Ten New Dwarf Planets and Candidates in Our Solar System

By Alton Parrish.

Do you know how many planets and dwarf planets are in our solar system. I thought I did, but I didn’t.

Eris, Haumea, Sedna, Makemake are dwarf planets. Other plutoids or planetoids that may one day be named dwarf planets include Quaoar, Orcus, Ixion, Varuna, Huya and 2002 TC302. These candidates are all large enough to be named dwarf planets and their candidacy is under consideration by the International Astronomical Union (IAU) as they are all larger than Ceres. Ceres is a dwarf planet, smallest of the five identified dwarf planets, originally classified as a planet, and later as the largest asteroid and now as a dwarf planet.

Dwarf Planets in our solar system beyond Neptune

Credit: NASA

Astronomers have detected over 500 bodies orbiting the sun well beyond the orbit of Neptune. Among these Trans-Neptunian Bodies (TNOs) are a growing number that rival Pluto in size. This caused astronomers to rethink how they should define the term planet.

In 2006 Pluto was demoted from a planet to a dwarf planet, joining the large asteroid Ceres in that new group. Several other TNOs also joined that group, which now includes five bodies shown highlighted in the table. A number of other large objects, called Plutoids, are also listed. The AU (astronomical unit) distance from the Earth to the Sun is one or 93 million miles (92,955,807.273 miles to be exact). Ceres which is between Jupiter and Mars was once considered the largest asteroid but is now considered a dwarf planet like Pluto.

Name AU Distance Years For Orbit

Credit: NASA
For the first time, in 2005 NASA’s Hubble Space Telescope distinctly saw the “tenth planet,” then nicknamed “Xena,” after the warrior princess in the TV show of the same name and found that it’s only slightly larger than Pluto. The announcement was made on March 11th, 2006. The name was later officially changed to Eris.

Eris, formerly known as Xena

An artist’s concept of the Kuiper Belt Object nicknamed “Xena,” now called Eris, with its moon dubbed “Gabrielle” just above. The sun can be seen in the upper left corner.

Credit: NASA, ESA, and A. Schaller (for STScI)

Though previous ground-based observations suggested that Eris’ diameter was about 30 percent greater than Pluto, Hubble observations taken Dec. 9 and 10, 2005, showed Eris’ diameter as 1,490 miles (with an uncertainty of 60 miles).Adding insult to injury for the former ninth planet, Brown has now determined that Eris is also more massive than Pluto. This new detail was determined by observations of Eris’ tiny moon Dysnomia. The Hubble Space Telescope and Keck Observatory took images of the moon’s movement, from which Brown precisely calculated Eris to be 27 percent more massive than Pluto. In fact, if you scooped up all the asteroids in the asteroid belt they would fit inside Eris, with a lot of room to spare.

Currently, Eris is more than three times farther from the Sun than Pluto. It is so cold out there that the dwarf planet’s atmosphere has frozen onto the surface as a frosty glaze. The coating gleams brightly, reflecting as much sunlight as fresh fallen snow. The path Eris takes around the Sun is shaped like an oval rather than a circle. In about 290 years, Eris will move close enough to the Sun to partially thaw. Its icy veneer will melt away revealing a rocky, speckled landscape similar to Pluto’s.

This is an artist’s concept of Kuiper Belt object Eris and its tiny satellite Dysnomia. Eris is the large object at the bottom of the illustration. A portion of its surface is lit by the Sun, located in the upper left corner of the image. Eris’s moon, Dysnomia, is located just above and to the left of Eris. The Hubble Space Telescope and Keck Observatory took images of Dysnomia’s movement from which astronomer Mike Brown (Caltech) precisely calculated Eris to be 27 percent more massive than Pluto.
Artist concept of Eris
Artwork Credit: NASA, ESA, Adolph Schaller (for STScI)

Aptly named after the Greek goddess of conflict, the icy dwarf planet Eris has rattled the general model of our solar system. The object was discovered by astronomer Mike Brown of Caltech in the outer reaches of the Kuiper belt in 2005. Its detection provoked debate about Pluto’s classification as a planet. Eris is slightly larger than Pluto.
“Hubble is the only telescope capable of getting a clean visible-light measurement of the actual diameter of Eris,” said Mike Brown, planetary scientist at the California Institute of Technology in Pasadena, Calif. Brown’s research team discovered Eris, officially cataloged as 2003 UB313, and its results have been accepted for publication in the Astrophysical Journal.

