A Planet with Two Suns

[Toby Frost]

Some characters are looking at a secretly-taken film of a villain. They need to deduce the rough location of the villain from the film (I'd rather they did this through observation rather than sci-fi tech).

Would I be right in thinking that a person on a planet with two suns would have two obvious shadows? And would a planet with two suns have two dawns and two sunsets?

 

[mosaix]

I suppose it depends on the relative brightness of the suns.

It's interesting though. Are you thinking that it's possible to determine the villain's location by the angle of his shadows?

 

[Toby Frost]

That hadn't been my original idea. I just thought that someone would say "Look, he's got two shadows - that must mean he's on a planet with two suns!" or something like that.

 

[HareBrain]

Would I be right in thinking that a person on a planet with two suns would have two obvious shadows?

This is true when someone is between two streetlights etc. I don't see why it would be different with suns.

 

[Venusian Broon]

Depends on the geometry of your system, some double sun with orbiting planet systems shouldn't really give you two distinct shadows at all, some might....but not all the time. One that gives you the sort of visage I think you are thinking about....probably isn't a very stable system - assuming the planet the villain is on needs the right amount of energy for life to be on it.

Also as HB points out, you don't need two suns to have two distinct shadows, you just need two relatively similar intense light sources. I'm not sure you could assume that a double shadow in a picture implies two suns (he might be next to a very strong light?)

Also double star systems aren't that rare. I believe that about 1/3 of all star systems in the milky way are double, triple and higher order star systems. So, cuts down your search a little bit, but not to 'a-ha!' levels.


But if you aren't too bothered about hard SF rigour....why not have your protagonists make such a finding and therefore find the planet?

 

[Dave]

would a planet with two suns have two dawns and two sunsets?

Someone better at planetary geometry and orbital mechanics than me should probably answer this, but I think the answer is that "it depends!" Interestingly, unlike our Sol System, most planetary systems that are being discovered by astronomers do have twin suns.

According to Google:

A circumbinary planet is a planet that orbits two stars instead of one. The two stars orbit each other in a binary system, while the planet typically orbits farther from the center of the system than either of the two stars.

In that case, if it is much further away from the two stars then they would appear almost together in the sky. There would be a double sunset and a double sunrise, however these would be very close together. You would not get one rising half a day after the other like on Tatooine. This means that the shadows would also be close together too. The "streetlights" is a good analogy, but we are standing so far up the street from the two streetlights that they are both in the same direction.

However, a planet could be locked stationary within the one of the two equilateral and stable Lagrange points - L4 or L5 - Not the L1, L2 and L3 points as they are not stable, so a planet couldn't stay there. (It would eventually be pulled away, like a marble balanced on top of a saddle.)

A planet that was locked would not turn, so it would have no sunrise or sunset. It would however, like the streetlights have light from two different directions, and so there would be two shadows. Incidently, the Moon is often bright enough to cast a shadow. However, the planet would have to be metastable - in exactly the right place. Also, would the heat from the two stars give the planet the correct "Goldilocks Zone" habitable temperatures at that particular point? It's all highly unlikely.

However, and this is where it gets more complicated and I need help. You can also have objects that orbit the L1, L2 and L3 Lagrange points in Halo orbits and in Lissajous orbits and these are more stable. The Halo orbit is periodic, the Lissajous orbit is not periodic, but neither are stable for long either, so I don't think you would find planets in these orbits, but smaller planetoids.

Much depends on if you want your story set on an Earth-like planet with an atmosphere, or if it could be a Belter mining colony, I guess.

Edit: VB replied already while I was writing this. I think what I said was correct then. He said much the same.

 

[msstice]

If this gets too sticky you could still use some cool "spacemark" like rings, distinct shapes and sizes of moons, or some vegetation that only grows in some world etc. The suns thing does look a bit technical, but I suspect most readers won't care if the science is a bit off, like "habitable planets can only exist so far from such a binary system that the suns appear as one" Presumably you already have FTL in your story, so your audience is onboard for incredible things to happen.

