For those who want to be able to work out a few useful statistics for their planets in a hurry, the following little utility might be of use. It takes some information about the local star, and the planet, and works out the remaining information such as gravity and length of year.
Applet
Usage
At its simplest, you can select a star or planet from the two menu lists, and information on the combination will be shown. Though the starting values are accurate (to the best of my knowledge), the calculated values are only approximations to reality. The maths is right, but since diameters, densities and distances are averages, some errors do creep in.
Alternatively, you can type your own values into the following fields:
- Star Mass
- This is the mass of the star relative to Sol.
- Distance
- The average distance of the planet from the star in millions of kilometres. Since orbits tend to be ellipses rather than circles, this is always an approximation.
- Density
- The average density of the planet, where the density of water is 1.0 (1 tonne per cubic metre).
- Diameter
- The diameter of the planet, in kilometres. Planets tend to be squashed (equatorial diameters have been used in the examples), and not very smooth (though the latter has minor effect).
When you click on calculate, the following fields are filled in:
- Star Mass (kg)
- This is the display field next to the solar mass. It is the mass of the star in kilograms.
- Period
- This is the orbital period of the planet, given in whatever units are most suitable.
- Planetary Mass
- Total mass of the planet, in kilograms.
- Surface Gravity
- The value for freefall at the planet's surface, in metres per second per second.
- Escape Velocity
- This is in kilometres per second. If an object is given this velocity (straight up) on the planet's surface, and no other forces act upon it (such as further thrust, or atmospheric drag), then the object will never be pulled back due to gravity.
Source Code
The source code to the applet is available under the BSD license.