World Around 2 Suns

[Landesire]

I'm writing a fiction novel, and 1/2 the story takes place on another planet. I've written about 8 chapters so far, but I'm getting to a point where I really need to have a handle on when it's daylight vs night time and what the seasons are like. I'm hoping someone can help me with developing realistic-ish calculations so I can determine how long a day is, and how long a year is.

This planet is much bigger than earth. I have been writing it as 10x bigger, but I'm not sure it can be and be inhabitable, so I'm willing to scale it down some. Instead of lakes and seas all over the place, it has 1 body of water around the center of the planet, like a belt. The planet rotates around 2 suns, one much bigger than the other, and in a figure 8 motion.

I haven't decided on moons yet, so they could be a factor in how the planet slingshots around both suns back and forth. If anyone can help me with this, I'd really love some help.

Thanks!

Landesire.

 

[pmmg]

You can certainly write it that way but i think this would be near impossible to find in nature. It would have to pass right on a line of equal gravity between the two every time.

It would aslo be blasted with light when between the two making for very long hot days.

 

[ThinkerX]

Ok...first issue...which leads to other issues.

A planet ten times Earths size would be what astronomers now call a 'Mini-Neptune' - basically the smallest size of gas giant. Not habitable.

'Super-Earths' are rocky, Earth-like planets that can have up to about three times the mass of the earth, and maybe twice the diameter. There are a fair number of unknowns here, but these worlds likely have a much lighter density than does Earth. Case in point, Earth and Venus are almost the same size, and almost the same density. Mars, though, is much less dense than Earth. This likely means that elements abundant on Earth would be much less abundant on a planet with a lower density. This might include such basics as iron. Not altogether gone, just scarce. Likewise, the heavier elements would probably be extremely scarce.

The second issue is the suns. If they are what is termed a 'contact binary' - that is, one orbits about the other in say...two or three days, then given the requisite distance for the planet to be habitable, what you end up with is a spectacular double sunrise/double sunset. (I believe, but am not certain, that astronomers have discovered planets in one or two such systems.)

Then there are 'wide binaries,' where one star orbits the other at least twenty astronomical units out (any closer and there would be issues with planetary orbits.) One Astronomical Unit = the distance between Earth and the Sun. In that case, the sunrise/sunset cycle is normal enough for the planet. The second star would be visible - even in daylight - but the light would be on the faint side - kind of a 'twilight' situation.

There are two more things to take into account with binary and multiple star systems that might affect your story.

First, the second star will almost certainly be far less massive than the primary. The vast majority of stars in our galaxy are not yellowish 'G' stars like our sun, but far fainter 'M' class red dwarfs. Even the brightest of M dwarfs put out less than 2% the luminosity of Sol; if the sun were to be represented by a 100 watt lightbulb, then the equivalent M dwarf would be a (red) LED instrument light. 'K' stars are intermediate between 'G' and 'K,' using the light bulb comparison, they'd be (orange) Christmas tree lights.

If your system is a wide binary with the planet orbiting a G primary and the second star is an M dwarf at an average of 30 Astronomical Units, it will literally be nothing more than a very bright star - roughly on a par with the full moon. If it is a K star...it might show a disc and will provide some light, but no heat to speak of.

Additionally, the wider binary star orbits tend to have 'eccentricities' of 30% of or more. Hence, at one point in its orbit, the star could be 30 Astronomical Units out, but at nearest approach it might be only 20 Astronomical Units. These orbits tend to take multiple decades, sometimes centuries. (I've seen a few with orbital periods estimated at several thousand years.) The sky watchers on your world would be well aware of this.

 

[Prince of Spires]

There are more galaxies out there then there are grains of sand on earth, and each has more stars in it than grains of sand. Which means that anything that can possibly occur will occur somewhere in the universe. Which gives you a pretty big space to play in.

Having said that, planets don't move in a figure 8 motion. I can't come up with a single scenario where they would. It would mean the planet would have to escape the gravitational pull of on star enough to change direction and do this on a constant basis.

