Exoplanet facts for kids

Four exoplanets of the HR 8799 system imaged by the W. M. Keck Observatory over the course of seven years. Motion is interpolated from annual observations.

Comparison of the size of exoplanets orbiting Kepler-37 to Mercury, Mars and Earth

An exoplanet or extrasolar planet is a planet that orbits a star outside our own Solar System. Thousands of these distant alien worlds have been found across space. The very first confirmed exoplanet was discovered in 1992 around an unusual dying star called a pulsar. Just a few years later, in 1995, astronomers spotted the first planet orbiting a normal main-sequence star like our Sun. A different world, first spotted in 1988, was fully confirmed in 2003. Scientists even realized in 2016 that a glass photo plate from 1917 held hidden evidence of an exoplanet system, an early clue called a precovery. As of 25 April 2024, there are 5,616 confirmed exoplanets in 4,174 planetary systems, with 948 systems having more than one planet.

Astronomers use clever methods of detecting exoplanets because these worlds are too far away to visit with spacecraft. Techniques such as transit photometry and Doppler spectroscopy have found thousands of them. Scientists calculate that about one in five Sun-like stars has an Earth-sized neighbor inside its habitable zone. Because our Milky Way galaxy holds around 200 billion stars, there could be 11 billion small rocky worlds at comfortable distances. That number rises to roughly 40 billion if we count planets around small red dwarf stars!

Exoplanets come in unbelievable shapes, weights, and sizes. The least massive exoplanet known is called Draugr, which weighs only about twice as much as our Moon. On the other end of the scale, giant worlds listed in the NASA Exoplanet Archive like HR 2562 b can weigh nearly 30 times as much as Jupiter. An object that heavy might undergo nuclear fusion of deuterium, meaning it could actually be a "failed star" known as a brown dwarf. Some exoplanets speed around their sun in less than an hour, while others take thousands of years.

The nearest exoplanets are only 4.2 light-years away around Proxima Centauri, the closest stellar neighbor to our Sun. Astronomers have even found clues pointing to extragalactic planets located far away in completely different galaxies. The hunt for these worlds has boosted the exciting search for extraterrestrial life. Everyone wants to know if life exists beyond Earth, especially on worlds with liquid water. Powerful space telescopes like the James Webb Space Telescope study each planet's atmosphere and environmental conditions to search for clues of planetary habitability. Some worlds, called rogue planets, do not even orbit a star. These lonely wanderers roam the galaxy entirely on their own!

What Is an Exoplanet?

Exoplanet HIP 65426b is the first discovered planet around star HIP 65426.

The official definition of the term planet created by the International Astronomical Union (IAU) originally applied only to bodies inside our own Solar System. Because astronomers kept finding strange objects around other stars, the IAU created working guidelines to decide what counts as an exoplanet. Under these rules, a true exoplanet must orbit a star, a brown dwarf, or a stellar remnant. It also needs to have a true mass below 13 Jupiter masses.

When a giant object grows heavier than 13 times the mass of Jupiter, the immense heat and pressure inside its center ignite thermonuclear fusion of deuterium. When this happens, astronomers classify the object as a brown dwarf instead of a planet. The IAU rules also explain that free-floating objects in space that lack a host star are called sub-brown dwarfs or rogue worlds. Furthermore, the object must be small enough compared to its host star to remain in a stable path, avoiding gravitational instability.

Some astronomers prefer different rules to classify planets. Instead of just checking weight, they look at how the object formed in the first place. Giant planets typically grow inside a swirling disk of gas and dust through core accretion. During this process, a rocky core gathers huge amounts of gas until it swells into a giant world. On the other hand, brown dwarfs collapse directly out of giant gas clouds, just like regular stars do. Because these two processes can overlap, some scientists prefer to set different mass limits.

Different astronomical groups use slightly different boundary lines. The Extrasolar Planets Encyclopaedia has listed objects up to 60 Jupiter masses. The Exoplanet Data Explorer includes candidate worlds up to 24 Jupiter masses. Meanwhile, the official NASA Exoplanet Archive keeps objects that measure up to 30 Jupiter masses. Other researchers suggest that the true boundary depends on whether internal pressure comes from Coulomb pressure or electron degeneracy pressure. Despite these debates, any world that circles a distant star without shining like a star fits the general spirit of an exoplanet!

