Dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.
The interest of dwarf planets to planetary geologists is that they may be geologically active bodies, an expectation that was borne out in 2015 by the Dawn mission to Ceres and the New Horizons mission to Pluto. Astronomers are in general agreement that at least the eight largest candidates are dwarf planets – in rough order of size, Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna and Ceres. Considering the ten largest candidates adds Orcus and Salacia;[b] of these ten, two have been visited by spacecraft (Pluto and Ceres) and seven others have at least one known moon (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia), which allows their masses and thus an estimate of their densities to be determined. Mass and density in turn can be fit into geophysical models in an attempt to determine the nature of these worlds. Only one, Sedna, has neither been visited nor has any known moons, making an accurate estimate of mass difficult. Some astronomers include many smaller bodies as well,[1] but there is no consensus that these are likely to be dwarf planets.
The term dwarf planet was coined by planetary scientist
History of the concept
Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter that for decades were considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid (from Greek, meaning 'star-like' or 'star-shaped') for the smaller bodies and began to distinguish them as minor planets rather than major planets.[5]
With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine major planets, along with thousands of significantly smaller bodies (
In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away.[11] Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, the plutinos. It became clear that either the larger of these bodies would also have to be classified as planets, or Pluto would have to be reclassified, much as Ceres had been reclassified after the discovery of additional asteroids.[12] This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets.[13][14] Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain classified as a planet.[15]
Eris (then known as 2003 UB313) was discovered in January 2005;[16] it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet.[17] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006.[18] The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi: They proposed an intermediate category for objects large enough to be round but that had not cleared their orbits of planetesimals. Beside dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.[19]
Although concerns were raised about the classification of planets orbiting other stars,[20] the issue was not resolved; it was proposed instead to decide this only when dwarf-planet-size objects start to be observed.[19]
In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution.[21] Campaigns included car bumper stickers and T-shirts.[22] Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight.[23]
NASA announced in 2006 that it would use the new guidelines established by the IAU.
Name
Names for large subplanetary bodies include dwarf planet, planetoid (more general term), meso-planet (narrowly used for sizes between Mercury and Ceres), quasi-planet and (in the transneptunian region) plutoid. Dwarf planet, however, was originally coined as a term for the smallest planets, not the largest sub-planets, and is still used that way by many planetary astronomers.
Alan Stern coined the term dwarf planet, analogous to the term dwarf star, as part of a three-fold classification of planets, and he and many of his colleagues continue to classify dwarf planets as a class of planets. The IAU decided that dwarf planets are not to be considered planets, but kept Stern's term for them. Other terms for the IAU definition of the largest subplanetary bodies that do not have such conflicting connotations or usage include quasi-planet[29] and the older term planetoid ("having the form of a planet").[30] Michael E. Brown stated that planetoid is "a perfectly good word" that has been used for these bodies for years, and that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution.[31] Indeed, the draft of Resolution 5A had called these median bodies planetoids,[32][33] but the plenary session voted unanimously to change the name to dwarf planet.[2] The second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session that 5A was passed.[31] Because of the semantic inconsistency of a dwarf planet not being a planet due to the failure of Resolution 5B, alternative terms such as nanoplanet and subplanet were discussed, but there was no consensus among the CSBN to change it.[34]
In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine, Spanish planeta enano, German Zwergplanet, Russian karlikovaya planeta (карликовая планета), Arabic kaukab qazm (كوكب قزم), Chinese ǎixíngxīng (矮行星), Korean waesohangseong (왜소행성 / 矮小行星) or waehangseong (왜행성 / 矮行星), but in Japanese they are called junwakusei (準惑星), meaning "quasi-planets" or "peneplanets" (pene- meaning "almost").
