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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Eris approaches opposition.

The dwarf planet 136199 Eris will reach opposition (i.e. be directly opposite the Sun seen from Earth) on Saturday 18 October 2025 at 7.10 am GMT. This means that it will both be at its closest to the Earth this year, about 94.64 AU (94.56 times the average distance between the Earth and the Sun, or about 14, 147 000 000 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, 136199 Eris is 'full' when directly opposite the Sun. 

The orbit and position of 136199 Eris at 7.00 am GMT on Saturday 18 October 2025. JPL Small Body Database Browser.

Although this coincides with a Waxing Cresent Moon, the prospects for viewing withough a very poweful telescope is not as good. The dwarf planet will be in the constellation of Cetus 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 136199 Eris will only have a Magnitude of 18.6, 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.

136199 Eris orbits the Sun on an eccentric orbit tilted at an angle of 44.1° to the plane of the Solar System, which takes it from 35.9 AU from the Sun (35.9 times the average distance at which the Earth orbits the Sun) to 97.5 AU from the Sun (97.5 times the average distance at which the Earth orbits the Sun. With an average distance of 67.74 AU, 136199 Eris completes one orbit around the Sun every 558 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Opposition only four days 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 orbit and position of 136199 Eris at 7.00 am GMT on Saturday 18 October 2025, showing the inclination of its orbit relative to that of the plain of the Solar System. JPL Small Body Database Browser.

136199 Eris was discovered on 5 January 2005 by a team led by Mike Brown of the Palomar Observatory in California. With a diameter of 2326 km it is considered to be the second largest dwarf planet in the Solar System (after 134340 Pluto) as well as the sixteenth largest body in the Solar System, excluding the Sun (though several moons, including our own, are larger). It is also the largest body in the Solar System never to have been visited by a spacecraft (again, with the exception of the Sun). 136199 Eris has a single moon, Dysnomia, which has a diameter of about 700 km and obits at a distance of about 37 350 km.

Labelled Hubble Space Telescope Image of 136199 Eris, showing its satellite Dysnomia. Captured on 12 June 2007. Mike Brown/NASA/European Space Agency/Wikimedia Commons.

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Ceres comes to opposition.

Dwarf Planet 1 Ceres will reach opposition (the point at which it is directly opposite the Sun when observed from the Earth) at 1.12 pm GMT on Thursday 2 October 2025, when it will also be at the closest point on its orbit to the Earth, at a distance of 1.96 AU (i.e. 1.96 times as far from the Earth as the Sun, or about 239 361 000 km), and be completely illuminated by the Sun. While it is not obvious to the naked eye observer, asteroids have phases just like those of the Moon; being further from the Sun than the Earth, 1 Ceres is 'full' when directly opposite the Sun. As 1 Ceres is only about 939.4 km in diameter, it will not be visible to the naked eye, but with a maximum Apparent Magnitude (luminosity) of 7.6 at opposition, it should be visible in the Constellation of Cetus to viewers equipped with a good pair of binoculars or small telescope, with the best visibility being at about midnight local time from anywhere on Earth.

The calculated orbit and position of 1 Ceres at 1.00 pm GMT on Thursday 2 October 2025.  JPL Small Body Database. 

Because Ceres is further from the Sun than the Earth, its orbital period is much longer than ours, with the Dwarf Planet completing one obit every 1683 days (4.65 years), on an eccentric orbit tilted at 10.6° to the plane of the Solar System. The orbit of Ceres places it within the inner part of the Main Asteroid Belt, but due to its large size, with a diameter of 939.4 km, it is considered to be a Dwarf Planet rather than an asteroid.

High resolution image of Ceres made on 20 September 2020, by the Dawn Space Probe. Wikimedia Commons/NASA/JPL/Caltech.

Ceres was discovered on 1 January 1801 by Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily. It was the first body to be discovered in the Main Asteroid Belt, and at the time when it was discovered an international search was underway for a presumed 'missing planet' between the orbits of Mars and Jupiter (although Piazzi was studying stars when he first observed Ceres, and initially presumed he had found a new comet). Ceres was for a long time considered to be the largest asteroid in the Solar System, but in 2006 was re-classified as a Dwarf Planet, as part of a revision of the classification of Solar System bodies driven by the discovery of a growing number of bodies in the Outer Solar System which are too large to be considered asteroids or comets yet to small to be considered to be planets. 