How Exoplanets Get Their Names

Naming an exoplanet follows a neat set of rules created by the International Astronomical Union. The name starts with the official catalog name of the host star, followed by a lowercase letter. The host star itself is considered letter "a," so the very first planet discovered in the system receives the letter "b." For example, the first world found around the star HIP 65426 is called HIP 65426b.

If astronomers find more planets circling the exact same star later on, they assign the next letters in the alphabet, such as "c," "d," and "e." If several worlds are discovered on the exact same day, letters are given based on how close they are to the host star. A planet that orbits two stars at once is called a circumbinary planet. A few special exoplanets have even received fun, official nicknames through public voting contests run by the IAU, known as the IAU-sanctioned proper names.

History of Exoplanet Discoveries

NASA graphic of present and future exoplanet missions.

Long ago, ancient thinkers wondered if other solar systems existed in deep space. During the 16th century, an Italian philosopher named Giordano Bruno supported the Copernican model of heliocentrism. Bruno boldly suggested that the stars in the night sky were actually distant suns surrounded by their own families of planets. In the 18th century, the famous scientist Isaac Newton wrote similar ideas in the General Scholium of his masterwork, the Principia. Newton reasoned that distant stars might naturally form planetary systems like our own.

Finding real evidence took centuries of technological progress. In 1855, an astronomer named William Stephen Jacob at the East India Company's Madras Observatory noticed strange wobbles in the binary star system 70 Ophiuchi. He believed a hidden planet caused the shaking. Later in the 1890s, Thomas Jefferson Jackson See from the University of Chicago and the United States Naval Observatory claimed the wobbles proved a dark companion orbited the star. However, scientist Forest Ray Moulton proved that such an orbit would quickly tear the system apart.

During the 20th century, astronomers made many exciting claims that turned out to be mistakes. In 1942, Kaj Aage Gunnar Strand at Sproul Observatory thought he detected a giant planet around the star 61 Cygni. Similar claims were made about a red dwarf star named Lalande 21185. Between the 1950s and 1960s, astronomer Peter van de Kamp at Swarthmore College announced planets around Barnard's Star. Careful checks later showed that tiny flaws in telescope lenses caused these false signals. In 1991, Andrew Lyne and Matthew Bailes reported a planet around PSR 1829-10, but they bravely retracted the claim after discovering a math error.

Real success arrived at last in the late 1980s and early 1990s. In 1988, Canadian scientists Bruce Campbell, Gordon Walker, and Stephenson Yang from the University of Victoria and the University of British Columbia noticed radial-velocity wobbles around Gamma Cephei. Their discovery was finally confirmed in 2003. The first undeniable confirmation came on January 9, 1992, when astronomers Aleksander Wolszczan and Dale Frail discovered two small rocky worlds orbiting the millisecond pulsar PSR B1257+12. These rare pulsar planets likely formed from debris left behind by a giant supernova explosion.

Exoplanet detections per year.

The modern era of astronomy truly began on October 6, 1995. Scientists Michel Mayor and Didier Queloz from the University of Geneva announced the discovery of 51 Pegasi b at the Observatoire de Haute-Provence. This world orbited a normal G-type star named 51 Pegasi every four days. It was a scorching gas giant, earning the nickname hot Jupiter. This discovery proved that planets exist around stars like our Sun, earning both scientists a share of the 2019 Nobel Prize in Physics.

Soon, astronomers spotted multi-planet systems around stars like Upsilon Andromedae and two-star systems like Kepler-16. NASA launched dedicated space observatories like the Kepler space telescope to hunt for thousands of new worlds. Kepler discovered famous worlds like Kepler-452b and showed that small rocky planets are common across the galaxy. Telescopes like the Chandra X-ray Observatory even searched beyond our galaxy, finding candidate worlds like M51-ULS-1b in the famous Whirlpool Galaxy.

How Astronomers Find Alien Worlds

Directly imaged planet Beta Pictoris b, with an edge-on orbit as seen from Earth

Planets do not shine with their own nuclear fire, making them millions or billions of times fainter than their host stars. Glare from the central star washes out the planet, making it hard to see directly. To overcome this hurdle, astronomers have developed brilliant direct and indirect detection methods.