IAU Resolution 6a of 2006
On June 11, 2008, the IAU Executive Committee announced a new term, plutoid, and a definition: all trans-Neptunian dwarf planets are plutoids.[36] Other departments of the IAU have rejected the term:
...in part because of an email miscommunication, the WG-PSN [Working Group for Planetary System Nomenclature] was not involved in choosing the word plutoid. ... In fact, a vote taken by the WG-PSN subsequent to the Executive Committee meeting has rejected the use of that specific term..."[34]
The category of 'plutoid' captured an earlier distinction between the 'terrestrial dwarf' Ceres and the 'ice dwarfs' of the outer Solar system,[37] part of a conception of a threefold division of the Solar System into inner terrestrial planets, central gas giants and outer ice dwarfs, of which Pluto was the principal member.[38] 'Ice dwarf' also saw some use as an umbrella term for all trans-Neptunian minor planets, or for the ice asteroids of the outer Solar System; one attempted definition was that an ice dwarf "is larger than the nucleus of a normal comet and icier than a typical asteroid."[39]
Since the Dawn mission, it has been recognized that Ceres is an icy body more similar to the icy moons of the outer planets and to TNOs such as Pluto than it is to the terrestrial planets, blurring the distinction,[40][41] and Ceres has since been called an ice dwarf as well.[42]
Criteria
Body | m/ME [†] | Λ [‡] | µ [§] | Π [#] | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mercury | 0.055 | 1.95×103 | 9.1×104 | 1.3×102 | ||||||||
Venus | 0.815 | 1.66×105 | 1.35×106 | 9.5×102 | ||||||||
Earth | 1 | 1.53×105 | 1.7×106 | 8.1×102 | ||||||||
Mars | 0.107 | 9.42×102 | 1.8×105 | 5.4×101 | ||||||||
Ceres | 0.00016 | 8.32×10−4 | 0.33 | 4.0×10−2 | ||||||||
Jupiter | 317.7 | 1.30×109 | 6.25×105 | 4.0×104 | ||||||||
Saturn | 95.2 | 4.68×107 | 1.9×105 | 6.1×103 | ||||||||
Uranus | 14.5 | 3.85×105 | 2.9×104 | 4.2×102 | ||||||||
Neptune | 17.1 | 2.73×105 | 2.4×104 | 3.0×102 | ||||||||
Pluto | 0.0022 | 2.95×10−3 | 0.077 | 2.8×10−2 | ||||||||
Eris | 0.0028 | 2.13×10−3 | 0.10 | 2.0×10−2 | ||||||||
Sedna | 0.0002 | 3.64×10−7 | < 0.07[c] | 1.6×10−4 | ||||||||
Planetary discriminants of the planets ( white ), and of the largest known dwarf planet ( light purple ) in each orbital population (asteroid belt, Kuiper belt, scattered disc, sednoids). All other known objects in these populations have smaller discriminants than the one shown. | ||||||||||||
|
The category dwarf planet arose from a conflict between dynamical and geophysical ideas of what a useful conception of a planet would be. In terms of the dynamics of the Solar System, the major distinction is between bodies that gravitationally dominate their neighbourhood (Mercury through Neptune) and those that do not (such as the asteroids and Kuiper belt objects). A celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight, which results in the body acquiring a round shape. Because this requires a much lower mass than gravitationally dominating the region of space near their orbit, there are a population of objects that are massive enough to have a world-like appearance and planetary geology, but not massive enough to clear their neighborhood. Examples are Ceres in the asteroid belt and Pluto in the Kuiper belt.[46]
Dynamicists usually prefer using gravitational dominance as the threshold for planethood, because from their perspective smaller bodies are better grouped with their neighbours, e.g. Ceres as simply a large asteroid and Pluto as a large Kuiper belt object.[47][48] Geoscientists usually prefer roundness as the threshold, because from their perspective the internally driven geology of a body like Ceres makes it more similar to a classical planet like Mars, than to a small asteroid that lacks internally driven geology. This necessitated the creation of the category of dwarf planets to describe this intermediate class.[46]
Orbital dominance
Stern & Levison (2000) introduced a parameter Λ (upper case lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit.[28] The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects.[43]
Using this parameter, Soter and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so.[28] Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared.[43]
Jean-Luc Margot refined Stern and Levison's concept to produce a similar parameter Π (upper case Pi).[45] It is based on theory, avoiding the empirical data used by Λ . Π > 1 indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.
There are several other schemes that try to differentiate between planets and dwarf planets,[21] but the 2006 definition uses this concept.[2]
Hydrostatic equilibrium
Graphs are unavailable due to technical issues. There is more info on Phabricator and on MediaWiki.org. |
Enough internal pressure, caused by the body's gravitation, will turn a body plastic, and enough plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape and may be rubble piles. Larger objects, where gravity is significant but not dominant, are potato-shaped; the more massive the body, the higher its internal pressure, the more solid it is and the more rounded its shape, until the pressure is enough to overcome its compressive strength and it achieves hydrostatic equilibrium. Then, a body is as round as it is possible to be, given its rotation and tidal effects, and is an ellipsoid in shape. This is the defining limit of a dwarf planet.[49]
If an object is in hydrostatic equilibrium, a global layer of liquid on its surface would form a surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. The body will have a spherical shape if it does not rotate and an ellipsoidal one if it does. The faster it rotates, the more
There are no specific size or mass limits of dwarf planets, as those are not defining features. There is no clear upper limit: an object very far out in the Solar System that is more massive than Mercury might not have had time to clear its neighbourhood; such a body would fit the definition of dwarf planet rather than planet. Indeed, Mike Brown set out to find such an object.[50] The lower limit is determined by the requirements of achieving and retaining hydrostatic equilibrium, but the size or mass at which an object attains and retains equilibrium depends on its composition and thermal history, not simply its mass. An IAU 2006 press release[51] question-and-answer section estimated that objects with mass above 0.5×1021 kg and radius greater than 400 km would "normally" be in hydrostatic equilibrium (the shape ... would normally be determined by self-gravity), but that all borderline cases would need to be determined by observation.[51] This is close to what as of 2019 is believed to be roughly the limit for objects beyond Neptune that are fully compact, solid bodies, with Salacia ( r = 423±11 km , m = (0.492±0.007)×1021 kg ) being a borderline case both for the 2006 Q&A expectations and in more recent evaluations, and with Orcus being just above the expected limit.[52] No other body with a measured mass is close to the expected mass limit, though several without a measured mass approach the expected size limit.
