Planetary system

A planetary system is a collection of non-stellar bodies that are gravitationally bound to a star or a star system.

These bodies move in orbits around their parent star. While planets are the most famous members, these systems often contain many other objects. These can include dwarf planets, asteroids, comets, and natural satellites. They may also contain planetesimals and circumstellar disks of dust and gas. The International Astronomical Union (IAU) defines a planetary system as planets orbiting one or more stars, brown dwarfs, or stellar remnants. Our own Solar System is a primary example. It consists of the Sun and all the objects bound to it by gravity.

Most planetary systems begin their lives within protoplanetary disks.

These disks form from material surrounding a newly forming star. As the star grows, gravity pulls material together within the disk. This process can lead to the creation of planets. However, the process is often chaotic. During formation, much material is scattered into distant orbits. Some planets are even ejected from the system entirely. These lonely, wandering worlds are known as rogue planets.

Systems can change drastically as their stars age. High-mass stars eventually end their lives in supernova explosions. These massive blasts can destroy a pre-existing planetary system. Planets might evaporate or be pushed out of their orbits by escaping gas. If the central star loses most of its mass, planets may escape its gravitational hold. Some planets are found orbiting pulsars, which are the remnants of high-mass stars. These planets might form from material that failed to escape the supernova. They could also form from an accretion disk of fallback matter.

Lower-mass stars create different kinds of systems. These stars tend to have less material in their protoplanetary disks. Because of this, it is unlikely for them to form gas giants. Instead, their systems often consist of rocky or ice-based bodies. These systems are also typically compact. This happens because lower-mass stars have lower temperatures. This causes protoplanets to form closer to the star. As stars evolve into red giants, they can even engulf their inner planets. If a star loses mass, planets that are not swallowed will move further away.

Human understanding of these systems has evolved over centuries. In the 3rd century BC, Aristarchus of Samos proposed heliocentrism. This is the model where the Sun is at the center. This was the opposite of geocentrism, which placed Earth at the center. In 1543, Nicolaus Copernicus published a mathematically predictive heliocentric model.

Later, scientists like Galileo Galilei and Isaac Newton helped prove these ideas. Newton showed that the same physical laws govern both Earth and the planets. In the 16th century, Giordano Bruno suggested that other stars were also suns with their own planets. This idea was quite controversial at the time.

We only began confirming the existence of exoplanets recently. An exoplanet is a planet orbiting a star other than our Sun. The first confirmed detection was in 1992. These were terrestrial-mass planets orbiting the pulsar PSR B1257+12. In 1995, astronomers found the first exoplanet orbiting a main-sequence star. This was a giant planet named 51 Pegasi b. It orbits its star in only four days. Since then, missions like Kepler have greatly increased our detections.

Scientists also look for the habitable zone. This is the region where liquid water could exist on a planet's surface.

Planetary systems have different layouts, called architectures. We can classify these into four main groups based on planet mass. "Similar" systems are the most common in our galaxy. In these, planets are similar in size and mass. They are often called "peas in a pod" because they display packing. This means smaller planets stay closer together and larger planets have more space.

"Mixed" systems show large variations in mass between planets. "Anti-ordered" systems would have massive planets close to the star, but none are known. Finally, "ordered" systems have mass increasing with distance from the star. Our Solar System is an ordered system.