Celestia Forums

I'm trying to make a list of stars that might reasonably be considered habitable by humans, and I'm not sure of the best criteria to use to make this list. I'd kind of arbitrarily adopted the criteria of 0.5 Rsun or larger, and of those stars, 4,000 K or hotter. Before I get too far into the list (I'm trying to get all the stars within maybe 50 ly of Sol), I thought I'd better make sure I'm using a good system of criteria.

It occurs to me first of all that temperature might actually be a lot more important than radius, for example.

I basically just want to be able to say something like, "There's a fairly good chance that a planet pretty similar to Earth might be able to orbit somewhere around this star," so the criteria really don't have to be too strict.

Can anybody here suggest a good set of criteria I should use?

Also, since I'm using Celestia to compile this list, should I worry about any innacuracies in the star data Celestia displays?

I have some *general* statistics that I use, although I say general because some of the statistics are slightly different for different types of stars. Basically, I use these when assuming a habitable world with a mature ecosystem (that is, an ecosystem that has made the world a blue and green marble such as Earth):

Age: 3.5 to 5.0 billion years
Spectral Type: F8 V to K3 V
Luminosity: 0.4 to 2.0
Metallicity: 0.3 on up (possible cut-off at 2.0)
Known Planets: No epistellar jovians

Now, some of these depend on one another. For instance, there are spome pretty good odds that a star with a luminosity of 1.8, for example, might be an F7 V spectral type. So it passes in one regard, and fails in another.

My advice is to use a list such as this, and then make your own judgments. After all, a star beyond K3 V could well have a habitable planet in a close orbit, as there are theoretical ways to get around the issue of tide locking. And in that case, much of the criteria list goes out the window. Also, you could theoretically have a habitable planet in orbit of a subgiant star, although it is unlikely that advanced multicellular life would have much time to evolve (think of Mars when the Sun begins to move off of the main sequence).

Hope this helps somewhat.

...John...

If you haven't already done so, you might take a look at http://www.solstation.com/habitable.htm.
It has a discussion of some of the relevant parameters.

Such discussions of habitability usually assume that red dwarf stars cannot have habitable planets. Red dwarf stars have the following limitations:

• Planets that are close enough to the red dwarf to be habitable would be tidally locked to the star. This could cause the atmosphere and water to freeze out on the night side.
• Many red dwarf stars have stellar flares that could be dangerous.
• Photosynthesis of Earth-type plants would be less efficient.

However, recent research suggests that red dwarf stars may not be such a bad place to live after all.

• A sufficiently thick atmosphere could allow enough heat to be carried to the nightside to prevent the atmosphere from freezing out. 15% of Earth's atmospheric pressure could be enough.
• If the nightside has deep ocean basins, the ice could melt at the bottom due to geothermal heat. This would allow water to return to the dayside.
• Some red dwarf stars don't flare. Flaring is primarily seen in younger stars, so the older a red dwarf star is, the less likely it will flare.
• The lack of photosynthetic efficiency would be mitigated by the plant being in constant light. This would not pose a problem for plants that don't care about the day/night cycle and the seasons.

Therefore, when preparing such a list, consider including nonvariable red dwarf stars. While the conditions for habitability may not make red dwarf stars into prime real estate, the time will come when habitable planets around red dwarf stars are all that remain after the larger stars have burned out.

You might want to consider SETI's HabCat, which can be obtained here.

In regards to the question of habitable red dwarfs....

If one assumes this (and I think there is some solid science behind it to indeed assume this), then that also should open up the way for habitable stars around all of the K-type stars as well. They may be rare in the sense of specific stars, but with the sheer number available in the galaxy they may be common indeed.

...John...

Just to reiterate (or make clear), not all stars are going to be conducive to habitability in a Terran sense- That is, stars like Proxima Centauri, which shoot off superflares, are going to be quite hostile.

It was National Geographic's Extraterrestrial that offers a proposal for a habitable, stable red dwarf.

(I hope this gets out on DVD, sometime.)

d.m.f.

Thanks a lot guys. This is all really helpful.

Does the Theory of Evolution matter in your habitable planets list. What I mean is; Are you looking for planets to host Earth's humans? The reason I ask is because the Theory of Evolution suggests that life will evolve and adapt to it's surroundings. Not vice-versa. If other planets have different types of atmosheres and different lengths of days, different temperatures, etc. Evolution suggests that life will evolve according to it's environment.
Also, everything we actually know about evolution is indigenous to this planet alone! Could it be possible for a reversal of habitat, say, human-like beings breathing carbon dioxide and plants breathing oxygen? Or intelligent beings that breath methane? All the standards by which our understanding of evolution operate, are based on what we know of our own planet. For instance; What is the possibilty that our instruments used in the detection of extraterrestrial life will fail, since they are set up to detect life as we know it here on Earth?! What if life on other planets doesn't consist of familiar norms that we have come to accept on this planet? What if single celled organisms have a different molecular structure that is governed by the elements and science of the solar system it evolved in? I apologize if I'm taking this deeper than it was meant to go but, these possibilities are relevant to me. I don't know the standards by which you must hold your survey but, these variables should widen the habitable field for you.

Well, what I'm trying to do is basically make a very rudimentary roadmap of the places humans might go if and when we start moving to other stars using Celestia. I'm making a lot of assumptions about things I don't really understand (which is fine with me for my purposes), such as how easy it would be for humans to adapt to slightly different atmospheric compositions, temperatures, and day length and rugularity. But yes, basically for this map I'm trying to make it human-specific.

But I also think the things you talked about are very interesting, and I do a lot of sporadic thinking on my own about just such topics. I'm still waiting to see an alien concept that isn't quite clearly based on some kind of stylization of creatures that live on Earth. I say, if it has two eyes above a mouth, it's not a real alien.