1. Dwarf Planets
And TNOs

2. The Problem of Pluto
Pluto was discovered in Named after the god of the dead. Its position was predicted by perturbations in Neptune’s orbit From the beginning it was unlike any other planet. Its orbit is on a far different plane than the other planets, at a 17° angle from them and… is so elliptical (30 – 49 AU) that it crosses Neptune’s orbit.

3. Its size is very small, less than half Mercury’s radius and its mass is even less at only .002 of Earth Its Composition was at first thought to be a terrestrial planet, though isolated beyond the Gas Giants, later its composition was discovered to be icy, more like a comet than Earth. Its “moon,” Charon is unusually large, at half Pluto’s radius and orbits very close at 19,600 km. All this made Pluto very unique until…

4. Eris, and a new type of planet
In 2005 another ‘planet’, called Eris after the Greek goddess of discord, was discovered
It was very much like Pluto, a little larger
with similar composition, a tilted (44°) and elliptical (39 – 97 AU) orbit, and a large moon.
By 2006, (6+) other similar objects have been found.
In August ‘06 Astronomers decided that
they and Pluto were a new category of celestial objects,
“Dwarf Planets”
A Dwarf Planet:
Orbits the Sun
Has enough mass so that gravity makes them roughly spherical and differentiated.
But have not cleared their orbit of other objects

5. Ceres
One Dwarf planet does have a composition similar to Earth…Ceres It was once considered an asteroid it orbits in the asteroid belt between Mars and Jupiter at 2.77 AU It is between 909 & 975km in diameter.

6. Trans-Neptune Objects (TNO’s) and other dwarf planets
Pluto, Eris, Sedna and Orcus are all icy dwarf planets that cross Neptune’s orbit or come close to it At their greatest distance they pass into the Kuiper belt where comets orbit, They are known as ”TNO’s” About 10 such objects are known There are probably many more dwarf planets orbiting far beyond Neptune.

7. Plutoids Plutoids are icy dwarf planets similar to Pluto.
They orbit in the Kuiper Belt and in composition resemble comets.
(Although exactly what gas the ice is varies)
They Include: Pluto
Eris
Makemake
and Haumea
Some such as Sedna, Quaoar, Orcus and Varuna await a further observation before determination

8. Pluto and Charon are a “binary planet” because:
Charon has a radius (635km) more than half that of Pluto.
They orbit each other and
are tidally locked
so that both always face the same side to each other
Two true moons Nix and Hydra orbit the pair.

9. Pluto’s Transient Atmosphere
Nitrogen is especially common on Pluto
It has an atmosphere of mostly nitrogen that only exists when its closer to the Sun.
It freezes when Pluto reaches aphelion and vaporizes again as it approaches perihelion

10. Pluto Update
In 2015 the “New Horizons” space probe flew by Pluto giving us our first good images of its surface.
Including a large heart-shaped light colored region
Or this “Icy spider” of fractures revealing a red material beneath the surface

11. Eris Eris, with a radius of 1200 km, is the largest of the Plutoids.
It takes over 500 years to complete its 68 AU average orbit.
Eris has one small moon
Dysnomia

12. Makemake
At a radius of 950 km Makemake is the third largest plutoid.

13. Haumea Debate raged over the status of Haumea As it is ellipsoidal
But Its elongated shape is the result of it fast spin.
At a period of 4 hours, the fastest known
So now it qualifies
It has two small moons,
Hi’iaka and Namaka