An Introduction to Solar System Astronomy
Prof. Scott Gaudi
Is Pluto a Planet?
Classical & Copernican definitions of a "planet"
- Discovery of Uranus by William Herschel
- Discovery of the Asteroids
- Prediction & Discovery of Neptune
- Discovery of Pluto by Clyde Tombaugh
- Discovery of the Kuiper Belt
- Discovery of Eris
2006 IAU Definition of Planets & Dwarf Planets
There seven wandering stars or
"planets" that moved relative to the "fixed stars" in classical times:
The Earth was fixed and unmoving at the center of this Geocentric
- The Sun
- The Moon
- Mercury, Venus, Mars, Jupiter, & Saturn
With the Copernican Revolution, the Sun took the
Earth's place, and the Earth became the 3rd planet.
In 1700, there were 6 planets:
- Mercury, Venus, Earth, Mars, Jupiter, & Saturn
A Missing Planet?
The Titius-Bode Law is a numerical formula
that "predicts" the positions of the planets.
The correspondence between the measured semi-major axes of
the planets and the "predictions" of the Titius-Bode law aren't that
close in all cases, but:
It also "predicted" a planet orbiting at a=2.8 AU between Mars and
Jupiter that was not observed.
Nobody paid much attention to it until 1781.
The Discovery of Uranus
On March 13, 1781, William Herschel first discovered
a new planet which was eventually named Uranus.
The new planet had a semi-major axis of a=19.2 AU. The prediction
from the Titius Bode Law was a=19.6 AU!
Astronomers began to wonder if there
was something to the Titius-Bode Law.
Was there really a "missing" 5th planet at a=2.8AU between
Mars and Jupiter?
The Discovery of Ceres
On January 1, 1801, Giuseppe Piazzi discovered
a new body on a circular orbit between Mars and Jupiter
with a semi-major axis of 2.8 AU, which
we now know as Ceres.
The Discovery of Pallas
On March 28, 1802, Olbers found
another object in the Mars-Jupiter gap with the same semi-major
axis as Ceres.
This second new planet was quickly named Pallas.
What is a Planet? Part 1
The discovery of Ceres and Pallas quickly led to a discussion in the
astronomical community about the nature of these new planets.
William Herschel proposed the name asteroid (literally, "star-like") for these
objects, suggesting that they were not planets in the same way as the
- They were much smaller than the other planets.
- They had orbits that were more elliptical and tilted relative to
the ecliptic than the other planets.
- Ceres and Pallas shared the same orbit.
Four New Planets
By 1807, another 2 new planets were found in the space
between Mars and Jupiter
Bringing the total to 4 new planets discovered in a brief
- Juno, discovered in 1804 by Karl Ludwig Harding
- Vesta, discovered in 1807 by Olbers
But after that, no new objects were to be discovered in this region of
the Solar System for another 38 years!
By 1838, an astronomy textbook listed 11 planets in the Solar System.
The Discovery of Neptune
Observations of the orbit of
Uranus showed that it deviated from its predicted path.
The deviation was ascribed to the
gravitational pull of an as-yet unseen massive planet orbiting somewhere
in the vicinity of Uranus.
Calculations done independently by Urbain Leverrier and John
Couch Adams suggested that there must be a massive 8th planet
Leverrier convinced the Berlin Observatory to look
where he predicted this 8th planet should be, and on September 23, 1846,
Johann Galle found the planet Neptune.
Neptune was as big as Uranus, and orbiting with a semi-major axis
of 30.6 AU.
The dramatic discovery of Neptune led to a
reconsideration of what it meant to be a planet.
In general, without much fanfare, the number of planets was reduced
to 8 (Mercury through Neptune).
All the minor planets were
designated as "asteroids" and dropped from the list of planets.
The Discovery of Pluto
In 1929, Lowell Observatory hired
Clyde Tombaugh to resume a search for
"Planet X" started a decade earlier by the observatory
founder, Percival Lowell.
On February 18, 1930, Tombaugh discovered a tiny moving object
that was soon named Pluto.
It was immediately hailed as the 9th planet
in the solar System.
Pluto & Planet X
The discovery of Pluto also almost immediately began to generate
controversy among astronomers.
Eventually it was shown that
Pluto was in fact smaller than the Moon.
The orbit of Pluto was very elliptical and tilted by more than
17° relative to the ecliptic, unlike any other planet.
In retrospect, the discovery of Pluto was somewhat fortuitous.
However, since nobody had
found any other Trans-Neptunian objects except for Pluto, so it remained
unique and therefore its designation as a planet
Discovery of Trans-Neptunian Objects (TNOs)
In 1992, Dave
Jewitt and Jane Luu discovered the first
small Trans-Neptunian Object.
Within a few years, many hundreds of TNOs had been discovered.
All, however, were much smaller than Pluto.
2003UB313 - A 10th Planet?
In January 2005, astronomers Mike Brown, Chad Trujillo, and
David Rabinowitz discovered a Trans-Neptunian
object that was larger than Pluto.
The preliminary designation was 2003UB313.
If 2003UB313 was larger than Pluto, then either it was a
planet as well, or Pluto was not a planet. Astronomers would
have to decide.
What is a Planet? Part 2
In late 2005, the International Astronomical Union (IAU)
decided on the following definition of a "planet":
A planet is any celestial body that, within the Solar System,
The third criterion is often known as orbital dominance.
- is in orbit around the Sun, and not a satellite of another planet.
- has sufficient mass for its self-gravity to overcome rigid body
forces and thus assume a spheroidal (nearly round) shape in
- has cleared its orbital neighborhood
The largest 8 planets satisfy all three criteria, and are thus
designated as Planets by the new IAU definition.
Pluto, 2003UB313, and Ceres satisfy the first 2
criteria, but are not the dominant objects in their orbits, so these
three objects have been designated as Dwarf Planets.
Eris and Friends
After the 2006 IAU meeting, 2003UB313 was officially renamed
The IAU also gave the Minor Planet Center (MPC) at Harvard the go-ahead
to re-designate Pluto (and Eris) using MPC numbers, like asteroids, thus
- Pluto is now 134340 Pluto
- 2003UB313 is now 136199 Eris
- There are now an additional two dwarf planets (Haumea and Makemake),
and about another dozen candidate Dwarf Planets.
And then there were 8...
For now, the Solar System consists of
- The Sun
- Eight Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune.
- Five Dwarf Planets: Ceres, Pluto, Eris, Haumea, Makemake
- Many Many Small Solar System Bodies
NASA has officially adopted the IAU nomenclature, as have many, but not
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