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Dwarf Planets
And TNOs
The Problem of Pluto
Pluto was discovered in 1930. 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
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
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…
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
One Dwarf planet does have a composition similar to
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.
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.
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
and Haumea
Some such as Sedna, Quaoar, Orcus and Varuna
await a further observation before
Pluto’s Transient
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
Pluto and Charon are
a “binary planet”
1. Charon has a
radius (635km)
more than half
that of Pluto.
2. They orbit each
other and
are tidally locked
so that both
always face the
same side to each
Two true moons Nix and
Hydra orbit the pair.
Eris, with a radius of
1200 km, is the
largest of the
It takes over 500
years to complete
its 68 AU average
Eris has one small moon
At a radius of 950
km Makemake is
the third largest
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
Hi’iaka and Namaka