Of the ten bodies currently classified as Dwarf Planets, only Ceres is located within the Main Asteroid Belt, with five lying in the Kuiper Belt (Orcus, Pluto, Salacea, Haumea, Quaoar, and Makemake), two lie within the Scattered Disk (Gonggong and Eris), and one within the Detached Region on the outer fringe of the Solar System (Sedna).

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Pluto approaches opposition.

The Dwarf Planet Pluto will reach opposition (be directly on the opposite side of the Earth as the Sun) at 6.24 am GMT on Friday 25 July 2025. This means that it will be at its closest to the Earth this year, about 34.3AU (34.3 times the average distance between the Earth and the Sun, or about 3 639 716 200 km), and completely illuminated by the Sun. While it is not obvious to the naked eye observer, the planets have phases just like those of the Moon; being further from the Sun than the Earth, Pluto is 'full' when directly opposite the Sun, although with an apparent magnitude of only 15.0 it will take a reasonably good telescope to see Pluto at all, and it will only be visible as a star-like point to those that can see it. 

The relative positions of Earth and Pluto at 6.00 am on 25 July 2025. JPL Small Body Database.

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.

The Dwarf Planet Pluto imaged by the New Horizons space probe in July 2015. NASA/JPL/Southwest Research Institute.

Pluto was downgraded from a planet to a dwarf planet by the International Astronomical Union in 2006, following the discovery of several similar small bodies in the Kuiper Belt beyond the orbit of Neptune. The term 'Dwarf Planet' is now used 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.

The comparative sizes of Pluto, its largest moon, Charon, and the continental United States of America. Calvin Hamilton/Cornell University.

Pluto has an 247.7 year orbital period and an eccentric orbit tilted at an angle of 17.1° to the plane of the Solar System, which takes it from 29.6 AU from the Sun (i.e. 2960% of the average distance at which the Earth orbits the Sun) to 49.4 AU from the Sun (i.e. 4940% of the average distance at which the Earth orbits the Sun). As a body which spends most of its time outside the orbit of the planet Neptune it is classed as a Trans-Neptunian Object, even though it does come inside the orbit of Neptune for part of its orbital period.

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2017 OF201: A potential Dwaf Planet with an extremely wide orbit.

The majority of the known sub-planetary bodies of the Solar System are found within the Main Asteroid Belt, but the total mass of the bodies here is thought to add up to only about 0.04% of the mass of the Earth. The Kuiper Belt, located between 30 and 50 AU from the Sun (i.e. between 30 and 50 times as far from the Sun as the Earth) is thought to contain a total mass of about 2% that of the Earth, including large icy bodies such as the Dwarf Planet Pluto. Beyond this, at a distance of between about 2000 and 200 000 AU from the Sun, the Oort Cloud may contain several Earth masses of material, largely in the form of icy comets, but also including dwarf planets, and possibly unidentified planets.

Other than the Dwarf Planet Pluto, which was discovered in 1930, all of the 5000 plus known trans-Neptunian objects (Solar System bodies outside the orbit of the Planet Neptune) have been discovered in the past three decades. Most of these have been discovered by surveys concentrating on the elliptic plane of the Solar System, with higher latitudes very poorly mapped. The limitations of the instruments used also mean that few objects beyond 60 AU from the Sun have been discovered.

Cosmological surveys (i.e. surveys of the deeper cosmos, outside the Solar System) have also detected trans-Neptunian objects, most notably the Dark Energy Survey, which has already discovered about 800 such bodies.

In a paper published on the arXiv database at Cornell University on 22 May 2025, Sihao Cheng of the Institute for Advanced Study and the Perimeter Institute, and Jiaxuan Li and Eritas Yang of the Department of Astrophysical Sciences  at Princeton University, detail the discovery of  a large and exotic trans-Neptunian object from data collected by the Dark Energy Camera Legacy Survey.

Cheng et al. searched data collected by thee Dark Energy Camera Legacy Survy, which utilises the Dark Energy Camera on the 4-meter Blanco telescope at Cerro Tololoin Chile, discovering the same object had been detected at three wavelength bands on ten occasions between 2014 and 2018, and that it was possible to connect these sightings and calculate a tentative orbit for the object. This object, identified as 2017 OF201 (a name which implies it was the 5031st object discovered in the second half of July 2017) had an extremely wide and excentric orbit, was about 85 AU from the Sun was detected, and had an apparent magnitude of about 22.6, making it the second brightest object yet discovered with an orbital distance greater than 80 AU.

