Journey beyond our solar system to discover thousands of alien planets orbiting distant stars
Exoplanets, or extrasolar planets, are worlds that orbit stars other than our Sun. They represent one of the most exciting frontiers in astronomy, offering glimpses into the incredible diversity of planetary systems throughout our galaxy.
Confirmed Exoplanets
Planetary Systems
Candidates Awaiting Confirmation
Potentially Habitable
Since the first confirmed detection of an exoplanet orbiting a sun-like star in 1995 (51 Pegasi b), our knowledge has exploded exponentially. The Kepler Space Telescope alone discovered over 2,600 confirmed exoplanets during its mission from 2009 to 2018. Today, we're discovering new worlds at an unprecedented rate, with ground-based observatories and space telescopes working in tandem.
These discoveries have revolutionized our understanding of planet formation, orbital mechanics, and the potential prevalence of life in the universe. We've found planets that challenge our theories, from "hot Jupiters" that orbit their stars in mere days to rogue planets drifting alone through interstellar space.
Astronomers Aleksander Wolszczan and Dale Frail discover the first confirmed exoplanets orbiting a pulsar (PSR B1257+12). These harsh, radiation-blasted worlds proved that planets could exist in extreme environments.
Michel Mayor and Didier Queloz discover 51 Pegasi b, the first exoplanet found orbiting a sun-like star. This "hot Jupiter" revolutionized planet formation theories and earned them the 2019 Nobel Prize in Physics.
NASA's Kepler mission begins its planet-hunting journey, using the transit method to discover thousands of exoplanets. It would go on to revolutionize our statistical understanding of planetary systems.
Kepler-452b is discovered - a planet roughly Earth-sized in the habitable zone of a sun-like star. The "Earth's cousin" captures public imagination about potentially habitable worlds.
Astronomers announce the discovery of seven Earth-sized planets orbiting TRAPPIST-1, with three in the habitable zone. This system becomes a prime target for studying potentially habitable worlds.
NASA's Transiting Exoplanet Survey Satellite (TESS) launches, designed to find exoplanets around nearby bright stars, making them ideal targets for atmospheric studies.
The James Webb Space Telescope captures its first exoplanet atmospheric spectra, revealing detailed chemical compositions including water vapor, clouds, and even hints of photochemistry.
Detecting planets light-years away requires ingenious techniques. Here are the primary methods astronomers use to discover these distant worlds:
How it works: When a planet passes in front of its star (from our perspective), it blocks a tiny fraction of the star's light, causing a measurable dip in brightness.
What we learn: Planet size, orbital period, and with follow-up observations, atmospheric composition.
Success rate: Responsible for ~75% of all confirmed exoplanets. Kepler and TESS missions use this method.
Limitations: Only works if the planet's orbit is aligned with our line of sight. Typically detects about 0.5-1% of planets around target stars.
How it works: A planet's gravity causes its host star to wobble slightly. This wobble shifts the star's light spectrum due to the Doppler effect - similar to how an ambulance siren changes pitch as it passes.
What we learn: Planet mass (or minimum mass), orbital period, and orbital eccentricity.
Success rate: ~20% of confirmed exoplanets. This was the first method to successfully detect exoplanets around sun-like stars.
Limitations: More sensitive to massive planets close to their stars. Requires very precise spectrographic measurements.
How it works: Using coronagraphs or starshades to block out a star's overwhelming light, allowing telescopes to photograph planets directly. Young, massive planets glow with infrared heat, making them easier to image.
What we learn: Direct information about the planet's atmosphere, temperature, and orbit. Can detect multiple planets in the same system.
Success rate: ~1% of confirmed exoplanets. Extremely challenging due to the vast brightness difference between stars and planets.
Best targets: Young, massive planets orbiting far from their host stars.
How it works: When a star (with a planet) passes in front of a more distant background star, gravity bends and magnifies the background star's light. A planet causes a distinct spike in the light curve.
What we learn: Can detect planets at great distances from Earth and planets very far from their host stars, including free-floating rogue planets.
Success rate: ~3% of confirmed exoplanets. Each event is unique and typically can't be repeated.
Special advantage: Can detect low-mass planets that other methods might miss.
How it works: Measures the precise position of a star in the sky over time. A planet's gravity causes the star to move in a small ellipse against the background of more distant stars.
What we learn: Planet mass and orbital characteristics with high precision.
Current status: Limited confirmations so far, but the Gaia space telescope is expected to detect thousands of exoplanets using this method.
Future potential: Next-generation telescopes will make this a powerful detection method.
How it works: When multiple planets orbit the same star, their gravitational interactions cause slight variations in transit timing. By analyzing these variations, we can infer the presence of additional planets.
What we learn: Can detect non-transiting planets and provide precise mass measurements.
Success rate: Relatively new technique that has confirmed dozens of planets, particularly in multi-planet systems.
Advantage: Can find planets that don't transit their stars themselves.
Exoplanets come in an astonishing variety of sizes, compositions, and environments. Here are the main categories:
Size: Similar to or larger than Jupiter
Composition: Gas giants (hydrogen & helium)
Characteristics: Orbit extremely close to their stars (closer than Mercury to our Sun), with orbital periods of just days. Surface temperatures exceed 1,000°C (1,800°F), hot enough to vaporize metals.
