Dwarf Planet 136108 Haumea approaches opposition.

The Dwarf Planet 136108 Haumea will reach opposition (i.e. be directly opposite the Sun seen from Earth) at 8.47 am GMT on Thursday 23 April 2026. This means that it will both be at its closest to the Earth this year, about 42.1 AU (42.1 times the average distance between the Earth and the Sun, or about 6 298 255 000 km), and completely illuminated by the Sun. While it is not visible to the naked eye observer, the planets have phases just like those of the Moon; being further from the Sun than the Earth, 136108 Haumea is 'full' when directly opposite the Sun. 

The orbit and position of 136108 Haumea and the planets of the Solar System at 9.00 am on Thursday 23 April 2026. JPL Small Body Database Browser.

At opposition, the Dwarf Planet will be in the constellation of Bootes and at its highest point in the sky at about midnight local time from anywhere on Earth (this is because the rising and setting of objects in the sky is caused by the Earth's rotation, not the movement of the object). (Even at it's very brightest 136108 Haumea will only have a Magnitude of 17.3, making it almost impossible to see with any but the largest of Earth-based telescopes, and where resolvable it will only be possible to see it as a point of light indistinguishable from a faint star.

136108 Haumea orbits the Sun on an eccentric orbit tilted at an angle of 28.2° to the plane of the Solar System, which takes it from 34.4 AU from the Sun (34.4 times the average distance at which the Earth orbits the Sun) to 51.5 AU from the Sun (51.5 times the average distance at which the Earth orbits the Sun). With an average distance of 43.0 AU, 136108 Haumea completes one orbit around the Sun every 282 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Opposition only one day later each year than the year before, and reaches Solar Conjunction (when it is directly on the opposite side of the Sun to the Earth), roughly six months later.

136108 Haumea was discovered on 28 December 2004 by a team led by Mike Brown of the Palomar Observatory in California, in images taken by them on 28 May 2004; on 27 July 2005 a team led by José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía reported that they had also discovered the Dwarf Planet, in images taken between 7 and 10 March 2003. With a diameter of 2100 km it is considered to be the third largest dwarf planet in the Solar System (after 134340 Pluto and 136199 Eris) as well as the eighteenth largest body in the Solar System, excluding the Sun (several moons, including our own, are larger).

Haumea has been calculated to be rotating once every 3.9 hours, far more rapidly than any other large body in the Solar System. Curiously for such a fast rotating body, it has not adopted a oblate spheroid (flattened sphere) shape, but is instead a triaxial ellipsoid (elongate flattened sphere, or flattened egg-shape). This implies that, although its surface is comprised of ice, it has a core of fairly dense rocky material. 

The Dwarf Planet Haumea is believed to rotate in just under 4 hours. This rapid rotation causes the Dwarf Planet to be elongated in appearance. Stephanie Hoover/Wikimedia Commons.

Although Haumea is only about a quarter the size of Pluto, it is thought to be large enough that it should have reached hydrostatic equilibrium (i.e. become approximately spherical due to its own gravity). The elongate shape of Haumea is at odds with this, something which, in combination with its high rotational rate, has been suggested as evidence of a major collision in Haumea's past. This has been supported by the discovery in 2017 of a ring surrounding the dwarf planet with a radius of about 2285 km. This is well within the Roche limit for Haumea (the distance below which a ring or other orbiting body should be disrupted by the parent body's gravity and either fall onto it or be ejected), suggesting that this is a temporary structure caused by a relatively recent event (although still potentially billions of years ago, given the weak gravitational forces involved).

136108 Haumea rotating within its ring system. Tom Ruen/Wikimedia Commons.

136108 has two small moons, both of which were discovered in 2005 by Darin Ragozzine and Michael Brown, at that time working at the W. M. Keck Observatory in Hawai'i. The larger of these, Hi'iaka, is roughly 370 km in diameter (although, like its parent body, it has been calculated to have an elongate, non-spherical shape) and orbits at a distance of 49 400 km, completing one orbit every 49.5 days. The smaller, Namaka, has been calculated to be about 150 km in diameter with an irregular shape, and to orbit at a distance of approximately 25 500 km.