Armed with this data, Cheng et al. searched the data archives of the 3.6 m Canada-France-Hawaii Telescope, the Subaru Telescope, and Gemini-North Telescope, recovering images of 2017 OF201 at the predicted positions in  nine 3.6 m Canada-France-Hawaii Telescope images from 2011 and 2012, but not detecting it in data from the Subaru or Gemini-North telescopes.

Trajectory of 2017 OF201 on the sky from 2011 to 2018. Individual detections from 13 nights are shown on top of the predicted trajectory based on the best-fit orbit, which describes the detections very well with a scatter of 0.13 for the Dark Energy Camera (DECam) and 0.03 for the Canada-France-Hawaii Telescope (CFHT) arcsec in each component, consistent with the estimated astrometric error. The insets show example images from DECam (r-band on 2017-09-17) and CFHT (r-band on 2011-08-31). Cheng et al. (2025).

2017 OF201 is calculated to have an orbital period of 24 256 years, with a perihelion distance (closest approach to the Sun) of 44.9 AU, an aphelion distance (furthest distance from the Sun) of 1632 AU, and a semi-major axis (average distance from the Sun) of 838.3 AU. The last perihelion of 2017 OF201 was in 1930, the year in which Pluto was discovered, however, even at perihelion 2017 OF201 would have been about four orders of mangnitude fainter than Pluto (i.e. roughly a ten thousandth as bright), quite beyond detection by the telescopes of the day. The orbit of 2017 OF201 is tilted at 16.2° to the plane of the Solar System.

The orbits & current positions of Neptune, Pluto, and 2017 OF201. Jiaxuan Li & Sihao Cheng/Institute for Advanced Study.

2017 OF201 has a longitude of perihelion of 306° (i.e. it reaches perihelion at an angle of 306° relative to the First Point of Aries, taken as a celestial reference point). This is noteworthy, as many previously discovered trans-Neptunian objects have longitudes of perihelion clustered around 60°, something which has been postulated to imply the presence of a ninth planet (termed 'Planet X') in the Outer Solar System, the gravity of which is pushing the orbits of trans-Neptunian objects towards a similar trajectory. The orbit of 2017 OF201 not only shows no signs of such influence, it appears to be incompatible with such an object existing at all. This suggests that the similarity seen in the orbits of trans-Neptunian objects discovered to date is due to sampling bias - we have discovered more objects with longitudes of perihelion close to 60° because we have been looking at that part of the sky.

Plan view of the orbits of trens-Neptunian objects (TNOs) with extremely wide orbits, including our newly discovered 2017 OF201, which has a distinct orbit is an outlier to the apsidal clustering of the others. For reference, the most probable orbit of Planet X is shown in black. Cheng et al. (2025).

Analysis of light from 2017 OF201 suggests that it has a reddish hue, within the colour range of other trans-Neptunian objects, but possibly one of the redder objects. 2017 OF201 is calculated to be about 700 km in diameter, at which size it is presumed that it would be roughly spherical in shape. It is estimated to have a density of about 1.7 grams per cm squared, which would give it a total mass of about 300 000 000 000 000 megatons, or roughly one twenty thousandth the mass of the Earth.

2017 OF201 forms part of the Scattered Disk, an area between the Kuiper Belt considered to contain far less mass than either. However, if 2017 OF201, and other Scattered Disk objects, such as 90377 Sedna, represent an examples of a population of similar objects (which is a more likely explanation than all such objects currently being on the inner part of their orbits where we can detect them), then it is likely that the total mass contained in the Scattered Disk may be as high as 10% of that of the Earth, compared to 1-2% for the Kuiper Belt.

The orbits and positions of 2017 OF201 and 90377 Sedna. Wikimedia Commons.

2017 OF201 is unlikely to have formed on its current, highly eccentric, orbit. Rather, Cheng et al. estimate that it formed closer in to the Sun, on a more circular orbit, and has been moved onto its current orbit by encounters with other bodies. This orbit isnot consistent with the 'Planet X' hypothesis which has been used to explain the highly eccentric orbits of other trans-Neptunian objects.  Instead, Cheng et al. suggest that 2017 OF201 was initially knocked onto a less eccentric orbit by one or more encounters with the planet Neptune, and that that orbit has subsequently been further modified by the action of the Galactic Tides, and possibly close encounters with other steller systems.

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