The first topic was "hot Jupiters", or gas giants orbiting very close to their parent stars. There is iron vapor and iron rain in the atmospheres of these planets. One of the planets has been called "Osiris."
Many planets are in highly elliptical orbits, and either the original planets or their moons could have extreme seasons, with water boiling away in the short summers and freezing for long winters; a caricature of the climate in Minnesota.
Rogue planets (like the "Planet X" in the 1951 movie) could carry moons and generate enough gravitational tides to support some kind of chemistry on its moons.
There may be billions of earth-sized planets in our galaxy, including "super earths". The smallest such planet found so far is about 5 times the mass of Earth and may have an ocean a thousand miles deep, with a layer of "ice 7" created by pressure.
Closer to the center of the galaxy there is more carbon, and there may exist earth-sized worlds with reducing atmospheres like Saturn's Titan, perhaps even with diamonds on the surface. Some astronomers predict carbon monoxide atmospheres.
The Earth at one time had a Mars-sized planet in its orbit with which it collided, to form the (large) Moon.
The program was followed by "Naked Science: Hawking's Universe". The program covered how gravity is a manifestation of space-time, and how the universe could develop from an essential singularity of pure gravity. The biography of Stephen Hawking, who was diagnosed with ALS at 21, was also covered. The weakening of gravity when the universe began is critical and is discussed.
Gravity seems to disburse into other dimensions or branes on a microscale. The Hadron collidor will cause the dimensions to collapse in extremely small nanospaces of collisions, leading to the fleeting formation of mini-black-holes, which should evaporate (as Hawling radiation).
Attribution link for artist's drawing on Wikipedia of Gliese 581d, from the article on extrasolar planets.
