Download Celestial Architect Spreadsheet 1.2 below:
EDIT: Updated to 1.1. Renamed the Spreadsheet and added another page dealing with the moon.
EDIT: Updated to 1.2 with a better temperature calculation and a sheet for albedo, all thanks to Orion.
Using The Sheet
The "Sheet" is a spreadsheet-- a file you can open with Microsoft Office, or free alternatives such as OpenOffice or it's Mac port, NeoOffice. All you need to do is enter numbers in the yellow boxes, & all the other figures will be instantly recalculated.
Now, as i've mentioned before, the most important variable is the mass of your sun, from which all it's other stats can be pretty accurately derived. The sheet assumes that your star is a main sequence star, like 90% of the universe. If you want to try a dwarf or giant star, you are on your own, but it shouldn't be too hard to plug in those numbers if you are familiar with spreadsheets. I think all you would have to do is change the way luminosity is calculated to something appropriate to your new star type.
Some things about stars to be aware of:
- A smaller star puts out a lot less heat & light than you might expect, & the opposite is true of larger stars.
- Smaller stars have habitable zones which are closer to the star, so the planet's year is shorter. Again the opposite is true of larger stars.
- As a star's size increases, so does the depth of the habitable zone, so with a bigger star you have more room to adjust your orbital position (& thus length of the year) or maybe fit in multiple habitable planets.
The Habitable Zone (also known as Goldilocks Zones) is the area in space where a planet that could supports "life as we know it" might be. It doesn't mean that a suitable planet will be there, or that life will exist, just that both are theoretically possible in the zone. As might be expected not everyone agrees exactly how big the habitable zone is. I've chosen a newer, more conservative estimate from this PDF by Tom E. Morris, which has great in-depth explanations.
If you are clever you may be able to think of an unusual circumstance that allows your planet to be habitable outside of the normal zone.
The Habitable Zone from Wikimedia Commons.
Length of Days & Years
The length of your planet's year is set by the mass of it's sun, & the distance of it's orbit. However the length of it's day can be pretty much whatever you want.
You might even want to tidally lock your planet to it's primary (as the Moon is to Earth), so that one side is eternally day, & the other night. Tidally locked planets are generally thought to be incompatible with "life as we know it", but a lot may depend on the specifics of the planet. At the very least the weather will be freakishly weird. Any planets in the habitable zone of an M-class star are believed to likely be tidally locked. Note "M-class" doesn't really mean "earthlike" as it did in StarTrek. M-class are generally red dwarf stars.
To really complicate the calender you can make your planet into a moon of a larger planet such as a gas giant. Or you could put multiple stars in your solar system. If so, good luck, that's more math than i want to deal with.
Heat & Light
Temperature is one of the variables that, in my opinion science fiction doesn't use to it's full potential very often. There are plenty of "desert planets" & "ice planets", but they are blandly monotone, & realistically uninhabitable. With just the right temperature you could instead have a hot planet where only the polar regions are cool enough for human life, with an deadly equatorial belt where water boils away. Or a cold planet where a narrow equatorial band is the only place where the ice ever melts, with massive ice caps dominating the planet.
Albedo is the measure of how white or reflective your planet is overall. Any radiation reflected away isn't going to warm your planet. You can find out more about the albedo of various materials here.
The "Greenhouse Effect" mentioned isn't about climate change apocalypses. It's more basic than that. A greenhouse traps some of the energy that would normally be reflected back to the sky. Our atmosphere does the same thing, which keeps the planet from getting too cold. It's the strength of the greenhouse effect that is critical, which (among other factors) makes Venus a deadly inferno.
Be aware that this is not the final word on your planet's temperature. There are lots of other factors like axial tilt, land/sea arraignment & coverage, & mountain placement which will effect what the temperature is at a particular time & place. We'll get to that later.
Finally i should thank "Geoff" for his page Creating an Earthlike Planet. Even though the equations are found elsewhere, i didn't understand many of them until i read his explanation.
Instructions for second page to be added...