Dwarf Planets

Illustration showing dwarf planet sizes compared to Earth.

An artist's concept showing the size of the best known dwarf planets compared to Earth and its moon (top). Eris is left center; Ceres is the small body to its right and Pluto and its moon Charon are at the bottom.

Dwarf Planets: Overview

While similar to planets in many ways, dwarf planets share their orbits around the sun with other objects such as asteroids or comets. The first five recognized dwarf planets are:

Ceres | Pluto | Eris | Haumea | Makemake

Dwarf Planets: 10 Need-to-Know Things

  1. Smaller Than Our Moon
    If the sun were as tall as a typical front door, Earth would be about the size of a nickel and dwarf planet Pluto, one of the larger known dwarf planets, would be the size of the head of a pin.
  2. Long Trip
    Dwarf planets orbit our sun, a star. Most are located in the Kuiper Belt, a region of icy and rocky debris beyond Neptune. Ceres, the closest dwarf planet, is the largest world in the main asteroid belt between Mars and Jupiter.
  3. What's in a Day?
    Days on dwarf planets vary. One day on dwarf planet Ceres, for example, takes about nine Earth hours (the time it takes for Ceres to rotate once). It takes Ceres about 4.5 Earth years to go around the sun.
  4. Hard Worlds
    Dwarf planets are solid rocky and/or icy bodies. It all depends on their location in the solar system.
  5. Moon Check
    Some dwarf planets have moons. Two - Ceres and Makemake - do not.
  6. Ringworld?
    There are no known dwarf planet rings. But with five nearby moons shedding impact debris, Pluto might have rings.
  7. Bring a Spacesuit
    Dwarf planets Pluto and Eris have thin atmospheres that are more active when they are closer to the sun.
  8. Far Traveler
    New Horizons is the first mission to explore the Kuiper Belt, home of most dwarf planets.
  9. Tough for Life
    Dwarf planets cannot support life as we know it.
  10. World Class
    Pluto was considered a planet until 2006. But the discovery of similar worlds prompted scientists to change the definition of a planet to better describe the different kinds of worlds of our solar system.
Diagram of the solar system showing Pluto and Eris' eccentric orbits.
A diagram showing solar system orbits. The highly tilted orbit of Eris is in red.

Dwarf Planets: In Depth

Dwarf planets are a lot like regular planets:

  • They both have enough mass and gravity to be nearly round - unlike odd-shaped asteroids.
  • They both travel through space in a path around the sun.

The big difference?

  • A dwarf planet's path around the sun is full of other objects like asteroids and comets.
  • A regular planet has a clear path around the sun. Most of the major impacts with other objects in its orbit happened billions of years ago. There is not much left over to get in the way.

There may be dozens of dwarf planets in our solar system. So far, we've classified just a handful:

Closest to home, Ceres is the largest and most unique resident of the main asteroid belt between Jupiter and Mars. It also is the first dwarf planet to be visited by a spacecraft - NASA's Dawn mission.

Pluto is the most famous dwarf planet. Discovered in 1930, it was long classified as our solar system's ninth planet. Pluto and its busy system of moons orbits the sun in the Kuiper belt, a region of icy debris beyond Neptune.

Astronomers discovered Eris, a Pluto-sized world, in 2003. It takes icy Eris 557 Earth years to complete a single orbit around our sun.

Two more confirmed orbit the sun in the icy zone beyond Neptune - Haumea and Makemake.

 

Dwarf Planets: Trivia

Little Rocks
Dwarf planets are very different from their larger planetary cousins. It would take about 30,000 of them to create a world the size of Earth.

Closest Dwarf Planet
Ceres is the only dwarf planet found outside of the Kuiper Belt.

Fast Football
Dwarf planet Haumea is one of the fastest rotating objects in our solar system. Haumea spins so fast that it has pulled itself into the shape of a squashed American football.

Bring a Flashlight
High noon on Pluto would look a lot like a moonlit night here on Earth. The sunlight that reaches Pluto is about 1,000 times dimmer than what we see here on Earth and provides little warmth.

Distance Runner
Even though it traveled at speeds 30 times greater than the fastest fighter jet, NASA's New Horizons spacecraft still took more than nine years to get to Pluto.

Camera-Shy Planet
Even our most powerful telescopes show little more than fuzzy blobs when pointed across billions of kilometers at Pluto and Charon. It took the New Horizons spacecraft nine and a half years of space travel to get the only close up images.

Mission Milestone
New Horizons is the only spacecraft to explore the Kuiper Belt up close.