Direct Imaging

Direct imaging means taking an actual photograph of an exoplanet. To do this, astronomers must block the blinding glare of the host star. They use special tools called coronagraphs inside powerful telescopes. For instance, the Hale Telescope used a vector vortex coronagraph to photograph three giant planets circling the star HR 8799. Instruments like the Gemini Planet Imager, VLT-SPHERE on the Very Large Telescope, and SCExAO capture images of huge gas giants. Because direct imaging requires extreme optothermal stability, it works best for giant planets orbiting far from their host stars.

Indirect Methods

Because planets are so difficult to photograph, most worlds are discovered using clever indirect techniques:

When a planet passes in front of its star, the star's light dims slightly.

• The Transit Method: When a planet passes directly between its star and Earth, it blocks a tiny fraction of starlight. This event is called an Astronomical transit. Telescopes measure the star dimming on a chart called a light curve. The bigger the planet, the more light it blocks. NASA's Kepler mission used this technique to find thousands of candidate worlds.
• Radial Velocity (The Wobble Method): As a planet circles a star, its gravity pulls the star back and forth in a tiny circle. Astronomers track this motion using Doppler spectroscopy. When the star moves toward Earth, its spectral lines shift toward the blue end of the spectrum due to the Doppler effect. When it pulls away, the light shifts toward the red end.
• Transit Timing Variations: In systems with several worlds, the gravity of one planet tugs on another, speeding it up or slowing it down. In the Kepler-19b system, these small timing changes revealed a hidden sister planet named Kepler-19c.
• Gravitational Microlensing: Albert Einstein showed that gravity bends light like a giant glass lens. When a star passes in front of a more distant background star, its gravity magnifies the background light. If the foreground star hosts an orbiting planet, the planet's gravity creates an extra spike in brightness. This method can spot worlds located several astronomical units away from their stars.
• Astrometry: This technique involves measuring a star's position in the sky with extreme precision. Astronomers watch for tiny side-to-side wobbles caused by an orbiting planet's gravitational pull.
• Pulsar Timing: A spinning pulsar flashes radio waves toward Earth like a cosmic lighthouse. If a planet circles the pulsar, the star moves back and forth, altering the arrival time of each pulse. This tiny time shift is called a Roemer delay.
• Debris Disks: Dusty disks created by colliding comets and asteroids absorb starlight and glow in infrared light. Gaps, warps, and clumps within these dusty rings reveal where hidden planets are gravitationally clearing their paths.

How Planetary Systems Form and Grow

The Morgan–Keenan stellar spectral classification

Artist's impression of an exoplanet orbiting two stars.

Planets are born inside a swirling cloud of gas and dust called a protoplanetary disk, which surrounds a newborn star. Over millions of years, tiny grains of dust collide and stick together. They build pebbles, boulders, and giant rocky cores. Giant worlds sweep up thick blankets of hydrogen and helium gas before the disk blows away.

Young planets change dramatically as they mature. A young world may begin with a puffed-up atmosphere that gradually shrinks as it cools. For instance, the young exoplanet Kepler-51b has twice the mass of Earth, yet its puffy atmosphere makes it almost as large as Saturn! Over time, intense starlight strips away light gases, leaving behind denser planets.

Types of Planet-Hosting Stars

Exoplanets orbit many different types of stars. Most known worlds circle main-sequence stars with spectral categories F, G, or K. G-type stars include yellow stars like our Sun. K-type stars are cooler orange dwarfs, while F-type stars are hotter and white.

Astronomers have also discovered many planets around cool, small red dwarf stars, known as M-type stars. Red dwarfs are the most common stars in our galaxy. Data shows that a star's metallicity—the amount of elements heavier than hydrogen and helium it contains—plays a big role in planet formation. Stars with high metallicity are far more likely to build giant, Jupiter-sized planets. Exoplanets can also orbit binary pairs, like worlds in the Kepler-64 four-star system!

Strange and Extreme Features of Alien Worlds

This color–color diagram compares Solar System worlds to HD 189733b. Silicate droplets scatter blue light in its atmosphere.

Because exoplanets form under different conditions, they display wild and surprising characteristics that look nothing like our own neighborhood.

Colors and Reflectivity

Every planet has an albedo, which measures how much starlight its surface and clouds reflect. In 2013, astronomers calculated the true visible color of an exoplanet for the first time. The giant planet HD 189733b has a deep cobalt-blue color! However, this blue color does not come from oceans. Instead, high winds whip droplets of molten silicate glass through its atmosphere, scattering blue light. Other worlds look equally strange: GJ 504 b glows with a gentle magenta-pink color, while Kappa Andromedae b appears fiery red.

