Pluto, officially designated as (134340) Pluto (international designation: ((134340) Pluto), is a dwarf planet, the largest known in the Solar System (2,372 km in diameter, compared to 2,326 km for Eris), and the second in terms of mass (after Eris). Pluto is thus the ninth largest known object orbiting directly around the Sun and the tenth by mass. The first trans-Neptunian object identified, Pluto orbits the Sun at a distance varying between 30 and 49 astronomical units and belongs to the Kuiper belt, of which it is (both in size and mass) the largest known member.
After its discovery by American astronomer Clyde Tombaugh in 1930, Pluto was considered the ninth planet in the Solar System. In the late twentieth and early twenty-first centuries, more and more similar objects were discovered in the outer Solar System, especially Eris, then estimated to be slightly larger and more massive than Pluto. This evolution led the International Astronomical Union (IAU) to redefine the notion of planet, Ceres, Pluto and Eris being since the 24 August 2006 classified as dwarf planets. The IAU also decided to make Pluto the prototype of a new category of trans-Neptunian object. As a result of this change in nomenclature, Pluto was added to the list of minor objects in the Solar System and assigned the number 134340 in the catalog of minor objects.
|Semi-major axis (a)||5,900,898,440.583 108,67 km
(39,445,069 7 AU)
|Perihelion (q)||4,436,824,613 km
|Aphelion (Q)||7,375,927,931 km
|Eccentricity||0.250 248 71|
|Period of revolution (Prev)||90,487,276 9 days
|Average orbital velocity (vorb)||4.74 km/s|
|Medium movement (n)||0.003 978 45°/d|
|Inclination (i)||17.089 000 9°|
|Longitude of ascending node (Ω)||110.376 956°|
|Perihelion argument (ω)||112.597 141 7°|
|Mean anomaly (M0)||25.247 189 7°|
|Date of last perihelion (Tp)||JJ 2,447,778,716 79
(May 8, 1989)
|Category||Plutoid (trans-Neptunian dwarf planet), plutino|
|Known satellites||5: Charon, Hydra, Nix, Kerberos, Styx|
|Terrestrial DMIO||28,603 1 AU|
|Weaver parameter (TJ)||5,228|
|Equatorial radius (Req)||1,185 ± 10 km|
|Volume (V)||6.97×109 km3|
|Mass (m)||(1.314 ± 0.018) × 1022 kg|
|Density (ρ)||(1,854 ± 11) kg/m3|
|Equatorial gravity at the surface (g)||0.625 m/s2|
|Release velocity (vlib)||1.22 km/s|
|Rotation period (Prot)||−6.387 d (retrograde)|
|Absolute magnitude (H)||−0.8|
|Temperature (T)||≈ 48 K|
|Oldest pre-discovery sighting||January 23, 1914|
|Date||February 18, 1930, from photographs from January 1930|
|Discovered by||Clyde W. Tombaugh|
|place||Lowell Observatory in Flagstaff, Arizona|
|Announcement||March 14, 1930|
|Named after||Pluto (Roman god)|
Pluto is mainly composed of methane rock and ice, but also water ice and frozen nitrogen. Its diameter is about two-thirds that of the Moon.
Pluto is the main body of the Plutonian system. The pair that Pluto forms with its large satellite, Charon (diameter 1,207 km), is often considered a double system, because the difference in mass between the two objects is one of the smallest of all the primary body/satellite pairs of the solar system (ratio 8:1) and the barycenter of their orbits is not inside one of the two bodies (it is slightly at the exterior of Pluto).
Four other natural satellites, much smaller and all in a roughly circular orbit (eccentricity < 0.006) outside Charon’s orbit, complete the system as currently known (in order of moving away): Styx, Nix, Kerberos and Hydra. All four were discovered with the help of the Hubble Space Telescope: the two largest, Nix and Hydra (respectively 54 × 41 × 36 km and 43 × 33 km), in 2005, Kerberos (about 12 × 4 km) in 2011 and Styx (about 7 × 5 km) in 2012. The latter two received their official names in July 2013. The dimensions mentioned correspond to measurements made after their discovery, and not to the first estimates that could be made.
The New Horizons spacecraft, launched in January 2006 by NASA, is the first probe to explore the Plutonian system; it crosses on July 14, 2015, at a minimum distance of 11,095 km from Pluto, after a journey of 6.4 billion kilometers. The probe does not detect any other satellite larger than 1.7 km in diameter for an albedo of 0.5.
Historical of Pluto
According to Greg Buchwald, Michel DiMario and Walter Wild, Pluto was photographed on August 21 and November 11, 1909 at the Yerkes Observatory of the University of Chicago. However, their coordinates do not appear in the list of fourteen other pre-discoveries of Pluto that are listed in the data of the Minor Planet Center. The very first officially identified is that of January 23, 1914, at the Königstuhl Observatory in Heidelberg.
Pluto was discovered in 1930 during the search for a celestial body to explain Neptune’s orbital perturbations, a hypothesis proposed by Percival Lowell as Planet X.
Having made his fortune in business, Lowell had an observatory built in 1894 at an altitude of more than 2,000 m in Arizona and undertook the search for a ninth planet beyond Neptune. He thought he was following the same method that had led to the discovery of the latter by studying its orbit, but the precision of the instruments of the time did not allow to accurately measure the orbital anomalies, he had to fall back on those of Uranus. Its planet (named “X”) would be located at 47.5 AU, would have a period of 327 years and a mass of two-fifths of that of Neptune.
In 1905, he launched a first three-year photographic campaign, but it did not give anything conclusive, in particular, as it was later demonstrated, because this program was focused on the ecliptic and that the strongly inclined orbit of Pluto placed it at that time outside the field of photographs. Lowell did not give up and decided to redouble his efforts, especially when he saw a competitor appear: William Pickering. He announced in 1908 the presence of a planet he named “O” of two Earth masses, at a distance of 52 AU and a period of 373 years.