Population of dwarf planets
There is no clear definition of what constitutes a dwarf planet, and whether to classify an object as one is up to individual astronomers. Thus, the number of dwarf planets in the Solar System is unknown.
The three objects under consideration during the debates leading up to the 2006 IAU acceptance of the category of dwarf planet – Ceres, Pluto and Eris – are generally accepted as dwarf planets, including by those astronomers who continue to classify dwarf planets as planets. Only one of them – Pluto – has been observed in enough detail to verify that its current shape fits what would be expected from hydrostatic equilibrium.[53] Ceres is close to equilibrium, but some gravitational anomalies remain unexplained.[54] Eris is generally assumed to be a dwarf planet because it is more massive than Pluto.
In order of discovery, these three bodies are:
- Ceres – discovered January 1, 1801 and announced January 24, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006. Confirmation is pending.[54]
- Pluto – discovered February 18, 1930 and announced March 13. Considered a planet for 76 years. Explicitly reclassified as a dwarf planet by the IAU with Resolution 6A on August 24, 2006.[55] Five known moons.
- Eris (2003 UB313) – discovered January 5, 2005 and announced July 29. Called the "tenth planet" in media reports. Considered a dwarf planet by the IAU since the adoption of Resolution 5A on August 24, 2006, and named by the IAU dwarf-planet naming committee on September 13 of that year. One known moon.
The IAU only established guidelines for which committee would oversee the naming of likely dwarf planets: any unnamed trans-Neptunian object with an absolute magnitude brighter than +1 (and hence a minimum diameter of 838 km at the maximum geometric albedo of 1)[56] was to be named by a joint committee consisting of the Minor Planet Center and the planetary working group of the IAU.[36] At the time (and still as of 2023), the only bodies to meet this threshold were Haumea and Makemake. These bodies are generally assumed to be dwarf planets, although they have not yet been demonstrated to be in hydrostatic equilibrium, and there is some disagreement for Haumea:[57][58]
- Haumea (2003 EL61) – discovered by Brown et al. December 28, 2004 and announced by Ortiz et al. on July 27, 2005. Named by the IAU dwarf-planet naming committee on September 17, 2008. Two known moons and one known ring.
- Makemake (2005 FY9) – discovered March 31, 2005 and announced July 29. Named by the IAU dwarf-planet naming committee on July 11, 2008. One known moon.
