Hypothetical Planet in the Alpha Centauri System (Book research)

[williamsusaf]

I am writing a science fiction book that includes the idea of a planet orbiting within the habitable zone of Alpha Centauri A and I'm hoping this is a good place to get some questions answered. It's science fiction, so I'm perfectly fine with stretching the limits of possibility, here, just want it to be somewhat believable.

As I understand it, a planet orbiting Alpha Centauri A would have B constantly in it's sky for much of the year, even during "night". So my first question, what would that be like? I know that B is less bright, so would it still get dark, like a night with a very bright moon? Maybe a state of constant twilight? How long would this last? Also, what would it do to the weather? Would B give off enough heat to make a significant different in the temperature? Would one sun be yellow and one red, or am I confused on their types? How would Proxima come into play?

Sorry for all of the questions, but thank you for any answers. I've looked at anything I could find on the internet, but since there haven't been any planets found in this system yet, there aren't many answers that I have been able to find.

 

[cephron]

A quick disclaimer: I have far less scientific prowess than the veteran users here, so take anything I say with a grain of salt. But I do love science fiction, and would be happy to give you my thoughts.

Reading your comments, the first questions that occurred to me were:
1. Is a stable orbit around one of these stars possible?
2. How close would a stable orbit be to the habitable zone?
So I went to Wikipedia...

Its individual component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of our Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 50.0% of its luminosity. During the stars' 79.91 year orbit about a common center, the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun.

Some thoughts on this:

1. Orbits that pass close to the midpoint between the stars will probably be less stable than orbits close into the stars. With Cen A and B sometimes only as far apart as the sun and Saturn, we want to squeeze your world's orbit as close into one star as possible. How far would be actually necessary to make it stable, I don't know.

2. If Cen A is 150% brighter than the sun, I believe that would push it's habitable zone further out than the sun's, meaning your planet would need a more distant orbit. Cen B, however, is significantly dimmer than the sun, meaning its habitable zone would be closer in. Would you perhaps consider having your world orbit B instead of A? I'm thinking that the closer your world is to its primary, the more believable a stable orbit would be.

Other thoughts:

3. Your world's orbit around its primary should be retrograde to the stars' orbit around each other. In other words, looking "down from above" on the orbital plane, you would see the stars orbiting around each other clockwise, and your world orbiting its primary counterclockwise (or vice versa). This is again for stability.

4. Supposing your world was orbiting Cen A, Cen B would be in the night sky for half the year (When the planet is moving between the stars), and in the day sky for the other half, but this is a simplification. From the perspective of a person on the surface, it would appear that the suns are "together", and "move apart" in opposite directions, until a year later, they have moved the whole way around the sky and met back up.

5. Because of the double suns, seasons would be present even without the complication of an axial tilt. When moving between the suns, the night side will always be somewhat warmed by the other star.
1/4 of a year later, when Cen A, Cen B and your world are at the corners of an L shape, there will be four parts to a single day cycle...supposing one rotation is 24 hours, then you'd have: 6 hours of night ->(strong sunrise of primary)-> 6 hours of day ->(weaker sunrise of other star)-> 6 hours of day with two suns ->(sunset of primary) -> 6 hours of day ->(sunset of other star) -> back to night again.
Finally, when your world is on the "outside" of its orbit, you have just night and double-sunned day (and single-sunned for a short time when the other star is eclipsed behing the primary, which happens annually).
So, in a nutshell, your seasons might look like this, starting each day cycle with the sunrise of the primary:

"Summer": Strong day -> Weak day
"Fall": Strong day-> Double day -> Weak day -> Night
"Winter": Double day -> Night
"Spring": Double day -> Strong day -> Night -> Weak day
(Strong day = only primary is in the sky;
Weak day = only the other star is in the sky;
Double day = both suns in the sky)

6. Having your world orbit Cen A would make the difference between strong day and weak day greater (Cen B is both dimmer and more distant), whereas orbiting Cen B would cause strong day and weak day to be more similar in intensity (Cen B is dimmer, but Cen A is more distant -> their apparent intensities would be closer).

A final note: If you don't know about them already, read up on Lagrange points, especially L4 and L5. :) These would be great places for objects (space stations, even other planets?) to orbit in a binary system. The problem is that they're way outside the habitable zone in Alpha Centauri, so you probably couldn't put your world there.EDIT: Oh, and I found this in an archived thread on another forum. It's really informative!

