Introduction to the various bodies that orbit the sun:
There are many chunks of matter orbiting the sun in the solar system, other than the planets:
Asteroids made of rock, metal or a mixture of both. Image of main asteroid belt at: http://www.rawscience.tv/wp-content/uploads/2014/09/asteroid-belt.jpg Image of other asteroid orbits at: http://www.newscientist.com/data/images/ns/cms/dn9846/dn9846-1_450.jpg
Dwarf planets, smaller than the main planets, but large enough for their gravity to pull them into a sphere. They are in the asteroid belt (e.g. dwarf planet Ceres), or in the Kuiper belt (which is beyond Neptune e.g. Pluto, Eris). The orbits of dwarf planets are more elliptical and tilted than major planets. Image of dwarf planet orbits at: http://astronomy2009.nasa.gov/images/topics/sep/LB/topic05LG.jpg
Comets are in the Kuiper belt, or beyond. There are billions of them. Image of some comet orbits at: http://www.wired.com/images_blogs/wiredscience/2013/08/Orbits_of_periodi... (Orbit times: Halley orbit is 76 years, Borrelly orbit is 7 years, Ikeya–Zhang orbit is 366 years and is the longest known orbital period.) Comets are snowballs of frozen gases, rock and dust the size of a small town. A comet warms up near the sun and develops a coma (atmosphere), a glowing head, hundreds of thousands of km across. It has two tails (which always point away from sun), one gas and one dust (which curves with sun’s gravity). Image of comet structure through its orbit: http://www.skyandtelescope.com/wp-content/uploads/Koehn_IZ_orbit_300dpi.jpg Comets may have brought water and organic compounds (the building blocks of life) to Earth and other bodies in our solar system.
Students use the gravity well to model the different orbit shapes around the sun:
The fabric represents space, and the funnel in the centre of the fabric represents the gravity of the sun.
Using the marbles, students can make the marbles orbit the "sun". By varying the shape of the orbit (e.g. circular or elliptical) students can model the orbits of the various bodies orbiting the sun.
Put up images of the various orbits (of planets, moons, dwarf planets, asteroids, Kuiper belt objects and comets) so that students can refer to their shapes as they experiment. Planets and asteroids have a circular orbit. Comets have an elliptical orbit.
Ask students to notice what happens to the speed of an orbit as the object approaches the gravity well of the sun. (It will speed up.) This happens to the real objects orbiting the sun.
Several marbles orbiting at once can represent the asteroid belt, with collisions that knock some of them out of orbit.
Discuss the ways that this is not the same as bodies orbiting the sun:
In this model, every marble inevitably loses energy as it rubs against the cloth and falls into the gravity well.
Planets stay in orbit around the sun. They are continually falling towards the sun, but their speed means that they curve around it and stay in orbit.
Using the gravity well to model stars orbiting a black hole:
Stars orbit black holes, so astronomers look for stars that appear to be orbiting around "nothing", as evidence for the presence of a black hole.
To modify the gravity well model to be more like the shape of the gravity well of a black hole, add a ring underneath to model the event horizon (the point past which nothing can escape from a black hole's gravity). See the last two photos.
Notes to add in:
Look at orbit shapes. Circle, elipse, long narrow elipse, star shape as the ellipses cycle round. Orbits in the galaxy are all these shapes. Astronomers discover black holes by looking for orbits around "nothing".