Both the rings of 136108 Haumea and the larger moon, Hi'iaka, follow orbital paths 1-3° offset from the equator of the dwarf planet, and are thought to be products of the same collision. The smaller moon, Namaka, has an orbit offset by 69° from the equator of Haumea. This moon is thought to have been formed in the same collision, but to have had it's orbit perturbed significantly by tidal interactions with the larger moon.

Dwarf Planet Haumea and its satellites, imaged by the Hubble Space Telescope's WFC2 camera from 12 May 2008 and 19 May 2008. The brighter dot orbiting Haumea is the larger outer moon Hi'iaka while the fainter dot is the smaller inner moon Namaka. This animation of the moons' orbits spans 7 days and the orbital plane of Namaka is oriented vertically. Hubble Space Telescope/Michael Brown/Wikimedia Commons.

136108 Haumea is thought to be a member of a collisional family of Kuiper Belt objects; the only collisional family identified from this part of the Solar System. This family has been named the Haumea, or Haumean, Family in reference to its largest known member. Collisional families are groups of bodies which appear to have been created in a single collision event, and whose orbital trajectories can in theory all be traced back to a single point of origin, although this is not the case for the Haumea Family, which are thought to have had their orbital paths modified over time by interactions with the gravitational field of Neptune. 

As well as 136108 Haumea and its ring and moons, this family is thought to include the Kuiper Belt Objects (19308) 1996 TO66, (24835) 1995 SM55, (55636) 2002 TX300, (86047) 1999 OY3, (120178) 2003 OP32, (145453) 2005 RR43, (202421) 2005 UQ513, (308193) 2005 CB79, (315530) 2008 AP129, (386723) 2009 YE7, (416400) 2003 UZ117, (523645) 2010 VK201, (543454) 2014 HZ199, (612620) 2003 SQ317, (653589) 2014 QW441, (671467) 2014 LO28, and (673087) 2015 AJ281, as well, presumably, as other as yet undiscovered bodies.

The orbits of the bodies of the Haumea Collisional Family (not all are shown). Tom Ruen/Wikimedia Commons.

As well as having similar orbital properties, the bodies of the Haumea Family all share a similar high albedo (with the exception of (202421) 2005 UQ513), which suggests a surface largely covered by reflective water ice, rather than the darker, reddish, tholins (frozen organic compounds) which cover the surface of most Kuiper Belt objects. (202421) 2005 UQ513 has a lower albedo and a reddish spectrum, suggesting that it has an outer surface covered with tholins, but is included within the Haumea Family due to the similarity of its orbit.

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Dwarf Planet 136108 Haumea reaches oposition.

The Dwarf Planet 136108 Haumea will reach opposition (i.e. be directly opposite the Sun seen from Earth) at 8.38 pm GMT on Sunday 21 April 2024. This means that it will both be at its closest to the Earth this year, about 49.10 AU (19.28 times the average distance between the Earth and the Sun, or about 7 345 255 000 km), and completely illuminated by the Sun. While it is not visible to the naked eye observer, the planets have phases just like those of the Moon; being further from the Sun than the Earth, 136108 Haumea is 'full' when directly opposite the Sun. The Dwarf Planet will be in the constellation of Bootes and at its highest point in the sky at about midnight local time from anywhere on Earth (this is because the rising and setting of objects in the sky is caused by the Earth's rotation, not the movement of the object). (Even at it's very brightest 136108 Haumea will only have a Magnitude of 17.3, making it almost impossible to see with any but the largest of Earth-based telescopes, and where resolvable it will only be possible to see it as a point of light indistinguishable from a faint star.

The orbit and position of 136108 Haumea at 9.00 pm on Sunday 21 April 2024. JPL Small Body Database Browser.