So the brightest the A star could get in the night sky of a planet around Alpha Centauri B is about 1/500th the brightness of the Sun; still bright, and equivalent to a thousand full moons- but barely bright enough to turn the sky dark blue. It would be very small- 3.5 arc minutes at the largest- about a tenth of the apparent diameter of the Sun (or Moon) in our own sky. Most of the time it would be smaller still- and not big enough to show a disk.

Proxima is about 4th magnitude from A+B so it isn't even a bright star; red dwarfs are very dim.

http://www.bautforum.com/archive/index.php/t-19288.html

 

[williamsusaf]

Cephron,

Thank you for such an informative reply! This will definitely help as I build the setting for my book.

My initial thought was to have the world orbit A, but I'm definitely not opposed to switching it to orbiting B. You've given me a lot to think about here! I might try to work out some models of some sort to get a good visual on it.

Thanks again for all of the information. I really appreciate the help!

I'm off to learn about Lagrange points!

 

[Janus]

One thing to consider is that the Centauri AB binary orbit is quite eccentric. The two stars can get as close as 11.2 AU and as far apart as 36.6 AU. These are about the equivalent distances of Saturn and Pluto from the Sun.

This would work out to a difference at conjunction of ~3.5 magnitudes or a factor of 25 in brightness.

 

[williamsusaf]

Thanks to you as well, Janus. This has been very helpful!

One more question - is there a computer program that would allow me model the system, something that would let me run a hypothetical orbit and maybe create a complete system to visualize? I've found several that model our Solar System and even a few that show the movement of Alpha Centauri A & B, but nothing that will let me customize the worlds.

 

[TheLetterEcks]

Several years ago while learning computer languages, I wrote a few computer programs that would simulate a gravitational system. I actually simulated a hypothetical planets around stars A and B.

Since A has 1.52 times the luminosity of our sun, by the inverse-square law an "Earth" should be about 1.23 AU away. I think I usually started by having the planet have a minimum distance (periapsis) at 1.25 AU and maximum distance (apapsis) at 1.30 AU. I found that the length of this semi-major axis (1.275 AU) of the planet didn't change much at all but the periapsis and apapsis would vary in a sinusoidal fashion with a period of perhaps a few thousand years. I cannot remember all the exact values and may do a little digging later this week but I think the periapsis did not dip much below 1.05 AU and the apapsis did not go much above 1.50 AU.

When the planet's orbit has such a high eccentricity, I would think that that would have a substantial effect on the seasons but a tilt like Earth's (23+ degrees) would have an even greater influence on seasons at regions far away from the equator.

I also remember the general results of simulating a planet around star B. By the inverse-square law, the planet needed to be much closer and as a result the tidal effects of the other star on the planet's orbit was less. That is, the eccentricity varied a lot less and the period of the variation was longer--several thousand years, I think.


Wikipedia gives the apparent brightness of the stars as viewed from a hypothetical habitable planet. I'm too tired to do any calculations right now to verify those results but they seem about right.

When star B is about 10 AU away from the hypothetical planet that orbits star A, it should contribute an extra 0.5 percent of radiation for heating. From my knowledge of blackbody radiation, that should amount to an extra 0.125 percent (or so) higher absolute temperature of the planet (or less than one degree Fahrenheit). The tilt and eccentricity should factor greater in the seasons.

 

[Chronos]

The planet's orbit would be unstable at 1AU, as Janus inferred. If it was near enough to Alpha C to maintain a stable orbit, it should be tidally locked.

 

[Janus]

Thanks to you as well, Janus. This has been very helpful!

One more question - is there a computer program that would allow me model the system, something that would let me run a hypothetical orbit and maybe create a complete system to visualize? I've found several that model our Solar System and even a few that show the movement of Alpha Centauri A & B, but nothing that will let me customize the worlds.

Try here:

http://www.orbitsimulator.com/gravity/articles/what.html

 

[williamsusaf]

Thanks for the link! I'll have to check it out when I get home. It looks like someone has created a binary system with planets around each star, so that will help give me a jumping off point.

Thanks again to everyone for all of the information and help!

 

[qraal]

The planet's orbit would be unstable at 1AU, as Janus inferred. If it was near enough to Alpha C to maintain a stable orbit, it should be tidally locked.

Where does Janus infer any such thing?