Five dwarf planets—Ceres, Pluto, Makemake, Haumea, and Eris—make up the official list of dwarf planets defined by the International Astronomical Union. The IAU defines dwarf planets as rounded objects that orbit the Sun and are massive enough to maintain a rounded and roughly spherical shape under their own gravity (hydrostatic equilibrium), but have not “cleared the neighborhood” of other significantly more massive objects. This definition was created in the wake of the discovery of the latter three objects within a few months in 2004–2005, particularly Eris, which is more massive than Pluto and similar in size.
More “possible” dwarf planets fit the criteria laid out by the IAU, and a few are nearly unanimously recognized as such by most astronomers and NASA. These are Quaoar, Sedna, Orcus, and Gonggong. Salacia is also recognized as a dwarf planet by almost all astronomers. A few more objects are probably also dwarf planets, and even more recently discovered bodies may or may not qualify but have yet to have their masses or radii accurately measured to determine such.
All of the dwarf planets appear as star-like points through a telescope, with the exception of Ceres, which we’ll get to. With the exception of Ceres, all of the known dwarf planets belong to the Kuiper Belt, a region of icy objects just beyond the orbit of Neptune that is home to many objects (KBOs) too small to be considered dwarf planets, some of which can be the progenitors of comets. The Kuiper Belt dwarfs the Asteroid Belt in size and mass, with a total mass of a couple percent of Earth’s or similar to that of the Moon. Some moons like Phoebe around Saturn or Triton around Neptune, are probably captured KBOs. The Kuiper Belt and regions beyond could contain over 100 dwarf planets.
With the exception of Pluto and its moon Charon, all of the Kuiper Belt objects were discovered after 1990, but many (particularly the top ten we’ve mentioned) showed up in photographic sky surveys in the 1950s and simply went unnoticed—nobody was checking for such slow-moving objects at the time.
When can I see Dwarf Planets?
The dwarf planets are all best around their oppositions, which don’t change much yearly but can be seen any time they’re not too close to the Sun in the sky to not be visible in total darkness. However, owing to their faintness, the dwarf planets are best seen when they are positioned high in the sky, allowing you to look through as little atmosphere as you can and preserve the tiny amounts of brightness from these dim objects.
How Good of a Telescope Do I need to see the Dwarf Planets?
Ceres can be seen with binoculars as a star-like point. A 10” or larger telescope can just barely resolve Ceres’ disk as “fatter” than a star. A 16” or larger telescope is needed to see Makemake and Haumea, while 25” or larger instruments are required to tease out the meager glow of the other dwarf planets against the void of space.
Dark skies are also required to see many of these objects; Ceres can be seen from light-polluted skies. The rest of the dwarf planets require dark and transparent skies owing to their dimness.
Dwarf Planet Occultations
Dwarf planets – as well as smaller icy bodies and asteroids in our Solar System – occasionally pass in front of, or occult, stars. These events are important, as they can help narrow down the orbits, physical shape, and find any atmosphere, orbiting moons or rings around objects too small to study directly with telescopes. Observers on different parts of the Earth will see the stars obscured for different amounts of time and at different times by fractions of a second, which allows the “shadow” of the occulting object to be found.
Occultations can be seen with any telescope big enough to show the target star, and imaging them and reporting data to the International Occultation Timing Association is easy to do with a planetary camera and any telescope that has tracking. You can help with studying distant and unknown worlds in the Solar System with just a few hundred dollars of equipment and a few minutes of your time by imaging occultations, so it’s certainly worth doing.
Imaging Dwarf Planets
Dwarf planets are dim, and many are beyond the reach of a typical background telescope, and light-polluted skies hide them from view. Fortunately, however, they can be imaged with any camera capable of taking long exposures. The key is to use a very large image scale (usually coinciding with a long focal length) so that the object’s movement over one or two nights appears as more than one pixel or so of distance in the frame of the images. This is the same technique used to discover dwarf planets.
The author knows of a successful attempt to image Eris’ movement by one pixel over the course of a few nights with a basic color astrophotography camera and 80mm f/6 refracting telescope, not exactly pushing the limits of cost or focal length.
- Ceres
Ceres is the only true dwarf planet to be discovered visually, without photographic aid, and the only one closer to the sun than Neptune. Ceres was discovered in 1801 by Giuseppe Piazzi. After its discovery, a wave of other star-like points of light orbiting the Sun were also discovered; they were initially defined as planets, but as their numbers creeped into the dozens, they were recognized to probably be a separate class of object and termed asteroids, Greek for “star-like”.
The asteroids 4 Vesta and 2 Pallas are big enough to be dwarf planets, but they have failed to round themselves under their own gravity, and thus Ceres stands alone as a dwarf planet in the Asteroid Belt. Ceres is considered the largest asteroid as well, owing to its rocky nature, though Vesta is the largest to not also be classified as a dwarf planet. Ceres and Vesta were both orbited and studied by NASA’s Dawn spacecraft in the mid-2010s, and Ceres was the first dwarf planet to be explored, as Dawn arrived shortly before the New Horizons spacecraft flew by Pluto in 2015. Multiple space agencies are planning to land on or even retrieve surface samples from Ceres in the future.
