General AstronomyThe Solar System

The Dwarf Planets

Defining a Planet

• A dwarf planet is a celestial body that– is in orbit around the Sun, – has sufficient mass for its self-gravity to

overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

– has not cleared the neighborhood around its orbit, and

– is not a satellite.

Ceres

Ceres• On January 1, 1801, Piazzi (a Sicilian

astronomer) noted a new object which he watched until February 11. He wrote Bode of his discovery, but by the time Bode looked for it, the object was too near the direction of the Sun for observation.

• In November, Gauss predicted the location of this object based on Piazzi's observations; it was found on Dec 31, 1801

• Piazzi named the object "Ceres" after the protecting goddess of Sicily

• The missing planet had been found at a distance of 2.77 AU

Ceres• By 1890, over 300 more "planets"

had been discovered in the same region (in regards to distance from the Sun) and Ceres was stripped of its planetary status.

• In 2006, Ceres was given a new designation of "dwarf planet"

Ceres

Ceres’ Statistics

Eccentricity 0.080

Orbital period 4.6 yr

Orbital Radius 2.7 AU

Inclination 10.6°

Size 975×909 km

Mass 9.5×1020 kg

Density 2.08 g/cm³

Escape velocity 0.51 km/s

Rotation 0.3781 d

Tilt of Axis ~4 °

Albedo 0.113

Mean surface temperature ~167 K

Max surface temperature 239 K (-34 ° C)

Ceres’ Interior

Ceres versus the Moon

Pluto

PlutoEccentricity 0.249

Orbital period 248 years

Orbital Radius 39.5 AU

Inclination 17.14 °

Size 2274 Km

Mass 9.5×1020 kg

Density 2.06 g/cm³

Escape velocity 1.2 km/s

Rotation 6.4 d

Tilt of Axis 119.6 °

Albedo 0.49–0.66 (varies by 35%)

Mean surface temperature

44 K

Max surface temperature 55 K

Discoverer Clyde Tombaugh, 1930

Pluto

Pluto

Pluto

Charon

Nix and Hydra• Two additional moons of Pluto were imaged by astronomers working with the

Hubble Space Telescope on May 15, 2005.• These small moons orbit Pluto at approximately two and three times the

distance of Charon– Nix at 48,700 Km– Hydra at 64,800 Km from the barycenter of the system.

• They have nearly circular prograde orbits in the same orbital plane as Charon

• Observations of Nix and Hydra are ongoing to determine individual characteristics.– Hydra is sometimes brighter than Nix, speculating that it either is larger in

dimension or different parts of its surface may vary in brightness. • Sizes are estimated from albedos. • The moons' spectral similarity with Charon suggests a 35% albedo similar to

Charon's– this results in diameter estimates of 46 kilometers for Nix and 61 kilometers for

brighter Hydra. – Upper limits on their diameters can be estimated by assuming the 4% albedo of

the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km respectively.

– At the larger end of this range, the inferred masses are less than 0.3% of Charon's mass, or 0.03% of Pluto's.

Pluto’s newest moons (2011 and 2012)

Eris and Dysnomia

Eris

Eris

Eris (2003 UB313)

Eccentricity 0.438

Orbital period 557 years

Orbital Radius 38.2 – 97.6AU

Inclination 44°

Size 2400 Km

Mass ?

Density ?

Escape velocity ?

Rotation > 8 h

Tilt of Axis ?

Albedo 0.86

Mean surface temperature ~ 30 K

Discoverer Chad Trujillo, Dave Rabinowitz and Mike Brown, 2002

Makemake

Makemake• Makemake is the creator of humanity

in the mythos of Rapa Nui• It is the third-largest known dwarf

planet in the Solar System• No satellites have been detected

around Makemake so far• The presence of methane and possibly

nitrogen suggests that Makemake could have a transient atmosphere

Makemake

Orbital characteristics

Aphelion 53.1 AU

Perihelion 38.5 AU

Semi-major axis 45.8 AU

Eccentricity 0.159

Orbital period 309.88 yr

Average orbital speed 4.4 km/s

Inclination 28.96°Physical characteristics

Dimensions 1300–1900 km

Mean radius 750 kmMass ~4 × 1021 kgMean density ~2 g/cm³Sidereal rotation period unknown

Axial tilt unknownAlbedo 78.2

Temperature 30–35 K

Apparent magnitude 16.7

Haumea

Haumea• The goddess of childbirth and fertility in Hawaiian

mythology. – The name is particularly apt as the goddess Haumea also

represents the element of stone and observations of Haumea hint that, unusually, the dwarf planet is almost entirely composed of rock with a crust of pure ice.

• Haumea is joined in its orbit by two satellites that are thought to have been created by impacts with it in the past. – The first and largest moon is to be called Hi'iaka, after the

Hawaiian goddess, the patron goddess of the island of Hawai'i.

– The second moon of Haumea is named Namaka, a sea goddess, also Haumea's daughter

Haumea

Orbital characteristics

Aphelion 51.5 AU

Perihelion 35.2 AU

Semi-major axis 43.3 AU

Orbital period 285.4 yr

Inclination 28.2°

Satellites 2

Eccentricity 0.188

Physical characteristics

Dimensions ~1960 × 1518 × 996 Km

Mass 4.2 ×1021 kg

Mean density 2.6–3.3 g/cm³

Rotation period 3.9 hr

Albedo 0.7

Temperature 32K

Apparent magnitude

17.3

MakemakeHaumea

Pluto

Other Candidates

Quaoar

Quaoar

• Distance from Sun = 50 AU• Orbital period = 285 years• Eccentricity = 0.04• Diameter = 1250 Km• Orbital Inclination = 8°• Discoverers: Chad Trujillo and Mike

Brown, 2002

Sedna

Sedna (2003 VB12)

Distance from Sun: 74 AU - 990 AULength of day: 10 hoursOrbital period: ~11,249 yearsEccentricity: 0.849Diameter: 1800 KmDiscoverers: Chad Trujillo, Dave Rabinowitz and Mike Brown

Year of Discovery: 2002

Sedna

Easterbunny Santa

Makemake Haumea