Somewhere in the Dim Outer Reaches...

Does a Tenth Planet Orbit Our Solar System?



NASA artist conception of the Tenth Planet Click to enlarge this NASA artist concept of the view from the Tenth Planet, looking inward at the distant Sun.

A body said to be as much as 1.5 times the size of the planet Pluto and located about 9.7 billion miles from the Sun is being called the tenth planet of the Solar System.

For decades, we have counted nine of the largest bodies of the Solar System as planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Earth is third outward from the Sun. Neptune is eighth. Pluto is ninth. In 2006, Pluto was downgraded in its classification to dwarf leaving eight planets of the Solar System described as major.

The new body, known to astronomers as 2003-UB313,has been named Eris by the International Astronomical Union. The California Institute of Technology astronomer who found it, Michael Brown, says it is the tenth planet.

Eris has a moon, which has been named Dysnomia.

Dwarf planets. Eris, Pluto and the big asteroid Ceres now are referred to by the IAU as dwarf planets. Two other Solar System bodies are being considered for the the category of dwarf planets.

Ceres has been known since 1801 and is the largest of the asteroids. It was the first asteroid ever discovered. Its mass is more than one-third of all the 3,000 recorded asteroids. Ceres is about 578 miles in diameter.

Michael Brown and his fellow astronomers – Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University – used the the 48-inch Samuel Oschin Telescope at the Palomar Observatory near San Diego to photograph the object on October 21, 2003. The Samuel Oschin Telescope is a wide-field Schmidt telescope designed for sky surveys.

It was spotted in the photograph on January 8, 2005. The discovery was announced by the Minor Planet Center at Cambridge, Massachusetts, on July 29, 2005.

The astronomers have proposed to the International Astronomical Union naming the suspected planet after a creation mythology figure.Their proposed name has not been released pending a decision by the IAU.

Distance. Brown describes it as the most distant object ever found to be orbiting the Sun.

At 9.7 billion miles, Eris is three times farther from the Sun than Pluto, which averages 3.6 billion miles from the Sun.

The newly-discovered object is more distant than the mysterious planetoid Sedna discovered in 2003. Sedna is designated 2003 VB12.

It takes 560 years for the newly-discovered body to orbit the Sun. Right now, it is on the far side of the Solar System at its farthest point from Earth. In 280 years, the planet will be as close as the planet Neptune is to Earth.

It seems to be in the Kuiper Belt swarm of icy objects orbiting the Sun way beyond Neptune. Astronomers think they are remnants of ancient materials that formed the Solar System. It is 97 astronomical units (AU) from the Sun.

NASA artist conception of Eris Click to enlarge this alternative NASA artist concept of the view from Eris, looking inward at the distant Sun.
Size. Brown said the newly-discovered body is larger than Pluto, which was discovered in 1930. That would make it the largest object found orbiting the Sun since the discovery of Neptune and its moon Triton back in 1846.

Calculations show the newly-discovered object to be at least the size of Pluto and probably half again as large. The body can't be more than 2,205 miles in diameter.

By comparison, Pluto is about 1410 miles in diameter, its moon Charon is about 727 miles in diameter, and Earth is 7,900 miles in diameter.

In the past, Pluto has been described by some astronomers as the largest Kuiper belt object. The new discovery would make Pluto second largest.

Composition. The surface of the discovered body seems to be mostly methane, like Pluto.

Trujillo used the near-infrared spectrograph (NIRI) on Gemini Observatory's eight-meter Gemini North Telescope on Mauna Kea, Hawaii, to record a spectrum of the surface of Eris on January 25, 2005.

It showed strong signatures of methane ice similar to the spectrum of Pluto.

Methane ice suggests a primitive surface unheated by the Sun since the Solar System formed 4.5 billion years ago. If Eris ever had been close to the Sun, the methane ice would have been boiled off.

The interior of the planet probably is a mix of rock and ice, like Pluto.

Previously, icy methane surfaces had been seen on Pluto and Neptune's moon Triton, but not on other Kuiper Belt objects.

Orbit. The elliptical orbit of Eris is tilted at a 45-degree angle away from orbits of the other planets.

The first eight planets of the Solar System are not like the new body and Pluto: Pluto and the newly-discovered object are different: Registry. The Minor Planet Center is an international information clearing house for the International Astronomical Union. It is based at the Smithsonian Astrophysical Observatory, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts.