In your scenario, where you have 1 star much larger than the other, the smaller star would in effect rotate around the larger one. Yes, the larger one would wobble significantly, but it would be more like Jupiter orbiting the sun. (they actually both would rotate around a shared point, but that point would be much closer to the heavier star).

I think there are 2 stable scenario's. The first is that the planet orbits both stars. As in, you have 2 stars relatively close together, and the planet is much farther out. In the other, stars are very far apart, and the planet passes between them. This would be more like our solar system, where we have Jupiter, except of course that the star would be bigger and farther away.

A scenario where the stars are relatively close together and the planet passes between them would simply see the planet be pulled apart by gravity.

Of course, it's your world, so you can do what you want.

 

[Penpilot]

Having said that, planets don't move in a figure 8 motion. I can't come up with a single scenario where they would. It would mean the planet would have to escape the gravitational pull of on star enough to change direction and do this on a constant basis.

I have no idea about the plausibility of a planet doing this, but I do have an example of a book that has a planet that orbits three stars. It's called the Three-Body Problem, by Liu Cixin. It's the first book in the Remembrance of Earth's Past series, which I believe is considered Hard Scifi. And part of the book/series deals with what kind of species/society would develop under such conditions. The conditions being incredibly unstable and chaotic.

Though I have mixed feelings about the series, it's has been incredibly well received.

 

[Landesire]

Wow, thank you all for the replies! This is so much information to help me make some decisions. Once I do, I will likely post back to see if you all can help with some ideas for size and speed, so I can create the length of a day, season, year, etc.

I have been planning to introduce some elements that are not on earth or known to people on earth. Their density and composition could help to create the conditions for a larger planet to be stable, but again this is not something I am an expert at, so I don't want to miss any important factors.

 

[Prince of Spires]

I have no idea about the plausibility of a planet doing this, but I do have an example of a book that has a planet that orbits three stars. It's called the Three-Body Problem, by Liu Cixin. It's the first book in the Remembrance of Earth's Past series, which I believe is considered Hard Scifi. And part of the book/series deals with what kind of species/society would develop under such conditions. The conditions being incredibly unstable and chaotic.

I haven't read that. Though the blurb seems to suggest that the world is placed in some kind of computer simulation.

A planet can for a very brief time orbit in a kind of figure-8 orbit, but that orbit would not be stable and quickly result in the planet being ejected into outer space, crashing into one of the stars, or being ripped apart by the gravitational pull of the two stars. And brief time here would be very brief on a cosmic scale (as in years). Not long enough for life to evolve.

There are 3 possible orbits for binary stars, either around both stars simultaneously, or around one of the two stars, with the other being far away. The third, surprisingly enough, is actually in the orbit of the smaller star. If the size difference between both stars is big enough, then you can have the planet gravitationally locked at the Lagrangian points, 60 degrees ahead of or behind the smaller star. Physics is funny...

 

[pmmg]

If its a computer sim, i would accept its figure 8 pattern. That would not be occurring in nature.

 

[Penpilot]

I haven't read that. Though the blurb seems to suggest that the world is placed in some kind of computer simulation.

It's not a computer sim. Though a computer sim plays a part in understanding the species that inhabits that crazy world.

I don't dispute the improbably/impossible nature of said planet. I'm just pointing out there is successful fiction out there that has this impossible element as a significant part of the story. I mean, to be fair, that same author wrote a book called the Wandering Earth. It was made into a movie. It basically involves the Earth escaping the Sun as it expands into a red giant by turning Earth into what is essentially a giant rocket ship by attaching 10k rockets to one side of the planet and using Jupiter's gravity to slingshot Earth to another star system.

A lot of stories require the reader to accept certain impossible things in the story's general premise in order for the story to function. If the reader can suspend disbelief for that, then the rest of the story will work. Eg. What if magic was real? What if people could have superpowers? What if we could travel faster than light? Stuff like that. What if a planet orbited two stars?

A lot of it, IMHO, is knowing the type of story you're telling and to not misrepresent the story as something it's not to the reader. That's what gets books thrown across the room.