In 1911, Lowell acquired a flashing comparator, a machine for photographic analysis allowing him to compare photographs much faster (two series of photos were taken a few days apart to identify the possible movement of a celestial body) and began a new series of photographs. This new failure will lead him to lose interest in his planet X.
Percival Lowell died in 1916 but left in his will enough to continue the research without worrying about money problems, although inheritance problems with his wife eventually reduced the budget of the observatory. Ten years later, however, the observatory must equip itself with a new instrument. Abbott Lawrence Lowell, Percival Lowell’s brother, agrees to donate ten thousand dollars for the construction of a 13-inch telescope that Clyde W. Tombaugh will be tasked with piloting the daunting task of meticulously mapping the sky in search of Planet X. Tombaugh rearranged his work plan and took three shots instead of two in order to increase the chances of perceiving the planet’s movement.
The third series of photographs ends on January 29, 1930, and then begins the analysis of the photographic plates. On February 18, 1930, he notices a point moving from one plate to another in two photographs taken on January 23 and 29. The Lowell Observatory team, after taking further photographs to confirm the discovery, telegraphed the news to the Harvard College Observatory on March 13, 1930. The discovery was announced on March 14, 1930 by a circular of the International Astronomical Union.
Many observatories then began to observe this new planet, in order to determine its orbit as accurately as possible. Using earlier images, Pluto is retroactively observed on photographic plates dating back to 1909.
The planet is named both in reference to the Roman god of the underworld and Percival Lowell whose initials form the first two letters of Pluto. His initials form the first astronomical and astrological symbol of Pluto: ♇ (This symbol is rare in astronomy today, but common in astrology). The name was suggested by Venetia Burney, an eleven-year-old girl from Oxford, England. Passionate about mythology and astronomy, Venetia Burney found it appropriate to associate the name of the god of the underworld with this dark and icy world. His grandfather, who worked at the Oxford University Library, told the astronomer Herbert Hall Turner, who passed the idea on to his American colleagues. The name Pluto was made official on March 24, 1930.
The very name attributed to Pluto as ruler of the Underworld has aroused the imagination of astrologers excessively at a time of troubled times when astrology has – as usual for periods of crisis – experienced an effervescence (at that time, it came out of hiding, bursting into the mass media). The specialist in the history of astrology Jacques Halbronn finds it curious that the name chosen by astronomers determined the symbolism adopted by astrologers.
Indeed, there was in the name “Pluto” the idea of the judge of souls, and therefore of a kind of Last Judgment. Only four years after the discovery of the star, the German astrologer Fritz Brunhübner, seeing in Pluto a super-evil star, claimed that “Pluto can be called the cosmic aspect at the origin of the Third Reich”. With a remarkable lack of hindsight for a star whose period of revolution is 249 years, Brunhübner then went so far as to attribute to Pluto the astrological mastery over the sign of Scorpio. However, there was no consensus: Alexander Volgin believed that Pluto ruled the sign of Sagittarius while Dane Rudhyar saw the star in analogy with the sign of Aries. Others have hypothesized an astrological mastery over the sign of Pisces!
Pluto and Planet X
Initially, the discovery of Pluto is linked to the systematic search for a planet to explain the perturbations observed in the orbits of Uranus and Neptune, but the doubt is quickly cast on the fact that Pluto would be the planet X that Percival Lowell was looking for.
At that time, Pluto was so distant that its diameter could not be determined precisely, but its low luminosity and lack of apparent disc suggest a rather small body, comparable in size to the terrestrial planets already known, probably larger than Mercury but no larger than Mars, it was thought at the time. So it quickly becomes clear that Pluto cannot be the source of the disturbances in the orbits of Neptune and Uranus. Clyde Tombaugh and other astronomers persevered in the search for Planet X for 12 years but discovered only asteroids and comets.
Astronomers are led to imagine that many other bodies similar to Pluto could orbit the Sun beyond Neptune. It is then thought that the solar system could be made up of several zones grouping celestial bodies by families, telluric planets, giant planets, “ultra-Neptunian objects”. This hypothesis would later be formalized during the 1940s and 1950s by Kenneth Edgeworth and Gerard Kuiper and is now known as the Kuiper belt.
The first satellite of Pluto was discovered on June 22, 1978, when James W. Christy realized that the image of Pluto appearing on photographic plates taken in the previous two months appeared to show a protuberance sometimes on one side, sometimes on the other. The protuberance was confirmed on other plates, the oldest of which dated back to April 29, 1965. Later observations of the protuberance showed that it was caused by a small body. The periodicity of the prominence corresponded to Pluto’s rotation period, which was known from its luminosity curve, indicating a synchronous orbit and suggesting that it was a real effect and not an observational artifact. The name Charon was given to the satellite.
In 1993, calculations of the flyby path of Neptune by the Voyager 2 probe in August 1989 showed that Neptune had a lower mass than previous assumptions, and taking into account this new measurement, the mathematician Myles Standish shows that the divergences in the motions of the planets Uranus and Neptune become negligible in the face of the uncertainty of the measurement related to the accuracy of the instruments. The hypothesis of a disruptive planet X no longer holds, and it is therefore on the basis of a false position prediction that Pluto was discovered.
Dwarf planet status
In the last decade of the twentieth century, the discovery of many trans-Neptunian objects (more than a thousand), some of which have an estimated size close to that of Pluto (for example Eris), leads to the questioning of its status as a planet.
Among these, many bodies are discovered that possess a period of revolution equal to that of Pluto and are like him in 2:3 resonance with Neptune.
Some scientists then proposed to reclassify Pluto as a minor planet or trans-Neptunian object. Others, such as Brian Marsden of the Minor Planet Center, lean toward assigning it both statuses, because of the historical significance of its discovery. Marsden announced on February 3, 1999, that Pluto would be classified as the 10,000th object in the catalog of 10,000 minor planets. The round number of “10,000” would be assigned to Pluto in his honor for the “celebration” of this reached count. The International Astronomical Union (IAU), the coordinating body of astronomy at the international level, responsible for the naming of celestial bodies as well as their status, then made a clarification, recalling that it alone was authorized to determine the status of Pluto.