You would not get one rising half a day after the other like on Tatooine

At least in the first movie (ANH) the two suns set close together and we only ever see one shadow.

 

[paranoid marvin]

Have you thought of having a constellation visible in the sky that is only attainable from a certain planet?

 

[Wayne Mack]

I'd guess there are two basic scenarios. One the planet orbits in a figure eight and swaps which sun it is orbiting. Two, would be an extended oval around both.

In either situation, there will be situations where two, one, or no suns would be visible. In cases where both are visible, one sun would be significantly closer and therefore larger than the other. Without also knowing a time, I'm not sure that one could easily determine the longitude and latitude of the villain. Know the time and the angle of either of the two suns or the villain's height and the length of at least one of his or her shadows would likely lead to a certain arc on which the villain would be located.

 

[tinkerdan]

Try this: Take two light sources aimed at some objects on a table and you should be able to draw some conclusions.
You can even Walk one source around the other to see the effect.
I just did that before answering--it's pretty interesting.
A professional photographer might be able to tell you how and why multiple sources can cancel out shadows when you don't want them.

 

[BAYLOR]

A planet with two suns is not likely to be hospitable to life as we know it . Even if the both suns were the class yellow suns like out sun, the orbiting planet get twice the solar output that Earth gets , which would cook the planet .

 

[Dave]

if the both suns were the class yellow suns like out sun, the orbiting planet get twice the solar output that Earth gets , which would cook the planet .

If both Stars were identical in size and solar output to our Sun, and if they were for practical purposes at the same point, then the average orbit of the planet would only need to be a further distance away, a distance calculated using from using the intensity as being inversely proportional to the square of the distance from the source.

 

[Swank]

You want to write a story where two suns make two shadows? You can. No astronomers will protest.

The same effect can come from standing near a glass building that reflects the sun from the opposite side of the person, so you may want to set the film away from large reflective objects.

 

[chrispenycate]

Lot of ways of getting a double shadow. If you got your binary system by heating up Jupiter until it became a small star, (as Arthur C. Clarke in his obelisk series), you'd almost certainly get differing colours of shadow as well as different intensities. But you can do the same thing with reflections - if you want to warm up a somewhat cold planet you might orbit a very big mirror (Kim Stanley Robinson's polychrome mars series, or Louise McMaster Bujold's 'Komarr') you'll get a double shadow some of the time, as you might with a moon with a higher albedo than ours. How about a ringed planet, or

But you don't even need orbital technology. A big, glass-windowed building in a city reflects enough light to make two quite distinct shadows of everything, or a rocket blasting off from a planet can generate as much light as is arriving from the star officially illuminating the world, just by proximity. So could a big enough meteor, or a relatively close supernova on a less well inhabited planet.

None of these are fixed effects, but they can all last long enough to be confused with each other on a short film extract, or a fixed photo.

 

[Wayne Mack]

I've come with a different answer than I gave before. With a rotating planet, I'm not sure that two suns give any longitudinal information; the same orientation of the suns would be visible . The angle of the shadows could help derive a latitude. The difference in solar angles would be greatest when the planet is nearest the dividing line between the two suns and non-existent at the far reaches of the orbit. To make things easier, assume the villain's photograph was taken when the planet was at the midpoint between the two suns (and it is not a figure eight orbit). At noon, on the equator, the two shadows would be inline in opposite directions. Further to the north, the shadows would twist towards each other angling northward. The same would be true to the south. By comparing the relative lengths of the shadows, one might also be able to determine the time of day.

Uneven surfaces would distort the measures, so the villain might need to be a flat, level surface; perhaps a street or a paved lot.

I don't think that shadows alone will be sufficient to identify the villain's location on the planet. It may provide some help, but additional data points would be needed. I don't think any sky-based phenomena would help as the planet's rotation would bring the same overhead view throughout the day.