Large Dark Planetoids Orbiting Our Solar System

NASA artist conception of the large Kuiper Belt object Sedna beyond Pluto The large Kuiper Belt object Sedna beyond Pluto
Click to enlarge this NASA artist concept
Yet another mysterious large body was found earlier in the outer reaches of the Solar System orbiting the Sun at the astonishing distance of 6.2 billion miles beyond Earth.

For a time, the newly discovered body, which was named Sedna, was the most distant object yet found orbiting our Sun. It is two times farther from the Sun than Pluto.

By comparision, Earth is only 93 million miles from the Sun, while Pluto's average distance from the Sun is 3.6 billion miles. Sedna is 2.5 billion miles beyond Pluto.

Sedna's orbit is an extraordinarily long path around the Sun. It currently is almost 90 times the distance from the Earth to the Sun. That distance is referred to as one astronomical unit (AU), so Sedna's distance is almost 90 AU.

Its elliptical path around the Sun may take it as much as ten times farther away at times. At its most distant, Sedna probably would be 84 billion miles from the Sun. That would be 900 times the distance from Earth to the Sun.

It probably takes this planetoid 10,500 years to complete one orbit around the Sun.

The discovery was announced on March 15, 2004.

Red and shiny. Sedna is red in color and very shiny. It is said to be the reddest object in the Solar System after the planet Mars.

The surface temperature on Sedna probably is about -400 degrees Fahrenheit. It would be even colder at times because it approaches the Sun only briefly during its lengthy journey around the Sun.

Astronomers had thought that, because Sedna seems to rotate more slowly than expected, it might have a natural satellite – a moon – orbiting it. Now they are not sure.

When they used the Hubble Space Telescope to look at Sedna, no moon could be seen, although it might have been out of sight behind Sedna.

Sizes Compared
In recent years, astronomers have found several big objects beyond Neptune. Here are the approximate diameters of some discovered objects:
in Miles
Earth 7,926
How big is it? At the time of its discovery, the rock and ice body Sedna was the largest object to be found circling the Sun since Pluto was discovered in 1930.

Astronomers using the Hubble Space Telescope were able to calculate an upper limit on the size of Sedna. It seem's to be about three-quarters the diameter of Pluto, which would make Sedna about 995 miles in diameter.

Even though Sedna is smaller than Pluto, it is greater in volume than all the asteroids in the Solar System combined. On the other hand, Sedna probably amounts to only about one-third the mass of the Asteroid Belt, because it's icy rather than rocky.

The finders. Astronomer Michael Brown of the California Institute of Technology led a team of astronomers that made the discovery on November 14, 2003. The official designation for the object is 2003 VB12.

The team, from the California Institute of Technology, Yale Observatory and the Gemini Observatory in Hawaii, used the 48-inch Samuel Oschin Telescope at Mount Palomar Observatory in California. Then the Spitzer Space Telescope and the Tenagra Observatory in Arizona were used to verify the object and measure its size.

They named the object Sedna after the mythical Inuit goddess of the sea. It is said she was thrown into very cold Arctic waters by her father. Naming the faraway body Sedna seemed appropriate to its discoverers as the planetoid may be the coldest object in the Solar System.

The Inuit are an aboriginal people on Earth living on fish and animals in northern Canada above the tree line in the Northwest Territories, and in Northern Quebec and Labrador.

Before Sedna

NASA artist conception of the large Kuiper Belt object Quaoar beyond Pluto The large Kuiper Belt object Quaoar beyond Pluto
Click to enlarge this NASA artist concept
Other large dark objects already had been found orbiting the Sun along the far distant reaches at the edge of our Solar System.

In 2002, Brown and Trujillo found a large body and called it Quaoar. Its official scientific designation is 2002 LM60.

Quaoar is pronounced "kwa-whar."

Quaoar discoverers Brown and Trujillo named the body they found after a creation god of the Native American Tongva tribe, the original inhabitants of the Los Angeles basin.

According to legend, Quaoar "came down from heaven; and, after reducing chaos to order, laid out the world on the back of seven giants. He then created the lower animals, and then mankind."

Quaoar and the other large planetoids dwell in the Kuiper Belt, an icy debris field of comet-like bodies extending billions of miles beyond the orbit of the distant planet Neptune.

The Quaoar Story

After Quaoar was detected by a telescope on Earth as simply a dot of light, astronomers aimed the Hubble Space Telescope's powerful camera at it. Quaoar at that time became the farthest object in the Solar System ever to be resolved by a telescope.

Astronomers think Quaoar is composed mostly of ices mixed with rock, something like the makeup of a comet, although 100 million times greater in volume than an ordinary comet.

