The Dwarf Planet is officially the Largest in Solar System

This image shows an artist impression of the Dwarf Planet Eris.

Since Eris is larger than the Dwarf Planet Pluto, it is presented as the tenth planet.

However, a long-lasting debate over the status of Pluto forced the International Astronomical Union (IAU) to develop a precise definition of the term planet. 

On August 24, 2006, the IAU adopted a resolution, under which both Pluto and Eris were classified as "dwarf planets" and subsequently added to the Minor Planet Catalogue.

Our universe is full of mysteries but there are a few things we know for certain.

For instance, that the Earth orbits the Sun and not vice versa, or that there are eight planets in the solar system. If you still believe in the latter, you probably have not heard of Eris.

This is an image of the dwarf planet Eris (center) and its companion satellite Dysnomia (at 9 o'clock position) taken with NASA's Hubble Space Telescope on Aug. 30, 2006. Hubble observations were obtained on Dec. 3, 2005 and Aug. 30, 2006 using the Advanced Camera for Surveys.

Credit: Hubblesite

Eris is the largest dwarf planet discovered in 2005 using the Hubble Telescope and was initially described by NASA as the Solar System's tenth planet.

Eris is 27% larger than Pluto, has a diameter of 2.3 kilometers and one companion satellite (moon) called Dysnomia.

The planet orbits the sun at a distance of 96.4 astronomical units, taking 557 years to complete one lap.

Whilst it sounds like a fully-fledged planet, the word 'dwarf' tends to instill confusion. Eris is what astronomers call a plutoid; a trans-Neptunian object located in the part of the solar system known as the Kuiper belt.

A dwarf planet is now officially defined as a "celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravity, but that unlike a planet has not cleared its orbit of other objects."

The number of known planets in the solar system was therefore reduced to eight, as it was before Pluto's discovery in 1930.

With the new status Eris was granted its present name. Previously, the newly discovered space object was informally called Xena after a character from the popular television series Xena: Warrior Princess, but given the discord it caused in the astronomical community, the name of the Greek goddess Eris, a personification of strife, suits this planet like no other.

NASA seeks Kuiper Belt Objects for New Horizon's post-Pluto mission

An artist’s conception of a KBO encounter by New Horizons. 

Credit: JHUAPL/SwRI.

Are you ready for the summer of 2015? A showdown of epic proportions is in the making, as NASA's New Horizons spacecraft is set to pass within 12,500 kilometres of Pluto, roughly a third of the distance of the ring of geosynchronous satellites orbiting the Earth, a little over a year from now on July 14th, 2015.

But another question is already being raised, one that's assuming center stage even before we explore Pluto and its retinue of moons: will New Horizons have another target available to study for its post-Pluto encounter out in the Kuiper Belt?

Researchers say time is of the essence to find it.

To be sure, it's a big solar system out there, and it's not that researchers haven't been looking.

New Horizons was launched from Cape Canaveral Air Force Station on January 19th, 2006 atop an Atlas V rocket flying in a 551 configuration in one of the fastest departures from Earth ever: it took New Horizons just nine hours to pass Earth's moon after launch.

The idea has always been out there to send New Horizons onward to explore and object beyond Pluto in the Kuiper Belt, but thus far, searches for a potential target have turned up naught.

A recent joint statement from NASA's Small Bodies and Outer Planets Assessment Groups (SBAG and OPAG) has emphasized the scientific priority needed for identifying a possible Kuiper Belt Object (KBO) for the New Horizons mission post-Pluto encounter.

The assessment notes that such a chance to check out a KBO up close may only come once in our lifetimes: even though it's currently moving at a heliocentric velocity of just under 15 kilometres a second, it will have taken New Horizons almost a decade to traverse the 32 A.U. distance to Pluto.

The report also highlights the fact that KBOs are expected to dynamically different from Pluto as well and worthy of study.

The statement also notes that the window may be closing to find such a favourable target after 2014, as the upcoming observational apparition of Pluto as seen from Earth, and the direction New Horizons is headed afterwards, reaches opposition this summer on July 4th.

But time is of the essence, as it will allow researchers to plan for a burn and trajectory change for New Horizons shortly after its encounter with Pluto and Charon using what little fuel it has left.

Then there's the issue of debris in the Pluto system that may require fine-tuning its trajectory pre-encounter as well.

