Exoplanet

An exoplanet or extrasolar planet is a planet outside the Solar System.

The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. There are many planetary systems with exoplanets, and some have more than one planet.

There are many methods of detecting exoplanets, with transit photometry and Doppler spectroscopy being the most common. Scientists think that about 1 in 5 Sun-like stars may have an "Earth-sized" planet in the habitable zone, where liquid water could exist. This means there could be billions of possibly habitable planets in the Milky Way.

The least massive exoplanet known is Draugr, about twice the mass of the Moon. The most massive known is HR 2562 b, about 30 times the mass of Jupiter. Some exoplanets orbit their stars very quickly, in less than an hour, while others take thousands of years. The nearest exoplanets are 4.2 light-years from Earth, orbiting Proxima Centauri, the closest star to the Sun.

The discovery of exoplanets has increased interest in the search for extraterrestrial life, especially on planets in the habitable zone. The James Webb Space Telescope is helping scientists learn more about these distant worlds and their composition. There may also be many rogue planets in the Milky Way—planets not orbiting any star at all.

Definition

An exoplanet is a planet that goes around a star that is not our Sun. It is far from our Solar System. Scientists have rules to decide if something is an exoplanet. The object must be small and not shine by itself. It also has to go around a star.

Scientists use many ways to find and check these planets. They often need to look more than once to be sure they have found a real planet.

Nomenclature

Exoplanets are planets outside our Solar System. They have special names set by the International Astronomical Union. When naming a planet that orbits one star, we start with the star’s name and add a lowercase letter. The first planet found is called "b", the next one "c", and so on. If several planets are found at the same time, the one closest to the star gets the next letter. Some exoplanets have official names, but most are known by these letter names. There are also rules for naming planets that orbit two stars together.

History of detection

For a long time, scientists and writers wondered if planets existed beyond our Solar System. But there was no way to prove it. Many early claims of finding such planets were later shown to be wrong.

The first confirmed discovery of an exoplanet was in 1992. Two planets were found orbiting a fast-spinning star called a pulsar. The first planet found around a normal star like our Sun was discovered in 1995. Most exoplanets are found indirectly by watching how they affect their stars, but some have been imaged directly by telescopes.

Early speculations

In the past, many thinkers believed other stars might have planets like our Sun does. For example, the Italian philosopher Giordano Bruno thought that stars were like our Sun and probably had planets orbiting them.

Discredited claims

Many early claims of finding exoplanets turned out to be mistakes. For example, some scientists thought they found planets around stars like 61 Cygni or Barnard's Star, but later observations showed these claims were wrong.

Confirmed discoveries

The first confirmed exoplanet around a normal star was found in 1995 orbiting the star 51 Pegasi. This discovery started a new era of finding many more exoplanets. Since then, many exoplanets have been confirmed. These planets come in many sizes and orbits.

Candidate discoveries

Space telescopes like Kepler and TESS have found many possible planets that need more study to confirm. Some of these candidates are about the size of Earth.

Detection methods

Main article: Methods of detecting exoplanets

We find planets outside our Solar System, called exoplanets, in many ways. One way is direct imaging. Special cameras block the bright light of a star. This lets us see the dimmer light from a faraway planet. This works best for big planets far from their stars.

Most exoplanets are found using indirect methods. The transit method looks for small drops in a star's brightness when a planet passes in front of it. The radial velocity method finds a star's small wobble caused by a planet orbiting it. Other ways include watching changes in a star’s position over time (astrometry) or seeing how a star’s light bends when another star passes by (microlensing). Each way helps scientists find planets of different sizes and places.

Formation and evolution

See also: Accretion (astrophysics), Nebular hypothesis, and Planetary migration

Planets can form just a few million years after their star is born. We can study planets at different ages by looking at different planetary systems. This helps us see how planets change over time. Young planets often start with thick layers of gas, but these layers get smaller as the planets grow older. Even small planets might have been much larger before losing their gas layers. One example is Kepler-51b, a planet only twice the size of Earth but as big as Saturn when it was young.

Planet-hosting stars

Main article: Planet-hosting star

Scientists have found that there is about one planet for every star. About one in five stars like our Sun have a planet the size of Earth where water might be able to exist. Most of the planets we know are around stars like the Sun, but some are around smaller, cooler stars called red dwarfs. Space telescopes like Kepler have found many planets around these smaller stars. Stars with more heavy elements are more likely to have big planets, like Jupiter. Some planets orbit two stars at once, and a few have been found around systems with three or four stars.

Orbital and physical parameters

Scientists study how far exoplanets are from their stars and how big they are. These details help us learn about what these planets might be like. By looking at these measurements, researchers can guess if an exoplanet could maybe support life or what the weather there might be like.

General features

The brightness of an exoplanet — a planet outside our Solar System — depends on how far it is from us, how shiny it is, and how much light it gets from its star. Planets that are closer to their stars or less shiny can look brighter than ones that are farther away and very shiny.

In 2013, scientists found the color of an exoplanet for the first time. The planet HD 189733b looks deep dark blue, while another, GJ 504 b, looks magenta. Some exoplanets, like TrES-2b, reflect almost no light, making them darker than coal.

Surface

We study the surfaces of exoplanets by looking at different kinds of light signals. Scientists use special tools to learn if a planet is made of rock or gas.

We can guess how hot an exoplanet is by how much light it gets from its star. For example, the planet OGLE-2005-BLG-390Lb is thought to be very cold. But these guesses can be hard because we don’t always know how much light the planet reflects. Some planets, like HD 189733b, have had their temperatures measured by watching changes in infrared light.

Habitability

See also: Astrobiology, Circumstellar habitable zone, and Planetary habitability

Scientists look for planets beyond our solar system that might be able to support life. For life as we know it, a planet needs liquid water. This means the planet must be at the right distance from its star—not too hot and not too cold—so water can stay liquid. This special distance is called the habitable zone.

The habitable zone can change depending on the type of star. Some planets might stay warm enough for liquid water even if they are far from their star, because of things like thick atmospheres or tidal heating, which makes heat through gravitational forces. Scientists have found several planets in habitable zones, such as Kepler-62f, Kepler-186f, and Kepler-442b, but we are still learning if these planets could truly support life.

Notable Exoplanets – Kepler Space Telescope
Comparison of small planets found by Kepler in the habitable zone of their host stars.

Planetary system

Main article: Planetary system

Exoplanets often belong to systems with multiple planets orbiting the same star. These planets can pull on each other with their gravity. This can create special patterns in how they move.

For example, the Kepler-223 system has four planets. Their orbits fit into a simple pattern of 8:6:4:3.

Some big planets, called hot Jupiters, orbit their stars in the opposite direction of the star's own spin. Scientists think these planets may form in crowded areas where the gravity of nearby stars can change their paths.

Search projects

Many projects help scientists find exoplanets. CoRoT is a space telescope that found the first rocky planet moving in front of its star. Kepler was a mission that looked for many exoplanets. It watched for stars that get dimmer when a planet passes in front of them.

TESS is another mission that searched for new exoplanets. It looked at stars all over the sky from 2018 to 2020 and found many possible exoplanets. Other tools like ESPRESSO and HARPS are powerful instruments on big telescopes that help study these distant worlds.