Even though Quaoar is smaller than Pluto, it is greater in volume than all the asteroids in the Solar System combined. On the other hand, Quaoar probably amounts to only about one-third the mass of the Asteroid Belt, because it's icy rather than rocky.

The discoverers. In 2002, Brown and Trujillo used the Palomar Oschin Schmidt telescope to see Quaoar as an 18.5-magnitude object creeping across the summer constellation Ophiuchus. Quaoar is less than 1/100,000 the brightness of the faintest star seen by the human eye.

Brown then made follow-up observations of the object using Hubble's new Advanced Camera for Surveys to measure the object's true angular size of 40 milliarcseconds. That corresponded with a diameter of about 800 miles.

Only the Hubble Space Telescope had the sharpness needed to resolve the disk of such a distant world. HST made possible the first-ever direct measurement of the true size of a Kuiper Belt Object (KBO).

The Orcus Story

Early in 2004, before Sedna, astronomers Brown, Trujillo and Rabinowitz announced they had found yet another large body using the telescope at the Palomar Observatory outside San Diego.

The mysterious object received the official scientific designation of 2004 DW. It appears to travel an elliptical orbit around the Sun, bringing it as close as 2.7 billion miles from the Sun and sending it as far out as 4.7 billion miles from the Sun. By comparison, Earth is a mere 93 million miles from the Sun.

It probably takes Orcus some 252 years to complete its orbit around the Sun.

Since 2001, Brown's team has found more than three dozen bright Kuiper Belt objects while surveying the outer range of the Solar System – first with the Samuel Oschin Telescope at Palomar Mountain in Southern California from Fall 2001, then with the Palomar QUEST camera from Summer 2003.

Orcus was announced in February 2004 and Sedna was announced in March 2004.

How Big Are These Mystery Objects?

NASA art comparing Sedna with Earth NASA composite image compares the relative sizes of Sedna, Quaoar, Pluto, Earth and Earth's Moon.
Click to enlarge NASA image
Measurements by NASA's Hubble Space Telescope previously had identified the icy world Quaoar as the largest object found anywhere in the Solar System since the discovery of the planet Pluto 72 years ago. Now, Eris and Sedna have overtaken Quaoar for the honor of being the largest.

The newfound planetoid Orcus is the 15th object larger than 300 miles in diameter to have been found in the Kuiper Belt. Sedna is the 16th.

In fact, some 800 objects have been spotted in the outer Solar System since 1992. Five of those could be larger than 600 miles in diameter.

Quaoar seems to be somewhere between 621 miles and 870 miles in diameter. By comparison, astronomers estimate that Orcus is between 522 miles and 1,118 miles in diameter. Sedna is larger at 800-1,100 miles in diameter.

That makes Orcus and Quaoar about the same size as each other, and just a bit more than half the size of Pluto, which is 1,413 miles in diameter.

Quaoar and Orcus are larger than Pluto's moon, Charon, which is 728 miles.

They Are Far Far Away

Planets: How Big, How Far?
in miles
Sun Distance
in miles
Mercury3,03236 million
Venus7,54367 million
Earth7,92693 million
Mars4,217142 million
Jupiter88,732483 million
Saturn74,975870 million
Uranus31,7631.8 billion
Neptune30,7752.8 billion
Pluto1,4293.7 billion
Sedna is one of the most distant object found orbiting our Sun so far. It is three times farther from the Sun than Pluto.

By comparision, Earth is only 93 million miles from the Sun, while Pluto is 3.7 billion miles from the Sun. Sedna is 6.2 billion miles beyond Earth and 2.5 billion miles beyond Pluto.

Quaoar is roughly four billion miles away from Earth. That is more than a billion miles farther away than Pluto.

Quaoar is so far away, it takes light from the Sun five hours to reach it.

Unlike Pluto, Quaoar's orbit around the Sun is circular. In fact, even more so than most of the planetary bodies in our Solar System.

The object Orcus is 4.4 billion miles from Earth.

Finding and measuring these distant Kuiper Belt objects gives us new insight into the origin and dynamics of the planets, and that mysterious population of bodies dwelling way out there at the Solar System frontier — the elusive, icy, Kuiper Belt of objects beyond Neptune and Pluto.

