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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 44:
Is Pluto a Planet?

Key Ideas:

Classical & Copernican definitions of a "planet"

Telescopic Discoveries

Trans-Neptunian Worlds

2006 IAU Definition of Planets & Dwarf Planets


Classical Planets

There seven wandering stars or "planets" that moved relative to the "fixed stars" in classical times:
The Sun
The Moon
Mercury, Venus, Mars, Jupiter, & Saturn
The Earth was fixed and unmoving at the center of this Geocentric system.

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 other 7:


    Four New Planets

    By 1807, another 2 new planets were found in the space between Mars and Jupiter
    Juno, discovered in 1804 by Karl Ludwig Harding
    Vesta, discovered in 1807 by Olbers
    Bringing the total to 4 new planets discovered in a brief 6-year period.

    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 beyond Uranus.

    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.


    Minor Planets

    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 remained unchallenged.


    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,

    1. is in orbit around the Sun, and not a satellite of another planet.
    2. has sufficient mass for its self-gravity to overcome rigid body forces and thus assume a spheroidal (nearly round) shape in hydrostatic equilibrium
    3. has cleared its orbital neighborhood
    The third criterion is often known as orbital dominance.

    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 Eris.

    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


    And then there were 8...

    For now, the Solar System consists of

    NASA has officially adopted the IAU nomenclature, as have many, but not all, astronomers.


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