Celestia Forums

Ok, this is something I've been working on for a while, and I wanted to make a few points here for all to see.

Allright, we all know that Celestia gives a temperature reading for planets orbiting stars. For earth the temperature is a rather underinflated 258K, but if we use this for the baseline, modeling planets in temperate zones around other stars should still be possible.

Now, in my own research I've discovered several incontravertable facts.

One, consider the apparent magnitude of the sun from the Earth, it is about -26.72. Now one way that alot of people use (including, it would seem, Celestia's developers) is to place the "temperate" zone of a star in the radius of the orbit at which a star would appear to be magnitude
-26.72.

This is, of course, in no way a bad thing to do, however as far as realism goes it is a bit of a catch-22. Here is why. For many stars, because of the differences in size, composition and radiation output the environment at which the star appears to be mag -26.72 may or may not be Earthlike. To make a rather extreme example, consider my favorite low luminosity star, Wolf 359.

Wolf 359 is a star of about 10% Solar Mass and about 14% Solar Diameter, it shines with an absolute magnitude of just 16.55 and is incredibly dim by any standard. The Brightness of Wolf 359 compared to Sol is an amazing .0000209 Sol, this is about 1/50,000 the brightness of the sun. Er-go, the zone at which a planet would recieve light from Wolf 359 on the order of magnitude -26.72 is just .0045754 AU, or about 684464 kilometers. The year for a planet in such an orbit would be only
.111 Days or a bit over 2.6 hours.

So far so good right? however here is the catch. The problem is at such close proximity to Wolf 359 the planet would be seared by intense infrared ration, on the order of 857 K, with an incoming flux of 122377 W/m^2. One can see the problem here. Placing a planet close enough to certain low mass, low luminosity stars results in a charred world. Conversely placing the planet far enough away in which the temperature would be an earthly 288K, Wolf 359's brightness falls to about mag -21.91.

So, my question is, why does Celestia (at the moment) NOT differentiate between the visual and temperature comfort zones, and are there any plans to change the setup in the future?

Note I am defining the Visual Comfort Zone as the area at which a star appears to be mag -26.72, and the Temperature Comfort Zone is the area where a planets ambient (black body) temperature would be 288 K.

Apollo7 wrote:So, my question is, why does Celestia (at the moment) NOT differentiate between the visual and temperature comfort zones, and are there any plans to change the setup in the future?

It makes the differentiation at the moment.
If I place a clone of the Earth in orbit around Barnard's Star at 0.0058AU, it has the same temperature as Earth in the solar system, but Barnard's appears with a visual magnitude of -24.53. If I move my Earth in to 0.025AU, Barnard's now appears at visual magnitude -26.3, but Earth's temperature is a scorching 390K, for exactly the reasons you've given.

(BTW, to seek orbits for Earth-like worlds, you should use the calculated temperature of Earth in Celestia, around 256K, which is the radiative equilibrium temperature at the cloud tops, not the greenhouse-enhanced surface temperature.)

Grant

thanks for the reply grant, I've had surgery recently so perhaps I'm not thinking as well as usual. At least I got the concept right. Indeed I do "normalize" to about 256K as that seems to be the liveable "zone" such as it is.

I got into this whole habitable zone thing long before I even heard of Celestia, and I quickly determined that there can be as many as three different 'earth-like' zones depending on what your looking for, and what measurements you make. You can key to the incoming solar flux, or the ambient temperature or the apparent brightness of the star.

I guess it took using Celestia for me to really start to see the differences. And I think I've even opined here as to weather extreme-low mass stars can even HAVE habitable zones, stars like Wolf 359, Van Biesbroeck 8 etc. In those situations you end up in a chicken-or-the-egg complex in which you sacrafice ambient heat for light, or vice versa. Either you accept scorching temperatures and sun-like lighting or you deal with a vast drop off in light and liveable temps. Either way with the red-light of those types of stars and the fact that most of them flare-up like hell, its an academic question (but interesting none the less).

So back to my current system, I'll be posting again later, thanks for the reply as always. Cheers.