These five bodies – the three under consideration in 2006 (Pluto, Ceres and Eris) plus the two named in 2008 (Haumea and Makemake) – are commonly presented as the dwarf planets of the Solar System, though the limiting factor (albedo) is not what defines an object as a dwarf planet.[59]
The astronomical community commonly refers to other larger TNOs as dwarf planets as well.[60] At least three additional bodies meet the preliminary criteria of Brown, of Tancredi et al., of Grundy et al., and of Emery et al. for identifying dwarf planets, and are generally called dwarf planets by astronomers as well:
For instance, JPL/NASA called Gonggong a dwarf planet after observations in 2016,[61] and Simon Porter of the Southwest Research Institute spoke of "the big eight [TNO] dwarf planets" in 2018, referring to Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna and Orcus.[62] The IAU itself has called Quaoar a dwarf planet in a 2022–2023 annual report.[63]
More bodies have been proposed, such as Orcus,
At the time Makemake and Haumea were named, it was thought that
Individual astronomers have recognized a number of objects as dwarf planets or as likely to prove to be dwarf planets. In 2008, Tancredi et al. advised the IAU to officially accept Orcus, Sedna and Quaoar as dwarf planets (Gonggong was not yet known), though the IAU did not address the issue then and has not since. Tancredi also considered the five TNOs Varuna, Ixion, 2003 AZ84, 2004 GV9, and 2002 AW197 to most likely be dwarf planets as well.[68] Since 2011, Brown has maintained a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets, based solely on estimated size.[69] As of 13 September 2019, Brown's list identifies ten
But in 2019 Grundy et al. proposed, based on their studies of Gǃkúnǁʼhòmdímà, that dark, low-density bodies smaller than about 900–1000 km in diameter, such as Salacia and Varda, never fully collapsed into solid planetary bodies and retain internal porosity from their formation (in which case they could not be dwarf planets). They accept that brighter (albedo > ≈0.2)[70] or denser (> ≈1.4 g/cc) Orcus and Quaoar probably were fully solid:[52]
Orcus and Charon probably melted and differentiated, considering their higher densities and spectra indicating surfaces made of relatively clean H2O ice. But the lower albedos and densities of Gǃkúnǁʼhòmdímà, 55637, Varda, and Salacia suggest that they never did differentiate, or if they did, it was only in their deep interiors, not a complete melting and overturning that involved the surface. Their surfaces could remain quite cold and uncompressed even as the interior becomes warm and collapses. The liberation of volatiles could further help transport heat out of their interiors, limiting the extent of their internal collapse. An object with a cold, relatively pristine surface and a partially collapsed interior should exhibit very distinctive surface geology, with abundant thrust faults indicative of the reduction in total surface area as the interior compresses and shrinks.[52]
Salacia was later found to have a somewhat higher density, comparable within uncertainties to that of Orcus, though still with a very dark surface. Despite this determination, Grundy et al. call it "dwarf-planet sized", while calling Orcus a dwarf planet.[71] Later studies on Varda suggest that its density may also be high, though a low density could not be excluded.[72]
In 2023, Emery et al. wrote that
In 2024, Kiss et al. found that Quaoar has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesised that Quaoar originally had a rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot.[75] If so, this would resemble the situation of Saturn's moon Iapetus, which is too oblate for its current spin.[76][77] Iapetus is generally still considered a planetary-mass moon nonetheless,[46] though not always.[78]
Most likely dwarf planets
The trans-Neptunian objects in the following tables, except Orcus and Salacia, are agreed by Brown, Tancredi et al., Grundy et al., and Emery et al. to be probable dwarf planets, or close to it. Orcus and Salacia have been included as the largest TNOs not generally agreed to be dwarf planets; they are borderline bodies by many criteria. Charon, a moon of Pluto that was proposed as a dwarf planet by the IAU in 2006, is included for comparison. Those objects that have absolute magnitude greater than +1, and so meet the threshold of the joint planet–minor planet naming committee of the IAU, are highlighted, as is Ceres, which the IAU has assumed is a dwarf planet since they first debated the concept.
The masses of given dwarf planets are listed for their systems (if they have satellites) with exceptions for Pluto and Orcus.
Name | Region of the Solar System |
Orbital radius (AU) |
Orbital period (years) |
Mean orbital speed (km/s) |
Inclination
to ecliptic |
Orbital eccentricity |
Planetary discriminant |