Some exoplanets are exceptionally dark. The scorching world TrES-2b is known as the darkest planet ever discovered. Its geometric albedo is so low that it reflects less than 1% of incoming starlight, making it darker than coal or black acrylic paint! Chemical compounds like vaporized sodium and potassium in its atmosphere soak up light instead of reflecting it.

Magnetic Fields

Earth's magnetic field protects our atmosphere from harsh space weather, generated by liquid iron swirling inside our planet's core. In 2014, astronomers detected the first magnetic field on an exoplanet around HD 209458 b by watching escaping clouds of hydrogen. Scientists have even found signs of a magnetic shield around a rocky planet in the YZ Ceti system.

Magnetic fields can make planets swell in size. When the fierce stellar wind strikes a planet's magnetosphere, strong electric currents flow through the planet. This process, known as Joule heating, pumps energy into the world, puffing up its radius. Inside giant worlds, crushing pressures squeeze hydrogen into liquid metallic hydrogen, a super-conductive fluid that powers enormous magnetic fields. Scientists look for planetary auroras using sensitive instruments like the LOFAR radio telescope.

Volcanoes and Geology

Some rocky worlds experience intense volcanic activity. The blazing super-Earth 55 Cancri e undergoes massive temperature swings. Astronomers suspect giant volcanic eruptions blast thick clouds of ash into the sky, blocking heat from escaping. On other planets, gravitational tugs from neighboring worlds cause tidal heating, melting the interior and fueling surface volcanoes.

Scientists also study whether large rocky planets have moving plate tectonics. Plate motions help regulate a planet's climate over millions of years by recycling carbon dioxide between the atmosphere and mantle. Some planets might be completely covered in endless oceans. These ocean worlds have so much water that rocky land never pokes above the waves.

Rings and Moons

Clear versus cloudy skies on two different exoplanets.

In our own Solar System, Saturn's rings are famous for their beauty. Extrasolar worlds can have rings too! In 2007, a distant body named J1407b was found with an enormous ring system hundreds of times larger than Saturn's. Another planet, HIP 41378 f, appears much larger than it should be for its mass, which suggests wide rings might be puffing up its silhouette.

Moons that orbit exoplanets are called exomoons. Although scientists have found candidates around planets like Kepler-1625b, none have been confirmed with certainty yet. In hot Jupiter systems like WASP-76b and WASP-49b, clouds of sodium gas might be blasted into space by volcanic moons similar to Jupiter's moon Io.

Alien Atmospheres

Studying sunsets on Saturn's moon Titan with Cassini helps scientists understand alien worlds.

Astronomers study alien air by watching starlight filter through a planet's atmospheric edges during a transit, a method called transmission spectroscopy. The first exoplanet atmosphere ever detected belonged to HD 209458 b in 2001. Since then, telescopes have spotted water vapor, methane, and carbon dioxide around dozens of distant worlds, including the giant world Tau Boötis b.

Some planets are so close to their parent stars that their atmospheres boil away into space! The small rocky world Kepler-1520b is evaporating, leaving behind a dusty tail like a giant comet. Similarly, the warm Neptune-sized planet GJ 436 b is trailed by a glowing cloud of hydrogen gas millions of miles long.

Worlds that are tidally locked show unusual weather patterns. One side permanently faces the star in perpetual daylight, while the other side faces cold, permanent night. These "eyeball planets" feature a scorching hotspot on the daytime side, surrounded by rings of ice on the dark side.

The Search for Habitable Worlds

potentials rocky worlds plotted across different star types within their habitable zones.

The discovery of distant worlds has supercharged the field of exoplanetology and astrobiology. Scientists want to answer one of humanity's biggest questions: does life exist beyond Earth? To find living organisms from light-years away, astronomers look for a biosignature. This is a chemical sign, like oxygen or ozone produced by photosynthesis, that shows life has transformed the planet's atmosphere.

The Goldilocks Zone

For living things to thrive, a planet must have the right conditions for liquid water on its surface. The area around a star where temperatures allow liquid water to pool without freezing solid or boiling away is called the habitable zone. Because it is neither too hot nor too cold, astronomers often nickname it the "Goldilocks zone."