Ceres orbits in a roughly circular path around the Sun halfway between Mars and Jupiter about 3 astronomical units (AU) from the Sun, or 3 times the Earth-Sun distance. It is about 950 kilometers in diameter.
Due to its proximity to us, Ceres hovers between magnitude 6.7 to 9, just barely within reach of the naked eye under dark skies at its brightest, and is within range of binoculars and finder scopes regardless of conditions or its current brightness. Ceres spans an angular diameter of 0.4 to 0.9 arc seconds, depending on its distance. The upper bound of this range is similar to the angular diameter of Jupiter’s moon Europa, which can be seen clearly as a disk with telescopes as small as 6 inches during transits. As such, it is possible to resolve Ceres as a goldish, albeit featureless, tiny disk appearing “fatter” than surrounding stars with telescopes of 10” or greater aperture and magnifications of around 400x or more, provided atmospheric conditions permit the use of such high magnifications. Ceres is the only dwarf planet that can be resolved as more than a point source with amateur telescopes.
Kuiper Belt Dwarf Planets
Our article on Pluto goes into detail on observing it, as it is the brightest and most well-known Kuiper Belt dwarf planet. It can be seen with 10” or larger telescopes.
All of the Kuiper Belt dwarf planets besides Pluto will require a large telescope of 16” or greater aperture to see. Only two of these are likely to be observed without truly monster instruments of 25” or greater aperture.
Makemake, discovered in 2005, is about ⅔ as big as Pluto at 1,400 kilometers in diameter but is in a similarly elongated orbit more distant from the Sun, with a perihelion of 38.1 AU, a bit further than Pluto’s current distance from the Sun, in the “classical” Kuiper Belt. Makemake is currently also around aphelion (52.7 AU) and reaches a brightness of just a bit better than magnitude 17, requiring 16” or larger telescopes to be seen under dark skies. It reaches opposition around early April for the foreseeable future.
Makemake is presently the brightest Kuiper Belt object after Pluto and usually holds this title, despite its currently dim apparent magnitude. Things will get better in the 2100s; by mid-century it will rival Pluto in brightness (both around magnitude 15.8) peaking at magnitude 15.5 in 2200, which will be doable with a 12” telescope if anyone is still doing visual astronomy by then. Makemake is strongly reddish due to its tholin-covered surface, and has one small moon, which currently lacks a name.
Makemake is of note in that due to its position near the ecliptic and brightness (being near perihelion at the time), it probably could’ve been discovered during Clyde Tombaugh’s search for “Planet X” in the 1930s, which culminated in Pluto’s discovery. Makemake was in the middle of a crowded star field at the time, however, so Tombaugh missed it.
Haumea, discovered in 2004, is not far in the sky from Makemake and has similar orbital parameters in inclination, the direction of its aphelion, and in its distance, ranging from 51.6 to 34.6 AU. Haumea is similar in mass and total size to Makemake but is egg-shaped owing to its fast rotation, with a long axis nearly as big as Pluto and a short axis about half the size of Makemake.
Haumea will peak in brightness at magnitude 15.8 in the mid-2100s. Right now, however, Haumea is near aphelion like Makemake and consequently at around magnitude 17.25, requiring a 16” or larger instrument to see, though if you get the chance you can probably go after both Haumea and Makemake in a single observation session with a monster telescope. Haumea reaches opposition in late April for the foreseeable future.
Haumea has two moons, H’iaka and Namaka, probably generated as the result of impact events along with Haumea’s ring system. Haumea was probably once a larger body, somewhere between the size of Pluto and the Moon, and smaller fragments of its former self have been found throughout the Kuiper Belt. One, an object known as 2002 TX300, may be a dwarf planet and can be seen with 32” or larger telescopes, hovering around magnitude 19.5. Haumea has a giant pinkish-red spot, presumably made of organic tholins, on its surface, which may be similar to Pluto’s two-toned surface or an impact crater.
If you have a 25” or larger instrument, or can access one under dark skies, a few more dwarf planets can be observed.
Quaoar is the brightest and next-largest of the Kuiper Belt dwarf planets after Haumea and Makemake, at magnitude 18.9 and similar in size to Charon at around 1,100 kilometers wide. It will require a 25”, but preferably 30”, or larger telescope to see. Quaoar is in the constellation of Scutum and comes to opposition every July for the foreseeable future. Quaoar has a moon, Weywot, and it has a surface with some tholins present and possibly active cryovolcanism. Quaoar may end up being the next dwarf planet to be visited by a spacecraft, as it has been proposed for flybys by numerous American and Chinese interstellar probe missions in the 2030s or 2040s due to its convenient placement. With a near-circular orbit, Quaoar’s brightness will never change much, and it is around 43 AU from the Sun, near Pluto’s aphelion distance.
