[katieHaylor (OP)]

Donald says:

Most solar systems have two suns. Why? Or how?
Is there a relationship between them like size, composition, spin, distance that makes it more probable?
And if our solar system formed with two suns, is there a candidate sun/star we could look for?

What do you think?

[evan_au]

Most solar systems have two suns. Why? Or how?

It is thought that stars and their (exo)planets form as groups of up to thousands, in cold dust clouds, in space.

Parts of the dust cloud collapse under their own gravity, forming a protoplanetary disk. For this to collapse to form a star, it must lose a lot of angular momentum - it is much easier to collapse if it collapses to 2 or 3 large orbiting masses, which go on to form stars. Much of the original angular momentum is retained in the orbiting stars, and less energy needs to be lost through collisions and radiation.

Because 3 or more orbiting stars are gravitationally unstable, the excess stars get ejected, leaving behind a pair of stars orbiting each other, a binary star.

Over time, the hundreds or thousands of stars formed from the same gas cloud drift apart as they orbit the galaxy, but binary stars will stay together.

Astronomers are looking for chemical signs of other stars that formed from the same gas cloud as the Sun, but they will be hard to find, as they will have drifted thousands of light-years apart in the roughy 20 orbits of the galaxy since this dust cloud dissipated. Depending on size, some of them will be dim red dwarfs and hardly changed since their formation, while larger ones would have exploded as supernova long ago.

See: https://en.wikipedia.org/wiki/Binary_star#Evolution

solar systems have two suns

As mentioned above, 3 or more orbiting bodies is gravitationally unstable - and that goes for planets, too.

A planet orbiting 2 (or more) stars is likely to be thrown out of its orbit, flung into the cold of interstellar space, or burnt up as it crashes into one of the stars. So exoplanet hunters were surprised to find some exoplanets orbiting multiple-star systems.

This can be somewhat stable if:
1. the two stars are in close orbit around each other, and the planets are orbiting far away from both of them; or...
2.the two stars are in orbits that put the two stars distant from each other, and the planets are orbiting close to one of the stars (or possibly planets around both of them).

Most solar systems

Technically, "Sol" is the Latin name for our Sun. So there is only one "Solar System", the one containing our Sun.
Planetary systems around other stars could be called "Stellar systems", or refer to exoplanets.

But I'm being pedantic - to the Romans, the Sun was without peer - but now we know that the Sun is just one star among many (and not a particularly bright one, at that).

And Planets referred to the bright objects seen moving across the star field throughout the year - but now we know that Earth is a planet (and not a particularly large one, either) and there are many exoplanets around other stars.

So "Solar Systems" may well become the common term...