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NEW - 17 September 2008 - Haumea (2003 EL61) newly-classified as a Dwarf Planet
(bringing the total to 5)
The past few years have been an exciting time for astronomy as improved telescopes, the use of computers to quickly compare parts of the sky at different times, and visits to various parts of the Solar System by robotic probes have all led to a mass of new discoveries. With what appears to have been an ever-increasing pace since the 1970's many new discoveries have been made, for instance:
On 24th August 2006 the International Astronomical Union (IAU) finally defined what a planet is (following much argument and debate).
The result of this was that Pluto was re-categorised, being moved from the
'planet' category to a new
category, 'dwarf planet', together with Ceres, the largest body in the Asteroid Belt
(which lies between the orbits of Mars and Jupiter), and Eris (formerly called by its 'discovery' number 2003 UB313 or
sometimes referred to as 'Xena' - this last however was simply a temporary name that was used by its
discoverers Mike Brown, David Rabinowitz and Chad Trujillo).
Comment: Note that Pluto has not been 'demoted', it's just had its category changed, the suggestion of 'demotion' is just media hype. This is no different from when horticulturalists find new evidence and change the categorisation of a plant species - which happens quite often. In this case the new evidence has accumulated over the past several years that Pluto is one of many similar bodies, and also that it is much smaller than originally thought.
In 2008, two further bodies were classified as dwarf planets - Makemake (formerly 2005 FY9) and Haumea (formerly 2003 EL61). Haumea is particularly interesting, as it has a very fast orbital spin that results in an elongated shape.
Eris, Haumea, Makemake and Pluto are also classified as Kuiper Belt Objects
(KBOs - bodies that circle the Sun in and around a 'belt' beyond Neptune, roughly 40 to 50 times further
from the Sun than the Earth), along with thousands of other smaller objects - Pluto is a Plutino (an object whose orbit is 'fixed' by the gravitational 'pull' of Neptune - Pluto orbits the Sun twice for every three times Neptune orbits the Sun), whilst Eris is part of the 'Scattered Disc'.
(Scattered Disc (or, Scattered Disk) Objects are objects beyond Neptune with highly elliptical orbits and/or orbits that are substantially inclined away from the plane in which the eight main planets orbit the Sun - some astronomers believe they are in unusual orbits because they have been 'thrown' there by Neptune's gravitational pull, and some astronomers also believe that Centaurs - small bodies that orbit the Sun between Jupiter and Neptune - are also Scattered Disc Objects, ones that have been 'thrown' inward rather than outward).
Haumea and Makemake are officially classified as 'Cubewanos' (sometimes Classical KBOs) (Cubewanos have orbits that are not highly inclined or highly elliptical, and not 'controlled' by Neptune). However, Haumea and Makemake have quite high orbital inclinations, and Haumea has been suggested by its discoverers to be a 'Near Scattered Disk Object', and there is a possibility that further study will reveal them to have an orbital resonance with Neptune, and thus be Plutinos!
In July 2008 the IAU announced that these objects should be termed 'Plutoids', this being the collective name they have decided upon for Dwarf Planets beyond the orbit of Neptune. (This has drawn a barrage of further criticism, and note the possible confusion with the term 'Plutino').
A number of other bodies would appear to meet the definition of 'dwarf planet', but information about them will be considered by the IAU before it confirms any of them as 'dwarf planets'. Note that astronomer Mike Brown, co-discoverer of 3 of the currently-recognised dwarf planets, estimates that eventually about 100 will be discovered in total, and lists 41 currently-known Kuiper Belt objects as being probable (and all are estimated larger than the 4th largest object in the asteroid belt).
Likely candidate dwarf planets include asteroid Vesta (and less likely, Hygeia and Pallas); Kuiper Belt Objects Ixion, Orcus, Quaoar, Varuna, 2002 AW197, 2002 TX300 and 2002 UX25; plus Sedna (and less likely, 2004 XR190 and 2005 RM43) (which all orbit the Sun wholly at a distance beyond the Kuiper Belt).
For the record , the EIGHT planets in the Solar System are:
(In order, nearest to the Sun first): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
(In order, largest first): Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury
(In order, most massive first): Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury
For the record , the FIVE recognised dwarf planets in the Solar System are:
(In order, nearest to the Sun first): Ceres, Pluto, Haumea, Makemake, Eris
(In order, largest first): Eris, Pluto, Makemake, Haumea, Ceres
(In order, most massive first): Eris, Pluto, Haumea, Makemake, Ceres
But note - size and mass are currently just estimates, so these orders could change - the estimated masses of Haumea and Makemake are particularly close. And don't forget, there are many more objects likely to be recognised as dwarf planets in the future - never just say 'there are five dwarf planets in the solar system'!
