An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30
2006 IAU Definition of Planets & Dwarf Planets
With the Copernican Revolution, and the rising favor of the Heliocentric model of the Solar System in the late 17th Century, the Sun took the Earth's place. and the Earth became the 3rd planet.
In 1700, the Solar system had 6 planets, in order from the Sun...
One interesting manifestation of this was an interest in explaining why the planets had the orbits they did. In 1766, Johann Daniel Titius noted that the semi-major axes of the known planets appeared to follow a simple numerical progression. The "Titius Law" was published, without attribution, in 1772 by Johann Elert Bode, then directory of the Berlin Observatory.
It became known thereafter as the Titius-Bode Law.
Most contemporaries thought the Titius-Bode Law was an interesting curiosity, but because there was no physics behind it (it is only a numerological exercise), few gave it much further thought.
After a few nights, he found it was moving slowly relative to the stars, and thus was not a star.
Within a few months, astronomers had confirmed it was not a comet, but was in fact a new planet orbiting beyond Saturn!
To appear resolved in his small telescope, it had to be bigger than the Earth.
The new planet was eventually named Uranus, and made Herschel's fame, starting his career as one of the most distinguished astronomers of the 18th Century.
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!
All of a sudden, astronomers began to wonder if there wasn't something to the Titius-Bode Law after all.
In particular, was there really a "missing" 5th planet at a=2.8AU between Mars and Jupiter?
Began an international program of systematically searching the ecliptic for this unseen planet. The astronomers in the "police" divided the ecliptic into 24 zones of 15° each and began the search.
Despite this organization, they were scooped by a most unlikely person.
By mid-February, the new object was lost behind the Sun, but a tentative calculation put the new body on a circular orbit between Mars and Jupiter with a semi-major axis of 2.8 AU!
Piazzi christened his new planet "Ceres Ferdinandea", Ceres for the Roman patron goddess of Sicily, and Ferdinandea as an homage to his patron, Sicilian King Ferdinand III.
We now know the body as just Ceres, continuing a tradition of naming planets after Greek and Roman gods and goddesses.
However, on March 28 of 1802, while searching for Ceres, Olbers found another object in the Mars-Jupiter gap. Its orbit was even more elliptical and tilted than Ceres', but it had the same semi-major axis!
This second new planet was quickly named Pallas.
William Herschel, writing to the Royal Society in May of 1802, reporting his observations of the two new planets, and his reflections on their nature.
In particular Herschel noted that neither showed a disk when viewed through his 10-foot focal-length telescope, instead appearing like stars.
He proposed the name asteroid (literally, "star-like") for these objects, suggesting that they were not planets in the same way as the other 7:
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 written John Herschel (William's son) listed 11 planets in the Solar System (curiously, he did not use the word "asteroid" coined by his father).
Calculations done independently by Urbain Leverrier in France and John Couch Adams in England suggested that there must be a massive 8th planet beyond Uranus.
Leverrier convinced the Berlin Observatory to look at the place on the sky 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.
Neptune, however, did not fit the "prediction" of the Titius-Bode Law (38.8 AU), which is one reason why the Titius-Bode Law quickly fell out of favor in the 19th century. Nobody has taken it seriously since.
By 1865, John Herschel's astronomy textbook listed "more than 50 planets", of which 8 were the large planets and more than 50 were "asteroids".
Some highlights in the change in nomenclature:
By the end of the 19th Century, the introduction of photography led to the discovery of many 100s of asteroids per year.
Percival Lowell (1855-1916) was a wealthy scion of one of Boston's leading families, whose interest in astronomy led him to use his fortune to build a private observatory near Flagstaff, Arizona (known today as the Lowell Observatory, it remains one of the premier private research observatories in the United States, best known for its solar system research).
Among Lowell's interests were his studies of the "Canals of Mars", which we now know are an optical illusion, but which Lowell thought proved that there was intelligent life on Mars.
However, Lowell was also interested in searching for new Trans-Neptunian planets. He started using orbital data for Neptune, and claimed that these data showed residual discrepancies between prediction and observations that were consistent with a gravitational tug from an as-yet unseen 9th planet.
Lowell began his systematic search for "Planet X" in 1909, and continued it from 1913-1916, using photographic techniques for his search.
Lowell died in 1916 before completing his search. Later re-analysis of his photographs showed that Lowell had actually photographed Pluto twice, but not noticed it (it is not clear how carefully he examined his own photographs at this time).