New Horizons will begin long range operations later this year in November, switching on permanently for two years of operations pre-, during and post- encounter with Pluto.

New Horizons spends its last days on Earth pre-encapsulation. Credit: NASA/KSC

And there currently isn't a short-list of "next best thing" targets for New Horizons post-Pluto encounter.

One object, dubbed VNH0004, may be available for distant observations in January of next year, but even this object will only pass 75 million kilometres, about 0.5 A.U. from New Horizons at its closest.

Ground based assets such as the Keck, Subaru and Gemini observatories have been repeatedly employed in the search over the past three years.

The best hopes lie with the Hubble Space Telescope, which can go deeper and spy fainter targets.

Nor could New Horizons carry out a search for new targets on its own. Its eight inch (20 cm in diameter) LORRI instrument has a limiting magnitude of about +18, which is not even close to what would be required for such a discovery.

New Horizons currently has 130 metres/sec of hydrazine fuel available to send it onwards to a possible KBO encounter, limiting its range and maneuverability into a narrow cone straight ahead of the spacecraft.

This restricts the parameters for a potential encounter to 0.35 A.U. off of its nominal path for a target candidate be to still be viable objective.

New Horizons will exit the Kuiper Belt at around 55 A.U. from the Sun, and will probably end its days joining the Voyager missions probing the outer solar system environment.

Like Pioneers 10 and 11, Voyagers 1 and 2 and the upper stage boosters that deployed them, New Horizons will escape our solar system and orbit the Milky Way galaxy for millions of years.

We recently proposed a fun thought experiment concerning just how much extraterrestrial "space junk" might be out there, littering the galactic disk.

Painted Stones video: Asteroids observed by the Sloan Digital Sky Survey

Alex Parker is an astronomer at UC Berkeley, where he researches minor planets—asteroids, Kuiper Belt Objects (giant iceballs orbiting past Neptune), and more.

He took the asteroids in the solar system observed by the Sloan Digital Sky Survey (over 100,000 of them) and created an animation showing their orbits, their relative sizes, and even their colors in the survey. 

The resulting video, “Painted Stones”, has been called 'simply wondrous.'

Some planet-like Kuiper belt objects don't fit "Nice" model

The bodies in the Kuiper Belt. Credit: Don Dixon

The Kuiper belt—the region beyond the orbit of Neptune inhabited by a number of small bodies of rock and ice—hides many clues about the early days of the Solar System.

According to the standard picture of Solar System formation, many planetesimals were born in the chaotic region where the giant planets now reside.

Some were thrown out beyond the orbit of Neptune, while others stayed put in the form of Trojan asteroids (which orbit in the same trajectory as Jupiter and other planets). This is called the Nice model.

However, not all Kuiper belt objects (KBOs) play nicely with the Nice model (the model is named after the city of Nice in France.)

A new study of large scale surveys of KBOs revealed that those with nearly circular orbits lying roughly in the same plane as the orbits of the major planets don't fit the Nice model, while those with irregular orbits do.

It's a puzzling anomaly, one with no immediate resolution, but it hints that we need to refine our Solar System formation models.

This new study is described in a recently released paper by Wesley Fraser, Mike Brown, Alessandro Morbidelli, Alex Parker, and Konstantin Batygin published in the Astrophysical Journal,.

These researchers combined data from seven different surveys of KBOs to determine roughly how many of each size of object are in the Solar System, which in turn is a good gauge of the environment in which they formed.

The difference between this and previous studies is the use of absolute magnitudes—a measure of how bright an object really is—as opposed to their apparent magnitudes, which are simply how bright an object appears.

The two types of magnitude are related by the distance an object is from Earth, so the observational challenge comes down to accurate distance measurements.

Absolute magnitude is also related to the size of an KBO and its albedo (how much light it reflects), both important physical quantities for understanding formation and composition.

Finding the absolute magnitudes for KBOs is more challenging than apparent magnitudes for obvious reasons: these are small objects, often not resolved as anything other than points of light in a telescope.

That means requires measuring the distance to each KBO as accurately as possible. As the authors of the study point out, even small errors in distance measurements can have a large effect on the estimated absolute magnitude.

In terms of orbits, KBOs fall into two categories: "hot" and "cold", confusing terms having nothing to do with temperature.

The "cold" KBOs are those with nearly circular orbits (low eccentricity, in mathematical terms) and low inclinations, meaning their trajectories lie nearly in the ecliptic plane, where the eight canonical planets also orbit.