Only a handful of images were required to determine Eris’ diameter. Located 10 billion miles from Earth with a diameter a little more than half the width of the United States, the object is 1.5 pixels across in Hubble’s view. That’s enough to make a precise size measurement.

Pluto’s diameter, as measured by Hubble, is 1,422 miles.

Located 10 billion miles away, but with a diameter that is a little more than half the width of the United States, Xena is only 1.5 picture elements across in Hubble’s Advanced Camera for Surveys’ view.

Credit: NASA, ESA, and M. Brown (CalTech)Because Eris is smaller than previously thought, but comparatively bright, it must be one of the most reflective objects in the solar system. The only object more reflective is Enceladus, a geologically active moon of Saturn whose surface is continuously recoated with highly reflective ice by active geysers.

Diagram shows orbit of Eris along with a photo of the dwarf planet

Eris’ bright reflectivity is possibly due to fresh methane frost on its surface. The object may have had an atmosphere when it was closer to the sun, but as it moved to its current location farther away this atmosphere would have “frozen out,” settling on the surface as frost.

Another possibility is that Eris leaks methane gas continuously from its warmer interior. When this methane reaches the cold surface, it immediately freezes solid, covering craters and other features to make it uniformly bright to Hubble’s telescopic eye.

Eris takes about 560 years to orbit the sun, and it is now very close to aphelion (the point on its orbit that is farthest from the sun). Brown next plans to use Hubble and other telescopes to study other recently discovered Kuiper Belt objects that are almost as large as Pluto and Eris. The Kuiper Belt is a vast ring of primordial icy comets and larger bodies encircling Neptune’s orbit.

These time-lapse images of a newfound planet in our solar system, called 2003UB313, were taken on Oct. 21, 2003, using the Samuel Oschin Telescope at the Palomar Observatory near San Diego, Calif. The planet, circled in white, is seen moving across a field of stars. The three images were taken about 90 minutes apart.Scientists did not discover that the object in these pictures was a planet until Jan. 8, 2005.

time-lapse image of planet 2003UB313, which is circled

Image credit: Samuel Oschin Telescope, Palomar Observatory

Since the discovery of Eris, the number of planetoids has grown and may to continue to grow.

Credit: NASA

Sedna Mystery Deepens With Hubble Images Of Farthest Planetoid

Astronomers studying 35 NASA Hubble Space Telescope (HST) images of the solar system’s farthest known object, unofficially named Sedna, are surprised the object does not appear to have a companion moon of any substantial size.

This unexpected result might offer new clues to the origin and evolution of objects on the far edge of the solar system.

Sedna’s existence was announced on March 15. Its discoverer, Mike Brown of the California Institute of Technology, Pasadena, Calif., was so convinced it had a satellite, that an artist’s concept of Sedna released to the media included a hypothetical moon.

Credit: Hubble Space TelescopeBrown’s prediction was based on the fact, Sedna appears to have a very slow rotation that could best be explained by the gravitational tug of a companion object. Almost all other solitary bodies in the solar system complete a spin in a matter of hours.“I’m completely baffled at the absence of a moon,” Brown said. “This is outside the realm of expectation and makes Sedna even more interesting. But I simply don’t know what it means.” Immediately following the announcement of the discovery of Sedna, NASA astronomers turned the HST toward the new planetoid to search for the expected companion moon. The space-based platform provides the resolving power needed to make such precision measurements in visible light. “Sedna’s image isn’t stable enough in ground-based telescopes,” Brown said.Surprisingly, the HST images, taken March 16 with the new Advanced Camera for Surveys, only show the single object Sedna, along with a faint, very distant background star in the same field of view.

Even with Hubble’s crisp view, it may just be barely resolving the disk of mysterious Sedna, Brown said. It’s equivalent to trying to see a soccer ball 900 miles away. This would place an upper limit in the object’s size of being approximately three-quarters the size of Pluto, or about 1,000 miles across.

But Brown predicted a satellite would pop up as a companion “dot” in Hubble’s precise view. But the object is not there. There is a very small chance, it might have been behind Sedna or transiting in front of it, so it could not be seen separately from Sedna in the HST images.

Brown based this prediction on his earlier observations of apparent periodic changes in light reflecting from Sedna’s mottled surface. The resulting light curve gives a rotation period of 40 days. If true, Sedna would be the slowest rotating object in the solar system after Mercury and Venus, whose slow rotation rates are due to the tidal influence of the sun.