136108 Haumea orbits the Sun on an eccentric orbit tilted at an angle of 28.2° to the plane of the Solar System, which takes it from 34.4 AU from the Sun (34.4 times the average distance at which the Earth orbits the Sun) to 51.5 AU from the Sun (51.5 times the average distance at which the Earth orbits the Sun). With an average distance of 43.0 AU, 136108 Haumea completes one orbit around the Sun every 282 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Opposition only one day later each year than the year before, and reaches Solar Conjunction (when it is directly on the opposite side of the Sun to the Earth), roughly six months later.

The Dwarf Planet Haumea is believed to rotate in just under 4 hours. This rapid rotation causes the Dwarf Planet to be elongated in appearance. Stephanie Hoover/Wikimedia Commons.

136108 Haumea was discovered on 28 December 2004 by a team led by Mike Brown of the Palomar Observatory in California, in images taken by them on 28 May 2004; on 27 July 2005 a team led by José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía reported that they had also discovered the Dwarf Planet, in images taken between 7 and 10 March 2003. With a diameter of 2100 km it is considered to be the third largest dwarf planet in the Solar System (after 134340 Pluto and 136199 Eris) as well as the eighteenth largest body in the Solar System, excluding the Sun (several moons, including our own, are larger).

Haumea has been calculated to be rotating once every 3.9 hours, far more rapidly than any other large body in the Solar System. Curiously for such a fast rotating body, it has not adopted a oblate spheroid (flattened sphere) shape, but is instead a triaxial ellipsoid (elongate flattened sphere, or flattened egg-shape). This implies that, although its surface is comprised of ice, it has a core of fairly dense rocky material. The Dwarf Planet also appears to be surrounded by a ring of icy material, and at least two moons, which have been named Hiʻiaka and Namaka.

Dwarf Planet Haumea and its satellites, imaged by the Hubble Space Telescope's WFC2 camera from 12 May 2008 and 19 May 2008. The brighter dot orbiting Haumea is the larger outer moon Hi'iaka while the fainter dot is the smaller inner moon Namaka. This animation of the moons' orbits spans 7 days and the orbital plane of Namaka is oriented vertically. Hubble Space Telescope/Michael Brown/Wikimedia Commons.

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Dwarf Planet 136108 Haumea reaches oposition.

The Dwarf Planet 136108 Haumea will reach opposition (i.e. be directly opposite the Sun seen from Earth) on Tuesday 19 April 2022 at 8.18 pm GMT. This means that it will both be at its closest to the Earth this year, about 49.28 AU (19.28 times the average distance between the Earth and the Sun, or about 7 327 183 000 km), and completely illuminated by the Sun. While it is not visible to the naked eye observer, the planets have phases just like those of the Moon; being further from the Sun than the Earth, 136108 Haumea is 'full' when directly opposite the Sun. The Dwarf Planet will be in the constellation of Bootes and at its highest point in the sky at about 1.40 am local time from anywhere on Earth (this is because the rising and setting of objects in the sky is caused by the Earth's rotation, not the movement of the object). (Even at it's very brightest 136108 Haumea will only have a Magnitude of 17.3, making it almost impossible to see with any but the largest of Earth-based telescopes, and where resolvable it will only be possible to see it as a point of light indistinguishable from a faint star.

 

The orbit and position of 136108 Haumea (2003 EL61) at 8.00 pm on Tuesday 19 April 2022. JPL Small Body Database Browser.

136108 Haumea orbits the Sun on an eccentric orbit tilted at an angle of 28.2° to the plane of the Solar System, which takes it from 34.4 AU from the Sun (34.4 times the average distance at which the Earth orbits the Sun) to 51.5 AU from the Sun (51.5 times the average distance at which the Earth orbits the Sun). With an average distance of 43.0 AU, 136108 Haumea completes one orbit around the Sun every 282 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Opposition only one day later each year than the year before, and reaches Solar Conjunction (when it is directly on the opposite side of the Sun to the Earth), roughly six months later.

