Pluto's natural satellites
Encyclopedia
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
has four known moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
. The largest, Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, is proportionally larger, compared to its primary, than any other satellite of a known planet or dwarf planet in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. The other moons, Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...
, Hydra
Hydra (moon)
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...
, and S/2011 P 1
S/2011 P 1
S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011...
("P4") are much smaller. The team that discovered S/2011 P 1 also found possible evidence of a couple of even fainter moons, but this needs more study to be confirmed.
History
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
, was discovered by James Christy on June 22, 1978, nearly half a century after Pluto. Two outer moons were imaged by the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
Pluto Companion Search Team in May 2005, and precovered
Precovery
Precovery is a term used in astronomy that describes the process of finding the image of an object in old archived images or photographic plates, for the purpose of calculating a more accurate orbit...
from Hubble images taken in June 2002. With the orbits confirmed, the moons have been given definitive names: Hydra
Hydra (moon)
Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...
(Pluto III, formerly S/2005 P 1) and Nix
Nix (moon)
Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :...
(Pluto II, formerly S/2005 P 2). The names were chosen in part because the initials (NH) allude to the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
mission. Further Hubble observations were made in February and March 2006. The possibility of rings
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
created by impacts on the smaller moons will be investigated by the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
probe. The fourth moon was announced in July 2011.
Characteristics
The Plutonian system is highly compact: The four known satellites orbit within the inner 3% of the region where prograde orbits would be stable.Pluto and Charon have been called a double planet
Double planet
In astronomy, double planet and binary planet are informal terms used to describe a binary system of two astronomical objects that each satisfy the definition of planet and that are near enough to each other to have a significant gravitational effect on each other compared with the effect of the...
because Charon is larger compared to Pluto (half its diameter and an eighth its mass) than any other moon is to a planet or dwarf planet; indeed, Charon is massive enough that, despite their proximity, Pluto orbits the system's barycenter at a point outside its surface. Charon and Pluto are also tidally locked
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
, so that they always present the same face toward each other.
Following Buie and Grundy's recent re-calculations taking into account older images, the orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0.4° and eccentricities less than 0.005. The diagram on the left shows the view from the axis of the moons' orbits (declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
0°, right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
133°), aligned with the HST
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
diagram above it. As seen from Earth, these circular orbits appear foreshortened into ellipses depending on Pluto's position.
When discovered, Hydra was somewhat brighter than Nix, and therefore thought to be larger by 20%, but follow-up observations found them to be nearly identical. It is likely that the change in brightness is due to the light curve
Light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band...
of Hydra, but whether this is due to an irregular shape or to a variation in surface brightness (albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
) is unknown. The diameters of objects can be estimated from their assumed albedos; the estimates above correspond to a 35% albedo like Charon, but the moons could be as large as 130 km if they have the 4% albedo of the darkest KBOs. However, given their color and suspected chemical similarities to Charon, it is likely that their albedos are similar as well and that the diameters are closer to the lower estimates.
Formation and resonances
It is suspected that the Plutonian satellite system was created by a massive collisionCollisional family
In astronomy, a collisional family is a group of objects that are thought to have a common origin in an impact . They have similar compositions, and most share similar orbital elements....
, similar to the "big whack"
Giant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...
believed to have created the Earth's Moon. In both cases it may be that the high angular momenta
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
of the moons can only be explained by such a scenario. The nearly circular orbits of the smaller moons suggests that they were also formed in this collision, rather than being captured Kuiper Belt objects. This and their near orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
s with Charon (see below) suggest that they formed even closer to Pluto than they are at present, and that they migrated outward as Charon achieved its current orbit. If Hydra and Nix turn out to be tidally locked, as Charon is, that will settle the issue, as tidal forces are insufficient to damp their rotations in their present orbits. The color of each is a lunar grey like Charon, which is consistent with a common origin. Their difference in color from Pluto, one of the reddest bodies in the Solar System due to the effects of sunlight on the nitrogen and methane ices of its surface, may be due to a loss of such volatiles during the impact or subsequent coalescence, leaving the surfaces of the moons dominated by water ice. Such an impact would be expected to create additional debris (more moons), but these must be relatively small to have avoided detection by Hubble. It is possible that there are also undiscovered irregular satellite
Irregular satellite
In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....
s, which are captured Kuiper Belt objects.