Historically, the first four asteroids discovered from 1801 to 1807 — (1) Ceres, (2) Pallas, (3) Juno and (4) Vesta — were also considered planets for several decades (at the time, their dimensions were not precisely known). Some astronomical texts from the early nineteenth century refer to eleven planets (including Uranus and the first four asteroids). The fifth asteroid (5) Astrée was discovered in 1845 shortly before the discovery of Neptune, followed by several others in the following years. In the 1850s, these increasing numbers of objects were no longer considered “planets” and called “asteroids” or “minor planets”.
The discovery in 2005 of (136199) Eris, with a comparable diameter and a mass slightly greater than those of Pluto, helped to revive the debate; since it is indeed a question of not reproducing the same scenario as what had happened for Ceres, Pallas, Juno and finally Vesta. The diameter of Eris, which had initially been estimated at 3,600 km (it then seemed significantly larger than Pluto) was still in 2006 of the same order of magnitude as that of Pluto, even after being revised downwards (2,400 ± 100 km). According to a study published in 14 June 2007, its mass would be about 27% greater than that of Pluto. Many other bodies were also discovered at this time, such as (136472) Makemake, (90482) Orcus or (90377) Sedna, regularly announced as the tenth planet of the Solar System.
The classification into nine planets becomes difficult to sustain. The last word goes to the IAU, which, at its 26th congress held on August 24, 2006, in the Czech Republic, decided after a week of debates to complete the definition of planet, saying that a planet eliminates from its vicinity all objects of a size comparable to it. This is not the case of Pluto, which shares its space with other trans-Neptunian objects and is reclassified as a dwarf planet. The Minor Planet Center assigned it the minor object number “134340” on September 7, 2006. 134340 Pluto became the official designation of the International Astronomical Union on September 13, 2006.
Nevertheless, following the vote, a petition gathering in five days the signatures of more than 300 planetary scientists and astronomers mainly American (Pluto having been the first planet discovered by an American) was launched to challenge the scientific validity of the new definition of planet that downgraded Pluto as well as its mode of adoption and invite reflection on another more appropriate definition.
It must be said that during the 26th Congress in Prague which was held from 14 to 25 August 2006, the vote on whether or not to demote Pluto took place only on August 24 and in the presence of about 400 members out of 6,000, which may call into question the validity of the decision. Nevertheless, Catherine Cesarsky, president of the IAU, closed the debate by deciding that the UAI assembly of August 2009 would not go back on the definition of a planet. Planetary scientists, however, continue to speak of Pluto as a planet in 2018, such as Alan Stern.
On September 18, 2014, the Harvard-Smithsonian Center for Astrophysics organized a debate bringing together three experts presenting three points of view on the definition of a planet: historical, the definition adopted by the IAU and finally the point of view of exoplanet researchers; the latter, presented by Dimitar Sasselov, president of the Harvard Origins of Life Initiative, is supported by experts, for whom Pluto would therefore be a planet.
Pluto retains its importance
About one hundred and fifty objects orbiting like Pluto with a 2:3 resonance with Neptune were identified in February 2006, which tends to show that Pluto is the largest representative of a vast family of more or less massive bodies. Astronomers David Jewitt and Jane Luu propose naming them “plutinos”.
A new subcategory, plutoids, was created by the IAU for dwarf planets that spent most of their orbital revolution outside the orbit of Neptune, of which Pluto was a part.
Hubble Space Telescope observations
The Hubble Space Telescope provided the most detailed images of Pluto’s surface before the arrival of New Horizons.
Pluto is a difficult target for space exploration, because of the great distance separating it from the Earth (about 4.8 billion kilometers), the high inclination of its orbit (17°) on the ecliptic and its very low mass.
For comparison, if the Earth were a football (70 cm in circumference), Pluto would be about the size of a golf ball. At this scale, a distance of 86 kilometers would separate the two planets or 20 laps of the Circuit Gilles-Villeneuve or the distance from Paris to Evreux.
The Voyager 1 probe could eventually have reached it, but the exploration of Titan (the largest satellite of the countless in Saturn) and Saturn’s rings was deemed more important, which had the effect of making its trajectory incompatible with a rendezvous with Pluto. Voyager 2 was not able to reach it because the theoretical trajectory of the probe to make this rendezvous would have assumed crossing the planet Neptune.
NASA studied a probe project to Pluto in 1991, which was scaled down in 1992 and abandoned in 1994. A new US-Russian project, the Pluto Kuiper Express mission, began in 1995. It would have been intended to flyby around 2012 of the couple Pluto / Charon, and at least one object of the Kuiper belt. NASA cancelled it in 2000, for budgetary reasons.
It was eventually replaced by a similar mission, New Horizons. The New Horizons probe, launched on 19 January 2006, is, therefore, the first space probe to visit Pluto, benefiting in February 2007 from the gravitational assistance of Jupiter to arrive closest to the dwarf planet on July 14, 2015, after a journey of 6.4 billion kilometers. Observations begin about five months before the nearest passage and are expected to continue about a month after. However, the flyby was so fast that only a hemisphere could be photographed with the highest resolution. The spacecraft carries imaging, spectroscopy, and other measuring instruments to determine the geological and morphological characteristics of Pluto and its moon Charon, as well as to map their surface elements and study Pluto’s atmosphere (composition and escape rate). The mission also includes a flyby of Kuiper belt objects until 2025.
Orbit of Pluto
Pluto’s orbit around the Sun has been observed for more than a century (the oldest image on which Pluto is spotted dates back to January 1914), a travel time of just over a third of its annual trajectory, but sufficient to accurately measure its orbital characteristics.
The semi-major axis of Pluto’s orbit is 39.88 AU, but because of the pronounced eccentricity of this orbit, the distance between Pluto and the Sun varies between 29.7 AU at perihelion and 49.5 AU at aphelion, and the Plutonian year lasts 248.1 Earth years.