---|---|---|---|---|---|---|---|
Ceres | Asteroid belt | 2.768 | 4.604 | 17.90 | 10.59° | 0.079 | 0.3 |
Orcus | Kuiper belt (resonant – 2:3) | 39.40 | 247.3 | 4.75 | 20.58° | 0.220 | 0.003 |
Pluto | Kuiper belt (resonant – 2:3) | 39.48 | 247.9 | 4.74 | 17.16° | 0.249 | 0.08 |
Salacia | Kuiper belt (cubewano) | 42.18 | 274.0 | 4.57 | 23.92° | 0.106 | 0.003 |
Haumea | Kuiper belt (resonant – 7:12) | 43.22 | 284.1 | 4.53 | 28.19° | 0.191 | 0.02 |
Quaoar | Kuiper belt (cubewano) | 43.69 | 288.8 | 4.51 | 7.99° | 0.040 | 0.007 |
Makemake | Kuiper belt (cubewano) | 45.56 | 307.5 | 4.41 | 28.98° | 0.158 | 0.02 |
Gonggong | Scattered disc (resonant – 3:10) | 67.49 | 554.4 | 3.63 | 30.74° | 0.503 | 0.01 |
Eris | Scattered disc | 67.86 | 559.1 | 3.62 | 44.04° | 0.441 | 0.1 |
Sedna | Detached | 506.8 | ≈ 11,400 | ≈ 1.3 | 11.93° | 0.855 | < 0.07 |
Name | Diameter relative to the Moon |
Diameter (km) |
Mass relative to the Moon |
Mass (×1021 kg) |
Density (g/cm3) |
Rotation period (hours) |
Moons | Albedo | H |
---|---|---|---|---|---|---|---|---|---|
Ceres | 27% | 939.4±0.2 | 1.3% | 0.93835±0.00001 | 2.16 | 9.1 | 0 | 0.09 | 3.33 |
Orcus | 26% | 910+50 −40 |
0.8% | 0.55±0.01 | 1.4±0.2 | 13±4 | 1 | 0.23+0.02 −0.01 |
2.19 |
Pluto | 68% | 2377±3 | 17.7% | 13.03±0.03 | 1.85 | 6d 9.3h | 5 | 0.49 to 0.66 | −0.45 |
(Charon) | 35% | 1212±1 | 2.2% | 1.59±0.02 | 1.70±0.02 | 6d 9.3h | – | 0.2 to 0.5 | 1 |
Salacia | 24% | 846±21 | 0.7% | 0.49±0.01 | 1.50±0.12 | 6.1 | 1 | 0.04 | 4.27 |
Haumea | ≈ 45% | ≈ 1560[58] | 5.5% | 4.01±0.04 | ≈ 2.02[58] | 3.9 | 2 | ≈ 0.66 | 0.23 |
Quaoar | 32% | 1086±4 | 1.9% | 1.2±0.05 | 1.7±0.1 | 17.7 | 1
|
0.11±0.01 | 2.42 |
Makemake | 41% | 1430+38 −22 |
≈ 4.2% | ≈ 3.1 | 1.9±0.2 | 22.8 | 1 | 0.81+0.03 −0.05 |
−0.20 |
Gonggong | 35% | 1230±50 | 2.4% | 1.75±0.07 | 1.74±0.16 | 22.4±0.2? | 1 | 0.14±0.01 | 1.86 |
Eris | 67% | 2326±12 | 22.4% | 16.47±0.09 | 2.43±0.05 | 15d 18.9h | 1 | 0.96±0.04 | −1.21 |
Sedna | 26% | 906+314 −258 |
≈ 1%? | ≈ 1? | ? | 10±3 | 0? | 0.41+0.393 −0.186 |
1.52 |
Symbols
Ceres
Exploration
As of 2024, only two missions have targeted and explored dwarf planets up close. On March 6, 2015, the Dawn spacecraft entered orbit around Ceres, becoming the first spacecraft to visit a dwarf planet.[85] On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons.
Ceres displays such evidence of an active geology as salt deposits and cryovolcanos, while Pluto has water-ice mountains drifting in nitrogen-ice glaciers, as well as a significant atmosphere. Ceres evidently has brine percolating through its subsurface, while there is evidence that Pluto has an actual subsurface ocean.
Dawn had previously orbited the asteroid Vesta. Saturn's moon Phoebe has been imaged by Cassini and before that by Voyager 2, which also encountered Neptune's moon Triton. All three bodies show evidence of once being dwarf planets, and their exploration helps clarify the evolution of dwarf planets.
New Horizons has captured distant images of Triton, Quaoar, Haumea, Eris, and Makemake, as well as the smaller candidates Ixion, 2002 MS4, and 2014 OE394.[86] One of the China National Space Administration's two Shensuo probes has been proposed to visit Quaoar in 2040.[87]
Similar objects
A number of bodies physically resemble dwarf planets. These include former dwarf planets, which may still have equilibrium shape or evidence of active geology; planetary-mass moons, which meet the physical but not the orbital definition for dwarf planet; and Charon in the Pluto–Charon system, which is arguably a binary dwarf planet. The categories may overlap: Triton, for example, is both a former dwarf planet and a planetary-mass moon.
Former dwarf planets
Evidence from 2019 suggests that Theia, the former planet that collided with Earth in the giant-impact hypothesis, may have originated in the outer Solar System rather than the inner Solar System and that Earth's water originated on Theia, thus implying that Theia may have been a former dwarf planet from the Kuiper Belt.[93]
Planetary-mass moons
At least nineteen
Charon
There has been some debate as to whether the Pluto–
See also
Notes
- ^ The hydrostatic equilibrium criterion of a dwarf planet cannot be confirmed unless a spacecraft directly visits the object.
- ^ Considerable uncertainty remains over whether Orcus and Salacia are likely to be dwarf planets. As such, Orcus and Salacia may be considered borderline cases
- ^ Calculated using the minimum estimate from 15 objects in its region with at least Sedna's mass, as estimated by Schwamb, Brown, & Rabinowitz (2009).[44]
- ^ The footnote in the original text reads: "For two or more objects comprising a multiple object system. ... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto–Charon a double planet."[20]
References
- ^ "Dwarf planets are planets, too: Planetary pedagogy after New Horizons" Archived June 27, 2021, at the Wayback Machine.
