Possible issue with surface temperature calculations

[Omnigeek6]

I tried applying the planet temperature calculations to some of my already-made systems. One of them seems to generate a nonsensical result.

My star has a luminosity of 0.50 suns and a radius of 0.66 M km.
My planet has a radius of 4800 km and is 115 M km from the star.

Using the default values, the surface temperature feature calculates an average surface temperature of 32 *C.

However, calculating the amount of light received by the planet... light received = star luminosity / orbital distance^2, right?

0.5/(115/150)^2 = 0.85.

In other words, this planet should be getting 85% as much radiation as Earth, and yet the program predicts a much higher temperature for it.

[smjjames]

Whats the mass of your star? It goes a little wierd for stars outside the main sequence.

I noticed this as well when I was setting up the star for the Kepler-22 system. While the habitable zone looked right when I used the stars settings (I think Dan's hab zone is a little wider, but then again, it's really a fuzzy zone than a sharp-cut line) but the planets temperature was 10 degrees C higher than it was believed to be. Without changing the atnosphere, I had to change the luminosity by quite a bit (from about 90 L to about 70 L) and it was near the center of the hab zone by the time the temp went to 22 degrees C.

I don't think it's the hab zone calculation, although in that 'if Sirius had planets' simulation' the hab zone for Sirius's compainion isn't correct as the planet that circled it was still minus C. I'm chalking it up to the simplified atmosphere and we'll just have to see how the more advanced one goes.

Anyways, I agree that there is something up with it.

[Dan Dixon]

Star
---
0.84 suns
660000 km
11.1 g/cm^3
7222 K
.55 L
(mass, diam, den, temp, lum)

The radius of the planet isn't considered in calculating the surface temperature.
And at a distance of 115 M km I get a surface temp of 9.48 C

The surface temperature calc considers the temperature and radius of the star and the distance of the planet. What temperature is your star?

Solar energy is calculated like this: http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

[smjjames]

You have the luminosity off. Also, While the interlinked values is cool, I'm finding that I have to wrestle with various values (unchecking the main sequence box helps) when trying to reach a known set of values and even then, some of the values are off.

Case in point, I used http://kepler.nasa.gov/Mission/discoveries/kepler22b/ to set the properties for the star. While I was able to get the mass, size and density about right, the luminosity and temp are off. In order to change the luminosity, the mass has to change. I may have forgotten to take into account the range of error for mass, but the luminosity and temp are beyond the range of error.

Edit while posting: Hrm, after some tweaking of the Kepler-22 system, it looks like I did forget to take into account the range of error for mass, although the hab zone seems a little off now.

Edit: I got those same numbers, although with 0.50 L, it's colder of course. Also, Omnigeek, what's the mass and density of the star in your simulation? Just to make sure that we have it right.

[Omnigeek6]

Problem solved. Verdict: user error on properties of star.

Here are the star's actual properties:

0.84 solar masses
Diameter 1.21 M km.
Luminosity 0.50 suns
Surface temperature 5209 K.

Originally I had diameter at 1.31 M km and temperature at 5570 K, which explains the bizarre result.

The updated values give me a surface temperature of 0.72 *C with Earth's albedo and emissivity (the planet's actual surface temperature is about 15 *C due to a stronger greenhouse effect.)


I do still have a question: what do I do if I want to find the surface temperature of an object like Venus?

[Dan Dixon]

I do still have a question: what do I do if I want to find the surface temperature of an object like Venus?

Sorry... you'll have to wait until the next version which will have this working by default.
Currently you can't get accurate values for Venus with the atmosphere model that Universe Sandbox is using.

[smjjames]

Problem solved. Verdict: user error on properties of star.

Here are the star's actual properties:

0.84 solar masses
Diameter 1.21 M km.
Luminosity 0.50 suns
Surface temperature 5209 K.

Originally I had diameter at 1.31 M km and temperature at 5570 K, which explains the bizarre result.

The updated values give me a surface temperature of 0.72 *C with Earth's albedo and emissivity (the planet's actual surface temperature is about 15 *C due to a stronger greenhouse effect.)


I do still have a question: what do I do if I want to find the surface temperature of an object like Venus?

As for Kepler-22, a little tweaking of stellar properties made the temperature the way I wanted it to, although the hab zone display still seemed off. Then again, the hab zone is really a diffuse zone rather than a cutoff point.

I do still have a question: what do I do if I want to find the surface temperature of an object like Venus?

Sorry... you'll have to wait until the next version which will have this working by default.
Currently you can't get accurate values with the atmosphere model that Universe Sandbox is using.

Looking forward to the more advanced atmospheric calculations and possibly accounting for armospheric density.  Also, did you know that Titans atmosphere is actually a bit denser than Earths? We wouldn't even need pressure suits. Not sure what the equivalent depth underwater Titans atmosphere would be, so pressure acclimation might be neccesary.