One easy way out of this dilemma is the possibility the rotation period is not as slow as astronomers thought. But even with a careful reanalysis, the team remains convinced the period is correct. Brown admits, “I’m completely lost for an explanation as to why the object rotates so slowly.”

Small bodies like asteroids and comets typically complete one rotation in a matter of hours. Pluto has a six-day period from being tidally locked to the revolution of its satellite Charon. The HST was the first telescope to resolve Pluto and Charon as two separate bodies. NASA’s forthcoming James Webb Space Telescope will provide a platform for further high-resolution studies of infrared light from such distant, cold bodies in our solar system.

Electronic images of Sedna and additional information are available on the Internet, at:
http://hubblesite.org/news/2004/14

Quaoar

NASA’s Hubble Space Telescope has measured the largest object in the solar system seen since the discovery of Pluto 72 years ago. The discovery was made October 7, 2002

Approximately half the size of Pluto, the icy world 2002 LM60, dubbed “Quaoar” (pronounced kwa-whar) by its discoverers, is the farthest object in the solar system ever to be resolved by a telescope. It was initially detected by a ground-based telescope as simply a dot of light, until astronomers aimed Hubble’s powerful telescope at it.

Quaoar is about 4 billion miles away from Earth, well over a billion miles farther away than Pluto. Unlike Pluto, its orbit around the Sun is circular, even more so than most of the planetary-class bodies in the solar system.

Sum of 16 Hubble exposures registered on Quaoar

Credit: NASA/HST

Although smaller than Pluto, Quaoar is greater in volume than all the asteroids combined (though probably only one-third the mass of the asteroid belt, because it’s icy rather than rocky). Quaoar’s composition is theorized to be largely ices mixed with rock, not unlike the makeup of a comet, though 100 million times greater in volume.

This finding yields important new insights into the origin and dynamics of the planets, and the mysterious population of bodies dwelling in the solar system’s final frontier: the elusive, icy Kuiper Belt beyond Neptune.

Michael Brown and Chadwick Trujillo of the California Institute of Technology, Pasadena, Calif. are reported the findings at the 34th annual meeting of the Division for Planetary Sciences of the American Astronomical Society in Birmingham, Ala. 50000 Quaoar (“Kwawar”) is a rocky trans-Neptunian object in the Kuiper belt with one known moon. Several astronomers consider it to be a dwarf planet, although the IAU has not formally recognized it as one.

Trujillo and Brown used the Palomar Oschin Schmidt telescope to discover Quaoar as an 18.5-magnitude object creeping across the summer constellation Ophiuchus (it’s less than 1/100,000 the brightness of the faintest star seen by the human eye). Brown had to do follow-up observations using Hubble’s new Advanced Camera for Surveys on July 5 and August 1, 2002, to measure the object’s true angular size of 40 milliarcseconds, corresponding to a diameter of about 800 miles (1300 kilometers). Only Hubble has the sharpness needed to actually resolve the disk of the distant world, leading to the first-ever direct measurement of the true size of a Kuiper Belt Object (KBO).

Like Pluto, Quaoar dwells in the Kuiper Belt, an icy debris field of comet-like bodies extending 7 billion miles beyond Neptune’s orbit. Over the past decade more than 500 icy bodies have been found in the Kuiper Belt. With a few exceptions all have been significantly smaller than Pluto.

This latest large KBO is too new to have been officially named by the International Astronomical Union. Trujillo and Brown have proposed naming it 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’s “icy dwarf” cousin, Pluto, was discovered in 1930 in the course of a 15-year search for trans-Neptunian planets. It wasn’t realized until much later that Pluto actually was the largest of the known Kuiper Belt objects. The Kuiper Belt wasn’t theorized until 1950, after comet orbits provided telltale evidence of a vast nesting ground for comets just beyond Neptune. The first recognized Kuiper Belt objects were not discovered until the early 1990s. This new object is by far the “biggest fish” astronomers have snagged in KBO surveys. Brown predicts, within a few years, even larger KBOs will be found, and Hubble will be invaluable for follow-up observations to pin down sizes.