With Lowell's death, and the inevitable fight over his estate, the search was not revived by the Lowell Observatory until 1928.
On February 18, 1930, using a pair of plates taken 6 nights apart in January of that year, Tombaugh discovered a tiny moving objects in Gemini. Subsequent observations showed that it was on an orbit beyond that of Neptune.
It was soon named Pluto, and immediately hailed as the 9th planet in the solar System.
The discovery of Pluto also almost immediately began to generate controversy among astronomers. It was clearly bigger than Ceres, and apparently larger than the Earth in some early estimates, but it was not large enough to explain the discrepancies in the motions of Neptune that Lowell had used to initiate the search.
Subsequent observations taken over the decades following its discovery began to sow doubt in astronomer's minds:
Within a few years, many hundreds of TNOs had been discovered, falling into one of three dynamical classes:
All, however, were much smaller than Pluto, but as time went on, progressively larger TNOs were discovered.
Now, to some astronomers, Pluto was not just a small planet with odd orbital and physical characteristics, but the largest of a new class of icy Trans-Neptunian worlds. This led some to question its continued classification as a "planet".
The team always gives whimsical code names to their large TNOs, this one was code-named "Xena", for the titular character in the TV series Xena: Warrior Princess.
By 2005, this team had perfected a technique for finding large TNOs, and had in fact discovered many of the TNOs larger than 1000km in diameter - more than any other team, of which there were now more than a half dozen, of which the largest by far were Pluto and 2003UB313.
If "Xena" was larger than Pluto, then either it was the 10th planet, or Pluto was not a planet but a large TNO.
In late 2005 the International Astronomical Union (IAU) established a committee to examine the issue and present recommendations to the IAU General Assembly Meeting scheduled to be held in Prague in August 2006. Note that the committee's charge was not to specifically address the status of Pluto, as it is often misrepresented, but to instead address the more generic question of a scientific definition of a planet.
The discussions at the Prague meeting were webcast, and rather contentious. The main axis of contention was between those who felt that the planet's properties itself were most important and those who felt that the planet's orbital dynamics must also play a role in the definition.
After many iteration, a final resolution was brought forward to a vote of the general assembly on the definition of a "planet":
A planet is any celestial body that, within the Solar System,
The largest 8 planets satisfy all three criteria, and are thus designated as Planets by the new IAU definition.
Pluto, 2003UB313 (aka Xena), 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.
There are about a dozen other candidate Dwarf Planets, mostly large TNOs, pending subsequent observations to confirm they are in hydrostatic equilibrium.
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
This plot, however, makes the point about as clearly as any I've seen. It's based on a figure from a recent paper by Steven Soter, and shows a variation on the figure we showed back in Lecture 31
This diagram shows better what is meant by "orbital dominance", and shows how Pluto and Eris don't even come close to meeting this criterion.
NASA has officially adopted the IAU nomenclature, as have many, but not all, astronomers.
A group of astronomers, primarily led by Alan Stern, Principal Investigator of the New Horizons mission currently en-route to Pluto and the Kuiper Belt, have expressed opposition to the IAU definition, so we haven't heard the last of this.
What do I think?
Personally, I think the new nomenclature for Solar System bodies is basically sound and makes physical sense. Pluto is more scientifically interesting in its new role as the largest of the resonant Kuiper Belt than it ever was as a small, oddball planet that didn't really fit with the other 8 planets. The contention about "clearing the neighborhood" can be resolved by adopting clearer language about what is meant by "orbital dominance". My opinion is that the imprecise language was a consequence of a misbegotten attempt on the part of some on the IAU committee to make the language of the resolution more "accessible" to the "general public". If astronomers are to make a major revision in the nomenclature of the Solar System, we have an obligation to explain ourselves to everyone, astronomer and non-astronomer alike, but to tweak the technical definition to make it more "accessible" was a mistake. Scientific definitions require precise language. All that this poor choice of language (which was not in the penultimate draft of the resolution) has accomplished is to sew confusion.
We have redefined what we mean by a "planet" many times before, and each time it was motivated by new knowledge that changed how we looked at the heavens. This is how science works - it is self correcting, and doesn't allow tradition or sentiment to dictate ideas. Pluto is no longer a planet because we have learned more about what it is, and to insist otherwise is to cling to old ideas that are no longer correct or relevant. Far from being "demoted" or "banished" to quote a number of new headlines, Pluto is now taking its rightful place among the largest of a fascinating new class of objects that have much to teach us about the evolution and origins of the Solar System.