If you have detected a match between the apparent visual magnitude of the parent star and the temperature of an orbiting planet, it may well date back to previous versions of Celestia, in which the bolometric correction wasn't taken into account when calculating stellar radii.
Current versions of Celestia do this much better, though a simple black body approximation is always going to be in error for very cool stars, with multiple spectral lines messing up the smooth curves of Planck's formula. And for big cool stars there's the additional problem that there is no definite edge to the stellar disc, and therefore no perfect cut-off of bright/not-bright.
Some day, hopefully, there'll be the option to specific a radius for a star of particular interest, while accepting Celestia's automatic estimate for thousands of others.

Grant

I have been a vocal proponent of being allowed to specify conditions for a star where the data is known, rathern than being forced to accept Celestia's sometimes inaccurate estimates. For example being able to specify temperature, radius and mass would be a good start. I know Chris is working hard on this so I'm sure in time the choices will be there. And I know there have been improvements made to Celestia's radius estimation system which should also help in the more immediate future of the program.

Anyway as long as development continues who can complain? Cheers!

Why not just use the normal blackbody temperature equation for the planet? It can be calculated as follows:

T = { [L.(1-A)] / [16*PI*s*(D^2)] } ^0.25

Where:

L = stellar luminosity in Watts (the sun = 3.846e26 W)
A = planet's Albedo (for a blackbody, A = 0)
PI = pi (3.14159etc)
s = Stefan-Boltzmann constant (5.67e-8 W m-2 K-4)
D = distance between planet and surface of star in metres.

The only problem is that you'd have to convert magnitudes to luminosities somehow - but it's possible to do, you'd just need to import the conversion to Celestia. Unfortunately, knowing a star's luminosity in magnitudes is utterly useless for calculating the physical properties of a system - you need to know the luminosity in watts or sols to get any practical use out of it.

You'd also have to rearrange the equation to use larger units (eg AU, solar luminosities), but either way that should give you unambiguous results. You've got the albedo in there if you need it, and the only other thing you need to add is the greenhouse effect and that should provide you with realistic temperatures.

Thanks for the informative reply, sorry for the lateness of my response. Anyway I DO have means of calculating temperature. On my spreadsheet I find many attributes of a system, as all of you know. Of those I do calculate temperature, and even adjust for albedo and greenhouse effect based upon some equations I got from a website some time ago. In any event the data I have is modeled correctly. Your point about luminosities in magnitudes being not very usefull is well taken.

I find the luminosity of stars in several ways, one via a simple equation which is as follows: 2.5^(4.85-X) where X is the absolute magnitude of the star. I also find the luminosity in Watts and ergs (when necessary). My spreadsheet is also setup to find the solar flux at a distance from a star, and I can solve for angular size and other goodies. I usually key to 288K as a comfortable mean temperature. And its not a big deal that Celestia doesn't get it exactly right, the stuff on my end works, Celestia is merely an excellent tool at representing what I create, that is, it makes it seem more real, than just the numbers on the page indicate. Cheers.

Glad it was some use

Though I would still rather that Celestia also listed the Luminosity of the stars (in Sols) by default (or at least, there'd be an option to switch between magnitudes and luminosities).

Evil Dr Ganymede wrote:Though I would still rather that Celestia also listed the Luminosity of the stars (in Sols) by default (or at least, there'd be an option to switch between magnitudes and luminosities).

Good suggestion . . . I can easily add this.

--Chris

chris wrote:

Evil Dr Ganymede wrote:Though I would still rather that Celestia also listed the Luminosity of the stars (in Sols) by default (or at least, there'd be an option to switch between magnitudes and luminosities).

Good suggestion . . . I can easily add this.

--Chris

Gawd bless ya, guv!

Evil Dr Ganymede wrote:The only problem is that you'd have to convert magnitudes to luminosities somehow ...

You can make the calculation based only on data that are currently used/provided by Celestia - you can go straight from stellar temperature to planet temperature, using the calculation Celestia currently employs:

Tplanet = Tstar * [(1-A)/4]^0.25 * [Rstar/a]^0.5

where Tstar and Rstar are the temperature and radius of the star, respectively, A is albedo and a is the semimajor axis of the planet's orbit.
The above is an estimate for a relative rapidly rotating body, or one that has good heat transfer from dayside to nightside (such as a hot Jupiter might have). If you have a body in synchronous or very slow rotation, which doesn't transfer heat effectively, dayside radiation predominates, so the equilibrium temperature tends towards:

Tplanet = Tstar * [(1-A)/2]^0.25 * [Rstar/a]^0.5

with the dayside and nightside temperatures obviously straddling this mean value.

Grant