Compared to the classical planets of the solar system, Pluto’s orbit is strongly inclined with respect to the plane of the ecliptic (17.14175°) and eccentric (0.24880766). The orbits of the classical planets are quasicircular and coplanar of the ecliptic (only Mercury has an inclined (7°) and eccentric (0.2) orbit significantly).
Comparison with Neptune
Pluto’s perihelion is located more than 8.0 AU above the plane of the ecliptic or 1.2 billion km, and it is near this position of its orbit that the dwarf planet is closer to the Sun than Neptune. This was the case for twenty years between February 7, 1979, and on February 11, 1999. In contrast, Pluto moves 13 AU below the plane of the ecliptic.
Crosses with other asteroids
Since Pluto’s orbit is very eccentric, it crosses that of many other objects; Among the numbered asteroids, these hadeocruisers counted (in July 2004) 10 inner cruisers (including (5145) Pholos), 24 outer brushers (including (19521) Chaos), 17 cruisers (including (38628) Huya) and 37 coorbital (including (20000) Varuna, (28978) Ixion and (50000) Quaoar).
Although Pluto is sometimes closer to the Sun than Neptune, the orbits of the two objects never intersect, due to the high inclination (about 17°) of Pluto’s orbit relative to the plane of the ecliptic. The nodes of Pluto’s orbit (the points where the orbit crosses the plane of the ecliptic) are located outside Neptune’s orbit.
Pluto is in resonance with Neptune of ratio 3:2, that is to say that over a period of 496 years, Pluto makes two revolutions around the Sun while Neptune makes three. This resonance is stable: a disturbance in Pluto’s orbit would be corrected by Neptune’s attraction. Because of this phenomenon, Pluto and Neptune are never closer than 18.9 AU, while Pluto can come within 12 AU of Uranus. When Neptune passes the point where the two orbits are closest, the resonance maintains an angular separation Neptune-Sun-Pluto greater than 50° and Pluto remains nearly 30 AU behind Neptune, or nearly 4.5 billion kilometers. The real point of approach is on the other side of the orbit. Neptune still “surpasses” Pluto some 30 years after the latter’s aphelion.
Other trans-Neptunian objects that orbit with a semi-major axis of 39.4 AU have such a 3:2 orbital resonance with Neptune and are called plutinos, with reference to Pluto. In 2009, there were more than 200.
Pluto physical characteristics
If the trajectory of Pluto could be determined without great difficulty, its physical characteristics (diameter, mass, and therefore density, reflective power, state of the surface) have long remained poorly known and controversial: its apparent diameter is less than 1/4 of an arc second, while the turbulence of the Earth’s atmosphere makes it difficult to observe details less than one second of arc. The finesse of observations increased in the 1980s, by the use of adaptive optics, spectrometry, and the Hubble Space Telescope.
The discovery in 1978 of a satellite of Pluto, Charon, offered additional means of investigation. However, in 2010, the published values still differ somewhat depending on whether one refers to NASA or to recent publications. The flyby in 2015 by the New Horizons mission and the gravitational effects of the Pluto-Charon couple on the probe will make it possible to adjust the values of its gravity field, according to the observation of the Doppler effect on the signals of the probe and the resulting deduction of the variations in its speed and acceleration induced by Pluto and Charon.
In 1955, it was observed that the variations in the luminosity of Pluto are of the order of 30% and are periodic. We deduce that Pluto rotates on itself in 6,387 days, or 6 days, 9 hours and 17 minutes. Its axis of rotation is inclined 57.5° from its orbital plane, which is rather high and unusual in the Solar System (only Uranus has a comparable inclination). At the solstice points of its orbit, Pluto exposes a pole to the Sun for many decades, and at equinox points, every 124 years, it rotates as on a pin facing the Sun, while the Earth sees vertically its equator line as well as the orbit of Charon, which passes alternately in front of and behind Pluto.
The action of the tidal forces forced the rotation period of Pluto to synchronize it with the period of revolution of its main satellite, Charon: the two periods being equal, Charon is therefore always vertical at the same point on the surface of Pluto, and Charon therefore appears motionless in the Plutonian sky.
Mass and dimensions
Pluto, with its mass of one-five-hundredth of that of the Earth and a diameter of 2,370 ± 20 km, is smaller and less massive than seven natural satellites in the Solar System: the Moon (3,476 km in diameter), the four Galilean satellites of Jupiter (Ganymede, 5,262 km; Callisto, 4,880 km; Io, 3,640 km; Europa, 3,122 km), the largest satellite of Saturn (Titan, 5,150 km) and that of Neptune (Triton, 2,706 km).
|Year||Radius and (diameter)||Notes|
|1993||1,195 (2,390) km||Millis, et al. (if no haze)|
|1993||1,180 (2,360) km||Millis, et al. (surface and haze)|
|1994||1,164 (2,328) km||Young & Binzel|
|1997||1,173 ± 23 (2,346 ± 46) km||Tholen and Buie|
|2006||1,153 ± 10 (2,306 ± 20) km||Buie, et al.|
|2007||1,161 (2,322) km||Young, Young, & Buie|
|2009||> 1,169-1,172 (> 2,338-2,344) km||Lellouch, et al.|
|2011||1,180 +20/-10 (2,360 +40/-20) km||Zalucha, et al.|
|2011||1,173 +20/-10 (2,346 +40/-20) km||Zalucha, et al.|
|2014||1,184 ± 4 (2,368 ± 8) km||Lellouch, et al.|
|2015||1,185 ± 10 (2,370 ± 20) km||New Horizons measure|
|2017||1,188.3 ± 1.6 (2,376.6 ± 3.2) km||New Horizons measurement|
Before its flyby by the New Horizons probe, Pluto’s diameter was one of the least known and most difficult to measure physical parameters, and the main source of uncertainty about other derived parameters such as density. Its very large distance combined with its small size make it impossible to resolve Pluto’s disk precisely, and therefore prevent “direct” measurements of its dimensions, either with the Hubble Space Telescope or with ground-based instruments with adaptive optics.