The Kuiper Belt

The Kuiper Belt
  • There are objects orbiting the Sun beyond the planet Neptune in a formation that astronomers call the Kuiper Belt.
  • Together, the numerous objects seem to form a disc or belt around the Sun.
  • More than 400 of these icy objects are known, and there may be many more.
  • They seem to be remnants of materials near the Sun as our Solar System formed.
The Oort Cloud
  • The large planetoids, such as Sedna and Quaoar, as well as comets and other smaller bodies, may be part of something astronomers call the Oort Cloud.
  • The Oort Cloud may have been formed by gravity from a rogue star near the Sun in the early days of the Solar System.
  • Our star may have been born into a cluster of stars, which means there would have been many stars close to the Sun back then.
  • The Sun and Earth are estimated to be 4.6 billion years old.
  • Sedna, Quaoar and the other large planetoids might represent the first actual detection of the previously suspected collection of icy bodies.
  • The belt of objects seems to be circling the Sun in a spherical orbit that extends as far as 3 lightyears out from our star.
  • That would be at the extreme edge of the Sun's gravitational influence.
  • The Oort Cloud may extend out from our star as far as half the distance to the nearest star.
  • Sedna is ten times closer to the Sun than the distance that had been predicted for the Oort Cloud, which raises a question of whether there is an inner and an outer Oort Cloud.
  • Sometimes comets from the Oort Cloud may be pushed by passing stars into the inner area of the Solar System near Earth.
The Sun is a star with several major bodies circling it along with numerous minor bodies such as planetoids, comets and asteroids. The Sun and the objects swarming around it and under its control are the Solar System.

For decades we have counted nine of the largest bodies as planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Earth is third from the Sun. Neptune is eight. Pluto was ninth. In 2006, Pluto was downgraded in its classification leaving eight planets of the Solar System described as major. Pluto now is classified with the Kuiper Belt planetoids like Sedna, Quaoar and Orcus.

Pluto, Sedna, Quaoar and Orcus dwell in the Kuiper Belt, an icy debris field of comet-like bodies extending seven or more billion miles beyond the orbit of the planet Neptune.

Overall, the Kuiper Belt is an orbital ring around the Sun. It's a dim, ancient region, far from the Sun, inhabited by small hard blobs of rock and ice. It is similar in some ways to the rocky debris found in the Asteroid Belt, which is an orbital ring between Mars and Jupiter. However, the Kuiper Belt consists of far more material than all of the asteroids together.

What's in the Kuiper Belt? Since the 1980s, hundreds of icy bodies have been detected in the Kuiper Belt. Almost all have been much smaller than Pluto.

Once its orbit is understood clearly, a small body in the Kuiper Belt is given an official number by the Minor Planet Center. Some of the objects also are given names. At least 84 objects have numbers and 19 of those have names. The names include Asbolus, Bienor, Chaos, Chariklo, Chiron, Cyllarus, Deucalion, Elatus, Huya, Hylonome, Ixion, Nessus, Okyrhoe, Pelion, Pholus, Quaoar, Rhadamanthus, Thereus, and Varuna.

The International Astronomical Union (IAU) Minor Planet Center (MPC) is at the Smithsonian Astrophysical Observatory. SAO is a research institute of the Smithsonian Institution headquartered in Cambridge, Massachusetts, where it and the Harvard College Observatory (HCO) form the Harvard-Smithsonian Center for Astrophysics (CfA).

Previous size record holders before Sedna, Quaoar and Orcus were a Kuiper Belt Object (KBO) named Varuna and a different object known as 2002 AW197. Each of those are more or less 550 miles across.

When it was discovered in 2002, Varuna, was recognized as the largest body ever seen in the Kuiper Belt up to that time, other than Pluto and its moon Charon.

Sedna, Quaoar and Orcus are by far the biggest fish ever snagged by astronomers in their KBO surveys.

Astronomers believe that there are many more Kuiper Belt Objects. They suggest even larger KBOs might be uncovered in the next few years in the cold, dark, outer reaches of the Solar System. The Hubble and Spitzer space telescopes are their valuable tool for observations leading to size calculations.

Hubble and Spitzer. Unlike the dimensions of other bodies derived from direct observation by Hubble, the diameter of one of these distant Kuiper Belt is deduced from measurement of an object's temperature and then calculation of its size based on assumptions about its reflectivity. That means, of course, that there is great uncertainty about the object's true size.

The Spitzer Space Telescope, formerly known as the Space Infrared Telescope Facility (SIRTF), was launched to Earth orbit in 2003. It already has become an important tool in measuring Kuiper Belt objects.

What Is Pluto?

NASA artist concept of New Horizons at Pluto NASA plans to launch an interplanetary probe called New Horizons in 2006 to explore Pluto. New Horizons would arrive in the vicinity of Pluto about 2015. The robot explorer would swing by Pluto and then enter the Kuiper Belt.    MORE ABOUT NEW HORIZONS
Click to enlarge NASA artist concept
The planet Pluto was discovered by the late U.S. astronomer Clyde Tombaugh in 1930 following a 15-year search for trans-Neptunian planets.