Orbits of Makemake (blue), Haumea (green), contrasted with the orbit of Pluto (red) and the ecliptic (grey). The perihelia and the aphelia (Q) are marked with the dates of passage. The positions on April 2006 are marked with the spheres illustrating relative sizes and differences in albedo and colour.
Credit: NASA

Makemake (2005 FY9)

Makemake, formally designated (136472) Makemake, is a dwarf planet and perhaps the largest Kuiper belt object (KBO) in the classical population,[nb 4] with a diameter that is probably about 2/3 the size of Pluto. Makemake has no known satellites, which makes it unique among the largest KBOs and means that its mass can only be estimated. Its extremely low average temperature, about 30 K (−243.2 °C), means its surface is covered with methane, ethane, and possibly nitrogen ices.

Makemake as seen by the Hubble Space Telescope

Credit: Hubble Space TelescopeInitially known as 2005 FY9 and later given the minor planet number 136472, it was discovered on March 31, 2005, by a team led by Michael Brown, and announced on July 29, 2005. Its name derives from the Rapanui god Makemake. On June 11, 2008, the International Astronomical Union (IAU) included Makemake in its list of potential candidates to be given “plutoid” status, a term for dwarf planets beyond the orbit of Neptune that would place the object alongside Pluto,Haumea and Eris. Makemake was formally classified as a plutoid in July 2008

Makemake map

Credit: HST

Artistic comparison of Eris, Pluto, Makemake, Haumea, Sedna, 2007 OR10, Quaoar, Orcus, and Earth. These eight trans-Neptunian objects have the brightest absolute magnitudes, although several other TNOs have been found to be physically larger than Orcus, and several more may yet be found.

In a letter written to the journal Astronomy and Astrophysics in 2006, Licandro et al. reported the measurements of the visible and near-infrared spectrum of Makemake. They used the William Herschel Telescope and Telescopio Nazionale Galileo and showed that the surface of Makemake resembles that of Pluto. Like Pluto, Makemake appears red in the visible spectrum, and significantly redder than the surface of Eris (see colour comparison of TNOs).The near-infrared spectrum is marked by the presence of the broad methane (CH4) absorption bands. Methane is observed also on Pluto and Eris, but its spectral signature is much weaker.

Spectral analysis of Makemake’s surface revealed that methane must be present in the form of large grains at least one centimetre in size. In addition, large amounts of ethane and tholins may be present as well, most likely created by photolysis of methane by solar radiation. The tholins are probably responsible for the red color of the visible spectrum. Although evidence exists for the presence of nitrogen ice on its surface, at least mixed with other ices, there is nowhere near the same level of nitrogen as on Pluto and Triton, where it composes more than 98 percent of the crust. The relative lack of nitrogen ice suggests that its supply of nitrogen has somehow been depleted over the age of the Solar System

The far-infrared (24–70 μm) and submillimeter (70–500 μm) photometry performed by Spitzer and Herschel telescopes revealed that the surface of Makemake is not homogeneous. While the majority of it is covered by nitrogen and methane ices, where the albedo ranges from 78 to 90%, there are small patches of dark terrain whose albedo is only 2 to 12%, and which make up 3–7% of the surface.

The presence of methane and possibly nitrogen suggests that Makemake could have a transient atmosphere similar to that of Pluto near its perihelion. Nitrogen, if present, will be the dominant component of it. The existence of an atmosphere also provides a natural explanation for the nitrogen depletion: since the gravity of Makemake is weaker than that of Pluto, Eris and Triton, a large amount of nitrogen was probably lost via atmospheric escape; methane is lighter than nitrogen, but has significantly lower vapor pressure at temperatures prevalent at the surface of Makemake (30–35 K),[nb 3] which hinders its escape; the result of this process is a higher relative abundance of methane.

No satellites have been detected around Makemake so far. A satellite having a brightness 1% of that of the primary would have been detected if it had been at the distance 0.4 arcseconds or further from Makemake. This contrasts with the other largest trans-Neptunian objects, which all possess at least one satellite: Eris has one, Haumea has two and Pluto has five. 10% to 20% of all trans-Neptunian objects are expected to have one or more satellites. Since satellites offer a simple method to measure an object’s mass, lack of a satellite makes obtaining an accurate figure for Makemake’s mass more difficult

Haumea

Haumea, formal designation 136108 Haumea, is a dwarf planet located beyond Neptune’s orbit. Just one-third the mass of Pluto, it was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory in the United States and, in 2005, by a team headed by J. L. Ortiz at the Sierra Nevada Observatory in Spain, though the latter claim has been contested and neither is official. On September 17, 2008, it was designated a dwarf planet by the International Astronomical Union (IAU) and named after Haumea, the Hawaiian goddess of childbirth.