Long, Strange Trip
Pluto is so far away it takes 248 years to orbit the sun -- so long it still hasn't finished one orbit since it was discovered in 1930. Pluto's orbit is so elliptical -- picture a squashed circle -- its distance from the sun varies more than 2.7 billion km (1.9 billion miles).

New Class
The discovery of Eris -- about the same size as Pluto -- far beyond Neptune sparked intense debate about the definition of a planet. In the end, Eris and Pluto were classified as dwarf planets.

Deep Freeze
Pluto is so cold even the air can freeze and fall to the ground like snow. The planet's average temperature is about -387 degrees Fahrenheit (-233 degrees Celsius). For comparison, the coldest spot on Earth gets down to -128 degrees Fahrenheit (-89 degrees Celsius).

Little Worlds, Big Numbers
Scientists now consider that dwarf planets may be the most common type of planet. There are likely hundreds more dwarf planets out there waiting to be discovered.

Faint Discovery
There are millions of stars brighter than dwarf planet Pluto in the night sky, but observant astronomer Clyde Tombaugh caught a faint image -- light equal to a candle seen at a distance of 480 km (300 miles) -- on a photographic plate. Tombaugh had discovered the mysterious Pluto.

 



Pluto

Color image of Pluto
Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. The images, taken when the spacecraft was 280,000 miles (450,000 kilometers) away, show features as small as 1.4 miles (2.2 kilometers). Image Credit: NASA/JHUAPL/SwRI

Pluto: Overview

Discovered in 1930, Pluto was long considered our solar system's ninth planet. But after the discovery of similar intriguing worlds deeper in the distant Kuiper Belt, icy Pluto was reclassified as a dwarf planet. This new class of worlds may offer some of the best evidence about the origins of our solar system.

Pluto: 10 Need-to-Know Things

  1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and dwarf planet Pluto would be about the size of the head of a pin.
  2. Pluto orbits our sun, a star, at an average distance of 3.7 billion miles (5.9 billion kilometers) or 39.5 AU.
  3. One day on Pluto takes about 153 hours. That's the time it takes for Pluto to rotate or spin once. Pluto makes a complete orbit around the sun (a year in Plutonian time) in about 248 Earth years.
  4. It is thought that Pluto has a rocky core surrounded by a mantle of water ice with other ices coating its surface.
  5. Pluto has five known moons. Pluto is sometimes called a double-planet system due to the fact that its moon Charon is quite large and orbits close to its parent planet.
  6. NASA's New Horizons spacecraft is the first mission sent to encounter the Pluto-system and other members of the Kuiper Belt.
  7. There are no known rings around Pluto.
  8. Pluto has a thin, tenuous atmosphere that expands when it comes closer to the sun and collapses as it moves farther away -- similar to a comet.
  9. Scientists do not think Pluto can support life as we know it. Although, some scientists believe it is possible Pluto could possess a hidden ocean under its surface.
  10. Pluto was considered a planet from 1930, when it was first discovered, until 2006. The discovery of similar-sized worlds deeper in the distant Kuiper Belt sparked a debate which resulted in a new official definition of a planet. The new definition did not include Pluto.

Pluto: In Depth

Pluto is classified as a dwarf planet and is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant realm is populated with thousands of miniature icy worlds, which formed early in the history of our solar system. These icy, rocky bodies are called Kuiper Belt objects or transneptunian objects.

Pluto is about two-thirds the diameter of Earth's Moon and probably has a rocky core surrounded by a mantle of water ice. More exotic ices like methane and nitrogen frost coat its surface. Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's Moon. Pluto is more massive than Ceres - the dwarf planet that resides in the asteroid belt between Mars and Jupiter - by a factor of 20.

Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the Sun. (One AU is the mean distance between Earth and the Sun: about 150 million km or 93 million miles.) From 1979 to 1999, Pluto was actually closer to the Sun than Neptune, and in 1989, Pluto came to within 29.7 AU of the Sun, providing rare opportunities to study this small, cold, distant world.

Since its orbit is so elliptical, when Pluto is close to the sun, its surface ices thaw, rise and temporarily form a thin atmosphere. Pluto's low gravity (about 6 percent of Earth's) causes the atmosphere to be much more extended in altitude than our planet's atmosphere. Pluto becomes much colder during the part of each orbit when it is traveling far away from the sun. During this time, the bulk of the planet's atmosphere is thought to freeze.

Pluto has a very large moon that is almost half its size named Charon, which was discovered in 1978. This moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 19,640 km (12,200 miles).

The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth. These photos showed that Charon is grayer than Pluto (which appears to have a slightly red hue), indicating that they have different surface compositions and structures.

Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days. Charon neither rises nor sets, but hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto -- this is called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus. Pluto's rotation is retrograde: it rotates backwards, from east to west (Uranus and Venus also have retrograde rotations).

It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or no magnetic field.

Because Pluto and Charon are so small and far away, they are extremely difficult to observe from Earth. In the late 1980s, Pluto and Charon passed in front of each other repeatedly for several years. Observations of these rare events allowed astronomers to make rudimentary maps of each body showing areas of relative brightness and darkness.

In 2005, scientists photographing Pluto with the Hubble Space Telescope in preparation for the New Horizons mission found two tiny moons orbiting in the same plane as Charon. These two moons, named Nix and Hydra, are two to three times farther away from Pluto than Charon.

In 2011 and 2012, scientists used Hubble to spot two more moons (originally designated P4 and P5). In 2013, the two moons were named Kerberos (P4) and Styx (P5).


How Pluto Got its Name

Pluto is the only world named by an 11-year-old girl. In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for the Roman god the underworld. He forwarded the name to the Lowell Observatory and it was selected. Pluto's moons are named for other mythological figures associated with the underworld. Charon is named for the river Styx boatman who ferries souls in the underworld; Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology; and Styx is named for the mythological river that separates the world of the living from the realm of the dead.

Pluto's place in mythology can get a little muddled, so we asked Dr. Elizabeth Vandiver, chair of the Department of Classics in Whitman College in Walla Walla, Washington, to clarify the origins of the name: "Pluto is the name of the Roman god of the Underworld, equivalent to the Greek Hades. However, the Greek name "Plouton" (from which the Romans derived their name "Pluto") was also occasionally used as an alternative name for Hades. But Pluto is definitely the Roman spelling."

The symbol for dwarf planet Pluto is a monogram made up of P and L in Pluto (and also the initials of Percival Lowell, who predicted its discovery).


Pluto: Significant Dates

  • 1930: Clyde Tombaugh discovers Pluto.
  • 1977-1999: Pluto's lopsided orbit brings it slightly closer to the sun than Neptune. It will be at least 230 years before Pluto moves inward of Neptune's orbit for 20 years.
  • 1978: American astronomers James Christy and Robert Harrington discover Pluto's unusually large moon, Charon.
  • 1988: Astronomers discover that Pluto has an atmosphere.
  • 2005: Scientists using the Hubble Space Telescope announce the discovery of two additional moons of Pluto. Named Nix and Hydra, the little moons may have formed at the same time as Charon did, perhaps all three splitting off from Pluto in a giant impact event.
  • 2006: NASA's New Horizons mission launches on a path to explore Pluto and the Kuiper Belt region. The spacecraft is scheduled to reach Pluto in 2015.
  • 2006: The International Astronomical Union classifies Pluto as a dwarf planet and recognizes similar worlds beyond the orbit of Neptune as plutoids.
  • 2011: Astronomers using the Hubble Space Telescope discovered a fourth moon, named Kerberos, orbiting the icy dwarf planet.
  • 2012: Astronomers using the Hubble Space Telescope to search for potential hazards to the New Horizon mission discovered a fifth moon, named Styx, orbiting the icy dwarf planet.

Pluto: Trivia

Scary Boat
Charon is named after the mythological boatman who ferried souls across the river Styx to Pluto for judgment.

Slow Communication
A radio signal moving at the speed of light takes about 4 hours to reach Pluto from Earth.

Big Companion
Charon is half of Pluto's diameter - making it the largest satellite relative to the planet it orbits.

Shifting Scene
Pluto's brightness changes as it rotates, revealing large light and dark regions.

Planet Hunter
Pluto was discovered in 1930 by American astronomer Clyde Tombaugh.

Cold Climate
Pluto's estimated surface temperature falls between --378 to --396 degrees F (-228 to -238 C).

Dark Mythology
Pluto is named after the Roman god of the underworld.

Tribute
The symbol for Pluto ("PL") is tribute to Percival Lowell, who started the search for the ninth planet in the early 1900s.

Close Companion
Charon is 20 times closer to Pluto than our moon is to Earth.

Moon Surprise
American astronomers James Christy and Robert Harrington discovered Charon in 1978.

Lightweight
A person on Pluto would weigh 1/15 what they weigh on Earth . The astronauts on the Moon had 1/6 of their Earth weight.

Small World
At 1,470 miles (about 2,370 kilometers) across, Pluto could fit between Washington, D.C. and Denver, Colorado.

How Pluto Got Its Name
Student Venetia Burney suggested the name Pluto in 1930.

Sideways
Like Uranus, Pluto rotates on its side.