Examples: 51 Pegasi b (the first discovered), HD 209458 b (Osiris)
Mystery: Their existence challenged planet formation theories - how did such massive planets end up so close to their stars?
Size: 1.5 to 2 times Earth's radius
Composition: Rocky planets with possible thick atmospheres
Characteristics: More massive than Earth but lighter than Neptune. Can be rocky, icy, or have thick atmospheres. Some may have oceans or continents. These are the most common type in our galaxy, though our solar system has none.
Examples: Kepler-452b, Proxima Centauri b, LHS 1140 b
Habitability: Prime candidates for finding life, especially those in habitable zones.
Size: 2 to 4 times Earth's radius
Composition: Rocky core with thick hydrogen-helium atmospheres
Characteristics: The most abundant type of exoplanet discovered. Smaller than Neptune but larger than Earth. May have water worlds beneath their atmospheres. Scientists debate whether they have rocky surfaces or are entirely gaseous.
Examples: Kepler-138d, K2-18b, TOI-270 d
Research focus: Understanding the "radius gap" - why planets of certain sizes are rare.
Size: Similar to Uranus or Neptune
Composition: Water, methane, and ammonia ices with rocky cores
Characteristics: These planets have thick atmospheres over mantles of icy materials. They're much less common in exoplanet surveys than hot Jupiters or super-Earths, possibly because they're harder to detect.
Examples: HAT-P-26b (warm Neptune), GJ 3470 b
Interesting fact: Some may have diamond rain in their interiors due to extreme pressure.
Size: Jupiter-sized or larger
Composition: Primarily hydrogen and helium
Characteristics: Similar to Jupiter and Saturn. Can orbit at various distances from their stars. "Cold Jupiters" orbit far from their stars like our Jupiter, while others orbit at intermediate distances.
Examples: HD 106906 b, Beta Pictoris b, 51 Eridani b
Role in systems: May protect inner rocky planets from asteroid impacts, like Jupiter does for Earth.
Size: Similar to Earth or smaller
Composition: Primarily rock and metal
Characteristics: Similar to Earth, Venus, Mars, and Mercury. May have thin atmospheres or none at all. These are the holy grail of exoplanet research for finding Earth-like life.
Examples: TRAPPIST-1e, Kepler-186f, Proxima Centauri b
Challenge: Small size makes them harder to detect than larger planets.
Size: Various, but typically rocky
Composition: Rock and metal with molten surfaces
Characteristics: Orbit so close to their stars that surface rock melts into lava oceans. Daytime temperatures can exceed 2,000°C (3,600°F). May have atmospheres of vaporized rock.
Examples: Kepler-10b, CoRoT-7b, K2-141b
Extreme conditions: Some may have one side permanently facing their star, with lava on one side and solid rock on the other.
Size: Typically super-Earth sized
Composition: Deep global oceans over rocky cores
Characteristics: Planets covered entirely by deep water oceans, potentially hundreds of kilometers deep. May have exotic high-pressure ice at the bottom of their oceans. Some scientists theorize these could harbor life in subsurface oceans.
Examples: GJ 1214 b, Kepler-22b (candidate)
Astrobiology interest: Could life evolve in perpetual darkness under thick ice?
Size: Various sizes
Composition: Any composition
Characteristics: Planets that don't orbit any star, drifting alone through interstellar space. Ejected from their original systems or formed independently. Could be numerous in the galaxy - possibly billions.
Examples: CFBDSIR 2149-0403, PSO J318.5-22
Speculation: Could some retain enough internal heat to support subsurface oceans and potentially life?
| Planet Type | Temperature Range | Comparison |
|---|---|---|
| Ultra-Hot Jupiters | 2,000°C - 4,000°C | Hotter than some stars! |
| Hot Jupiters | 1,000°C - 2,000°C | Hot enough to melt iron |
| Lava Worlds | 1,200°C - 3,000°C | Surface is molten rock |
| Habitable Zone Planets | -50°C - 50°C | Similar to Earth's range |
| Ice Giants | -200°C - -100°C | Colder than Antarctica |
| Rogue Planets | -230°C or colder | Nearly absolute zero |
The habitable zone (also called the "Goldilocks zone") is the region around a star where conditions might be just right for liquid water to exist on a planet's surface - not too hot, not too cold. This is considered the primary criterion for habitability as we know it.
The habitable zone's location depends on the star's temperature and brightness:
For our Sun, the habitable zone extends from about 0.95 AU (just inside Earth's orbit) to 1.37 AU (between Earth and Mars).
A thick greenhouse atmosphere can warm a planet beyond the outer edge of the habitable zone. Conversely, a thin atmosphere might not trap enough heat even within the zone.
Protects atmosphere from being stripped away by stellar wind. Without it, even a habitable-zone planet could lose its water and become barren like Mars.
Volcanoes and plate tectonics recycle carbon and regulate climate over geological timescales, potentially extending habitability.