Keck image of Haumea and its two moons. Hiʻiaka is above Haumea (centre), and Namaka is directly below.

Haumea’s extreme elongation makes it unique among known dwarf planets. Although its shape has not been directly observed, calculations from its light curve suggest it is an ellipsoid, with its major axis twice as long as its minor.

Artist conception of Haumea and its moons

Nonetheless, its gravity is believed sufficient for it to have relaxed into hydrostatic equilibrium, thereby meeting the definition of a dwarf planet. This elongation, along with its unusually rapid rotation, high density, and high albedo (from a surface of crystalline water ice), are thought to be the results of a giant collision, which left Haumea the largest member of a collisional family that includes several large trans-Neptunian objects (TNOs) and its two known moons.

Haumea is the largest member of its collisional family, a group of astronomical objects with similar physical and orbital characteristics thought to have formed when a larger progenitor was shattered by an impact. This family is the first to be identified among TNOs and includes—beside Haumea and its moons

Brown et al. proposed that the family were a direct product of the impact that removed Haumea’s ice mantle, but a second proposal suggests a more complicated origin: that the material ejected in the initial collision instead coalesced into a large moon of Haumea, which was later shattered in a second collision, dispersing its shards outwards. This second scenario appears to produce a dispersion of velocities for the fragments that is more closely matched to the measured velocity dispersion of the family members.

The presence of the collisional family could imply that Haumea and its “offspring” might have originated in the scattered disc. In today’s sparsely populated Kuiper belt, the chance of such a collision occurring over the age of the Solar System is less than 0.1 percent.

The family could not have formed in the denser primordial Kuiper belt because such a close-knit group would have been disrupted by Neptune’s migration into the belt—the believed cause of the belt’s current low density. Therefore it appears likely that the dynamic scattered disc region, in which the possibility of such a collision is far higher, is the place of origin for the object that generated Haumea and its kin.

Because it would have taken at least a billion years for the group to have diffused as far as it has, the collision which created the Haumea family is believed to have occurred very early in the Solar System’s history.

Huya

38628 Huya is a plutino and dwarf-planet candidate discovered in 2000.

With an old Spitzer size estimate of 532 ± 25 km, it was a dwarf planet candidate (icy trans-Neptunian objects with a diameter above around 400 km are expected to be spherical) although the IAU has never classified it as such. Light-curve-amplitude analysis, which shows only small deviations, suggests that it is likely a spheroid with small albedo spots. Tancredi (2010) thinks that Huya is very probably a dwarf planet.

It was discovered in March 2000 by Ignacio Ferrin and announced on 24 October 2000. It was assigned the name Huya, after Juyá, the Wayuu rain god, in August 2003 by the International Astronomical Union (IAU).

Artist’s impression of 38628 Huya

Orcus

90482 Orcus, a huge planetoid, is discovered in 2004 by the Near Earth Asteroid Tracking survey team. In 2252, the planetoid Orcus will have completed one orbit of the Sun since its discovery in 2004, based upon current orbital measurements which give it a period of 248 Earth years.

Orcus and its moon Vanth

Credit: Wikipedia

Vanth, is the single known natural satellite of the plutino and likely dwarf planet Orcus. It was discovered by Mike Brown and T.-A. Suer using discovery images taken by the Hubble Space Telescope on November 13, 2005. The discovery was announced on 22 February 2007 in IAUC 8812.

2007 OR10

2007 OR10 is a very large trans-Neptunian object. It is the largest body in the Solar System without a name, estimated to be between Haumea and Sedna in size. Its mass has never been measured, and its diameter is not well determined. It has not been formally recognized as a dwarf planet by the IAU, although some astronomers consider it to be one and others consider it likely to be one.

Artist impression of 2007 OR10

File:1880-CT MBrown SPOTLIGHT medium.jpg

Varuna

Varuna and an object called 2002 AW197 are each approximately 540 miles across (900 kilometers). Unlike dimensions derived from Hubble’s direct observations, these diameters are deduced from measuring the objects’ temperatures and calculating a size based on assumptions about the KBOs’ reflectivity, so the uncertainty in true size is much greater.

Artistic conception of Varuna

Varuna is named after the Hindu deity, Varuṇa. Varuṇa was one of the most important deities of the ancient Indians, and he presided over the waters of the heaven and of the ocean and as the guardian of immortality.] Due to his association with the waters and the ocean, he is often identified with Greek Poseidon and Roman Neptune. Varuna received the minor planet number 20000 because it was the largest cubewano found so far, and was believed to be as large as Ceres.