Orcus is currently at magnitude 19.2, requiring a 32” or larger instrument to see. It occupies a similar orbit to Pluto but is aimed in the opposite direction and has a similarly massive moon similar to Charon, named Vanth, though both Orcus and Vanth are smaller than Charon, both under 1,000 kilometers in diameter. Like Quaoar, Orcus is one of the more likely dwarf planets to be visited by a space probe sometime in the next few decades. Orcus is named for the Etruscan equivalent of Pluto because of their many similarities and is nicknamed the “anti-Pluto” as a result of their twin orbits. Orcus will brighten up to around magnitude 17.3 near perihelion around 2150, similar to the brightness of Haumea today and within reach of a 16-18” telescope.
Ixion, Salacia, Varda, and 2002 AW197, at magnitude 20, can be observed with very large telescopes of 36” or greater aperture, though to this date no recorded visual observations exist. All are small, no more than 850 kilometers in diameter, or about ⅓ that of Pluto. Varda and its moon Ilmarë are named for characters from the The Lord of the Rings mythology and form a similar binary system to Pluto and Charon, while Salacia has a moon as well, Actea.
2002 MS4, 2013 FY27, and 2003 AZ84 are all below magnitude 20 in brightness, invisible with pretty much any telescope that can accept an eyepiece, even 60-80” monster observatory instruments. 2013 FY27 has a small moon, as does 2003 AZ84. All three are around 700–800 kilometers in diameter, and little is known about them.
Scattered Disk Dwarf Planets
The Kuiper Belt has a “cliff” at 47.8 AU beyond which few objects spend much of their time; the more distant but tenuous “scattered disk” consists of objects that can stray within the Kuiper Belt but with elongated orbits poking far beyond and sometimes extending out to a bit over 100 AU from the Sun. Further “detached objects” of unknown origin never get closer than the edges of the Kuiper Belt, and their origins are unclear.
Eris, discovered alongside Makemake in 2005, is currently near its aphelion very far from the Sun at around 95 AU away, in a highly elongated orbit. Eris is one of the most distant objects known to be within the gravitational influence of the Sun, besides a handful of comets, the Voyager and Pioneer spacecraft, and a few small scattered disk and detached objects.
Eris is larger than Pluto in mass and possibly diameter, as well as being more reflective. Its discovery, combined with those of Makemake and Haumea within a span of about 6 months, led to a controversy and the ultimate demotion of Pluto. Eris is literally named for the Greek goddess of controversy and strife as a result. Eris is extremely reflective, with over 96% of the sunlight that hits it bouncing off its surface. It was originally thought to be bigger than Pluto as a result; accurate measurements of its radius are unlikely to be able to nail down its brightness without a visit by a space probe, though current estimates say it’s a few tens of kilometers smaller in size than Pluto.
The discovery of Eris’ one moon, Dysnomia, allowed Eris’ mass to be calculated, and Eris is now known to be more massive than Pluto, making it the 9th most massive object orbiting the Sun. At magnitude 18.7, Eris is visually out of range of anything other than a monster 25-30” or larger telescope for the time being. Despite this, a few people have actually seen it with their eyes with such instruments. Immediately after Eris’ discovery, a few amateur astronomers observed it with the 82” telescope at McDonald Observatory. Eris reaches opposition in mid-October in the constellation Cetus for the foreseeable future.
As with some of the other dwarf planets, observers in the far future will have better luck with Eris. Eris will be as bright as Haumea and Makemake are today by the late 2100s, and reach its 38.3-AU perihelion in the mid-2200s at a peak of magnitude 14.7, within the range of a 12” telescope.
Gonggong, formerly known as 2007 OR10, is around 1,200 kilometers in diameter, similar to Quaoar and Charon in size, and is nearly as far from the Sun as Eris at 89 AU away, with an aphelion of 101 AU that it has yet to reach – beating out Eris’ aphelion of 97 AU. It is currently dimmer than magnitude 21, far too dim to see with any telescope. It will reach perihelion around the year 2400 and brighten to magnitude 17. Gonggong has a tiny moon, Xianglu, and makes three trips around the Sun for every ten Neptune does.
Sedna is a “detached object” or “Sednoid” with an orbit bringing it as far as 937 AU – a few percent of a light-year from the Sun – and a perihelion of 76 AU, which it will reach in 2076, brightening to magnitude 20.4 from its current magnitude of 20.75 – too dim to observe, but possible to photograph. Sedna is around 1,000 kilometers in diameter, reddish, and has no moons. The low probability of Sedna’s discovery, given that for most of its orbit it would be too dim to detect, suggests many more objects similar in size or even bigger – perhaps as large as Pluto – exist. The probability of more massive undetected objects, combined with the elongated orbits of Sedna and many other smaller objects that don’t get close enough to the Sun to be influenced by the giant planets, seems to be evidence for a potential ninth planet several hundred AU from the Sun, dragging these objects’ aphelions to aim in similar directions. Whether one exists is unknown, and it would be too dim to observe with a backyard telescope even if it were near the size of Neptune; anything larger would have been detected by now with sky surveys. Alternatively, they could have been displaced by or captured by passing stars.