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View Full Version : If a planet is in a two-star system then.....



Plat
2003-Nov-10, 03:59 AM
its daytime all the time, right?

that must suck

Jpax2003
2003-Nov-10, 04:34 AM
It depends on how close the two stars are. If they are so far away that the planet is locked between them at a lagrange point then they would have sunlight from both directions. However, it's probably more likely that the planet would orbit the two stars more distantly, and the stars' center of gravity would be treated as one point. This way, it would have a day-night type cycle (cf. Tatooine in Star Wars, twin setting suns). It would also probably have 2 or more cooling cycles when the suns eclipse each other by planetary revolution. But then, as the stars revolve around each other it may be more frequent. Both assume the stars revolutions are on the same plane as the planet's orbit. But based on distance necessary for earth like life, polar tilt would still be more important.

Unless the two stars were really far apart (several AU) I would not want to be on a planet locked at their L1 point, I also wonder what would be the effect of two solar winds colliding at that point. Would it create a magnetic flux or something that would preclude life on a planet in that region?

Perhaps it would be better to orbit at the L4 or L5 point, then you would have an extended day, but at least some night (1/3 rotation "darkness"). And if that planet had a moon, you might get two eclipses in a few weeks time. Better yet, an earthlike planet could orbit a larger jupiter type and then it could have a dark week or more. Of course this all depends on orbital period, distance and inclination. effects of which, I am sorry to say, I am not an expert on.

I would very much like to know what a more qualified person's thoughts are on orbital dances such as these. I would be interested in writting stories predicated on such a solar system.

Andromeda321
2003-Nov-10, 01:23 PM
If it WAS daytime all the time life would adapt pretty easily anyway. Anyone here ever read Asimov's Nightfall?

R.A.F.
2003-Nov-10, 01:35 PM
Asimov? who's he? Just kidding. :)

Actually the "good doctor" wrote a science essay addressing this exact point. IIRC, In that essay he had the 2 Suns separated by a distance equal to the distance of our Sun from the Planet Uranus. The 2nd Sun appeared (only) as a very bright point of light. I'll have to dig up that book and take another look at it to be sure.

Kaptain K
2003-Nov-10, 01:52 PM
There are two basic viable scenarios; Close binary and wide binary.

For a planet orbiting a close binary pair, There would be normal day and night, but objects would cast double shadows (assuming that the stars were of roughly equal brightness).

For a planet orbiting one of the stars of a wide binary, There would also be a normal day and night, but for part (most) of the year, the night would not be totally dark, because of the bright star in the sky.

Madcat
2003-Nov-10, 02:33 PM
I don't mean to duck the question, but I've heard it said that binary systems aren't very good places to have a planet with a stable orbit and temperature. I'm not sure that one would be habitable long enough for life to form. :-? Of course I guess that the way the system forms in the first place has a lot to do with that.

Jpax2003
2003-Nov-12, 02:58 AM
There are two basic viable scenarios; Close binary and wide binary.

For a planet orbiting a close binary pair, There would be normal day and night, but objects would cast double shadows (assuming that the stars were of roughly equal brightness).

For a planet orbiting one of the stars of a wide binary, There would also be a normal day and night, but for part (most) of the year, the night would not be totally dark, because of the bright star in the sky.

Hey I had forgotten about that. I was stuck on lagrange points or a distant orbit, I forgot that a binary might be wide enough that the planet might orbit one star. Would the other star be stable, kinda like a big Jupiter or the proposed Nemesis. How would it affect the orbits of the planets. Would the second star have planets of it's own? Would these orbit the opposite direction of the first star's planets? Or would their orbits be head on (Imagine such a collision!) If they both had large enough planets close enough and in, say, counter-clockwise revolutions, would we be able to have a smaller planet orbiting at the Stars' L1 point with transfer of momentum to the L1 planet by the perturbations cause by the jupiter type planets' tidal effects in a synchronicity?

Just a thought.

Sister Ray
2003-Nov-12, 02:24 PM
I'd like to mention that in the Asimov story, it wasn't a binary system. There were either five or six stars. I don't think that all of them were ever out at once, though. He mentions in the story three or four being out at once, but I suppose the rotation of the planet kept at least one start out of the sky.

Haven't they discovered planets in a few binary systems? Whether they could hold life or not I have no idea.

frenat
2003-Nov-12, 02:53 PM
Nightfall by Asimov involved a planet with 6 suns of which, though the orbits are never described, 5 could be in the sky at the same time. The last one would occasionally be alone. There was also a moon that remained unknown as it was drowned out by the glare of the suns that once every few thousand years would eclipse this sun on one of the occasions it happened to be alone in the sky. For a population used to constant light darkness was a deeply ingrained fear. Great book, you should all give it a read.

eburacum45
2003-Nov-12, 03:45 PM
An example of a nearby star with a planet and a second star is Gamma Cephei;
the second star is a red dwarf at a distance of 21 au, 21 times the distance of the Sun from the Earth;
This star is small must be at least eighty times the mass of Jupiter, and it will probably be several times the brightness of the moon.

More interesting would be any possible planets around a double star like Alpha Centauri;
(quote from here http://www.solstation.com/stars/alp-cent3.htm )

In a binary system, a planet must not be located too far away from its "home" star or its orbit will be unstable. If that distance exceeds about one fifth of the closest approach of the other star, then the gravitational pull of that second star can disrupt the orbit of the planet. Recent numerical integrations, however, suggest that stable planetary orbits exist: within three AUs (four AUs for retrograde orbits) of either Alpha Centauri A or B in the plane of the binary's orbit; only as far as 0.23 AU for 90-degree inclined orbits; and beyond 70 AUs for planets circling both stars (Weigert and Holman, 1997). Hence, under optimal conditions, either Alpha Centauri A and B could hold four inner rocky planets like the Solar System: Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AUs). (/quote)
So both stars could have Earth-like planets;
from a planet around the bright star, the other one would appear mag -21;
from a planet around the dimmer star, the other one would appear mag -22

our own sun appears at mag -26.7 to us, which is very much brighter on this scale.

Jpax2003
2003-Nov-13, 02:54 AM
Would it be possible to have a stable figure-8 orbit with two stars and only one planet in that exchange? Would we call that a covalent orbit?

Even if we had inernal planets, and even one figure-8, could we also have distant planets orbiting both stars? Could we have all this plus Lagrange point planets? Would a figure-8 preclude an L1 orbital location? Would the figure-8 be highly eccentric? Would there need to be a large L2 and/or L3 planet to assist the figure-8 planet in transition?

just wondering.

Eroica
2003-Nov-13, 08:58 AM
This was discussed recently in another thread (http://www.badastronomy.com/phpBB/viewtopic.php?t=8361), if you're interested.

Bad Fisherman
2003-Nov-16, 11:48 PM
[quote="Jpax2003"]Would it be possible to have a stable figure-8 orbit with two stars and only one planet in that exchange?


No, as the acceleration that would occure in the transition from one to the other, being that one would have to have a much greater mass than the other in that the requient mass required to 'capture' an orbiting object from a 'neighbor' would preclude this from happening more than once. At least this sounds good to me. :roll: