A rose by any other name… Dwarf Planets

Prior to 2006 When Pluto was discovered in 1930, it was automatically called a planet. It was thought to be larger than Mercury. However, when Pluto’s moon Charon was discovered, it was used to determine Pluto’s mass, which ended up being 1/20th Mercury’s mass. Two other problems for Pluto: Large eccentric orbit Large orbital inclination

August 2006 The IAU (International Astronomical Union) adopted the new category. Brought about by discoveries of trans- Neptunian objects that rival Pluto in size. Three definitions resulted.

However, the final decision did not come easily… The original draft included Charon, Ceres and Eris (more later) as planets…so we almost had 12!

Planet: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape, and (c) has cleared the neighborhood around its orbit.

Dwarf planet: A “dwarf planet” is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

Small Solar System Bodies All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar System Bodies.” This would include most asteroids, comets, and Kuiper belt objects.

What is hydrostatic equilibrium? In hydrostatic equilibrium (gravitational relaxation) there are no gravitational imbalances in its surface. A global layer of liquid placed on this surface would form a liquid surface of the same shape, apart from small-scale surface features such as craters and fissures. Doesn’t mean that the object is a sphere.

Currently included as dwarf planets… Ceres Pluto Makemake Haumea Eris

Others? It is highly suspected that at least 40 other known KBOs could be determined to be in this category. Once properly explored, there may be as many as 200 such objects. Outside the Kuiper belt, maybe as many as 2000 could meet the criteria.

Ceres discovered Discovered on January 1, 1801 by Giuseppe Piazzi, (45 years before Neptune.) Considered a planet for half a century before reclassification as an asteroid. Classified as a dwarf planet on September 13, 2006.

Enhanced Hubble pic of Ceres

Rotation and Revolution of Ceres Rotational period: 9 hrs 4 mins Revolutionary period: 4.6 Earth years

Ceres Characteristics Diameter of about 950 km (590 mi.) Ceres is by far the largest and most massive body in the asteroid belt, and contains almost a third (32%) of the belt's total mass. Recent observations have revealed that it is spherical. The surface of Ceres is probably a mixture of water ice and various minerals such as carbonates and clays. Ceres appears to be differentiated into a rocky core and ice mantle. It may harbor an ocean of liquid water underneath its surface.

Ceres will be visited… By the spacecraft Dawn (launched in September 2007 by NASA) in 2015. It will visit Vesta before it arrives at Ceres. One last interesting note…in 1803 a newly discovered element was named cerium after Ceres!

The Discovery of Pluto Discovered due to perturbations of Neptune. Pluto’s position was calculated by Lowell. It was actually discovered by Tombaugh using Lowell’s calculations. Actually, it is too small to perturb Uranus and Neptune…so really, its discovery was just luck!

Physical Data Very eccentric orbit. At perihelion it is about 30 AU from the sun (and inside the orbit of Neptune). At aphelion it is about 50 AU from the sun! It is inclined 17.2˚ to the orbit of the other planets. Orbital period: 248 years

Interesting Note…(to me!) Its orbital period is 1.5 times that of Neptune. So it has resonance with Neptune…the number is _____ 3:2 Also, Pluto falls in several categories Dwarf planet KBO (Kuiper belt object) TNO (trans-Neptunian object) Plutoid (trans-Neptunian dwarf planets) Plutino (TNO’s with that 3:2 resonance with Neptune)

Physical Properties Orbital period: 6.4 days Mass: 1.3 x 1022 kg or 0.0021 Earth mass Has one large moon, Charon, that is tidally locked in orbit with Pluto. Probably made up of water ice. Also has frozen methane. It may have a methane atmosphere on one side. In 2005 it was discovered to have two smaller moons, Nix and Hydra.

The Origin of Pluto May have formed like the other planets, along with its moon. More likely, it is a member of the Kuiper belt. Some of the moons of Uranus and Neptune may also be captured Kuiper belt objects. Pluto is still considered a planet in New Mexico, Tombaugh’s home state! This gives us the 2006 Word of the Year Plutoed: to be demoted or dismissed.

Could Pluto be re-categorized? Since the barycenter of Pluto and Charon’s orbit lies outside of Pluto, some astronomers think that they should be considered a binary (dwarf) planetary system.

Pluto and Charon

Makemake discovered Discovered on March 31, 2005, by a team at the Palomar Observatory, led by Michael Brown. Announced to the public on July 29, 2005. Brown originally called it “Easterbunny” due to it’s discovery being close to Easter.

Makemake – Official naming Kuiper belt objects, per IAU rules, must be named for creation mythology. Makemake is the Rapanui deity of creation and fertility. The Rapanui are the native people of Easter Island…so the connection to “Easter” remained. What do you know about Easter Island? If anything, probably this…

The moai of Easter Island

Makemake’s Characteristics Distance from sun: 39 – 54 AU Orbital period: 310 years Rotational period: 7.8 hours Currently the 2nd brightest KBO after Pluto Its size is not precisely known. It has no satellites. It may have a transient atmosphere of nitrogen. Surface is composed of solid nitrogen, methane, ethane and tholins.

Haumea’s discovery It was discovered in 2004 by a team headed by Mike Brown at the Palomar Observatory in the United States and, in 2005, by a team headed by J. L. Ortiz at the Sierra Nevada Observatory in Spain. There has been controversy over who actually discovered it.

Haumea’s Discovery Controversy The Spanish group accessed Brown’s groups observation logs at Caltech the day before Brown made the discovery announcement. Ortiz’s group used that information to find Haumea in photos that they took in 2003 (earlier than Brown’s group.) Thus, they believed that they were the official discoverers. Both groups are listed as the discoverers.

Haumea’s naming It was originally named “Santa” due to being discovered close to Christmas time. It was officially named after the Hawaiian goddess of childbirth. This was proposed by Brown, as it was discovered using the Keck telescopes in Hawaii. Haumea’s two moons were named Hiʻiaka (first nicknamed “Rudolph”) and Nāmaka (“Blitzen”), after Haumea’s two daughters.

Keck image of Haumea and its moons

Haumea’s Characteristics It has 1/3rd the mass of Pluto. Orbital period: 283 years Distance: 35-50 AU Rotation rate: 4 hours It is made of water ice and organics and possibly hydrogen cyanide. Its moons and other objects in the area are likely pieces of Haumea that formed from a collision.

Most interesting Haumea characteristic It has an ellipsodial shape. Its semi-major axis is twice as long as its semi-minor axis.

Haumea and its moons (artist representation)

Eris discovered First identified in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity verified later that year. It is a TNO that is part of the “scattered disk” that is out from the tighter Kuiper belt.

Eris’ naming Eris’ size (larger than Plut0) brought about the whole “Pluto is not really a planet” controversy. As a result, Brown’s group suggested the name Eris, after the Greek goddess of discord. It had the informal name “Xena” prior to being officially named. They had saved the name for the first planet larger than Pluto, “Planet X.”

Eris’ naming continued Brown originally wanted to name the object "Lila", after a concept in Hindu mythology that described the cosmos as the outcome of a game played by Brahma. The name was very similar to "Lilah", the name of his newborn daughter. When the name leaked out, he decided to remove it from consideration.

Eris’ Characteristics Orbital period: 557 years Very elliptical orbit: 38-98 AUs Has a highly inclined orbit – 44° It is 27% more massive than Pluto It has one moon – Dysnomia (named after Eris’ daughter, goddess of lawlessness) It’s surface may have methane ice

Eris, Dysnomia and the sun

What about other candidate dwarf planets? Sedna Discovered in 2003 by Brown. Named after the Inuit goddess of the sea, who was believed to live in the cold depths of the Arctic Ocean, which is quite fitting at the distance it is from the sun (88 AU now.) Orbital period is calculated at between 10,500 and 12,000 years. Considered a “detached” object, instead of a scattered disk object like Eris.

What about other candidate dwarf planets? Quaoar Discovered by Brown (and Trujillo) from Caltech in 2002. It was named for the Tongva creator god. The Tongva are the native people of the Los Angeles area where the discovery was made. It’s half the size of Pluto. It has one moon, Weywot, named after the son of Quaoar.

Quaoar and Weywot

What about other candidate dwarf planets? Orcus Discovered in 2004 by….Brown, Trujillo and Rabinowitz. It’s a plutino with a 247 year orbital period. Plutinos are named after underworld deities, as Pluto is. Orcus is the Etruscan god of the underworld. It is said to sometimes be the anti-Pluto. Its moon was named by readers of Brown’s weekly column . The name Vanth was chosen, after the Etruscan goddess who guided the souls of the dead to the underworld.

Pluto-red, Orcus-blue, Neptune-gray

Artist’s representation of Orcus and Vanth