Measurements based on Pluto’s star occultations and Charon’s occultations of Pluto do not agree exactly, and explanations for these differences depend on the models used to analyze the data, including the dwarf planet’s atmosphere. The value and margin of error generally used of 2,306 ± 20 km in diameter actually include the differences in the results of the different measurement methods. On July 13, 2015, the New Horizons probe makes it possible to slightly reassess the diameter of Pluto slightly upwards to 2,370 ± 20 km (i.e. a radius of 1,185 ± 10 km), the uncertainty of this value being due to the presence of a planetary atmosphere. In 2017, the re-analysis of New Horizons data made it possible to refine this result: 2,376.6 ± 3.2 km (radius: 1,188.3 ± 1.6 km).
|1931||1 Land||Nicholson & Mayall|
|1948||0.1 (1/10) Earth||Kuiper|
|1976||0.01 (1/100) Earth||Cruikshank, Pilcher, & Morrison|
|1978||0.002 (1/500) Earth||Christy & Harrington|
|2006||0.00218 (1/459) Earth||Buie et al.|
|2015||0.00220 (1/455) Earth||New Horizons|
Pluto’s mass, as well as its diameter, have been vastly overestimated in the decades following its discovery. Percival Lowell hoped to find a planet comparable to Neptune, on the order of ten times the Earth’s mass. The observed magnitude being lower than expected, the assessment was lowered to a land mass. Estimates that assumed a size between those of Mercury and Mars have been continuously revised downwards with the improvement of observation instruments.
In 1976, the analysis of the light of Pluto made suppose an icy surface, thus a brightness provided by a smaller surface, and a mass reduced to one-hundredth of that of the Earth. The discovery of Charon in 1978 made it possible, by application of Kepler’s third law, to determine much more precisely the total mass of the planetary couple. The mass of Pluto is estimated in 2006 at 1.314 × 1022kg, 5.6 times less than that of the Moon or five hundredth of the Earth’s mass. By extrapolating this continual decline, two facetious astronomers went so far as to announce the complete disappearance of Pluto.
Physical geography and geological features
Pluto does not have a significant atmosphere. But according to the laws of physics, the ice on its surface must be in thermodynamic equilibrium with gaseous phases, so it would be surrounded by a thin envelope of gas that would be composed of nitrogen (N2) at 90% because it is the most volatile element among those detected on the surface, and 10% carbon monoxide (CO), as well as traces of methane (CH4). In addition, scientists on the New Horizons mission noted that this atmosphere is escaping at a rate of about 500 tons per hour because of the dwarf planet’s weak gravitational pull.
Pluto’s atmosphere was discovered during a stellar occultation in 1985, and confirmed by another occultation in 1988. When an object without an atmosphere passes in front of a star, this background star disappears abruptly; in the case of Pluto, the brightness of the masked star gradually decreased. From the evolution of this luminosity curve, a thin atmosphere of 0.15 Pa was determined, about 700,000e of that of the Earth. This atmosphere could exist only when the planet is close to perihelion, and freeze when it moves away from the Sun. Indeed, the energy of the Sun received by Pluto varies quite strongly between perihelion and aphelion, because of its marked orbital eccentricity.
The temperature changes by about 10 K between these two points. When Pluto deviates from perihelion, part of its atmosphere freezes and falls to the surface. When it gets closer, the surface temperature rises and the nitrogen sublimates. Like sweat evaporating on the skin, this sublimation tends to cool the surface, and research has shown that Pluto’s temperature is 10 K lower than expected (mean surface temperature: −228 °C); unlike Charon which, without an atmosphere, has a surface temperature consistent with its albedo.
In 2002, another stellar occultation by Pluto was observed by several teams led by Bruno Sicardy, Jim Elliot and Jay Pasachoff. Surprisingly, atmospheric pressure has been estimated at 0.30 Pa, although Pluto is farther from the Sun than in 1988, and therefore cooler. The preferred hypothesis at present is that the south pole of Pluto would have emerged from the shadows in 1987 for the first time in 120 years and that a surplus of nitrogen would then have sublimated part of the south polar cap. This excess nitrogen is likely to take decades to condense at the other pole, in a cyclical manner.
The flyby of Pluto by New Horizons allows a direct measurement of the pressure on the ground: 11 μbar (1.1 Pa), 100,000 times less than on Earth but three times more than the previous highest estimate. This atmosphere escapes 500 to 1,000 times slower than previously expected, and it has a significant presence up to several hundred kilometers above sea level, with dozens of layers of haze but no clouds. On October 8, 2015, NASA announced that, seen from Pluto, the sky appears blue due to the scattering of light by particles (which would be rather gray or red), resembling soot, called tholines.
Albedo and surface
Variations in Pluto’s brightness testify to an uneven brightness between the different regions on its surface. Pluto reflects sunlight with an albedo of 58% on average, which is a high value (it is 31% for Earth, and rises to 72% for Venus thanks to its cloud layer). The North Pole is particularly bright, with an estimated albedo of 80%, the South Pole is a little less bright, while the equator has a dark band 5 times less reflective, and the intermediate areas marked contrasts.
Areas of high albedo are interpreted as parts covered with snow or ice of recent formation, not yet obscured by deposits of impurities, while dark parts could be carbon compounds. The mapping of these areas was refined by the analysis of light variations during Charon’s passages in front of Pluto, and confirmed in 1994 by direct Hubble observations. The overall image, taken with the Faint Object Camera, remains very blurry, however, because it consists of only a hundred pixels, each measuring 200 km on each side. A new Hubble equipment, the Advanced Camera for Surveys, provided in 2002-2003 complete views of Pluto, still blurred but showing changes in coloration compared to previous images.