Long Year
Pluto orbits the Sun once every 248 years. That's longer than the entire history of the United States of America.

Far From the Sun
Pluto's last time at aphelion - the farthest point in its orbit - was 1865. It will next reach aphelion in 2112.



Kuiper Belt

An artist's concept of the dwarf planet Eris and its moon Dysnomia. The sun is the small star in the distance.
Artist's concept of Eris and its moon. The sun is in the distance. Image credit: Robert Hurt (IPAC).

Kuiper Belt: Overview

The Kuiper Belt is a disc-shaped region of icy bodies - including dwarf planets such a Pluto - and comets beyond the orbit of Neptune. It extends from about 30 to 55 AU and is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets. The first Kuiper Belt Object was discovered in 1992.

Kuiper Belt: 10 Need-To-Know Things

  1. The Kuiper Belt and the Oort Cloud are regions of space. The known icy worlds and comets in both regions are much smaller than Earth's moon.
  2. The Kuiper Belt and the Oort Cloud surround our sun, a star. The Kuiper Belt is a doughnut-shaped ring, extending just beyond the orbit of Neptune from about 30 to 55 AU. The Oort Cloud is a spherical shell, occupying space at a distance between five thousand and 100 thousand AU.
  3. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud. Short-period comets (which take less than 200 years to orbit the Sun) originate in the Kuiper Belt.
  4. There may be are hundreds of thousands of icy bodies larger than 100 km (62 miles) and an estimated trillion or more comets within the Kuiper Belt. The Oort Cloud may contain more than a trillion icy bodies.
  5. Some dwarf planets within the Kuiper Belt have thin atmospheres that collapse when their orbit carries them farthest from the sun.
  6. The Kuiper Belt and the Oort Cloud are regions of space and therefore do not have moons. However, several dwarf planets within the Kuiper Belt have moons.
  7. The Kuiper Belt and the Oort Cloud are regions of space and therefore do not have rings. No objects within these two regions are known to have rings.
  8. The first mission to the Kuiper Belt is New Horizons. New Horizons will reach Pluto in 2015.
  9. These two regions of space are not capable of supporting life as we know it.
  10. Fact: Both the Kuiper Belt and the Oort Cloud are named for the astronomers who predicted their existence during the 1950s: Gerard Kuiper and Jan Oort.

Kuiper Belt: In Depth

The Kuiper Belt is a disc-shaped region beyond Neptune that extends from about 30 to 55 astronomical units (compared to Earth which is one astronomical unit, or AU, from the sun). This distant region is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets.

Dwarf planet Pluto may be the best known of the larger objects in the Kuiper Belt. Comets from the Kuiper Belt take less than 200 years to orbit the sun and travel approximately in the plane in which most of the planets orbit the sun. .Objects in the Kuiper Belt are presumed to be remnants from the formation of the solar system about 4.6 billion years ago.

The first of these strange bodies, which astronomers call Kuiper Belt Objects (KBOs), came to light in 1992, discovered by Dave Jewitt and Jane Luu -- a pair of scientists who didn't believe the outer solar system was empty. Beginning in 1987 they had doggedly scanned the heavens in search of dim objects beyond Neptune. It took five years, looking off-and-on through the University of Hawaii's 2.2 m telescope, but they finally found what they were after: a reddish-colored speck 44 AU from the Sun -- even more distant than Pluto! Jewitt (University of Hawaii) and Luu (UC Berkeley) wanted to name their find "Smiley," but it has since been cataloged as "1992 QB1."

That discovery marked our first glimpse of the long-sought Kuiper Belt, named after Gerard Kuiper who, in 1951, proposed that a belt of icy bodies might lay beyond Neptune. It was the only way, he figured, to solve a baffling mystery about comets: Some comets loop through the solar system on periodic orbits of a half-dozen years or so. They encounter the Sun so often that they quickly evaporate -- vanishing in only a few hundred thousand years. Astronomers call them "short-period comets," although "short-lived" is more to the point. Short-period comets evaporate so quickly compared to the age of the solar system that we shouldn't see any, yet astronomers routinely track dozens of them. It was a real puzzle.

Kuiper's solution was a population of dark comets circling the Sun in the realm of Pluto -- leftovers from the dawn of our solar system when planetesimals were coalescing to make planets. The ones beyond Neptune, Kuiper speculated, never stuck together, remaining instead primitive and individual. Nowadays they occasionally fall toward the Sun and become short-period comets.

Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how reflective the object's surface is. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.

One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun. The parent body, Haumea, apparently collided with another object that was roughly half its size. The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours. It spins so fast that it has pulled itself into the shape of a squashed American football. Haumea and two small moons -- Hi'iaka and Namaka -- make up the family.

