Download Small Solar System Bodies

How Many
Planets…
In Our
Solar System?
With the advent of powerful new
telescopes on the ground and in
space, recent new discoveries
have been made of objects in the
outer regions of our Solar System
that have sizes comparable to
and larger than Pluto.
These discoveries have rightfully
called into question whether or
not they should be considered as
new ‘planets.'
How Many
Planets…
In Our
Solar System?
The International Astronomical Union
(IAU), have concluded two years of
work defining the difference between
“planets”, “dwarf planets” and “small
solar system bodies” such as comets
and asteroids and other objects.
The astronomers gathered August
14-25, 2006 at the IAU General
Assembly in Prague, where they
announced voting results on the
definition of a planet.
How Many
Planets…
In Our
Solar System?
The new definition of a planet has 3 parts:
1. A Planet is a celestial body that is in orbit
around the Sun.
2. It has sufficient mass that it’s self-gravity forms
the body into a (nearly) spherical shape.
3. It has cleared the neighborhood around its
orbit. This means that it has enough mass to
“sweep” out debris, such as small asteroids,
from its orbital path.
How Many
Planets…
In Our
Solar System?
According to this definition, the Solar System contains 8 classical planets:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
In addition, the IAU voted to create a new class of objects called Dwarf
Planets.
In Our
Solar System?
How Many
Planets…
The first objects to be classified as
Dwarf Planets have been Pluto, Eris,
Ceres, Haumea, and Makemake.
Dwarf Planets are defined as:
Artist concept of Eris. Image
credit: NASA/JPL-Caltech
Ceres – From Dawn Spacecraft
Image credit: NASA
Pluto – From New Horizons
Spacecraft.
Image credit: NASA
1. A celestial body in orbit around the
Sun.
2. Has sufficient mass for its self-gravity
to form it into a (nearly) spherical
shape.
3. Has not cleared the neighborhood
around its orbit.
4. Is not a satellite.
How Many
Planets…
In Our
Solar System?
Pluto and Charon by New
Horizons during approach
on July 13 and July 14,
2015. The relative
reflectivity, size,
separation, and
orientations of Pluto and
Charon are
approximated.
Image credit: NASA
Because Pluto’s moon, Charon, is about half the diameter of Pluto,
there was much discussion of classifying the system as a “double
planet”. The IAU did not re-classify Charon, so for now it remains a
satellite of Pluto. Charon may later be classified as a Dwarf Planet.
How Many
Planets…
In Our
Solar System?
Currently a dozen candidate “Dwarf Planets" are listed on IAU's "watchlist"
which keeps changing as new objects are found and the physics of the
existing candidates becomes better known.
How Many
Planets…
In Our
Solar System?
Small Solar System Bodies
is the IAU classification name of all
other objects in the Solar System
except satellites. This includes most
comets, asteroids, and Kuiper Belt
objects.
Credit: NASA
Credit: Lynette Cook and W. M. Keck Observatory
How Many
Planets…
In Our
Solar System?
Dwarf Planets and Small Solar System Bodies typically have orbits that
are highly tilted with respect to the classical planets (technically referred
to as a large orbital inclination).
They also typically have orbits that are far from being perfectly circular
(technically referred to as having a large orbital eccentricity). All of these
distinguishing characteristics are scientifically interesting in that they
suggest a different origin from the 8 classical planets.
How Many
Planets…
In Our
Solar System?
Most Dwarf Planets, including Pluto, reside in the Kuiper Belt, in the outer region of our Solar
System.
These objects, which orbit beyond Neptune, are known as Trans-Neptunian Objects (TNOs)
Pluto has much more in common with other recently discovered TNOs than with the planets,
which led to its designation as a “Dwarf Planet”.
It is expected that many more Trans-Neptunian Objects will be discovered in the coming years.
How Many
Planets…
In Our
Solar System?
These 3 newly defined classification systems are not without controversy in the
astronomy community!
The International Astronomical Union (IAU) is an organization of over 8,500
professional astronomers from all over the world. The IAU is recognized as the
official authority in astronomy, and is responsible for naming all planets,
asteroids, and other celestial bodies and phenomena.
Despite the large number of members, only about 5% of members were
involved in voting on the 2006 resolution which narrowed the definition of a
planet.
Many astronomers have pointed out ambiguities in the definition. For
instance, several planets, including Earth, have not fully “cleared their
neighborhood” of debris. Many astronomers are also clearly
disappointed that Pluto was not classified as a double planet.
What Do You Think??