 

The Dwarf Planet Haumea is believed to rotate in just under 4 hours. This rapid rotation causes the Dwarf Planet to be elongated in appearance. Stephanie Hoover/Wikimedia Commons.

136108 Haumea was discovered on 28 December 2004 by a team led by Mike Brown of the Palomar Observatory in California, in images taken by them on 28 May 2004; on 27 July 2005 a team led by José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía reported that they had also discovered the Dwarf Planet, in images taken between 7 and 10 March 2003. With a diameter of 2100 km it is considered to be the third largest dwarf planet in the Solar System (after 134340 Pluto and 136199 Eris) as well as the eighteenth largest body in the Solar System, excluding the Sun (several moons, including our own, are larger).

Haumea has been calculated to be rotating once every 3.9 hours, far more rapidly than any other large body in the Solar System. Curiously for such a fast rotating body, it has not adopted a oblate spheroid (flattened sphere) shape, but is instead a triaxial ellipsoid (elongate flattened sphere, or flattened egg-shape). This implies that, although its surface is comprised of ice, it has a core of fairly dense rocky material. The Dwarf Planet also appears to be surrounded by a ring of icy material, and at least two moons, which have been named Hiʻiaka and Namaka.

 

Dwarf Planet Haumea and its satellites, imaged by the Hubble Space Telescope's WFC2 camera from 12 May 2008 and 19 May 2008. The brighter dot orbiting Haumea is the larger outer moon Hi'iaka while the fainter dot is the smaller inner moon Namaka. This animation of the moons' orbits spans 7 days and the orbital plane of Namaka is oriented vertically. Hubble Space Telescope/Michael Brown/Wikimedia Commons.

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Searching for bright objects in the Outer Solar System.

For over a century Pluto was the only known bright object in the Solar System beyond Neptune, but in the past two decades a large number of such objects have been discovered. The difficulty in seeing finding such objects lies not in observing them at all, but rather in detecting their movement, which is what reveals them to be a part of the Solar System rather than remote astronomical objects. Typically the movement of a body in the Inner Solar System can be detected from observations made hours apart, in which Outer Solar System objects appear effectively motionless. Only with the advent of long-term wide-field surveys, which enable the tracking of such objects over several months, has enabled the detection of many more such objects, though once their orbits and magnitudes were established, it has been possible to locate many of these objects in photographic plates taken by observatories dating back to the 1950s.

In a paper published on the arXiv database at Cornell University Library on 6 January 2015, Mike Brown of the California Institute of Technology, Michelle Bannister of the University of Victoria and The Australian National University, Andrew Drake, George Djorgovski, Matthew Graham, Ashish Mahabal and Ciro Donalek of the California Institute of Technology, Steve Larson, Eric Christensen and Ed Beshore of  the Lunar and PlanetaryLaboratory at The University of Arizona and Rob McNaught of The Australian National University, describe the results of a study of data collected by the Catalina Sky Survey in the Northern Hemisphere and Siding Spring Survey in the Southern Hemisphere, with the objective of discovering previously undetected bright objects on the Outer Solar System.

The Catalina Sky and Siding Spring Surveys are designed to search for Near Earth Objects, but are also capable of detecting other bright objects in the sky. 

The Catalina Sky Survey uses the 0.7 m Catalina Schmidt telescope at the Catalina Observatory in Arizona. It is capable of detecting objects with magnitudes of up to about 19.5 (in astronomy larger magnitudes equate to dimmer objects; the human eye can detect objects with magnitudes of up to about 5, while the Sun, Moon and inner planets have negative magnitudes, i.e. magnitudes of less than zero), with each image covering 8.1 square degrees of sky. Brown et al. used data from the Catalina Sky Survey collected between 6 December 2005 and 21 April 2012, covering approximately 19700 square degrees of sky between declinations of -25 and +70 (where zero is directly over the equator, +90 is directly over the North Pole and -90 is directly over the South Pole).  