Infrared spectroscopy analyses have identified several types of ice on the surface of Pluto: methane ice in 1976, then from 1992, nitrogen ice, the most abundant with a proportion of about 98%, carbon monoxide ice, water ice and ethane ice. The average ground temperature is estimated to be −223 °C, with variations between zones, −213 °C for dark areas and between −238 °C and −233 °C for the most reflective parts.
On its surface, ice of methane (CH4) and nitrogen (N2) has been detected at the poles by an infrared observation, in ice caps whose size varies according to the distance of the planet from the Sun. As of February 5, 2010, some experts have noticed that the ice at the North Pole has become brighter, while that at the South Pole has darkened. Beneath the Plutonian crust is likely an icy mantle.
In recent years, Pluto’s color has taken on a red hue 20 to 30 percent higher than in 2000, when it had not changed from the entire period from 1954 to 2000. This change in hue would be due to methane, a compound present on the dwarf planet. The hydrogen contained in methane, hit by solar winds, would release the carbon constituting the other part of the methane, producing shades of red and black on the surface of Pluto.
The June 26 and 27, 2015 photographs taken by New Horizons show “a series of intriguing spots at the equator, evenly spaced. Each of these spots is about 480 km in diameter”. On October 8, 2015, NASA announced the detection of water ice on the surface of Pluto by New Horizons.
Geography of Pluto
The flyby of Pluto by the New Horizons probe revealed a geography and geology much more diverse than expected: vast nitrogen glaciers (800,000 km2 for Sputnik Planitia, the largest of them), chaotic and mountainous terrain resulting from the dismantling of ancient glaciers, blocks of frozen methane and methane snow caps, a set of methane ice towers (over 300 m high) hundreds of kilometers long, and fault systems also stretching for hundreds of kilometers.
- The surface density of the impact craters on the surface of Pluto is extremely variable from one region to another, which reflects very varied ages: from less than 30 million years for the Sputnik plain to more than 4 billion, through areas of average age (between 100 million and one billion).
- Many ice volcanoes are also present, of relatively small age (100 to 300 million years).
- In addition to impact craters and volcanic craters, there are thousands of craters (up to 10 km in diameter) of uncertain origin but plausibly related to the sublimation of nitrogen ice.
Several major regions or geological features are known to date:
- the Tombaugh region, nicknamed the “Heart”, a large clear area more than 2,000 km wide. The western lobe of the heart is covered with carbon monoxide ice. This lobe includes the Sputnik Plain to the northeast, the Hillary Mountains to the west, and the Tenzing Mountains to the south;
- the Cthulhu macula, nicknamed “the Whale”, a large dark area more than 3,000 km long at the equator, so more than 40% of the circumference of Pluto, possibly covered with ice less volatile than that of nitrogen, such as that of methane;
- Krun, Ala, Balrog, other dark regions;
- the polar region, of intermediate luminosity;
- a polygonal structure measuring about 200 km in its largest dimension;
- A relatively complex strip of land running diagonally across the dwarf planet.
The existence of cryovolcanism on Pluto is considered. Thus, two geological structures on its surface, Mount Piccard and Mount Wright, are approximately circular with a depression at their center and could be two cryovolcanoes. On March 29, 2022, scientists confirmed that ice volcanoes had been recognized thanks to images captured by NASA’s New Horizons probe. According to scientists, it is very likely that these ice volcanoes are still active (Kelsi Singer – Southwest Research Institute – Boulder, Colorado).
The internal composition of Pluto is currently unknown. If there has been planetary differentiation, there could be a rocky core. If Pluto is given a density of 2, the approximate value, the density close to 1 of the ice detected on the surface must be compensated by a rock mass, of the density of the order of 4 or 5, in a proportion equal to the ice of water and volatile elements (nitrogen, methane, carbon monoxide). These rocks could outcrop on the surface without being visible because they lack characteristic spectral signatures, or be covered by a mantle of ice.
With a water ice content of the order of 50% or more for the mass of Pluto, the presence of liquid water at depth under the effect of high pressure is conceivable in the deep layers, coexisting with ice under high pressure. Simulations based on data from the New Horizons probe on the Sputnik Plain have reinforced the presumption of the existence of an internal ocean with a depth of a hundred kilometers. To explain that Pluto can maintain an underwater ocean while possessing an outer layer of very cold ice, it has been argued that there is probably an insulating layer of clathrates above the inner ocean, which is assumed to consist of water and methane.
|Type||Main object of the Plutonian system|
|Semi-major axis (a)||2,390 km
from the system barycentre
|Eccentricity||0.000 00 ± 0.000 07|
|Period of revolution (Prev)||(6.387 230 4 ± 0.000 001 1) d
(6 d 9 h 17 min 36.7 s ± 0.1 s)
|Inclination (i)||0° (relative to the equator of Pluto)|
|Size||(2,370 ± 20) km (diameter)|
|Rotation period (Prot)||6.387 230 4 d
|Date||February 18, 1930|
|Discovered by||Clyde W. Tombaugh|
The search for a satellite of Pluto was based on the assumption that a possible satellite must be much smaller than its planet, as is the case in the rest of the solar system, and therefore less luminous than Pluto. Shots taken in the 1950s and 1960s very overexposed by long break times gave no results. Gerard Kuiper’s theory that Pluto was an ancient satellite of Neptune ejected from orbit implied that Pluto probably could not have a moon, which did not encourage its search. The discovery of a satellite nearly 50 years after that of Pluto was therefore fortuitous.
Pluto has five known natural satellites, the largest being Charon which was identified as early as 1978. Two smaller satellites were discovered in 2005 and named Hydra and Nix (known until June 2006 by their provisional designations S/2005 P 1 and S/2005 P 2). The fifth member of the system, provisionally named S/2011 (134340) 1 and informally P4, was discovered in 2011. The discovery of the last satellite, provisionally known as S/2012 (134340) 1 and informally nicknamed P5, was announced on July 11, 2012. The New Horizons probe detects no other satellite larger than 1.7 kilometers in diameter for an albedo of 0.5 as it passes through the Plutonian system.
