The dawn on a planet in a binary system?

JaggedJimmyJr · Oct 20, 2010

I started one of my stories with a dawn scene on a planet in a binary system. The story is far from being a science fiction one; it tends to be more on the side of fantasy. The exact details about such a system aren’t vital etc, but the geek in me doesn’t want it to be um- a factual disaster.

Initially, I wrote it as the planet was orbiting both of these stars in the system. If I remember right (Sorry, if I’m butchering science here. I’m rather uneducated in this area.), that is sometimes called a close binary system... like Tatooine I think. And it's one of the stable position for a planet in a binary system.

Then I thought this particular dawn played an important part for the MC and therefore should be something a bit more than just two suns bundled up together. After some researching I thought it could be that the planet orbits only one of the stars.

The problem is I’m having trouble imagining the dawn on such a planet. Any of you -hopefully an astronomer, thought about this before? How would it look?

A bit off topic, but how far would you as a reader be willing to suspend your disbelief in a story like this? For example, I’m thinking of a scene with two sun eclipsing each etc. But factually for a stable orbit, one of the stars should be so far that it will most likely look like a very bright star. (Unless I’m mistaken…)

GeorgeK · Oct 20, 2010

That font that you are using is difficult for me to read. The astrophysics type people tend to hang out in the Science Fact subforum of the section Science Fiction, so if you don't get many responses you might try there. I would imagine one star relatively far away all the time, but bright enough to see during the day, and one star that is periodically massive by comparison. (My point of reference is imagining being on a Earth like moon of a much larger Saturn where Jupiter is a small and not terribly hot star and Sol is the dominant star of the binary system. Next some physicist will wander by and say, "Who let him in?"

So still imagining, I think there's three possible dawns, one with Jupiter rising, one with Sol rising and one with both dawning simultaneously. Of course maybe there's a three month darkness when moonEarth is opposite the stars on the other side of Saturn? Or, would the darkness necessarily have to be much longer?)

JaggedJimmyJr · Oct 20, 2010

Argh.. sorry about the awful font. (Fixed) It indeed is hard to read. I don't how that happened.

thothguard51 · Oct 20, 2010

I wonder if a planet could do a figure 8 orbit between the two suns.

Still, even if the planet only orbits one sun, there will be a time when the planet is between the two suns, and thus a season where there is no night, or limited night during this time period, which could be quarter of the year or more.

JaggedJimmyJr · Oct 20, 2010

I read that it is possible that a planet can do 8 figure orbit, but it's highly unstable and if I remember right, the planet is likely to be ejected from system.

thothguard51 · Oct 20, 2010

I think both suns would have to have equal influence not to tare the planet apart, provided the planet was at equal distance from both suns...

blacbird · Oct 20, 2010

Forget a figure 8 orbit. Not really feasible. But a planet in a stable orbit around a close binary star system is possible. Dawn on the planet would depend on the relative positions of the stars to each other at the moment of planetary rotation from night to day, which in turn would depend on the a person's location on the planet itself.

Drachen Jager · Oct 20, 2010

Think of the second sun as Jupiter. That's the normal pattern. Your planet would be either orbiting the second sun (which in turn is orbiting the first sun, the planet in question would be like a moon around the second sun). Or as you seem to be saying now it would be orbiting as a planet with one sun orbiting the other.

In most binary systems the second sun would not be nearly as bright as the first, it might be more like the moon. The orbital difference would make for odd sunrise/sunset, for 10% of the year or so the small sun would be blocked or so close to the primary that you wouldn't know it was there. For the remainder of the year it would probably be a bit like the moon, smaller but providing a similar amount of light to a full moon, of course the closer it is in orbits to your planet the closer it would come to being up in the middle of the night.

Drachen Jager · Oct 20, 2010

thothguard51 said:
I wonder if a planet could do a figure 8 orbit between the two suns.

Not for long enough to evolve life. At least not without some intervention.

That kind of orbit would be inherently unstable.

Lhun · Oct 21, 2010

thothguard51 said:
I think both suns would have to have equal influence not to tare the planet apart, provided the planet was at equal distance from both suns...