The Siding Spring Survey operates from the 0.5 m Uppsala Schmidt telescope at Siding Spring Observatory in Australia, and is capable of detecting objects with magnitudes of 19.0 or less, with each image covering 4.2 square degrees of sky. Brown et al. used data collected by the Siding Spring Survey between 20 February 2005 and 29 April 2012, covering approximately 14100 square degrees of sky from declination -80 to 0. Since there is some overlap between the two surveys, the total area covered is approximately 29700 square degrees, although it is impossible to survey for objects at galactic latitudes of less than 10 degrees (i.e. within 10 degrees of the plain of the galaxy) due to the density of the stars in this part of the sky.

(a) Coverage of the Catalina Sky Survey (CSS). The shading shows the number of opposition seasons in which each field has been covered 4 or more times. The equal area plot is centred at an RA of 180 degrees and a declination of 0 degrees with grid marks placed every 30 degrees of RA and declination. Contours of galactic latitude of 10 and 20 degrees are shown as well as a line showing the ecliptic. (b) Coverage of the Siding Spring Survey (SSS). All parameters are as in (a) except that the plot is centredat an RA of 0 degrees. Brown et al. (2015).

The Catalina Sky and Siding Spring Surveys are intended to search for Near Earth Objects (asteroids and comets whose paths bring them close to the Earth) as such the data produced by them has already been filtered for moving objects closer than 25 AU from the Sun (i.e. 25 times as far from the Sun as the Earth or less).  An object moving in a circular prograde orbit (the same direction as the planets, forwards) at a distance of 25 AU will appear to be moving in a retrograde orbit (the opposite direction to the planets, backwards) at a speed of 4.9 arcseconds per hour (the total sky, imagined as a sphere, is divided into 360 degrees, each of which can be subdivided into 60 arcminutes, each arcminute comprises 60 arcseconds); objects more distant from the Sun than the Earth appear to be moving in a retrograde orbits as they are moving more slowly than the Earth. For this reason Brown et al. looked for candidate objects sighted four times within a night within 4.9 arcseconds of one-another, counting these observations as a single sighting.

Some of these sightings were found to repeat at the same location on subsequent occasions. Such sightings are assumed to have been of undocumented astrophysical sources outside the Solar System. Objects which were not subsequently resighted were also excluded, being assumed to be either optical effects or one off astrophysical events. Objects which were subsequently observed within 4 arcseconds of the original sighting were retained as possible sightings of bright trans-Neptunian objects. This resulted in 1.2 million candidate objects in the Catalina Sky Survey Field and 2.3 million candidate objects in the Siding Spring Survey Field.

(a) The locations of transients in the Catalina Sky Survey. To allow viewability only every 20^th transient is shown. (b) The locations of transients in the Siding Spring Survey.Every 20^thtransient is shown. Brown et al. (2015).

Such candidate objects were suspiciously common around the edges of the fields, particularly on the Siding Spring Survey and around the northern extent of the Catalina Sky Survey, making it likely that a lot of false sightings were present in these areas, probably stationary stars being recorded as moving objects. The comparatively high number of sightings in the Siding Spring Survey also made analysing this data more difficult. However a known bright Kuiper Belt objects, such as the dwarf planet Makemake, could be identified within the data, suggesting that the method was capable of detecting real objects.

Since objects in the Outer Solar System must follow Keplerian orbits, Brown et al. next looked for objects for which such orbits could be defined. This required a minimum of three separate sightings, since three orbital parameters can be derived from each sighting, and a minimum of seven parameters are needed to define a Keplerian orbit. Three randomly selected points will not usually resolve to a Keplerian orbit, ruling out sets of sightings of different objects. In order to make the data manageable, only groups of sightings made from pairs separated by less than 120 days were considered. However this resulted in a total of 160 billion potential triplets in the Catalina Sky Survey Data and 18 trillion potential triplets in the Siding Spring Survey data, which would take a prohibitive amount of computational time to process.

