August General Meeting
Sedna, Quaoar and Pluto – Comets, Asteroids.
Speaker: Charles Morgan, PhD Student and Tutor
School of Mathematical Sciences at Monash University
He sat under the shade of an apple tree one late autumn day, looking absentmindedly at the gibbous moon in a pale-blue afternoon sky when it happened. Without a sound the apple detached itself from the tree and accelerated toward the top of his head. Ouch! That’s when the thought struck him, gravity! Gravity can stop our moon from flying out into space. What if the same force that makes the apple fall towards the centre of the Earth reaches out to the Moon and delicately balances the moon’s forward momentum into a circular orbit?
I wonder, would Newton really have reached such a (then) radical supposition on the basis of this apparently unrelated incident, without having already a proposition in his mind as a basis for such reasoning, a hypothesis? – Hypotheses non fingo, I do not feign hypotheses, he writes in the Principia. But that is hard to believe. The human mind is much more receptive (axons make new connection) to impressions it finds useful in satisfying an underlying thirst for knowledge; so much so that irrelevant objects in plain view sometimes don’t even register. It is in the nature of a hypothesis then, once conceived, to nourish itself and grow stronger with everything you see, hear, read and understand. Every impression the brain receives is constantly measured for a fit into the hypothetical jigsaw puzzle in the mind. – Bingo!
Einstein was a master at developing his hypotheses into complete Gedanken-Experimente, mind pictures that allowed him to run complex experiments on hand of such visualisations on the back of an envelope, or completely within his mind: “The aim of all good science is, of course, to cover the greatest number of facts by deduction from the smallest number of hypotheses...”. Are we to understand Newton did not make use of this endowment?
I was fascinated the way Charles Morgan lead us from a completely new angle into the structure and evolution of the solar system. It suddenly clicked with me, theories of planetary formation can not be considered complete unless they account for the emergence and presence of all the matter in the solar system, and that includes asteroids, planetoids, meteors and comets and dust in the solar system's newest “final” frontier: the elusive Kuiper belt. The Kuiper Belt is named after the Dutch / American astronomer Gerard P. Kuiper, who in 1951 first suggested such a belt as the source of many of the recurring comets in the solar system. It is an icy debris field of comet-like bodies extending 7 billion miles beyond Neptune's orbit. Its pristine environment may yet provide the most direct evidence of the process of planet building, which took place 4.6 billion years ago. With the advent of advanced telescope technology 1000 new planetary fragments have now been discovered there, the first (1992 QB1) 12 years ago. Some of them have diameters up to 1500 km, approaching the dimensions of Pluto, with 2,225 km. Pluto in fact shares a number of characteristics with the objects in the Kuiper Belt. It appears to have a similar composition to many KBOs, a sizeable fraction of KBOs have similar orbits to Pluto locked in orbital resonance with Neptune such that they complete two orbits of the Sun in the time it takes Neptune to make three. Theory has it that due to this ongoing harmonic balancing act one ejected comet from the Kuiper belt will hit Jupiter every 400 years, and Earth every 13 million years or so (which would make an awful mess).
When planetoid Quaoar was discovered in 2002 and the NASA's Hubble Space Telescope measured its angular size at 40 milli-arcsecond, it became the largest new object found in the solar system (1300 km) since the discovery of Pluto 72 years ago. Quaoar is about 4 billion miles away from Earth, well over a billion miles farther than Pluto and its orbit is very circular, more so than most of the planetary-class bodies in the solar system. The newest planetoid found just recently, in an area three times farther away again than Pluto, is dubbed Sedna after the Inuit sea goddess. Based on the number of objects found in the Kuiper belt so far astronomers estimate there could be upwards of 30,000 fragments larger than 50 km
The question that immediately comes to mind is why, with so much mass around, why is there no large planet in the belt position? Why only fragments and rubble? The clue may lie in the elementary composition of these objects. It is too heavy to fit the standard planetary cloud theory. The objects seem to fit better in-between the orbits of Uranus and Neptune. Here is were Charles’ ingenuity comes to the fore: He demonstrated a hypothetical program devised at Monash, simulating gravitational interaction between planets, and how this tussle can disrupt not only the initial formation of a planet between Neptune and Uranus (similar to Jupiter’s effect on the asteroid belt), but at the same time displace any floating fragments there gradually (over millions of years) from their gravitationally inconsistent locations into the relatively stable 2/3 and 3/5 harmonic multiples of Neptune’s orbit. As I said, it was fascinating to watch and see an idea take shape and become reality. The humble hypothesis is, if not the mother then at least the accoucheuse of inventions.
A more thorough understanding of the processes involved in the evolution of our own planetary system will complement new research into other planetary nebula, such as the HR4796A system. There the disk of dust, about three times the size of Pluto's orbit around the Sun, surrounds a star roughly 220 light-years from Earth, which, according to some theories, has the right age to be forming planets now.