1) Gallifrey was once pictured next to Earth, which shows that it’s at least twice the size. That makes it a super-Earth, right? From what we know of super-Earths out there, what are their compositions like compared to Earth’s?
When you mentioned how Gallifrey was pictured next to Earth, the first thing I thought of was an artist’s representation of an exoplanet discovered in 2004, classified as a “super Earth” called “55 Cancri e” depicted about twice the size of Earth, and like our planet orbiting its own star. One of the main things about Super Earths is that their mass is larger than that of our planet, so it is logical to assume that its physical characteristics will vary compared to our blue marble. Interestingly enough when people think of alien worlds many think of its inhabitants as almost human-like in appearance, we see this in Sci Fi all the time. When I think about other worlds my imagination goes completely wild, because even if another planet let’s say a Super Earth had similar characteristics to those that support life like: liquid water, an atmosphere that gives protection for life to thrive on land, and even our almost perfect position in relation to our star so we don’t burn to death… no life in other worlds would be/look like us unless that planet goes through the exact same changes Earth has from the smallest to the extinction level ones. So when you think about it any other world would need an identical timeline of events to even begin to be called “Earth-like”. A super Earth’s composition can only be determined based on what we know about earth geologically and chemically, but what about what we don’t know? That’s where the fun is.
2) The surface of Gallifrey is rust-colored. What does that tell us about its surface geology, and perhaps its atmosphere?
I’m no geologist, but if I think of Gallifrey in terms of only its appearance I’d say that the rusty color comes from iron within its dust reacting to the oxygen. Although that’s once you’re on the planet. From space Gallifrey is actually said to look more vivid yellow and orange as if it was still engorged in the fires and devastation of its war times.
3) The Tenth Doctor describes it as having a second Sun. How would a binary system affect Gallifrey? (One of them is described as large and red.) The leaves on the trees were silver: what kind of light are they hoping to absorb from the stars above based on that?
Since Gallifreyans/Time Lords once thrived on the planet it is safe to assume that a binary system is just what was normal to them and didn’t affect them in a negative way, just like how we survive our sun.
As for the leaves being silver… silver doesn’t absorb visible light but instead it absorbs ultraviolet light. So I don’t know if there was a point to the leaves being silver or that was just one of the charming features of Gallifrey, but I sure wish I lived there pre-war when the planet lived in all its glory so I could’ve seen what happens at night.
4) The mountains on Gallifrey are said to “shine” – would this mean they’re composed of a certain type of mineral?
There are a number of minerals that have a certain metallic “shine” to them, if I had to pick which one makes Gallifreyan Mountains I’d say Pyrite, because it’s common in rocks and some people confuse it with gold. Gallifrey to me always seemed like such a regal place, I like to think of even its former mountains as something that would captivate and amaze.
5) The planet is mentioned as having at least one season: autumn. What does this reveal about its rotation around the stars?
Well we know Earth’s season changes happen because its axis (an imaginary pole that runs through the planet from top to bottom) is tilted, so if Gallifrey has but one season we could assume that is because Gallifrey’s axis is almost straight, by how much? I have no idea but it would be awesome to find out?!
6) Gallifrey, at the moment, is located billions of years in the future when the universe is pretty much headed towards heat death. How would that affect the planet – and the night sky?
Well since we already know that Gallifrey is headed towards heat death as you said, then it is safe to assume that fire will be one of the main devastating forces as the planet approaches its doom, from then on it is really anyone’s guess what things would look like. That’s the fun part.
7) It’s been said that Gallifrey is pretty close to the center of its own galaxy. How would being so near to a super massive black hole influence life on the surface?
Given that the 10th Doctor told Rose that his people invented black holes, I assume in their universe it wouldn’t affect Time Lords the way it would affect us or even like in general. Omega (also known as Peylix) a Time Lord got trapped inside a black hole and survived… then again he was exceptionally powerful and said to be the only one to ever lived in the anti-matter universe. That didn’t go too well for him though 😉