Expert: Courtney Seligman - 7/15/2011

Question
Hello,

I have been using the build your own solar system gimmick on this website:-

http://janus.astro.umd.edu/orbits/ssbuild.html

I created various Earth-size planets at various distances from an imaginary Sun like ours in size. However, the statistics, I think, are suspect. For a yellow star, surface temperature of 5500 C(like ours, right?), the projected average global temperature of the 1st planet(at 0.6 AU, no eccentricity of orbit) was 87 degrees Celsius, citing it habitable, the 2nd planet, at 0.8 AU distance, was at 39 degrees celsius for average global temperature, which was also cited as being habitable, with the 3rd planet being claimed as uninhabitable at only 1.2 AU from the Sun,and a -17 degree C average global temperature. That 3rd planet doesn't seem to make sense to me, as it seems way too cold at only 1.2 AU from the Sun, and the 1st planet should have been hotter than boiling temperature, I suspect. What do you think?

Thanks,
Geoff

Answer
The numbers are "off", as are those for a 4-planet system more or less identical to Mercury, Venus, the Earth and Mars, which I used to test the Java applet. The three closer planets all had temperatures that were too low, and "Mars" had a temperature that was too high.

The applet may not be correctly accounting for the balance of stellar and planetary radiation at different distances from the star, or (less likely but also possible) it may be providing a temperature which is an average of day and night temperatures; and it certainly doesn't take into account the effects of an atmosphere for planets with atmospheres. So I wouldn't use it for anything but entertainment, as it stands.

However, the site does say that there is an upgrade in progress, so if that is true, perhaps the next version of the applet will work correctly.