Expert: James Gort - 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
Hello Geoff,

I agree with you. A few things don't seem right. If you put Earth's characteristics in the model, you come up with 6 degrees, rather than the actual 14-15 degrees. But I think the model does what it sets out to do - provide a bit of an education about orbital mechanics in a fun way. It's not meant to be a scientific model - it's a tutorial model. But I think there's an important parameter missing from estimating a planet's climate - that is, the composition of its atmosphere. How much CO2 (a greenhouse gas), how much dust, etc. comprises the atmosphere? The chemical composition has much influence on the global climate. But that would complicate the model for many people.

There's one other consideration. I don't think Dr. Hamilton is an exobiologist. He's a physicist and planetary astronomer. I personally feel that planets can be habitable in much more extreme climates than his model predicts. But I'm not an exobiologist, either. It's just my best guess.

Hope that helps.

Prof. James Gort