- ^ a b c d IAU (August 24, 2006). "Definition of a Planet in the Solar System: Resolutions 5 and 6" (PDF). IAU 2006 General Assembly. International Astronomical Union. Archived (PDF) from the original on June 20, 2009. Retrieved January 26, 2008.
- S2CID 240071005. Retrieved May 30, 2022.
- ^ "In Depth | 4 Vesta". NASA Solar System Exploration. Archived from the original on February 29, 2020. Retrieved February 29, 2020.
- ^ Mauro Murzi (2007). "Changes in a scientific concept: what is a planet?". Preprints in Philosophy of Science (Preprint). University of Pittsburgh. Archived from the original on June 11, 2019. Retrieved April 6, 2013.
- ^ Mager, Brad. "Pluto Revealed". discoveryofpluto.com. Archived from the original on July 22, 2011. Retrieved January 26, 2008.
- ^ Cuk, Matija; Masters, Karen (September 14, 2007). "Is Pluto a planet?". Cornell University, Astronomy Department. Archived from the original on October 12, 2007. Retrieved January 26, 2008.
- S2CID 119386667.
- ISBN 978-3-540-37683-5. Archived from the original(PDF) on May 25, 2006. Retrieved February 10, 2008.
- ISBN 978-0-691-12348-6.
- ^ Phillips, Tony; Phillips, Amelia (September 4, 2006). "Much Ado about Pluto". PlutoPetition.com. Archived from the original on January 25, 2008. Retrieved January 26, 2008.
- ^ Brown, Michael E. (2004). "What is the definition of a planet?". California Institute of Technology, Department of Geological Sciences. Archived from the original on July 19, 2011. Retrieved January 26, 2008.
- ^ Eicher, David J. (July 21, 2007). "Should Pluto Be Considered a Planet?". Astronomy. Archived from the original on November 28, 2022. Retrieved November 28, 2022.
- ^ "Hubble Observes Planetoid Sedna, Mystery Deepens". NASA's Hubble Space Telescope home site. April 14, 2004. Archived from the original on January 13, 2021. Retrieved January 26, 2008.
- ^ Brown, Mike (August 16, 2006). "War of the Worlds". The New York Times. Archived from the original on February 13, 2017. Retrieved February 20, 2008.
- ^ "California Institute of Technology, Retrieved 4-12-2015". Archived from the original on May 17, 2012. Retrieved April 12, 2015.
- ^ "Astronomers Measure Mass of Largest Dwarf Planet". NASA's Hubble Space Telescope home site. June 14, 2007. Archived from the original on August 7, 2011. Retrieved January 26, 2008.
- ^ Brown, Michael E. "What makes a planet?". California Institute of Technology, Department of Geological Sciences. Archived from the original on May 16, 2012. Retrieved January 26, 2008.
- ^ a b Britt, Robert Roy (August 19, 2006). "Details Emerge on Plan to Demote Pluto". Space.com. Archived from the original on June 28, 2011. Retrieved August 18, 2006.
- ^ a b c "The IAU draft definition of "planet" and "plutons"". International Astronomical Union. August 16, 2006. Archived from the original on April 29, 2014. Retrieved May 17, 2008.
- ^ from the original on July 23, 2011. Retrieved January 26, 2008.
- ^ Chang, Alicia (August 25, 2006). "Online merchants see green in Pluto news". USA Today. Associated Press. Archived from the original on May 11, 2008. Retrieved January 25, 2008.
- ^ Brown, Michael E. "The Eight Planets". California Institute of Technology, Department of Geological Sciences. Archived from the original on July 19, 2011. Retrieved January 26, 2008.
- ^ "Hotly-Debated Solar System Object Gets a Name" (Press release). NASA. September 14, 2006. Archived from the original on June 29, 2011. Retrieved January 26, 2008.
- ^ Stern, Alan (September 6, 2006). "Unabashedly Onward to the Ninth Planet". New Horizons Web Site. Archived from the original on December 7, 2013. Retrieved January 26, 2008.
- ^ Wall, Mike (August 24, 2011). "Pluto's Planet Title Defender: Q & A With Planetary Scientist Alan Stern". Space.com. Archived from the original on August 14, 2012. Retrieved December 3, 2012.
- ^ a b "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. May 14, 2010. Archived from the original on May 5, 2012. Retrieved November 4, 2011.
- ^ (PDF) from the original on September 23, 2015. Retrieved January 26, 2008.
