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Introduction to Astrophysics Lecture 6: Inventory of the Solar System This shows the planets to scale (though obviously not the orbit sizes to scale!!)

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Presentation on theme: "Introduction to Astrophysics Lecture 6: Inventory of the Solar System This shows the planets to scale (though obviously not the orbit sizes to scale!!)"— Presentation transcript:

1 Introduction to Astrophysics Lecture 6: Inventory of the Solar System This shows the planets to scale (though obviously not the orbit sizes to scale!!)

2 Basic facts  There are known planets, plus comets and asteroids.  99.8% of the mass of the Solar System is in the Sun.  Jupiter is more massive than the other planets put together.  Only planets out to Saturn were known in ancient times, and Pluto was not discovered until 1930.  The planets are usually divided into the terrestrial planets (Mercury, Venus, Earth, Mars) and the giant planets (Jupiter, Saturn, Uranus and Neptune). Pluto is a rogue which doesn’t fit this classification …  … and it’s not the only one!

3 Mercury The closest planet to the Sun, with an orbital radius about 40% of the Earth’s and a mass about one twentieth of the Earth’s. It is heavily cratered and geologically inactive. It used to be believed that its rotation was locked to its orbit around the Sun, like our moon with one face always pointing towards the Sun. In 1965 this was established not to be true; the rotation period is two-thirds the orbital one.

4 Venus Venus is often thought of as the sister planet of the Earth, due to its similar size. However, it is a victim of a runaway greenhouse effect with a surface temperature around 450 Celsius. Magellan probe radar image animation

5 Earth You know this one quite well... One of the peculiarities of the Earth is that its moon is so large; only Pluto has a relatively bigger moon.

6 Mars The most famous planet, thought by many to be the mostly likely planet (other than the Earth!) to host life. Movie created from Viking probe images. Mars has two small moons, Phobos and Deimos, which are basically just lumps of rock about 10 km across.


8 Jupiter The largest planet in the solar system. It is mostly gaseous, with an inner shell of metallic hydrogen and a core of ice and rock. Sometimes said to be a failed star, but it would need to be at least ten times heavier to burn. The great red spot is a storm which has persevered for at least 300 years!

9 Jupiter’s moons Jupiter is famous for its moons. There are four large ones discovered by Galileo, plus at least 59 smaller ones (most of which were found only in the last few years). Ganymede is the largest moon in the solar system, just beating Saturn’s moon Titan. Jupiter also has a modest system of three or four discernible rings.

10 Saturn Everyone knows of Saturn because of its rings. It also has 48 recognised and named satellites, including the large moon Titan which has its own atmosphere. It is currently under intensive scrutiny by the Cassini satellite, which was launched in 1997 and arrived there in July last year after a complex series of fly-bys.

11 Shadows cast by Saturn’s rings, and its moon Tethys

12 The cratered icy surface of Tethys

13 Uranus Uranus has 27 listed moons, and also has quite a substantial ring system. Uranus has an unusual rotation, where the axis is almost in the plane of its orbit. It therefore appears to rotate `on its side’.

14 Neptune Not too much is known about Neptune, though it is the third most massive planet and has at least 13 moons including the large moon Triton. It too has a modest ring system. It was discovered in 1846 after its position was predicted from its influence on Uranus.

15 Pluto Pluto is the rogue planet which fails to fit into the classification. It has several oddities. Its orbit is by far the most eccentric of the planets, and indeed it was closer to the Sun than Neptune for 20 years until February 1999 (something that won’t happen again in our lifetime). Its orbit is also at a much higher angle of inclination to the plane of the solar system than the other planets, 17 degrees. Its moon Charon is over half its diameter, and was only discovered in 1978. It’s the only case in the solar system where the centre of mass is outside the parent object, and argues against Pluto being a lost moon of Neptune.

16 Asteroids: For the most part located between the orbits of Mars and Jupiter. 10,000 asteroids are classified, and it is believed that there are at least 100,000. Ceres is the largest, and only Vesta is naked-eye visible. Many have orbits which cross the Earth’s orbit. Comets: Basically icy rocks from the outer solar system whose disturbed orbits send them into the inner solar system to burn up. Some are seen many times, such as the famous 76 year Halley’s comet. Kuiper Belt objects: A swarm of minor bodies orbiting the Sun in the outer solar system. The first such object was discovered in 1992, and there are probably at least tens of thousands of them. Miscellaneous `junk’ Comet Kohoutek

17 Quaoar and Sedna: new planets? Quaoar is a Kuiper belt object discovered by Trujillo and Brown in 2002 with the Palomar Telescope. It orbits outside Pluto and was the largest Solar System object discovered since Pluto in 1930. Its diameter is about 1300km (half the size of Pluto), and it is on a very circular orbit currently one billion miles outside Pluto. Sedna is a similar object that is even further away, and takes over 10,000 years to orbit the Sun. It was discovered in 2004 by the same astronomers.

18 2003UB313, aka Xena Xena and its moon Gabrielle, imaged by the Keck telescope. Earlier this year, the inevitable finally happened; a Kuiper-belt object was found which is bigger than Pluto. It even has its own moon! Its orbital period is 560 years on a highly- inclined orbit. Although colloquially known as Xena, it is called 2003UB313 until an official name is decided.

19 Quaoar, Sedna and 2003UB313: new planets? So will they be called new planets? The International Astronomical Union (IAU) is currently in a tizz trying to decide. Apparently under consideration are …  Pretend it all hasn’t happened and there are still nine planets.  Demote Pluto to a minor planet and henceforth have an eight-planet solar system.  Give Pluto an honorary title as a `historical planet’.  Define some criterion for inclusion as `planets’ and accept all newcomers to give a solar system of who-knows-how-many planets.


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