On February 11, 2013, the SETI Institute launches the Pluto Rocks! which allows Internet users to vote for the names they would prefer to see assigned to P4 and P5. The site also made it possible to propose names as long as they respected the rules of the International Astronomical Union. The campaign ends after garnering nearly 450,000 votes. The most popular name is Vulcan, proposed by former Star Trek actor William Shatner, followed by Cerberus. However, other objects already bearing these names and to avoid confusion, the Greek spelling Kerberos is preferred to its Latin version Cerberus, and Styx, third in the ranking, is preferred to Vulcan. On July 2, 2013, the International Astronomical Union confirms the names Kerberos for P4 and Styx for P5.
A peculiarity of the Plutonian system is that the barycenter of the Pluto/Charon pair is not located inside the first but in the void between the two bodies.
The distribution of Pluto’s satellites is concentrated in the center of the system. Potentially, a satellite could orbit Pluto up to 53% of the radius of its Hill sphere (about 6 × 106 km) in the direct direction and 69% in the retrograde direction, but the Plutonian system is constricted in the inner 3% of this area. For comparison, Psamathea orbits Neptune at 40% of the radius of its Hill sphere. In the words of the discoverers of Nix and Hydra, the Plutonian system is “highly compact and largely empty”.
Charon was discovered in 1978, during an astrometry campaign to refine the measurement of Pluto’s position. James Christy noticed on the luminous spot of the pictures of Pluto an outgrowth placed differently according to the pictures, the examination of which revealed a periodicity of one week. Christy announced her discovery on July 7, 1978, and proposed naming her Charon.
Compared to Pluto, Charon is a very large satellite (its radius of about 600 km is half that of Pluto, estimated at 1,170 km), and the barycenter of both bodies lies beyond the surface of Pluto (just over two Plutonian radii). It is the largest system of its kind in the Solar System (some binary asteroids also possess this trait, such as 617 Patroclus; the barycenter of the Sun and Jupiter is also located outside the first) and is sometimes referred to as a binary asteroid system.
Under the effect of the gravitational tide, Pluto and Charon are both in synchronous rotation, with a period of 6.387 days: Charon always presents the same face to Pluto and Pluto the same face to Charon, an unusual fact in the Solar System for two objects of this size (but not exceptional, some binary asteroids have this property).
The discovery of Charon made it possible by exploiting from 1985 to 1990 the occultations of Charon by Pluto and the transits of Charon in front of Pluto to specify the total mass of the double system and to determine that it was lower than previous estimates. In fact, it has caused astronomers to completely revise their estimate of Pluto’s size. Originally, Pluto was thought to be larger than Mercury (it was given about 6,800 km in diameter) and smaller than Mars, but the calculations were based on the fact that only one object was observed (Charon was not distinguished from Pluto). Once the dual system was discovered, the estimate of Pluto’s size was revised downwards. It is possible today, with modern instruments, to distinguish the disk of Pluto separately from that of Charon (see the image established by Hubble in 2006).
As a result, Pluto’s albedo also had to be recalculated and revised upwards: since the planet was much smaller than early estimates, its ability to reflect light must have been greater than previously thought. Current estimates give it an average value of 58%, while Charon with 36% appears much darker. Charon did not retain methane, only water ice and ammonia were detected.
Observations made by the New Horizons probe in July 2015 led to the discovery of a dark area north of this satellite, nicknamed “Mordor” by the NASA team.
Hydra and Nix
Pluto has two other satellites, which were photographed on 15 May 2005 during an observation campaign of the Hubble Space Telescope, temporarily named S/2005 P 1 and S/2005 P 2 then named Hydra (named after the monster the Hydra) and Nix (from Nyx, mother of Charon). They were spotted by a team from the Southwest Research Institute on images taken to prepare for the new Solar System far-field exploration mission, New Horizons. Their existence was confirmed by an examination of photographs taken by Hubble and dating from the 14 June 2002.
According to initial observations, the semi-major axis of Nix’s orbit measures 49,000 km with a period of 24.9 days and that of Hydra’s orbit is 65,000 km with a period of 38.2 days. The two satellites appear to orbit retrograde in the same plane as Charon and are two and three times farther away than Charon, with an orbital resonance close to (but not equal to) 4:1 and 6:1.
Observations continue to determine the characteristics of the two celestial bodies. Hydra is sometimes brighter than Nix, either because it is larger or because the brightness of its surface varies depending on the area. The spectrum of the satellites is similar to that of Charon, suggesting a similar albedo of about 0.35; in this case, the diameter of Nix is estimated at 46 km and that of Hydra at 61 km. An upper limit can be determined by assuming an albedo of 0.04 similar to the darkest Kuiper Belt objects: 137 ± 11 km for Nix and 167 ± 10 km for Hydra. In this case, the mass of the satellites would be 0.3% of that of Charon (0.03% of the mass of Pluto).
Other objects orbiting Pluto
Pluto has a quasi-satellite named 15810 Arawn.
Observations by the Hubble Space Telescope have placed limits on the existence of additional satellites in the Plutonian system. With a 90% probability, no moon larger than 12 km and with an albedo similar to that of Charon (0.38) exists in a 5″ zone around Pluto. For a darker albedo of 0.041, this limit is increased to 37 km. With a probability of 50%, this limit drops to 8 km.
In an article published in the journal Nature, a team of American scientists led by S. Has. Stern (of the Southwest Research Institute) announced that Nix and Hydra most likely formed during the same giant impact that gave birth to Charon. The team hypothesized that other large binary objects in the Kuiper belt could also harbor small moons and that those orbiting Pluto could generate rings of debris around the dwarf planet. At present, data from Hubble‘s advanced prospecting camera suggests that no ring exists. Otherwise, it is a tenuous ring like those of Jupiter or less than 1,000 km wide.