In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system. The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU -- instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.

In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto.

The discovery of Eris -- orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) -- forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category.

In 2015, NASA's New Horizons spacecraft flew past Pluto, making the first up-close exploration of a Kuiper Belt Object. The spacecraft is continuing deeper into this region of icy debris and may be able to explore at least one more object.


How the Kuiper Belt Got Its Names

The region is named for the astronomer who predicted its existence -- Gerard Kuiper. It is sometimes called the Edgeworth-Kuiper Belt, recognizing the independent and earlier discussion by Kenneth Edgeworth. Objects discovered in the Kuiper Belt get their names from diverse mythologies. Eris is named for the Greek goddess of discord and strife. Haumea is named for a Hawaiian goddess of fertility and childbirth. Comets from both regions are generally named for the person who discovered them.


Significant Dates
  • 1943: Astronomer Kenneth Edgeworth suggests that a reservoir of comets and larger bodies resides beyond the planets.
  • 1950: Astronomer Jan Oort theorizes that a vast population of comets may exist in a huge cloud on the distant edges of our solar system.
  • 1951: Astronomer Gerard Kuiper predicts the existence of a belt of icy objects just beyond the orbit of Neptune.
  • 1992: After five years of searching, astronomers David Jewitt and Jane Luu discover the first KBO, 1992QB1.
  • 2002: Scientists using the 48-inch Oschin telescope at Palomar Observatory find Quaoar, the first large KBO hundreds of kilometers in diameter. This object was photographed in 1980, but was not noticed in those images.
  • 2004: Astronomers using the 48-inch Oschin telescope announce the discovery of Sedna (2003VB12).
  • 2005: Astronomers announce the discovery of 2003UB313. This object, later named Eris, is slightly larger than Pluto.
  • 2008: The Kuiper Belt object provisionally known as 2005FY9 ("Easterbunny") is recognized in July as a dwarf planet and named Makemake (pronounced MAHkeh-MAHkeh) after the Polynesian (Rapa Nui) creation god. In September, 2003EL61 ("Santa") was designated a dwarf planet and given the name Haumea after the Hawaiian goddess of fertility and childbirth.


Oort Cloud

Oort Cloud: Overview: Giant Space Bubble

The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our sun to the next start -- between 5,000 and 100,000 astronomical units. Earth is about one astronomical unit from the sun (roughly 93 million miles or 150 million kilometers).

Oort Cloud: 10 Need-To-Know Things

  1. The Oort Cloud is a region of space. The known comets in this region are much smaller than the Earth's moon.
  2. The Oort Cloud surrounds our sun, a star. The Oort Cloud is a spherical shell, occupying space at a distance between five and 100 thousand AU.
  3. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud.
  4. The Oort Cloud may contain more than a trillion icy bodies.
  5. Comets that originate from the Oort Cloud gain atmospheres (the coma) when they near the sun. This atmosphere collapses when the comet's orbit carries it farthest from the sun.
  6. There are no known moons of Oort Cloud objects.
  7. The are no known rings around objects in this region of space.
  8. There have been no missions sent to the Oort Cloud.
  9. The Oort Cloud is not capable of supporting life as we know it.
  10. The Oort Cloud is named for the astronomer who predicted its existence during the 1950s: Jan Oort.

Oort Cloud: In Depth

The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our sun to the next start -- between 5,000 and 100,000 astronomical units. Earth is about one astronomical unit from the sun (roughly 93 million miles or 150 million kilometers).

Consider this: At its current speed of about a million miles a day, NASA's Voyager 1 spacecraft won't reach the Oort Cloud for about 300 years. And it will take about 30,000 years to reach the other side.

Dutch astronomer Jan Oort first proposed the idea of this region of space to explain the origins of comets with that take thousands of years to orbit the sun. These are called long-period comets and most have been seen only once in recorded history. More frequent visitors to the inner solar system are called short-period comets.

There may be hundreds of billions, even trillions, of icy bodies in the Oort Cloud. Every now and then, something disturbs one of these icy worlds and it begins a long fall toward our sun. Two recent examples are comets C/2012 S1 (ISON) and C/2013 A1 Siding Spring. ISON was destroyed when it passed too close the the sun. Siding Spring, which made a very close pass by Mars, will not return to the inner solar system for about 740,000 years.


How the Oort Cloud Got its Name

This region of space is named for Dutch astronomer Jan Oort who predicted its existence in 1950.


Discovery

The Oort Cloud still hasn't been discovered. It is just a theory, although we have studied several comets believed to have come from this distant region of our galaxy.