In order to further reduce the data volume, Brown et al. then looked for triplets which were moving on linear paths through the solar system perpendicular to the Earth-object vector. This reduced dataset still included known objects such as Makemake, and reduced the number of candidate triplets to 140 million in the Catalina Sky Survey data and 3 billion in the Siding Spring Survey data.

Brown et al. next calculated Keplerian orbits for these remaining triplets, ruling out those that could not be resolved. This reduced the number of candidate triplets to 4.8 million triplets in the Catalina Sky Survey data and 235 million in the Siding Spring Survey data. However this data was still likely to contain a considerable number of false sightings, requiring further data reduction to be used.

Brown et al. therefore decided to use quadruplets (sets of four sightings that could be determined to be the same object) rather than triplets. This reduced the number of candidate objects, with 1192 quadruplets detected in the Catalina Sky Survey data and 5515 quadruplets in the Siding Spring Survey data. This was a sufficiently small number to begin comparisons of the quadruplets, establishing whether the same objects had been detected multiple times.

The 1192 quadruplets recovered from the Catalina Sky Survey data yielded eight distinct objects, all of which were previously known (Makemake, Haumea, Orcus, Eris, 2002 TX300, Nereid, Huya and 2002 VE95). It was therefore concluded that requiring four observations per opposition season (an opposition season being the period when an object outside the Earth’s orbit is on the same side of the Sun as us, and therefore on the opposite side of the Earth to the Sun, the only time when faint objects visible only from reflected sunlight are visible) was sufficient to exclude false positives from the data.

The eight objects detected in the Catalina Sky Survey. Each of theobjects is found to be a previously discovered object in the Outer Solar System. Brown et al. (2015).

This means that the Catalina Sky Survey data found every object in the Solar System beyond 25 AU brighter than magnitude 19.1 (though Uranus and Neptune were very bright objects which swamped observations and were therefore excluded from the data processing) except Pluto, which is known to have been in an area of the galactic plane excluded from the study for the entire period and Quaoar, which was actually observed twice in the first season, but then also moved into the galactic plane.The detections included seven Kuiper Belt Objects in heliocentric Keplerian Orbits, plus Nereid, an irregular satellite of Neptune. This strongly suggests that there were no undetected objects in the field survey area brighter then magnitude 19.1.

However none of the 5515 quadruplets observed by the Siding Spring Survey could be resolved to one another, suggesting that for this field raising the number of detections to four was insufficient to remove the false positives, probably due to the far larger number of original detections. Attempting to raise the bar to five detections also resulted in an observation rate of zero.  Thus while it is highly likely that Siding Spring was observing objects, a way to recover these from the data was not found.

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Observations of New Horizons candidate bodies with the Hubble Space Telescope.      The New Horizons Spacecraft was launched in January 2006, and is due to pass through the Pluto System (Pluto is now recognized to be...

Pluto at opposition.

The dwarf planet Pluto will reach opposition slightly before 3.00 am GMT on Friday 4 July 2014; this means that it will be directly opposite the Sun in the sky when viewed from Earth, on this occasion in the...

The shape of Kuiper Belt Object 2003 SQ317.

The Kuiper Belt is a region of the Solar System extending from the orbit of Neptune at about 30 AU (i.e. 30 times the distance between the Sun and the...

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The shape of Kuiper Belt Object 2003 SQ317.

The Kuiper Belt is a region of the Solar System extending from the orbit of Neptune at about 30 AU (i.e. 30 times the distance between the Sun and the Earth) out to about 50 AU from the Sun it contains a very large number of bodies known as Kuiper Belt Objects, which are thought to be left over from the formation of the Solar System, elements of the original proto-planetary disk that were to widely scattered, and had to high an angular momentum, to assemble into planets under their own gravity. Some of these objects are large enough to be considered dwarf planets, notably Pluto (formerly considered to be a planet), Makemake and Haumea.