Giant planets in outer orbits can shield inner planets from comets and asteroids, while also delivering water during formation.
Distance: 4.24 light-years
Discovery: 2016
Mass: ~1.27 Earth masses
Orbital period: 11.2 days
This rocky planet orbits within the habitable zone of Proxima Centauri, the closest star to our Sun. However, it faces challenges: the star is a red dwarf that produces powerful flares, potentially stripping away the planet's atmosphere. If it has retained an atmosphere and magnetic field, it could potentially harbor life.
Distance: 41 light-years
Discovery: 2017
Mass: ~0.69 Earth masses
Orbital period: 6.1 days
Part of a remarkable seven-planet system, TRAPPIST-1e is considered one of the most Earth-like exoplanets discovered. It sits squarely in the habitable zone with a density suggesting a rocky composition. The James Webb Space Telescope is studying its atmosphere for signs of water vapor and other biosignatures.
Distance: 1,800 light-years
Discovery: 2015
Size: ~1.5 Earth radii
Orbital period: 385 days
Orbiting a sun-like star in a 385-day orbit (similar to Earth's year), Kepler-452b is located in its star's habitable zone. The star is 1.5 billion years older than our Sun, giving life (if it exists) more time to develop. However, the planet is significantly larger than Earth and may be a mini-Neptune rather than a rocky super-Earth.
Distance: 49 light-years
Discovery: 2017
Mass: ~6.6 Earth masses
Orbital period: 24.7 days
This dense rocky super-Earth orbits a quiet red dwarf star, making it an excellent candidate for atmospheric studies. Its high density suggests an iron-rich core. Recent observations hint at a possible atmosphere, making it a priority target for the James Webb Space Telescope.
Distance: 124 light-years
Discovery: 2015
Mass: ~8.6 Earth masses
Orbital period: 33 days
In 2023, JWST detected potential biosignature gases in K2-18 b's atmosphere, including dimethyl sulfide - a molecule on Earth produced only by life. Water vapor and carbon dioxide were also confirmed. However, scientists remain cautious as this mini-Neptune may not have a solid surface, possibly being more ocean world than Earth-like planet.
Multiple space missions and ground-based observatories are dedicated to discovering and studying exoplanets:
Status: Mission ended
Achievements: Discovered 2,662 confirmed exoplanets by continuously monitoring 150,000 stars in a single patch of sky. Revolutionized our statistical understanding of planetary systems.
Key findings: Planets are incredibly common - likely more planets than stars in our galaxy. Small planets are more common than large ones.
Legacy: The K2 extended mission continued until 2018, discovering hundreds more planets.
Status: Active
Mission: Transiting Exoplanet Survey Satellite surveys 85% of the sky, focusing on nearby bright stars. Makes exoplanets ideal targets for follow-up atmospheric studies.
Discoveries: Over 400 confirmed exoplanets and 6,000+ candidates.
Advantage: Targets nearby stars, making planets easier to study with other telescopes.
Status: Active
Mission: While not primarily an exoplanet hunter, JWST's infrared capabilities allow unprecedented atmospheric characterization of known exoplanets.
Breakthroughs: Detailed atmospheric spectra revealing molecules like water, carbon dioxide, methane, and potential biosignatures.
Impact: Transforming exoplanet science from detection to detailed characterization.
Key players: ESO's VLT, Keck Observatory, Gemini, Subaru
Methods: Radial velocity, direct imaging, and spectroscopic follow-up
Advantages: Can be upgraded with new instruments, adaptable to new discoveries, cheaper than space missions.
Future: Extremely Large Telescopes (ELTs) currently under construction will revolutionize exoplanet direct imaging.
Status: In development
Mission: Will use gravitational microlensing to discover thousands of exoplanets, including many too far from their stars for other methods to detect.
Expected discoveries: Thousands of planets including analogs to Jupiter, Saturn, and potentially rogue planets.
Unique capability: First mission optimized for microlensing surveys.
Status: ESA mission in development
Mission: PLAnetary Transits and Oscillations of stars will discover and characterize Earth-sized planets around sun-like stars.
Goal: Find Earth-like planets in habitable zones and characterize their host stars with asteroseismology.
Duration: Planned 4-year mission with possible extension.
How much have you learned about exoplanets? Take this quiz to find out!
On HD 189733 b, it likely rains molten glass sideways at 7,000 km/h (4,350 mph) winds. The planet's deep blue color comes from silicate particles in its atmosphere.
Many exoplanets are tidally locked - one side permanently faces their star (eternal day) while the other faces eternal night. This creates extreme temperature differences.
55 Cancri e may have a surface made largely of diamonds. With temperatures around 2,400°C and extreme pressure, carbon could crystallize into diamond.
Kepler-16b orbits two stars in a binary system - real-life Tatooine! About 40% of stars are in binary systems, potentially hosting these circumbinary planets.
GJ 1214 b might be covered in a deep ocean over 1,000 km deep. The water would exist in exotic high-pressure ice forms at the bottom.
Hot Jupiters like HD 209458 b may experience lightning storms millions of times more powerful than Earth's, with planet-wide electrical storms.