Ixion

Ixion was originally discovered on May 22, 2001, by a group of American astronomers lead by Robert L. Millis of the Lowell Observatory in Flagstaff, Arizona. (M. Buie (Lowell), E. Chiang (IAS), J. Elliot (MIT), S. Kern (MIT), D. Trilling (U. Penn.), R. M. Wagner (LBT Obs.), L. H. Wasserman (Lowell)]. Furthering this work, Finnish and Swedish astronomers, using the La Silla Observatory in Chile, calculate Ixion to have a diameter of about 1065 ±165 kilometers. Thus, Ixion is similar in size to Ceres (the largest of the four primary asteroids and also now considered a dwarf planet) and is one of the brightest of the Kuiper objects. Ixion’s orbital period is 250.05 years and it has a 19.598° orbital inclination

Ixion formerly known as 2001 KX76

Credit: ESO

28978 Ixion is a plutino (an object that has a 2:3 orbital resonance with Neptune). It is considered very likely to be a dwarf planet, although the IAU has not officially classified it as such. Light-curve-amplitude analysis shows only small deviations, which suggests that Ixion is a spheroid with small albedo spots and hence a dwarf planet. Tancredi (2010) thinks that Ixion is very probably a dwarf planet. Its diameter of 650 km estimated by Spitzer makes it about the fifth largest plutino. It is moderately red in visible light and has a surface made of a mixture of tholin and water ice.

Other than Pluto, Ixion was the first TNO discovered that was originally estimated to be larger than asteroid Ceres, Even in 2002, a year after its discovery, Ixion was still believed to be more than 1000 km in diameter, though the 2002 estimate was a result of a spurious detection at 250 GHz that was not confirmed by later observations. More recent estimates suggest that Ixion has a high albedo and is smaller than Ceres. Observations of Ixion by Spitzer Space Telescope in the far-infrared part of the spectrum revealed that its size is about 650 km.

Ixion is moderately red (slightly redder than 50000 Quaoar) in the visible light. It also has a higher albedo (>0.15) than the mid-sized red cubewanos.[12] There may be an absorption feature at the wavelength of 0.8 μm in its spectrum, which is commonly attributed to the alteration of surface materials by water. In the near-infrared the spectrum of Ixion is flat and featureless. Water ice absorption bands at 1.5 and 2 μm are absent.

This is in contrast to Varuna, which has a red spectral slope in the near-infrared as well as prominent water absorption bands.[15] Both visible and infrared spectroscopic results indicate that Ixion’s surface is a mixture of water ice, dark carbon and tholin, which is a heteropolymer formed by irradiation of clathrates of water and organic compounds.The Very Large Telescope (VLT) has checked Ixion for cometary activity, but did not detect a coma. Ixion is currently about 41 AU from the Sun,[1] and it is possible that Ixion could develop a coma or temporary atmosphere when it is closer to perihelion.It was discovered on May 22, 2001. Ixion was discovered by the Cerro Tololo Inter-American Observatory (807). It is named after Ixion, a figure from Greek mythology.

This diagram shows the orbits of Ixion (green), Pluto (red) and Neptune (grey). The current positions of Ixion and Pluto are indicated (as of April 2006).

Ixion and Pluto follow similar but differently oriented orbits: Ixion’s perihelion is below the ecliptic whereas Pluto’s is above it. Uncharacteristically for bodies locked in resonance with Neptune (such as Orcus), Ixion approaches Pluto with less than 20 degrees of angular separation. Ixion is currently crossing the ecliptic heading below, and will reach its perihelion in 2070. Pluto has passed its perihelion (1989) and is descending toward the ecliptic. Ixion’s orbital period is almost 250 Earth years, about 0.5% larger than Pluto’s. Ixion does demonstrate some regular changes in brightness, which are thought to be caused by its rotation. As of 2010, however, the rotation period of Ixion remains undetermined

Kuiper Belt objects

Finding that the largest known Kuiper Belt object is a virtual twin to Pluto may only further complicate the debate about whether to categorize the large icy worlds that populate the belt as planets. If Pluto were considered to be the minimum size for a planet, then Xena would fulfill this criterion, too. In time, the International Astronomical Union will designate the official name.

The image shows the relative sizes of the largest known Kuiper Belt Objects, including Pluto and its moon, Charon.

Credit: ESO

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Ten New Dwarf Planets and Candidates in Our Solar System

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