In a paper published on the online arXiv Database at Cornell University Library on 6 September 2013,  and in the Monthly Notices of the Royal Astronomical Society on 3 December 2013, Pedro Lancerda and Andrew McNeill of the Astrophysics Research Centre at Queen's University Belfast and Nuno Peixinho of the Center for Geophysics and Geophysical and Astronomical Observatory of the University of Coimbra, describe attempts to model the shape of one particular Kuiper Belt Object 2003 SQ317.

2003 SQ317 was discovered in September 2003 (the designation 2003 SQ371 indicates that it was the 7941st object discovered in the second half of September 2003). It has an eccentric 279 year orbit inclined to the plane of the Solar System, with an average distance of 42.7 AU from the Sun. It is considered to be a Haumea Family Object, a body with a similar orbital path and albedo to the dwarf planet Haumea, possibly formed by an ancient collision involving the proto-Haumea and another body, though modeling a scenario that could start in such a collision and end with the current pathways of these objects has proved elusive. In 2010 it was noted that 2003 SQ317 had a notably variable albedo, dimming and brightening by a factor of 14 over a period of 3.7 hours, suggesting that it might have a highly irregular shape.  

The calculated orbit of 2003 SQ317. JPL Small Body Database Browser.

Based upon additional observations of 2003 SQ317 using the ESO New Technology Telescope located at the La Silla Observatory in Chile, Lancerda et al. attempted to build a model of the object that fit with the observed pattern of brightening and darkening. 

They were able to come up with two, equally plausible models to account for this. Firstly 2003 SQ317 could be an flattened elongate body (Jacobi ellipsoid) spinning so that the body alternatively presents long and short sides to the Earth. Secondly the object could in fact be a pair of gravitationally bound bodies (a Roche binary pair), rotating one in front of the other, so that alternately one and two bodies can be seen.

Jacobi ellipsoid model that best fi ts the lightcurve of 2003 SQ317. Lancerda et al. (2013).

Roche binary model that best fi ts the lightcurve of 2003 SQ317. Lancerda et al. (2013).

2003 SQ317 is too small and too distant for its shape to be resolved visually with any current telescope. It would in theory be possible to differentiate between the two models it the mass of the object were known (the binary pair would need to be considerably more dense than the Jacobi ellipsoid), though there is no easy way to do this at the current time. 2003 SQ317 is likely to be a rubble pile type object rather than one or two large rocks, which would allow it quite a range of possible densities, and make it hard to determine its density based upon its mineralogy (which can sometimes be assessed from albedo).

See also Asteroids in retrograde orbits, Four more asteroids found to be co-orbitals of Neptune, Neptune's Trailing Trojans, The stability of Neptune's Trojan Asteroids and Pluto gains a fourth moon.

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Pluto gains a fourth moon.

NASA scientists announced this week (20 July 2011) that they had discovered a new moon around the dwarf planet Pluto. The discovery was made by scientists using the Hubble Space Telescope in a set of five long exposure pictures taken over a period of two months and is referred to as 'P4' or 'S/2011 p1'. The moon has since been identified in earlier Hubble images, from 2006 and 2010.

The Pluto system as we currently understand it, incorporating the new moon P4.

Pluto was discovered in 1930 by Clyde Tombaugh, a young astronomer working at the Lowell Observatory in Arizona; its existence had been predicted as early as 1909, due to anomalies in the orbit of Neptune. At the time it was assumed that Pluto was a planet of some size, capable of disturbing the orbit of Neptune. Pluto spends part of its 248 earth year orbit inside the orbit of Neptune; this is not the same on every orbit, but alternates between a 20 and a 14 year stay.