Because of the way gravity works, a gravitational equilibrium is never stable.

One thing to not is that even a sun that is positioned outside the planetary orbit (like Jupiter) would appear very tiny in the sky. It could be as bright as the moon, but it wouldn't be much bigger than other stars. (Well, about as big as Jupiter) Having big objects visible in the sky doesn't really work, since those would cause tremendous tidal forces. About the only scenario where you get something like that is when looking up from a moon of a gas giant.

PeterL · Oct 21, 2010

I agree with Drachen. There aren't many binary systems with both stars of equal size, and if there were, then the stable orbit would be around the center of gravity of the syste, rather than around one, or both, of the stars. For a planet to orbit both stars, the stars would have to be very small.

Lhun · Oct 21, 2010

PeterL said:
I agree with Drachen. There aren't many binary systems with both stars of equal size, and if there were, then the stable orbit would be around the center of gravity of the syste, rather than around one, or both, of the stars. For a planet to orbit both stars, the stars would have to be very small.

If you have two stars in close orbit, the planet(s) can orbit the centre of mass at a higher distance than either star, thus orbiting them both. Similarly, if you have a small star orbiting a big one, the orbit of the star can be closer (or farther) than that of the planet. Although in both these cases temperatures on the planet would probably be to low for liquid water.

PeterL · Oct 21, 2010

Lhun said:
If you have two stars in close orbit, the planet(s) can orbit the centre of mass at a higher distance than either star, thus orbiting them both. Similarly, if you have a small star orbiting a big one, the orbit of the star can be closer (or farther) than that of the planet. Although in both these cases temperatures on the planet would probably be to low for liquid water.

You are right. I didn't bother writing everything down. There is also the issue of whether the planet is capable of having life, and having a planet orbit outside both stars would mean that the planet would be too far away to be at a reasonable temperature, unless the stars were very small but if the stars were that small, then they would be cool.

I think that the Centauri system might serve as a model of most of the possibilities.

JaggedJimmyJr · Oct 21, 2010

Lhun said:
Because of the way gravity works, a gravitational equilibrium is never stable.

One thing to not is that even a sun that is positioned outside the planetary orbit (like Jupiter) would appear very tiny in the sky. It could be as bright as the moon, but it wouldn't be much bigger than other stars. (Well, about as big as Jupiter) Having big objects visible in the sky doesn't really work, since those would cause tremendous tidal forces. About the only scenario where you get something like that is when looking up from a moon of a gas giant.

Well, that's not good news for the story.

I was initially thinking of scenario where a major celestial figure (a second sun hopefully) would come close enough (every ?? year or so) to this planet that it has a considerable effect on their society.

Considering it won't be a SciFi story (although not exactly a high fantasy either... no magic etc, but plenty of weird stuff.), could I get away with a background like that even though it contradicts with scientific facts?

Lhun · Oct 21, 2010

StarTrek got away with shooting holes in the event horizon of a black hole to escape it, and it even presents itself as Science Fiction (which it isn't), so yeah, sure you can get away with it.

And a weird sky certainly fits very well with all kinds of fantasy, not only the swords and sorcery kind. One easy example for an actual use would be astrology. If the sky is actually weirdly non scientific, astrology might actually work.

Kudos for asking anyway, it is always better to know what is realistic, even if you ignore it, than it is to just make stuff up and to not know how silly it might be. (This is where i always think of the movie Sunshine, or that weird TV film where the moon broke apart)

Drachen Jager · Oct 22, 2010

Actually it could work. Mars is approximately twice it's minimum distance from Jupiter when it's at it's maximum distance. That's quite a bit of movement and it doesn't have a significant tidal effect. So if you took Jupiter and scaled it up ten or twenty times or so it would have a significant tidal impact once a year but it wouldn't be any more destructive than our moon.

It wouldn't provide the same light as the primary star but it could be significant, when it's at it's closest orbit to your Earth it would mean no real night.

As I said before the other option is to put your planet in a 'moon' orbit around a Jupiteresque sun. It would have an earth-like relationship with it's sun but more like what you're looking for with the system's primary sun.