During a new observation campaign using the Hubble Space Telescope, a new moon was observed, on June 28, 2011. This observation was confirmed by others on 3 and 18 July. The small moon named Kerberos (sometimes Frenchized as Cerberus; provisionally S/2011 (134340) 1 or P4) and whose size must be between 13 and 34 km, has an orbit inscribed between those of Nix and Hydra.
A new moon called Styx (tentatively S/2012 (134340) 1 or P5), was discovered between June 26 and July 9, 2012, it was baptized by the International Astronomical Union, on July 2, 2013.
After a first inspection of the surroundings of Pluto on the 11th and on May 12, 2015, during which the LORRI instrument of the New Horizons probe took 144 photos of 10 minutes each in order to identify any object that could be dangerous to the probe during its passage through the Plutonian system, no new satellites were spotted. If they exist, the additional satellites of Pluto have a maximum size of 5-15 kilometers (interval corresponding to different albedos). Similarly, no rings of matter have been spotted, meaning that, if they exist beyond Charon’s orbit, they are either extremely thin — less than 1,000 km wide — or extremely low reflective (they would reflect less than five-millionth of the incident sunlight).
Theories on the origin of the Plutonian system
Various theories have been formulated to explain the origin of the Plutonian system, including the small size of Pluto, comparable to that of satellites of the neighboring giant Neptune.
- The British mathematician Raymond Lyttleton envisioned in 1936 that Pluto and Triton orbited Neptune together, and that a gravitational perturbation had ejected Pluto out of planetary orbit, while it placed Triton back into a retrograde orbit. This idea was around long enough, and Gerard Kuiper took it up again when he saw in Pluto an ancient satellite of Neptune; Triton also seems to share some atmospheric and geological features with Pluto. Even if these points have been raised to support a Neptunian origin of Pluto, the current consensus is that the latter was never part of Neptune’s satellites.
- Triton’s retrograde orbit suggests that it was originally a Kuiper belt object in a solar orbit and was captured by Neptune. According to the most commonly accepted solar system formation scenario (Nice model), Pluto and Charon formed by accretion at the same time as other bodies, and were then driven beyond Neptune’s orbit by the gravitational influence of giant planets. While some of these bodies were ejected to the outer reaches of the Solar System, those that remained form the Kuiper Belt. The bodies that orbited in 2:3 resonance with Neptune kept a stable orbit, among them Pluto. The Pluto-Charon couple would have formed by mutual capture during a brush or collision between these two objects, and Nix and Hydra would perhaps be vestiges of this encounter.
- The determination of the size and density of Pluto and Charon by the New Horizons probe now makes it possible to specify their composition. Assuming that it is a mixture of rocks (formed by condensation of the solar nebula) and ice, Pluto would have about 2/3 of rocks (by mass) and Charon 3/5: these two bodies have densities closer to each other than other large Kuiper objects (≳ 1,000 km in diameter), while the four smaller satellites (Hydra, Nix, Kerberos and Styx) are much richer in ice. These results tend to favor the formation of Pluto and Charon by the impact, at moderate speed, of two partially differentiated precursor bodies. This theory is consistent with what is known about ancient dynamical conditions in the Kuiper belt but implies that the two precursor bodies accreted late. The ice richness of the small satellites is not compatible with a direct formation of the Plutonian system by gravitational destabilization of the solar nebula, a mechanism that has been proposed for the formation of binary Kuiper objects.
Pluto in fiction
- In 1922, in the anticipatory serial novel The Ring of Light: Great Scientific Adventure Novel, by L. Miral and A. Viger (pseudonyms of the novelist Ernest Jacob 1858-1942 and the popularizer of science Alphonse Berget, 1860-1934), episode No. 57 published in Le Petit Parisien of 3 January 1922 (novel published in volume under the title L’Anneau de feu, Hachette, 1922, p.275), Pluto is named the trans-Neptunian planet to be discovered (discovered in the story by the astronomers of the planet Mars, who call it “Zooh” in their language): “This unknown planet, suspected, sensed by Le Verrier who without seeing it had given it the name of Pluto, this planet was there, before their eyes. His image, weak but sharp, was projected on a screen” [Le Verrier does not seem to have named this still unknown planet Pluto; this name seems to have been given for the first time to this star in 1897, by an astrologer, Fomalhaut (pseudonym of Charles Nicoullaud, 1854-1923), in his Manuel d’Astrologie sphérique et Judiciaire (Vigot, 1897)]. More surprisingly, Miral and Viger’s account states that Pluto has a large satellite: “And around its mass, we saw gravitating a single but enormous satellite, a satellite as big as a quarter of the planet itself”, curious prescience of Charon.
- In 1930, in the short story, The One Who Whispered in the Darkness, H. P. Lovecraft features creatures from a planet named Yuggoth and recently discovered by humans, i.e. Pluto.
- In 1931, in the film The Hunt for Elk, Walt Disney named one of his animated film characters in honor of the newly discovered planet. This is the dog Pluto.
- In 1962, in the comic strip The Diabolical Trap, Edgar P. Jacobs places on the planet Pluto the base of armed resistance against the Earth dictatorship of the LIth century.
- In Joe Haldeman’s 1974 novel The Eternal War, the initial training of soldiers takes place on Charon, Pluto’s main satellite.
- In the 2010s, in the Rick and Morty series, the people of Pluto are terrified of losing their planet status. The decrease in Pluto’s diameter is explained by the mining of the planet’s core.
- In 2017, in the short novel Agents of Dreamland, Caitlín R. Kiernan uses the New Horizons ‘ approach to Pluto as one of the two central elements of his plot following Him Who Whispered in the Darkness.
Pluto in music
- The Creatures, Siouxsie Sioux’s second band, released the album Boomerang in 1989. The 7th song entitled Pluto Drive invites the listener to come and take a ride on Pluto, touting its clear atmosphere, cold temperature and methane oceans. The binary and throbbing rhythm can possibly evoke the Pluto-Charon couple in its continual movement.