In other words, these objects have nearly planet-like orbits. The "hot" KBOs have elongated orbits and higher inclinations, behavior more akin to comets.


The authors of the new study found that the hot KBOs have the same distribution of sizes as the Trojan asteroids, meaning there are the same relative number of small, medium, and large KBOs and similarly sized Trojans.

That hints at a probable common origin in the early days of the Solar System. This is in line with the Nice model, which predicts that, as they migrated into their current orbits, the giant planets kicked many planetesimals out beyond Neptune.

However, the cold KBOs don't match that pattern at all: there are fewer large KBOs relative to smaller objects.

To make matters more strange, both hot and cold seem to follow the same pattern for the smaller bodies, only deviating at larger masses, which is at odds with expectations if the cold KBOs formed where they orbit today.

To put it another way, the Nice model as it stands could explain the hot KBOs and Trojans, but not the cold. That doesn't mean all is lost, of course.

The Nice model seems to do very well except for a few nagging problems, so it's unlikely that it's completely wrong. As we've learned from studying exoplanet systems, planet formation models are a work in progress—and astronomers are an ingenious lot.

The Kuiper Belt Objects (KBO) at 20

Some planets and objects, of the Kuiper Belt (NB: Eris is actually smaller than Pluto). Credit: sollunaterra.webs.com

Planetary science is celebrating the 20th anniversary of the discovery of the Kuiper Belt.

That came in 1992, when the first Kuiper Belt Object (KBO) was discovered.

Actually, of course, the first object in the Kuiper Belt was discovered in 1930—Pluto itself; and the second such object, Pluto's giant moon Charon, was discovered in 1978.

New Horizons hopes to explore beyond Pluto, into the ancient and unexplored Kuiper Belt. Credit: NASA

The Kuiper Belt was first postulated—most famously by Gerard Kuiper—by planetary scientists back in the 1930s, '40s and '50s.

But it took until 1992 for technology to mature sufficiently enough to find another object (outside the Pluto system) orbiting the Sun beyond Neptune.

Since 1992, more than 1,000 KBOs have been discovered. But only a tiny fraction of the sky has been surveyed for KBOs.

It is estimated that more than 100,000 KBOs exist with diameters of 100 kilometers or larger, along with billions of smaller objects down to the size of cometary nuclei, just a kilometer or two across.

By comparison, Pluto is huge—its diameter is almost 2,400 kilometers, making a drive around its equator as far as from Manhattan to Moscow!

Most of the known KBOs are just 100 to 300 kilometers across, about one-tenth of Pluto's diameter. But some are smaller than 100 kilometers across, and some are larger than 300 kilometers across.

In fact, there is great diversity among KBOs:

• Some are red and some are gray; 
• The surfaces of some are covered in water ice, but others (like Pluto) have exotic volatile ices like methane and nitrogen; 
• Many have moons, though none with more known moons than Pluto; 
• Some are highly reflective (like Pluto), others have much darker surfaces; 
• Some have much lower densities than Pluto, meaning they are primarily made of ice. Pluto's density is so high that we know its interior is about 70% rock in its interior; 
• a few known KBOs are more dense than Pluto, and even rockier!

But I don't consider this surprising assortment of KBOs to be the most important contribution to our knowledge of the Solar System that has come from telescope exploration of the Kuiper Belt.

The three greatest solar system lessons we've learned from the Kuiper Belt are:

•  That our planetary system is much larger than we used to think. In fact, we were largely unaware of the Kuiper Belt—the largest structure in our solar system—until it was discovered 20 years ago. It's akin to not having maps of the Earth that included the Pacific Ocean as recently as 1992! 
• That the locations and orbital eccentricities and inclinations of the planets in our solar system (and other solar systems as well) can change with time. This even creates whole flocks of migration of planets in some cases. We have firm evidence that many KBOs (including some large ones like Pluto), were born much closer to the Sun, in the region where the giant planets now orbit. 
• And, perhaps most surprisingly, that our solar system, and very likely very many others, was very good at making small planets, which dominate the planetary population! Today we know of more than a dozen dwarf planets in the Solar System, and those dwarfs already outnumber the number of gas giants and terrestrial planets combined. But it is estimated that the ultimate number of dwarf planets we will discover in the Kuiper Belt and beyond may well exceed 10,000. Who knew? (And which class of planet is the misfit now?)