Look at http://www.cuug.ab.ca/kmcclary/ for some orbital mechanics simulators etc. They might give you a better idea of which way to move.

JaggedJimmyJr said:
Well, that's not good news for the story.

I was initially thinking of scenario where a major celestial figure (a second sun hopefully) would come close enough (every ?? year or so) to this planet that it has a considerable effect on their society.

Considering it won't be a SciFi story (although not exactly a high fantasy either... no magic etc, but plenty of weird stuff.), could I get away with a background like that even though it contradicts with scientific facts?

Lhun · Oct 22, 2010

Drachen Jager said:
Actually it could work. Mars is approximately twice it's minimum distance from Jupiter when it's at it's maximum distance. That's quite a bit of movement and it doesn't have a significant tidal effect. So if you took Jupiter and scaled it up ten or twenty times or so it would have a significant tidal impact once a year but it wouldn't be any more destructive than our moon.

Scaling Jupiter up 10 times will still only give you a view of a small bright dot in the sky, and by then you have it up to 1% of solar mass. Not sure you can still have normal orbits around the sun in that case, especially between the sun and Jupiter. Keep in mind that Jupiter already masses more than twice as much as all other planets combined.

Drachen Jager · Oct 22, 2010

You forgot I said a Mars orbit. That would double it so 20 times, plus I said 10-20 not 10.

So, scale it up 40 times and the tides wouldn't be too crazy. Which would put it at about the size of a full moon (but much brighter of course)

http://en.wikipedia.org/wiki/Angular_diameter

Lhun · Oct 23, 2010

While i'm not sure how orbits work out if you increase jupiters mass by ten times, i'm pretty sure they're not going to work out if you increase it by 40 times. Especially not if you try to stick a planet between jupiter and the sun. Heck, the sun being orbited by a gas giant with 5% its mass might actually get a visible wobble.

PeterL · Oct 23, 2010

Lhun is right. Increasing the size of Jupiter and leaving it the same distance from the Sun would require it to orbit faster. That might not be a problem for the story, until it became much, much larger. The faster orbital speed would almost require that the diameter of the orbit be larger. The other matter is that the line between gas-giant planets and brown dwarf stars is around 10 times the mass of Jupiter. I think that having the seconnd of the inary be an over-sized gas-giant that produces some heat and light would be inteesting, especially if there are planetoids orbitting it.

blacbird · Oct 23, 2010

Real science butts in:

http://www.msnbc.msn.com/id/39803010/ns/technology_and_science-space/

Drachen Jager · Oct 25, 2010

Lhun said:
While i'm not sure how orbits work out if you increase jupiters mass by ten times, i'm pretty sure they're not going to work out if you increase it by 40 times. Especially not if you try to stick a planet between jupiter and the sun. Heck, the sun being orbited by a gas giant with 5% its mass might actually get a visible wobble.

Would you read what I said before replying sometimes? It's getting annoying.

Smiling Ted · Oct 25, 2010

A couple of points.

1. There's software that allows you to construct orbits for solar systems, so you could play with that yourself and see what you can set up. (I had a little program for my Apple a decade ago; a creative Google search would turn it up now.)

2. For an evocative description of a day on the habitable moon of a gas giant, check out Vernor Vinge's "A Deepness in the Sky."

The dawn on a planet in a binary system?

JaggedJimmyJr

GeorgeK

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JaggedJimmyJr

thothguard51

A Gentleman of a refined age...

JaggedJimmyJr

thothguard51

A Gentleman of a refined age...

blacbird

Drachen Jager

Professor of applied misanthropy

Drachen Jager

Professor of applied misanthropy

Lhun

New kid, be gentle!

PeterL

Lhun

New kid, be gentle!

PeterL

JaggedJimmyJr

Lhun

New kid, be gentle!

Drachen Jager

Professor of applied misanthropy

Lhun

New kid, be gentle!

Drachen Jager

Professor of applied misanthropy

Lhun

New kid, be gentle!

PeterL

blacbird

Drachen Jager

Professor of applied misanthropy

Smiling Ted

Ah-HA!