Fun With: Alpha Centauri & habitable planets

[rhoenix]

This thread is all about just how feasible it would be to a) find a habitable planet in this 2.5 star system (I call it that since it's technically a binary star with a smaller sibling star tagging along), and b) what that planet might be like.

* Alpha Centauri (the biggest star) is about 10% more massive than our own sun is, with a 23% larger radius.
* Beta Centauri is 90% the total mass of our sun, and has a 14% smaller radius.
* Proxima Centauri is the tag-along star, orbiting the two larger (Alpha & Beta). It is effectively a red dwarf in size, being about 40 times more dense than our sun, but has a mass 12.5% of our sun, and a diameter one seventh that of our sun.

There several places where a theoretical habitable planet might reside around these three stars, but the end result would be pretty much the same. That planet would receive much more sunlight than anything on Earth would.

So, here are the scenario questions about the theoretical habitable planet(s) in orbit within this system:

1. On such a planet (or planets), what would surface conditions of this planet be like (given relatively similar characteristics to Earth in size, mass, and composition), due to the increase in sunlight?

2. Would this planet's composition be more or less likely to have heavy metals on this planet, given the proximity to the stellar bodies and space-borne phenomena (such as comets, asteroids, etc.)?

3. Would the habitable planets be more likely to orbit Alpha Centauri, Beta Centauri, both Alpha & Beta, or Proxima? Is it possible that this planet would even be passed around between Alpha and Beta in terms of orbit?

4. Would finding native plant life on this planet, given some semblance of atmosphere on this planet, be more or less likely due to the conditions? What about animal life?

5. What are likely characteristics of such habitable planets (in terms of detail not covered by questions 1, 2, 3 or 4)?



My thanks in advance.

[Vanas]

From what I recall, A and B don't come much closer than 40AU, so something in the haitable zone of either star would still have clearly differentiated days and nights even when the stars are at closest approach. Those nights would be rather brighter than a full moon though.
According to the wikipedia article:

During this hypothetical planet's year of 0.6(3) years, would see the intensely bright companion star circle an ecliptical path around the sky, but its illumination would not significantly affect climate nor influence plant photosynthesis.

So, I'd guess for an earth-like planet in either's habitable zone, things would be similar, but you can read more easily at night.

As for anything around Proxima, as I recall the habitable zone of similar stars is within the distance for them to get tidally locked, so you've got one of those fun 'thin stip of useful land' things going on.

[Lord of the Abyss]

As for anything around Proxima, as I recall the habitable zone of similar stars is within the distance for them to get tidally locked, so you've got one of those fun 'thin stip of useful land' things going on.

Maybe. Something I've wondered about ever since extrasolar planets actually started to be discovered. One of the surprises has been that existence of gas giants close to the star, unlike our own system. And such large planets, as we see in our own system can have planet sized moons.

So, if there's a gas giant orbiting in the habitable zone of a dim star, with an Earth sized moon, would the moon be tide locked to the gas giant instead of the star ? And if so, wouldn't that prevent the one-face-world syndrome ?

[GrandMasterTerwynn]

Alpha Centauri A and B. Don't call them Alpha and Beta Centauri, because Beta Centauri is a different starsystem entirely.

With that out of the way . . . Alpha Centauri A and B orbit a common barycenter once every 80 years or so, and never get any closer to each other than the orbit of Saturn here in the Solar System. Alpha Centauri C (Proxima Centauri,) lies roughly 1/5th of a light-year away from the binary.

So, for a planet orbiting either Alpha Centauri A or B, the answers to your questions would go thusly:
1) The companion star would be hundreds to thousands of times brighter than the full Moon on Earth. For a significant part of a habitable planet's orbit, visual astronomy would be entirely impossible. Beyond this, there would be no climatic effects.

2) The Alpha Centauri stars have higher metallicity than Sol does. So yes, it's possible that terrestrial planets orbiting them may have somewhat higher concentrations of metals than Earth does.

3) Given that Alpha Centauri B's optimal region of habitability sits about where the orbit of Venus is here in the Solar System, and A's habitable zone is centered halfway between Earth and Mars, Alpha Centauri B would be a slightly better bet. However, both stars could easily hold onto terrestrial planets as the Sun does. A planet orbiting far out enough to orbit between Alpha Centauri A and B would be an uninhabitable ball of ice. It is rather unlikely that a planet is orbiting within habitable distance of Alpha Centauri C, as the star has been studied for the last seven years, and there's nothing over 2.5x the mass of Earth orbiting within the habitable zone.

4) Depends on how much water is present. While either Alpha Centauri A or B would serve to draw comets towards each other's terrestrial planets, the wide orbit both stars describe around their common barycenter may have disrupted and dispelled the common system's Oort Cloud quite early on in the system's history, making less water available. Assuming there's enough water around, plant and animal life could have readily evolved on a habitable planet in this system.

5) The presence of a companion star would not provide enough light to affect native plant photosynthesis. The cycles that plant and animal life adopt, however, would probably be tied to the location of the companion star in the sky; since for part of the year, the primary and companion stars would appear to be close enough together that the planet will have an Earth-normal night, and increasing amounts of super-twilight until both stars appear to be opposite of one another in the sky. At which point, the concept of 'night' as we know it goes away for a while, and slowly starts to return as the two stars approach conjunction once more. Though, since the time of the planetary year at which conjunction and opposition of the two stars will change with respect to where the two stars are in their 80 year orbit, life outside the planetary tropics may adopt more Earthly seasonal cycles.

[rhoenix]

Everyone, and GrandMasterTerwynn in particular, thank you for your posts. I now have enough information to visualize the system, and life within it.

Further comments and discussion are welcome, however.

[Swindle1984]

Larry Niven tends to base his stuff on hard science (and then run away with it to amusing extremes, so it's basically hard sci-fi with a lot of space opera thrown in), and he has a planet named Wunderland in the Alpha Centauri system (plus an inhabited asteroid belt). You might look it up and see what he thinks of a hypothetical inhabitable planet in the system.

[Slacker]

They've modeled the Alpha Cent system pretty heavily, there's actually sufficent zones of stability around both of the primary stars for earth-like planets to have developed.