Kuiper Belt Wider and More Massive than Asteroid belt

Kuiper Belt is a region in space initially predicted by astronomer Gerard P. Kuiper.

Now, this was way back in 1951. It took more than four decades before the said region was finally discovered.

In fact, it took five years of searching before David Jewitt and Jane Luu were finally successful.

Well, you can say that there are similarities between Kuiper Belt and the asteroid belt, both being composed of celestial objects.

Recent discoveries reveal that the Kuiper is much much larger than originally thought, about 20 times wider and perhaps 200 times more massive.

Also, while the asteroid belt is composed mainly of asteroids and meteoroids, which are basically made up of rock and metal, Kuiper is made up of frozen volatiles (methane, ammonia, and water). That’s right, the objects there are pretty much like comets.

Now, how did a “planet” like Pluto get to be reclassified into a KBO? First, Pluto is found right inside Kuiper. Furthermore, there are lots of objects in there that are pretty much of the same size (only slightly smaller actually) as Pluto. Finally, there are objects outside (but near) Kuiper that are larger than Pluto.

One of these objects is Eris, which is estimated to be 27% more massive than Pluto. Another object which is larger than Pluto and also believed to have been once a KBO is Triton, Neptune’s natural satellite.

We’ve mentioned earlier that most of the objects found in Kuiper have similar composition to those of comets. It would be therefore tempting to conclude that Kuiper and the Oort cloud are one. The Oort cloud is a hypothetical region believed to be the “dwelling place” of comets.

Scientists however think the Oort cloud is much much farther away. While the Kuiper Belt is in the region between 30 AU to 55 AU from the Sun, the Oort cloud is believed to be 50,000 AU from the Sun. That’s nearly one light-year away.

A deeper understanding of Kuiper should emerge once the spacecraft New Horizons reaches Pluto in July 2015. New Horizons was launched on January 19, 2006 for the purpose of exploring the Kuiper Belt.

Snow White: Astronomers Find Ice and Possibly Methane

Astronomers at the California Institute of Technology (Caltech) have discovered that the dwarf planet 2007 OR10-nicknamed Snow White-is an icy world, with about half its surface covered in water ice that once flowed from ancient, slush-spewing volcanoes.

The new findings also suggest that the red-tinged dwarf planet may be covered in a thin layer of methane, the remnants of an atmosphere that's slowly being lost into space.

"You get to see this nice picture of what once was an active little world with water volcanoes and an atmosphere, and it's now just frozen, dead, with an atmosphere that's slowly slipping away," says Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, who is the lead author on a paper to be published in the Astrophysical Journal Letters describing the findings. The paper is now in press.

Snow White-which was discovered in 2007 as part of the PhD thesis of Brown's former graduate student Meg Schwamb-orbits the sun at the edge of the solar system and is about half the size of Pluto, making it the fifth largest dwarf planet.

At the time, Brown had guessed incorrectly that it was an icy body that had broken off from another dwarf planet named Haumea; he nicknamed it Snow White for its presumed white color.

Soon, however, follow-up observations revealed that Snow White is actually one of the reddest objects in the solar system. A few other dwarf planets at the edge of the solar system are also red.

These distant dwarf planets are themselves part of a larger group of icy bodies called Kuiper Belt Objects (KBOs). As far as the researchers could tell, Snow White, though relatively large, was unremarkable-just one out of more than 400 potential dwarf planets that are among hundreds of thousands of KBOs.

"With all of the dwarf planets that are this big, there's something interesting about them-they always tell us something," Brown says.

"This one frustrated us for years because we didn't know what it was telling us." At that time, the Near Infrared Camera (NIRC) at the Keck Observatory-which Caltech professor of physics Tom Soifer and chief instrument scientist Keith Matthews helped design in the 1990s-was the best instrument astronomers had to study KBOs, according to Brown. But NIRC had just been retired, so no one could observe 2007 OR10 in detail. "It kind of languished," he says.

NASA - What Is Pluto?

Pluto was discovered in 1930 by an astronomer from the United States. An astronomer is a person who studies stars and other objects in space.

Pluto was known as the smallest planet in the solar system and the ninth planet from the sun.

Today, Pluto is called a "dwarf planet." A dwarf planet orbits the sun just like other planets, but it is smaller. A dwarf planet is so small it cannot clear other objects out of its path.

