Planetary system

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

Scientists have found many planetary systems beyond our own. 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. 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 orbit 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 pulled by the gravity of one or more stars.

History

Heliocentrism

Heliocentrism is an idea about space where the Sun is at the center, not Earth. This idea was first suggested a long time ago in Western philosophy and Greek astronomy by a person named Aristarchus of Samos, but not many people agreed 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 then, 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 begin from protoplanetary disks that form around stars when they are born.

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

As stars change and grow, they can sometimes pull in the closest planets. If a star is near 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 some types of stars and can be affected by their surroundings.

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.

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 "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.

Young stars often have rings of dust and gas around them. Our Solar System has several of these, like the asteroid belt. Other stars have similar rings 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.

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. It’s not too close to the star, and not too far away. The exact place of this zone changes based on the star’s size and age, and the planet’s atmosphere.

Venus zone

The Venus zone is the area around a star where a planet might have very hot conditions, like Venus. Like the habitable zone, its place 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.

Galactic distribution of planets

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

The Milky Way is very big, about 100,000 light-years across. Most of the planets we know are within 2000 light years of Earth. Scientists use a method called microlensing to find planets far away. A new telescope, the Nancy Grace Roman Space Telescope, may help us learn more.

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