September General Meeting Report
September was again time for one of the Society’s annual exchange lectures, this time with Steven C. Raine from the Astronomical Society of South Australia. His topic covered ‘Newly Discovered Exoplanets, Ice Dwarfs and more”.
The story of “Goldilocks and the three Bears" is a popular fairy tale in the English language, and the “just right” aspect from that story has been adopted in as unrelated and unlikely subjects as economics and astronomy. A Goldilocks economy describes one which sustains moderate growth and low inflation allowing for a market friendly monetary policy, while a Goldilocks planet is a planet orbiting a Sun-like star, neither too close nor too far away to rule out a life-supporting environment. Hunting for Earth-sized Goldilocks planets has been going on since the first Exoplanet was discovered in 1995 (see The Count Rises).It is a key part of the current Kepler Mission, which uses a space telescope (launched on 7 March 2009 UTC) to survey some 100,000 stars and compile the characteristics of habitable-zone planets (refer presentation by Dr Sarah Maddison in CRUX No 2). Most of the 380 Exoplanets discovered to date are massive, Jupiter-size short orbit-time planets, as most of the current detection methods are biassed that way (see 51 Pegasi). Although the extra-solar planet 70 Virginis b was initially nicknamed "Goldilocks" because it was thought to be within the star's habitable zone, it is now believed to be far too warm to be "just right" for life. Gliese 581d, a rocky planet lying within the habitable zone of its star (see Gliese 581 system) could fit the bill.
The “just right” criteria is of course very much a user oriented philosophy. We tend to classify as “normal” what fits our present understanding and our needs, and give other labels to extraordinary conditions and behaviour, in planets as well as in people, were different cognitive styles and skills are not necessarily a disorder or a disability. With an uncanny grasp of facts and figures Steven presented us with a grand tour through our solar system, defining its components from the known planets to the icy worlds of their moons out into the vast, as yet mostly unexplored realm of the comets. For years researchers have suspected that the planetary system is surrounded by a vast reservoir of debris, left over from the Solar System formation period. Gerard Kuiper (rhymes with "viper"), 1905-1973, the son of a tailor in the village of Tuitjenhorn in North Holland, had an early interest in astronomy and an extraordinarily sharp eyesight allowing him to see magnitude 7.5 stars with the naked eye, about four times fainter than those visible to “normal” eyes. Kuiper (and earlier Edgeworth) postulated that the region beyond Pluto was the origin of existing comets. The Kuiper Belt, as it is now known, was initially thought to have been depleted, emptied out over the ages, but has since been found to be a heavily populated area extending from the orbit of Neptune at 30 AU to approximately 55 AU from the Sun. Similar to the Asteroid Belt between Mars and Jupiter, it is far larger – 20 times as wide and up to 200 times as massive (see The Kuiper Belt). While the Asteroid Belt is composed primarily of rock and metal, the Kuiper Belt objects are most likely composed of frozen volatiles (termed "ices"), such as methane, ammonia and water, that for some reason either failed to condense into a large planet or are the remnants of a disrupted, larger planetary body. The belt is home to at least three dwarf planets – Pluto, Haumea and Makemake. Since the first discovery beyond Pluto in 1992, the number of known Kuiper Belt objects has increased rapidly to over a thousand, and more than 70,000 objects over 100km in diameter are believed to reside there. Studies since the mid-1990s have shown that the Kuiper Belt is actually dynamically stable and most likely not the source of long-period comets.
As early as 1932 Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud extending to the outermost edge of the Solar System. In 1950 the idea was independently revived by Dutch astronomer Jan Hendrik Oort, who inferred the existence of the cloud from the physical evidence of long-period comets entering our planetary system. Oort, who in the 1920's also determined the rotation of the Milky Way galaxy, interpreted the comet orbital distribution on hand of 19 well-measured orbits and successfully recognized where these comets came from. Additional data gathered since then has confirmed his studies, establishing and expanding our knowledge of this distant cloud (now known as the Oort Cloud) as the point of origin for comets like Hale-Bopp, with orbits lasting thousands of years (see Oort Cloud). Sedna with a diameter 1,200-1,800km, discovered in 2003 at a perihelion of 76AU, is also a possible candidate. Believed to consist of two separate (inner and outer) regions, the cloud extends outwards from 2,000 to 50,000AU from the sun. That is almost a lightyear, a thousand times more distant than the Kuiper Belt, and nearly a quarter of the distance to the nearest star, Proxima Centauri. The total mass of the Oort Cloud was at one time estimated at 300 times the Earth, but is now believed to be about five Earths.
It was a most informative evening, presented on hand of numerous slides by a speaker with an extraordinary eye for details in the subject of the talk. Following an extensive question-time the vote of thanks was given by the President, Perry Vlahos, to general acclaim.
The Astronomical Society of South Australia was founded in 1892 and is the oldest society of its kind in Australia. Membership is open to people of all ages and professions and the Society has at present some 500 members. Steven Raine was diagnosed with Aspergers syndrome (a mild form of autism) soon after entering Australia as a child. A science fiction fan from an early age he is a member of the Blackwood Writers' Group and has written a number of short fiction stories, some of which have been published online, as well as several non-fiction articles for the Astronomical Society of South Australia