- ^ Service, Tom (July 15, 2015). "Sounds of the solar system: probing Pluto's predicted score". The Guardian. Archived from the original on December 26, 2019. Retrieved December 26, 2019.
- Fundamental Astronomy(5 ed.). Springer.
- ^ a b Brown, Mike (2010). How I Killed Pluto and Why It Had It Coming. Spiegel & Grau. p. 223.
- ^ Bailey, Mark E. "Comments & discussions on Resolution 5: The definition of a planet – Planets Galore". Dissertatio cum Nuncio Sidereo, Series Tertia – official newspaper of the IAU General Assembly 2006. Astronomical Institute Prague. Archived from the original on July 20, 2011. Retrieved February 9, 2008.
- ^ "Dos uruguayos, Julio Fernández y Gonzalo Tancredi en la historia de la astronomía:reducen la cantidad de planetas de 9 a 8 ...&Anotaciones de Tancredi" (in Spanish). Science and Research Institute, Mercedes, Uruguay. Archived from the original on December 20, 2007. Retrieved February 11, 2008.
- ^ .
- IAU. August 24, 2006. Archivedfrom the original on April 29, 2014. Retrieved August 10, 2021.
- ^ IAU. Paris. June 11, 2008. Archivedfrom the original on November 23, 2020. Retrieved August 10, 2021.
- OCLC 441945398 – via Internet Archive.
- OCLC 562529871 – via Internet Archive.
- ^ Darling, David (ed.). "Ice dwarf". Encyclopedia of Astrobiology, Astronomy and Spaceflight. Archived from the original on July 6, 2008. Retrieved June 22, 2008.
- ^ "Ice Volcanoes and More: Dwarf Planet Ceres Continues to Surprise". Space.com. September 2016. Archived from the original on October 12, 2019. Retrieved December 19, 2019.
- ^ Castillo-Rogez, J. C.; Raymond, C. A.; Russell, C. T.; et al. (September 12, 2017). "Dawn at Ceres: What Have we Learned?" (PDF). Committee on Astrobiology and Planetary Science. Archived (PDF) from the original on October 8, 2018. Retrieved October 12, 2019.
- ISBN 978-3-030-28120-5.
- ^ S2CID 14676169.
- S2CID 15072103.
- ^ S2CID 51684830.
- ^ a b c Lakdawalla, Emily; et al. (April 21, 2020). "What is a planet?". planetary.org. The Planetary Society. Archived from the original on January 22, 2022. Retrieved August 19, 2021.
- ^ Brown, Mike. "The eight planets". gps.caltech.edu. Caltech. Archived from the original on July 19, 2011. Retrieved January 26, 2008.
- ^ Jewitt, David. "Classification of Pluto". ess.ucla.edu. UCLA. Archived from the original on August 19, 2021. Retrieved August 19, 2021.
- ISBN 9780977574032. Archived(PDF) from the original on March 10, 2023. Retrieved August 11, 2023.
- Caltech astronomer Michael E. Brown.
- ^ a b "'Planet definition' questions & answers sheet" (Press release). International Astronomical Union. August 24, 2006. Archived from the original on May 7, 2021. Retrieved October 16, 2021.
- ^ (PDF) from the original on April 7, 2019.
- ^
Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. S2CID 44935431.
- ^ a b Raymond, C.; Castillo-Rogez, J.C.; Park, R.S.; Ermakov, A.; et al. (September 2018). "Dawn Data Reveal Ceres' Complex Crustal Evolution" (PDF). European Planetary Science Congress. Vol. 12. Archived (PDF) from the original on January 30, 2020. Retrieved July 19, 2020.
- ^ 'Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects'
- ^ Dan Bruton. "Conversion of Absolute Magnitude to Diameter for Minor Planets". Department of Physics & Astronomy (Stephen F. Austin State University). Archived from the original on March 23, 2010. Retrieved June 13, 2008.
- (PDF) from the original on November 7, 2020. Retrieved January 14, 2022.
- ^ a b c
Dunham, E. T.; Desch, S. J.; Probst, L. (April 2019). "Haumea's Shape, Composition, and Internal Structure". The Astrophysical Journal. 877 (1): 11. S2CID 90262114.
- ^ "Dwarf Planets and their Systems". Working Group for Planetary System Nomenclature (WGPSN). July 11, 2008. Archived from the original on July 14, 2007. Retrieved September 12, 2019.
- arXiv:1905.12320.
- ^ Dyches, Preston (May 11, 2016). "2007 OR10: Largest Unnamed World in the Solar System". Jet Propulsion Laboratory. Archived from the original on November 23, 2020. Retrieved September 12, 2019.
- ^ Porter, Simon (March 27, 2018). "#TNO2018". Twitter. Archived from the original on October 2, 2018. Retrieved March 27, 2018.