When Tombaugh discovered planet number nine, Pluto, he had been searching for what he called Planet X. He continued looking out there for a tenth planet, Planet X, even after he discovered Pluto.

It wasn't realized until much later that Pluto actually was the largest of the known Kuiper Belt objects.

In fact, the concept of a Kuiper Belt wasn't imagined by astronomers until 1950. They realized something was there after comet orbits provided telltale evidence of a vast nesting ground for comets beyond Neptune.

The first recognized Kuiper Belt Objects were not discovered until the early 1990s.

Along with the roughly 10,000 small asteroids and comets counted across the Solar System to date, Pluto is one among dozens of so-called Trans-Neptunian Objects (NTOs).

NTOs bear that label because they cross the orbit of Neptune as they orbit the Sun.

The recent discoveries of large bodies in the Kuiper Belt drove astronomers to see if Pluto was not the only planet out there. They imagined more bodies larger and more distant than Pluto would be found, and they were.

Such objects are hard to find. Because the illumination of the Sun is so far away, light reflecting from them is extremely faint. Also, they have very dark surfaces reflecting very little light.

NASA fired an interplanetary probe called New Horizons toward Pluto in 2006. The spacecraft will arrive in the vicinity of Pluto about 2015. The robot explorer would swing by Pluto and then travel on out into the Kuiper Belt.

While the probe will have the capability to visit one or more Kuiper objects, some of the recently-discovered mysterious bodies probably won't be in a convenient position for a flyby.      MORE ABOUT NEW HORIZONS »

In 2006, the International Astronomical Union downgraded Pluto's classification to minor status leaving only eight planets of the Solar System described as major. Eris, Pluto and the big asteroid Ceres now are referred to by the IAU as dwarf planets.


How Do Planets Get Names?
  • When a large object is found, the International Astronomical Union decides what it will be named officially.
  • Planets, moons and minor planets are given names from Greek and Roman mythology.
  • For instance, Venus was named for the Roman god of love because it was said to be the most beautiful planet. Mars was named for the Roman god of war because of its blood red color.
  • If the IAU were to name 2003 VB12 something other than Sedna, it wouldn't be the first time names proposed by astronomers have been changed.
  • For instance, astronomer William Herschel was court astronomer for the English king George III. In 1781, when Herschel found the planet we now know as Uranus, he tried to name it Georgian Sidus – the Georgian Star – after the king. Astronomers didn't like the name and the planet eventually was named after the Greek god of the sky, Uranus. The Greek name for the god was Ouranos. Mars, Jupiter, Saturn, and Uranus were a sequence of generations in mythology.
  • Of course, there are only so many names in classical mythology, so another source of names will be needed as more minor planets, stars, asteroids and comets are found.
  • There is an interest in making astronomy inclusive of more of Earth's races and cultures by using non-traditional names.
  • Sedna's official number, 2003 VB12, was composed from the discovery year, month and date.
Planetoids, or minor planets, are clumps of matter in space. These pieces of debris orbiting the Sun are small, roughly-shaped, usually-rocky bodies.

While Quaoar is beyond the farthest known planets, some 95 percent of all known minor planets are closer in an orbit known as the Asteroid Belt, which lies between the orbits of the major planets Mars and Jupiter.

Large planetesimals were created as the new Sun's heat acted on nearby metal grains and chunks of rock during formation of the Solar System.

The new planetesimals were tiny worlds, up to 60 miles in diameter. Over time, some collided and stuck together. Over hundreds of million years, the planetesimals smashed into each other as well as the major planets. Some would have been deflected, even out of the Solar System.

Looking through telescopes at sunlight reflected from minor planets, astronomers say there may be more than 100,000, but only 10,000 small asteroids and comets have been catalogued so far. The 3,000 or so small bodies known as asteroids probably are not the remains of an exploded planet as, together, they total only about four-ten thousandths of the mass of Earth. More likely, they are debris from collisions of many small bodies at the time the Solar System was forming around 4.6 billion years ago.


Read more about the Solar System . . .
Star: The Sun  
Inner Planets: Mercury Venus Earth Mars  
Outer Planets: Jupiter Saturn Uranus Neptune Pluto
Other Bodies: Moons Asteroids Comets Kuiper Belt  
Beyond: Pioneers Voyagers  
Solar System STO cover Search STO Exploring the Solar System Questions E-mail © 2005 Space Today Online