In 1979 the first moon of Pluto was discovered by James Christy at the United States Naval Observatory. Dubbed 'Charon' it enabled scientists to make an estimate of the size of Pluto; to their surprise they found it has only 2% of the mass of Earth or 18% of that of the moon, making it far to small to affect the orbit of Neptune, re-starting the hunt for new planets in the outer solar-system. In fact it is now understood that (due to orbital effects) Pluto passes closer to Uranus than it ever does to Neptune. Charon is so large (relative to Pluto) that their mutual centre of gravity, the point about which they both rotate, is 2040 km above the surface of Pluto; they are the only known planet/moon system for which this is true (671 Patroclus is a pair of asteroids orbiting a mutual centre of gravity at Jupiter's trailing Lagrangian Point, but these are nothing like planets), and for this reason some scientists suggest they should be referred to as a pair of binary planets rather than a planet and its moon. Charon has a mass 2% that of the moon (1520 × 10¹⁸ kg) and a diameter of 1205 km. It orbits the systems centre of gravity at a distance of 17 530 km.

The Pluto/Charon system.

In 2004 another Pluto-like object was located in the outer solar system by a team led by Mike Brown at the California Institute of Technology (this is disputed; José Luis Ortiz Moreno of the Instituto de Astrofísica de Andelucía and his team at the Sierra Nevada Observatory in Grenada, Spain). Haumea has only a third the mass of Pluto, and orbits slightly further out, but it confirmed what scientists were beginning to suspect, that the outer solar system might contain a number of such objects.

An artists impression of Haumea. It is ellipsoid in shape and has a distinctive red patch. These cannot be directly imaged, but are the best interpretation of the available data on Haumea.

In 2005 Mike Brown's team working at Caltech's Palomar Observatory discovered two further objects in the outer solar system.

Makemake is similar to Haumea, roughly third the mass of Pluto, and further out, but the other new object, Eris, was more interesting. Eris is half again as far from the sun as Pluto - and a third again as big. Clearly if Pluto is a planet, then Eris is too.

Hubble images of Eris and it's moon Dysnomia.

Since it was likely that there are many more objects of this size in the outer solar system many astronomers were becoming uncomfortable with the term 'planet' to describe them. Thus in 2006 the International Astronomical Union settled on the term 'Dwarf Planet', to designate objects large enough to form a roughly spherical shape under their own gravity, but no so massive as to have cleared the area around their orbit of all other objects. Pluto, Haumea, Makemake, and Eris were placed in this category, as was Ceres in the asteroid belt. Ceres had also been classified as a planet at the time of its discovery in 1801, as were a number of other asteroids until the mid-nineteenth century, when it became clear that asteroids were too abundant to be classed as planets.

Also in 2005 two more moons of Pluto were discovered by the Hubble Space Telescope Pluto Companion Search Team. Nix and Hydra are further out and smaller than Charon, but orbit the same centre of gravity, so logically if Charon should be considered a planet, then so should Nix and Hydra. This is rather more problematic, as Nix is only 91 km in diameter and Hydra 114 km. Nix orbits the system's centre of gravity at 48 708 km and Hydra at 64 749 km.

The new moon, S/2011 p1, orbits the same centre of gravity as the rest of the system and therefore logically could also potentially be considered a planet, despite having a radius of between 14 and 34 km. It orbits between Nix and Hydra, at a distance of about 59 000 km.

Clearly the Pluto system is something very different to anything in the inner solar system, a cloud of objects more than a planet with satellites. Some scientists theorize that this may be the result of a collision early in the solar system's history, in the same way that Earth's moon (larger in comparison to its parent body than any other moon in the solar system except Charon) is thought to be the result of a collision between the early Earth and a Mars-sized object.

The images we have at the moment, even those taken with our best telescopes, are still pretty faint. In July 2015 NASA's New Horizon spacecraft is due to reach Pluto and will hopefully bring us far more information, and probably a good few more surprises.

See also Visiting Vesta and Dwarf Planets on Sciency Thoughts Youtube.