The size and distance of this zone depends on the host star:

• Hot, bright stars: The habitable zone sits much farther out because the star pumps out huge amounts of energy.
• Cool red dwarf stars: The habitable zone is located very close to the star.
• Atmospheric effects: A planet's atmosphere also matters. A dry desert planet with little water vapor has a weaker greenhouse effect, allowing it to sit closer to its star without overheating. Rocky planets with thick hydrogen envelopes can retain heat much farther out.

Potentially Habitable Worlds

Astronomers have identified several rocky planets that orbit inside their star's habitable zone. In 2014, scientists announced the discovery of Kepler-186f, a rocky planet only 1.2 times the radius of Earth orbiting inside the sweet spot of its red dwarf star. Another exciting world is Kepler-442b, located over 1,100 light-years away. Closer to home, Proxima Centauri b circles the nearest star to our Sun at a distance of just 4.2 light-years. Telescopes like TESS have added worlds like TOI-700 d to this growing list.

Notable Exoplanets – Kepler Space Telescope
Small planets found by Kepler in the habitable zones of their host stars.

Challenges for Life on Alien Worlds

Living in the habitable zone does not guarantee that a planet is completely comfortable. Many factors can make a world hostile to life:

• Snowball States: If a planet cools down, growing ice sheets reflect more starlight into space. This cooling cycle can freeze the entire world into an ice age. Only volcanic carbon dioxide gas can produce enough greenhouse effect warming to melt the planet again.
• Tidal Locking: Close-in worlds around red dwarfs often lock into synchronous rotation, leaving one side blistering hot and the other side frozen solid.
• Fierce Stellar Flares: Young red dwarf stars blast powerful ultraviolet radiation and flares into space, which can strip away a planet's protective ozone layer.
• Tidal Heating: Strong gravitational pulls can trigger tidal heating, turning an otherwise pleasant world into a volcanic wasteland.

Great Exoplanet Telescopes and Missions

Plot showing exoplanet populations discovered by various missions.

Discovering thousands of alien worlds has required an international fleet of telescopes based on mountain peaks and in space:

• Kepler: NASA's legendary space telescope that stared at over 150,000 stars to find transiting planets. Kepler proved that small rocky worlds are common throughout the galaxy.
• TESS: A NASA satellite launched to scan nearby bright stars across the entire sky. TESS has cataloged thousands of exciting new planetary candidates.
• CoRoT: A pioneering space mission led by the French Space Agency that detected the first transiting rocky world.
• HARPS: A famous high-precision spectrograph mounted on the ESO 3.6 m Telescope at La Silla Observatory in Chile. It uses the radial-velocity technique to detect wobbling stars.
• ESPRESSO: An advanced spectrograph installed on the giant Very Large Telescope at Cerro Paranal in Chile's Atacama Desert, built to search for Earth-sized worlds.
• ANDES: A powerful instrument planned for the massive Extremely Large Telescope, designed to inspect the atmospheres of alien worlds for signs of habitability.

Images for kids

• 

  A direct image of star AB Pictoris with its faint companion object.

• 

  Direct image showing three exoplanets orbiting the star HR 8799.

• 

  A giant planet or sub-brown dwarf orbiting the brown dwarf 2MASS J044144.

• 

  Artist concept showing a temperature inversion in the sky of hot Jupiter WASP-33b.

• 

  Chart showing known exoplanet sizes and orbital periods.

• 

  Diagram explaining the common sizes of small exoplanets.

• 

  Habitable zone exoplanets discovered by NASA's Kepler mission.

• 

  Animation showing how an orbiting planet tugs its parent star back and forth.

See also

In Spanish: Planeta extrasolar para niños

• Detecting Earth from distant star-based systems
• Extrasolar Planets Encyclopedia
• Extrasolar planets in fiction
• Habitable zone for complex life
• Lists of exoplanets
• NASA Exoplanet Archive
• Planetary capture
• List of planet types
• Desert planet
• Carbon planet
• Lava planet
• Eyeball planet
• Ocean world
• Ice planet
• Hycean planet
• Chthonian planet

Images for kids

• 

  The three known planets of the star HR8799. The light from the central star was blanked out by a vector vortex coronagraph.

• 

  2MASS J044144 is a brown dwarf with a companion about 5–10 times the mass of Jupiter. It is not clear whether this companion object is a sub-brown dwarf or a planet.

• 

  Coronagraphic image of AB Pictoris showing a companion (bottom left), which is either a brown dwarf or a massive planet. The data were obtained on 16 March 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris.