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What Is a Dwarf Planet? The IAU Definition Explained
The three tests behind the label, the extra rule about moons, and how dwarf planets compare with planets, asteroids and satellites.
How Many Dwarf Planets Are There?
Five names are official as of November 2026. Here is who is on the list, who might join it, and why astronomers give different totals.
Why Pluto Is a Dwarf Planet
Pluto is not too small to be a planet. It failed one specific test. Here is the 2006 vote, the rule behind it and the arguments that continue.
What New Horizons Found at Pluto
The 2015 flyby showed a Pluto with glaciers, mountains, dunes and layered haze. This guide sorts the main findings and what is still uncertain.
Charon and Pluto's Four Small Moons
Pluto has five known moons as of November 2026. See how Charon stacks up against Pluto and why the four small moons tumble instead of spinning steadily.
Ceres: The Dwarf Planet in the Asteroid Belt
Ceres holds about a quarter of the belt's mass. Learn why it counts as an asteroid and a dwarf planet, and what Dawn found there from 2015 to 2018.
Eris: The Dwarf Planet That Reopened the Debate
Found in old images and announced in 2005, Eris forced a vote on what a planet is. Here is its story, its moon Dysnomia and how it measures up to Pluto.
Haumea: The Fast-Spinning, Egg-Shaped Dwarf Planet
A day on Haumea lasts under four hours. See how that spin shaped it, how a 2017 occultation revealed a ring and why astronomers still argue about its discovery.
Makemake: A Bright, Frozen World Beyond Neptune
Makemake is the second-brightest Kuiper Belt object seen from Earth. This guide covers its discovery, surface, tiny moon and the 2025 Webb detection of methane gas.
What Are Asteroids? Types, Sizes and Where They Live
A plain-language guide to asteroids, from the 530 km giant Vesta to rocks under 10 m, with the main compositional classes and the zones where they gather.
The Asteroid Belt Is Mostly Empty Space
Where the main belt sits, how little material it holds, what carves its Kirkwood gaps, and why Jupiter kept a planet from forming there.
Vesta: The Giant Asteroid Visited by Dawn
Vesta has a core, mantle and crust, a basin nearly as wide as itself, and a family of meteorites that landed on Earth. Here is what Dawn showed in 2011 and 2012.
Near-Earth Asteroids: What They Are and How They Are Tracked
How astronomers define, count and follow asteroids whose orbits come close to Earth, and why a label like "potentially hazardous" is not a prediction.
Planetary Defense Explained: DART and What Comes Next
What agencies actually do to watch for hazardous asteroids, the numbers from the DART impact on Dimorphos, and the missions that will check the results.
OSIRIS-REx and Bennu: Bringing Asteroid Dust Home
The mission's key dates, how the touch-and-go grab worked, how much material came back, and the findings from amino acids to sugars, as of November 2026.
Hayabusa and Hayabusa2: JAXA's Asteroid Sample Missions
How two Japanese spacecraft reached near-Earth asteroids and returned samples, what the grains revealed, and the 2026 Torifune flyby of Hayabusa2's extended mission.
Lucy and the Trojan Asteroids of Jupiter
What Trojan asteroids are, why they ride along Jupiter's orbit, and a dated tour of Lucy's flybys, from the main-belt rehearsals to the Patroclus and Menoetius pair.
Asteroid Apophis and Its 2029 Close Flyby
Why Apophis worried astronomers in 2004, how radar data in 2021 ended the concern, and which spacecraft and telescopes will watch the April 2029 flyby.
What Is a Comet? Anatomy of a Dirty Snowball
The parts of a comet, from the frozen nucleus to the coma and two tails, and how close-up visits changed the old snowball picture.
Comet Tails Explained: Ion Tail, Dust Tail and Anti-Tail
Why a comet grows two tails of different color and shape, why both point away from the Sun, and how a tail can seem to point the wrong way.
Comet vs. Asteroid: How to Tell Them Apart
How composition, orbit and behavior separate comets from asteroids, and why active asteroids, main-belt comets and dark comets keep the line fuzzy.
The Kuiper Belt: Home of Pluto and Short-Period Comets
Where the Kuiper Belt sits, what lives in it, how it feeds short-period comets, and what New Horizons found at Pluto and the snowman-shaped Arrokoth.