I could go into detail about the planets and other objects in the Solar System here on this site, but excellent resources for this already exist - the first six websites listed in 'Links' below and the first listed 'astronomer site' in particular - so I'll not try to 're-invent the wheel' here.
With such a large number of known moons in the Solar System (162 as at the end of 2006 just for the six planets with moons), it is possibly difficult to know which ones to teach about. I'd suggest that teaching should concentrate on the 19 that are of planetary mass (Planemos) listed below, plus select some others that are particularly interesting for various reasons - the Trojan moons that accompany Saturn's moons Dione and Tethys, and also Saturns moons Janus and Epimetheus, which are sometimes referred to as 'dancing moons' because they occupy the same orbit and exchange places every four years.
Planemos (see definitions page) in the Solar System that are neither planets nor dwarf planets are the satellites of a number of the planets and currently confirmed dwarf planets, as follows:
Another nice site is 'Views of the Solar System'
Sol Station is an excellent site giving details of both Solar System objects and star systems within 100 light years of the Solar System (and they have a 3D 'near star' map), plus an excellent analysis of the likelihood of stars having habitable planets.
Solar Space Station - great site about the Solar System by Chris Thomas
The Moons of the Solar System site. A great resource, with quite a lot of the moons and planets having links to rotating maps.
Astronomy Workshop - This site has great 'viewers' showing the movement of the planets round the sun and showing the movement of moons round the various planets.
UCAR (The University Corporation for Atmospheric Research) have a site, "Windows on the Universe", with a good Solar System section, as well as sections on the Earth, Space Weather and much more.
NASA's Space Science site
Ask An Astronomer is a great resource, with a collection of answers to many questions students and teachers might have, all answered by astronomers volunteering their time (most are attached to or connected with Cornell University). And if you can't find an existing answer to your question, you really can 'ask an astronomer'.
Mike Brown, co-discoverer of Eris, has a variety of articles about dwarf planets on his home page, including WHY a new definition of 'planet' was needed and a list of 'candidate' dwarf planets. His blog Mike Brown's Planets also has interesting articles on the subject.
Chad Trujillo has an article on what you need to survey for KBOs, plus articles on Orcus (2004DW) and Quaoar.
Comet and Kuiper Belt expert David Jewitt's Kuiper Belt page, Quaoar page, Comet pages, and Damocloids page are each full of interesting facts.
Asteroids - An Overview, by J.Bakkelund.
Home page of brown dwarf expert Gibor Basri
Astronomer Scott Sheppard's page detailing all the known satellites of the giant planets (Jupiter, Saturn, Uranus and Neptune) plus details of Jupiter and Neptune's Trojans and Jupiter-family comets - here
The home page of atronomer Paul Weigert has links to his pages about quasi-satellites, including his discovery of one of Earth's quasi-satellites, Cruithne, plus his work on how planets might behave in binary-star systems.
Dan Green has written an interesting article about the changing status of Pluto, Ceres and other Solar System objects.
RECONS (Research Consortium on Nearby Stars) have a list of the 100 nearest stars
The Extrasolar Planets Encyclopedia lists known extrasolar planets and
California & Carnegie Planet Search lists known extrasolar planets of nearby stars (but the data is not as up to date).
Detail about the discovery of Varuna by the Spacewatch Project (who also discovered 2002 UX25) can be found here.
The Extrasolar Planets Encyclopedia lists known extrasolar planets and California & Carnegie Planet Search lists known extrasolar planets of nearby stars (but the data is not as up to date).
Website for the Deep Ecliptic Survey, the discoverers of Ixion, here
An article about Scott Sheppard and Chad Trujillo's discovery of new Trojans in Neptune's orbit here.
Article from New Scientist about the discovery of 2004 XR190 is here
An article from Science Daily about the Gliese 581 star system
Hartmut 'Spider' Frommert runs a page with regularly updated information about past, current and future Mars exploration
NASA's Dawn Mission launched in 2007 and should visit visit Vesta in 2011 and Ceres in 2015, whilst their New Horizons mission to Pluto and hopefully other parts of the Kuiper Belt launched in 2006 and is scheduled to reach Pluto in 2015.
NASA's Astronaut Biographies pages and Human Spaceflight pages are also of interest. (UK educators might like to note that current British-born NASA astronauts are Mike Foale, Piers Sellers and Nick Patrick. The first (and only other) Briton in space was Helen Sharman).
Heavens Above gives details of when and how to observe the International Space Station and other man-made objects cross the sky (on the best days, the International Space Station is quite bright and easy to spot).
In the meantime I will be adding information to the site as best I can and with what time I have available (not much) and adding links to information about the subject(s).