- ^ "Report of Division F "Planetary Systems and Astrobiology": Annual Report 2022-23" (PDF). International Astronomical Union. 2022–2023. Archived (PDF) from the original on December 8, 2023. Retrieved December 8, 2023.
- S2CID 119522310.
- ^ a b Brown, Michael E. "The Dwarf Planets". California Institute of Technology, Department of Geological Sciences. Archived from the original on July 19, 2011. Retrieved January 26, 2008.
- ^ a b Brown, Mike. "How many dwarf planets are there in the outer solar system?". CalTech. Archived from the original on October 18, 2011. Retrieved November 15, 2013.
- ^ Stern, Alan (August 24, 2012). "The PI's Perspective". Archived from the original on November 13, 2014. Retrieved August 24, 2012.
- .
- ^ Brown, Michael (August 23, 2011). "Free the Dwarf Planets!". Mike Brown's Planets. Archived from the original on October 5, 2011. Retrieved August 24, 2011.
- 2005 QU182(albedo between 0.2 and 0.5).
- S2CID 133585837. Archived from the original(PDF) on January 15, 2020. Retrieved October 26, 2019.
- S2CID 221095753.
- .
- S2CID 260926329. A167.
- .
- ^ Cowen, R. (2007). Idiosyncratic Iapetus, Science News vol. 172, pp. 104–106. references Archived 2007-10-13 at the Wayback Machine
- doi:10.1016/j.icarus.2010.01.025. Archived from the original(PDF) on December 23, 2018. Retrieved September 25, 2015.
- S2CID 119114880.
- ^ Bode, J.E., ed. (1801). Berliner astronomisches Jahrbuch führ das Jahr 1804 [The Berlin Astronomical Yearbook for 1804]. pp. 97–98. Archived from the original on December 14, 2023. Retrieved October 19, 2022.
- ^ Slipher, V.M. (1930). "The trans-Neptunian planet". Popular Astronomy. Vol. 38. p. 415. Archived from the original on December 11, 2021. Retrieved October 19, 2022.
- ^ Anderson, Deborah (May 4, 2022). "Out of this World: New Astronomy Symbols Approved for the Unicode Standard". unicode.org. The Unicode Consortium. Archived from the original on August 6, 2022. Retrieved August 6, 2022.
- ^ a b c d Miller, Kirk (October 26, 2021). "Unicode request for dwarf-planet symbols" (PDF). unicode.org. Archived (PDF) from the original on March 23, 2022. Retrieved October 19, 2022.
- ^ "Alchemical Symbols" (PDF). unicode.org. The Unicode Consortium. 2022. Archived (PDF) from the original on April 2, 2020. Retrieved October 19, 2022.
- ^ "What is a Dwarf Planet?". Jet Propulsion Laboratory. NASA. April 22, 2015. Archived from the original on December 8, 2021. Retrieved September 24, 2021.
- ^ Landau, Elizabeth; Brown, Dwayne (March 6, 2015). "NASA Spacecraft Becomes First to Orbit a Dwarf Planet". NASA. Archived from the original on March 7, 2015. Retrieved March 6, 2015.
- . 95.
- ^ Jones, Andrew (April 16, 2021). "China to launch a pair of spacecraft towards the edge of the solar system". SpaceNews. SpaceNews. Archived from the original on September 29, 2021. Retrieved April 29, 2021.
- .
- Bibcode:2012LPI....43.2600A. Archived(PDF) from the original on October 20, 2013. Retrieved July 15, 2015.
- (PDF) from the original on July 15, 2015. Retrieved July 15, 2015.
- S2CID 4420518. Archived from the original(PDF) on October 14, 2016. Retrieved August 29, 2015.
- ^ Cook, Jia-Rui C.; Brown, Dwayne (April 26, 2012). "Cassini Finds Saturn Moon Has Planet-Like Qualities". Jey Propoulsion Laboratory. Pasadena, California: NASA. Archived from the original on July 13, 2015.
- S2CID 181460133.
- S2CID 119338327. Archived from the original(PDF) on July 31, 2013.
- ^ "Pluto and the Solar System". iau.org. International Astronomical Union. Archived from the original on April 17, 2020. Retrieved July 10, 2013.
External links
- NPR: Dwarf Planets May Finally Get Respect (David Kestenbaum, Morning Edition)
- BBC News: Q&A New planets proposal, August 16, 2006
- Ottawa Citizen: The case against Pluto (P. Surdas Mohit) August 24, 2006
- James L. Hilton: When Did the Asteroids Become Minor Planets?
- NASA: IYA 2009 Dwarf Planets