The Oort Cloud: The Solar System's Distant Comet Reservoir
What the Oort Cloud is, how far it reaches, why astronomers infer it from comet orbits instead of seeing it, and how a comet gets nudged toward the Sun.
Halley's Comet: Facts, History and the Next Return
Facts about Halley's Comet: its 76-year orbit, sightings back to 240 BCE, the Giotto flyby of 1986, the May and October meteor showers, and the 2061 return.
Great Comets in History: Hale-Bopp, Hyakutake and NEOWISE
What makes a comet "great", and the stories of Hyakutake in 1996, Hale-Bopp in 1997, NEOWISE in 2020 and the uncertain forecast for Tsuchinshan-ATLAS in 2024.
How to See a Comet With Your Eyes or Binoculars
How to check forecasts, time your night, choose binoculars, read a sky chart and know what a comet looks like at the eyepiece, from NASA skywatching advice.
Rosetta and Comet 67P: The First Comet Landing
How Rosetta reached Comet 67P, what happened to the Philae lander, and what the mission learned about the comet's two-lobed shape, porous body, water and gases.
Meteor, Meteoroid or Meteorite? The Difference Explained
A plain guide to the three words, with the extra terms fireball, bolide and micrometeorite placed where they belong.
How Meteor Showers Work: Comet Debris, Radiants and Dates
Why meteors seem to fan out from one spot, which parent bodies feed the best-known showers, and why peak dates barely change from year to year.
How to Watch a Meteor Shower: Dates, Gear and Tips
A table of the best-known showers, a packing checklist and the Moon conditions for the next peaks, as of November 2026.
Types of Meteorites: Stony, Iron and Stony-Iron
How scientists sort meteorites, what chondrites and achondrites say about the early Solar System, and which well-known specimens represent each type.
Fireballs and Bolides: Why Some Meteors Are So Bright
What the brightness cutoffs mean, why a few fireballs bring a sonic boom minutes later, and a checklist of what to write down if you see one.
Is It a Meteorite? How to Check a Suspicious Rock
A step-by-step screen for a possible meteorite, the Earth rocks that fool people most often, and why a lab exam, not a photo, settles the question.
Impact Craters on Earth: Meteor Crater to Chicxulub
From a 1.2 km bowl in the Arizona desert to a 180 km structure under Mexico, this covers how impacts leave marks and how scientists prove the cause.
The Tunguska Event of 1908: What Exploded Over Siberia
The airburst explanation, the size and energy estimates with their uncertainty, and what the event taught scientists about small impactors.
The Chelyabinsk Meteor of 2013: Airburst Over Russia
What an airburst about 24 to 30 km up did to a city, how videos and sensors reconstructed it within hours, and why it changed how scientists watch for small asteroids.
How Minor Planets Are Discovered, From Blink Plates to Sky Surveys
A minor planet is found when a dot drifts against fixed stars. Here is the path from first image to confirmed orbit, official designation and credit.
How Asteroids and Comets Get Their Names and Designations
A provisional code, a permanent number and an approved name are three separate steps. This guide decodes each one for asteroids and for comets.
How to Read an Orbit: Eccentricity, Inclination and More
Six numbers describe where a small body travels. This guide explains each one in plain words and uses real asteroids and comets to show how to read them.
How to Use the JPL Small-Body Database and Horizons
Enter an object's number or code into a public JPL tool and get orbit, size and discovery data. This guide shows each field and how to find a body's sky position.
Missions to Asteroids, Comets and Dwarf Planets: A Timeline
Forty years of spacecraft visits, from comet flybys to sample returns and an asteroid impact test. Every entry has a date and a mission type.
The Telescopes That Find and Study Small Bodies
Some instruments find new objects and others measure size, shape or makeup. Here is what each major facility is good at, with its status as of November 2026.
Citizen Science for Asteroids and Comets: How to Take Part
Several NASA-listed projects invite the public to review telescope images, and amateurs can report measurements. Here is what each involves as of November 2026.
Stellar Occultations: Measuring Worlds by Their Shadows
When a distant body hides a star for a few seconds, the shadow measures it better than most telescopes can. Here is how the method works and who takes part.
How Do We Know What Asteroids Are Made Of?
No single measurement identifies an asteroid's makeup. Scientists combine reflected light, density, radar, meteorite matches and returned samples to check one another.