planets. The Sun is one such star. These stars usually have regular paths around the center of the Milky Way. Older stars, with fewer metals, are found in different parts of the galaxy, like near its center or in its outer areas.