Nix, S/2011 P 1 and Hydra are very close to a 1:4:5:6 orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with the Charon-Pluto orbital period: Nix is within 2.7% of resonance, S/2011 P 1 is apparently within 0.6%, while Hydra is within 0.3%, though none appear to be in an exact resonance. It may be that these orbits originated as forced resonances when Charon was tidally boosted into its current geosynchronous orbit, and then released from resonance as Charon's orbital eccentricity was tidally damped. Today the Pluto–Charon pair continue to produce strong tidal forces, with the gravitational field at the outer moons varying by 15% peak to peak. At the lower estimated size range, Nix should have no significant precession
Precession
Precession is a change in the orientation of the rotation axis of a rotating body. It can be defined as a change in direction of the rotation axis in which the second Euler angle is constant...
, while Hydra should have a precession period of 15 years. However, at their maximum projected masses (assuming an albedo of 4%), the two moons may be in a 3:2 orbital resonance with each other, with libration
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...
periods of 400 to 450 days, though this may already be ruled out by the low eccentricity of Charon. Thus accurate orbital data can help resolve the sizes of these moons.
However, it was recently calculated that a resonance with Charon could boost either Nix or Hydra into its current orbit, but not both: boosting Hydra would have required a near-zero Charonian eccentricity of 0.024, while boosting Nix would have required a larger eccentricity of at least 0.05. This suggests that Nix and Hydra were instead captured and migrated inward until they were trapped in resonance with Charon.
Table
The Plutonian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsedHydrostatic equilibrium
Hydrostatic equilibrium or hydrostatic balance is the condition in fluid mechanics where a volume of a fluid is at rest or at constant velocity. This occurs when compression due to gravity is balanced by a pressure gradient force...
into a spheroid
Spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....
are highlighted in light purple. Pluto has been added for comparison, for it orbits a point outside itself. The team that discovered S/2011 P 1 also found possible evidence of a couple of even fainter moons, but this needs more study to be confirmed.
| Name (pronunciation) |
Image | Mean diameter (km) |
Mass (×1021 kg) | Semi-major axis (km) |
Orbital period (days) |
Eccentricity | Inclination (to Pluto's equator) |
Discovery date |
||
|---|---|---|---|---|---|---|---|---|---|---|
| Pluto | 2390 | 13.05 ± 0.07 | 2 035* | 6.387 230 | 0.0022 | 0.001° | 1930 | |||
| Pluto I | Charon Charon (moon) Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I... |
/ˈʃærən/, /ˈkɛərən/ |
1207 ± 3 | 1.52 ± 0.06 | 17 536 ± 3* | 6.387 230 | 0.0022 | 0.001° | 1978 | |
| Pluto II | Nix Nix (moon) Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :... |
/ˈnɪks/ | 46–137 | < 0.002 | 48 708 | 24.856 ± 0.001 | 0.0030 | 0.195° | 2005 | |
| Pluto IV | S/2011 P 1 S/2011 P 1 S/2011 P 1 is a small natural satellite of Pluto whose existence was announced on July 20, 2011... |
13–34 | ? | ~59,000 | 32.1 | ~0 | ? | 2011 | ||
| Pluto III | Hydra Hydra (moon) Hydra is the second outermost known natural satellite of Pluto. It was discovered along with Nix in June 2005, and is to be visited along with Pluto by the New Horizons mission in July 2015.- Discovery :... |
/ˈhaɪdrə/ | 61–167 | < 0.002 | 64 749 | 38.206 ± 0.001 | 0.0051 | 0.212° | 2005 | |