I read a reasonable amount of science fiction, and it’s always intriguing to attempt to visualize what having X in the sky instead of our own Sun and Moon might be like.
Authors will place terrestrial-type planets “huddling close to a red dwarf” or at Neptune-like distances from a bright, hot star, which appears as a point light source in the sky but delivers enough energy to give you comfortable surface temperatures. Or perhaps an Earth-size body in close orbit around a brown dwarf, which delivers nearly no light but quite enough infrared to keep an ecosystem working. The object, of course, is to come up with something “different” that delivers about as much energy to its satellite (planet/moon) as the Sun does to Earth.
I know there are probably formulae to calculate this sort of stuff, but I have no idea what they might be. Anyone with a working knowledge of this stuff who’d care to explain how this sort of thing is figured and what the limiting factors might be?
I think what you’re looking for is the “Goldilocks Zone,” the region around a star where the equilibrium temperature on your basic earthlike planet will be between the freezing point and boiling point of water. (Get it? This porrige is too hot! This porridge is too cold!) This is also known less poetically as the Habitable Zone.
The temperature of a planet is determined by radiative equilibrium: the amount of energy that the planet radiates as a blackbody is equal to the energy it recieves from its sun. If you want to calculate the distance from a star, d, for which a planet will have a certain temperature, T (in Kelvins), that’s:
d=sqrt((1-A)*L/(4 pi sigma T^4))
where A is the planet’s albedo (the fraction of sunlight it reflects), L is the luminosity of the star, sigma is the Stephan-Boltzmann constant.
This ignores the greenhouse effect, though, because the greenhouse effect decreases the efficiency of the blackbody radiation of the planet, but you could get some approximate numbers from scaling. Double the luminosity of the star, and you double the distance of the inner and outer borders of the Goldilocks zone.
Wow. I never thought about that kind of planetary system before, but this has sparked a fasincating image in my mind. I bet if the planet was close enough to the star, and if their relative sizes were right, tidal friction would also heat the planet and keep a plentiful source of geothermal energy going.
(Stupid hitting-the-submit-button-too-soon) … I’m also very interested in tools for figuring this kind of thing out – I can’t really figure out that plot that Podkayne linked to. Would it really even be practical to keep a planet warm enough to keep water liquid without visible light?
I’ve often thought that a generalized science fiction writing tool would be great – something that would let you do relativistic calculations of all sorts for stories set on slower-than-light ships, plus include this kind of data for creating extrasolar planets. Does anything like that exist, does anybody know?
The program Celestia automatically calculates the apparant magnitude of a star at whatever distance away from it you are. It only has data for (some) stars in our galaxy, but I believe you can insert other stars by modifying a text file with star data, if you know the right variables for the star.
Two questions: is that commercial software? And, can you create your own custom (fake) stars, like, say, a brown dwarf (or even a black hole!) for a story?
On a related note, I remember a fun little program that I used over a decade ago when I had a Macintosh. It was called Gravity (I think it was Gravity 4.0, but I’m not sure) and you could set up any number of bodies (planets, I thought of them as) and set them orbiting each other. How things turned out depended on various initial parameters you set like the velocity of each object, the mass of each object, and the distance between the bodies … which it allowed you to set graphically, by changing the size and position of the objects (I don’t recall how you set the velocity, but I think it was with an arrow that you dragged to set both initial direction and speed by the size and direction of the arrow). It was a lot of fun to set up little solar systems and watch them move. Is anybody else familiar with that program, and is there a version of it that you can run on a Windows machine?