On average, Pluto is more than 3.6 billion miles (5.8 billion kilometers) away from the sun. That is about 40 times as far from the sun as Earth.

Pluto orbits the sun in an oval like a racetrack. Because of its oval orbit, Pluto is sometimes closer to the sun than at other times. At its closest point to the sun Pluto is still billions of miles away.

Pluto is in a region called the Kuiper (KY-per) Belt. Thousands of small, icy objects like Pluto are in the Kuiper Belt.

Pluto is only 1,400 miles (2,300 kilometers) wide. That's about half the width of the United States. Pluto is slightly smaller than Earth's moon. It takes Pluto 248 years to go around the sun. One day on Pluto is about 6 1/2 days on Earth.

Pluto was named by an 11-year-old girl from England. The dwarf planet has three moons. Its largest moon is named Charon (KER-ən). Charon is about half the size of Pluto.

Pluto's two other moons are named Nix and Hydra. They were discovered in 2005. NASA's Hubble Space Telescope took pictures of the two new moons. Nix and Hydra are very small. The moons are less than 100 miles (160 kilometers) wide.

A drawing of the solar system shows Pluto's tilted orbit. Pluto's orbital path angles 17 degrees above the line, or plane, where the eight planets orbit. Credit: NASA

In 2003, an astronomer saw a new object beyond Pluto. The astronomer thought he had found a new planet. The object he saw was larger than Pluto. He named the object Eris (EER-is).

Finding Eris caused other astronomers to talk about what makes a planet a "planet." There is a group of astronomers that names objects in space. This group decided that Pluto was not really a planet because of its size and location in space. So Pluto and objects like it are now called dwarf planets.

Pluto is also called a plutoid. A plutoid is a dwarf planet that is farther out in space than the planet Neptune. The three known plutoids are Pluto, Eris and Makemake (MAH-kee-MAH-kee). Astronomers use telescopes to discover new objects like plutoids.

Scientists are learning more about the universe and Earth's place in it. What they learn may cause them to think about how objects like planets are grouped. Scientists group objects that are like each other to better understand them. Learning more about faraway objects in the solar system is helping astronomers learn more about what it means to be a planet.

Kuiper Belt Objects: Coordinated Stargazing

Coordinated Stargazing

The high albedo suggests that the KBO's surface is made of reflective water-ice particles, and that would support a theory about how the KBO formed.

Many researchers believe there was a collision that occurred one billion years ago between a dwarf planet in the Kuiper Belt known as Haumea and another object that caused Haumea's icy mantle to break into a dozen or so smaller bodies, including 55636.

Far beyond the orbit of Neptune in a region of the outer solar system known as the Kuiper Belt float thousands of icy, moon-sized bodies called Kuiper Belt objects (KBOs). Astronomers think they are the remnants of the bodies that slammed together to form the planets more than 4 billion years ago.

Unlike Earth, which has been continually eroded by wind and water since it was formed, KBOs haven't changed much over time and may hold clues about the early solar system and planet formation.

Until now, astronomers have used telescopes to find KBOs and obtain their spectra to determine what types of ices are on their surface. They have also used thermal-imaging techniques to get a rough idea of the size of KBOs, but other details have been difficult to glean.

While astronomers think there are about 70,000 KBOs that are larger than 100 kilometers in diameter, the objects' relatively small size and location make it hard to study them in detail.

One method that has been has been proposed for studying KBOs is to observe one as it passes briefly in front of a bright star; such events, known as stellar occultations, have yielded useful information about other planets in the solar system.

By monitoring the changes in starlight that occur during an occultation, astronomers can determine the object's size and temperature, whether it has any companion objects and if it has an atmosphere.

The trick is to know enough about the orbit of a KBO to be able to predict its path and observe it as it passes in front of a star.

This was done successfully for the first time last October when a team of 18 astronomy groups led by James Elliot, a professor of planetary astronomy in MIT's Department of Earth, Atmospheric and Planetary Sciences, observed an occultation by an object named "KBO 55636."

As Elliot and his colleagues report in a paper published in Nature, the occultation provided enough data to determine the KBO's size and albedo, or how strongly it reflects light.

The surface of 55636 turns out to be as reflective as snow and ice, which surprised the researchers because ancient objects in space usually have weathered, dull surfaces.