The solar system as a former binary-star system ?

Statistically, most stars form in groups or clusters.

Our sun's closest stellar neighbor is actually a group of 3 stars.

http://www.solstation.com/stars/alp-cent3.htm

Suppose our Sun formed with a second star like itself.

Let's say they orbited each other separated by 4 billion miles

with a period of 1,000 years.

You would be forced to answer the question - what caused the second star to leave?

The run away star should be receding from us by about 12 million miles a year or 1 light year every 500,000 years

Even if it left 5 billion years ago (the known age of the Sun)

it could only have only gotten 10 light years distance and be easily observed.

It would also have no apparent motion (i.e. no up or down or left or right motion) just motion away from us. Its spectral light would be red shifted.

At 10 LY or less it would still be in our stellar neighborhood.

I don't believe any star matching that description has been observed.

If it went nova or supernova there would be a ring or planetary nebula moving away from us in our neighborhood.

The most likely scenario is that our Sun formed as a lone star.

We've known about multi-star systems for a while and the speculation that our own solar system is/was binary has been around for a while.

How would we know our solar system used to consist of two or more close proximity stars when it was very young? I'm not sure. No doubt it would have been a very long time ago and we'd have to find some evidence that our current sun is either the result of a collision of smaller stars or that another star got flung out of the system never to be seen again, or was it?

There may be motions of our sun and solar that can be detected that implies that it is gravitationally locked with another star. This star would be dim and difficult to spot in the vastness of space. It may have a highly elliptical orbit which only brings it "near" our own sun every few million years and "near" still means very far.

Jupiter was never a candidate to become a star. It would need at least 80 times more mass to sustain nuclear fusion.

This NASA site gives a possibility of stellar systems with both planets and stars. But our Solar system is simple: only one star. Yes? Well, our system may be considered double as the mass center between Jupiter and the Sun falls outside the Sun. Although Jupiter don't radiate light, but radiate more energy (as IR) that the received by the Sun. Ganymede (Jupiter satellite) is larger than Mercury. So our system is not so far from a real double system.

If the question is about a long-time-ago real double system, Sun-Jupiter, I don't believe that. The smallest is a star, the longest is its life to be radiating light, so Jupiter is a big (quasi star) planet, but not a died star.

If the question is about a former system double or multiple, that was destroyed, how this system become simpler? Why the planets have almost circular orbits? The circular orbits look a result of a simple system.

I am not sure. My first guess has to do with the most commonly accepted theory about the origins of our solar system. Basically, a solar system starts out as a collection of dust and gases. The gases and dust coallesce into progressively larger balls according to a feedback cycle: the larger the dust and gas ball, the more dust and gas it can collect. It is like a snowball effect. In order to become a star that has the capacity for fusion, a ball of dust and gas must reach a certain size. If our solar system had another star, It is difficult to see what could have happened to a body of that size. Perhaps it could have escaped the gravity of the solar system and gone its separate way. It does not seem likely though, but I guess it can not be ruled out.

I don't think there is any evidence that our solar system was ever binary. There are different types of star systems with two or even more stars, but others, like our own, has only one sun.

There is a very slight chance that another star can come close enough to our sun that it will start orbiting us (or we to it)

I think you can rule it out! A binary star system would come into existence at the same time, there is no evidence of the remains of a star in our solar system. Jupiter came close to being a small star, but close only counts in horseshoes.

There's been unsubstatiated claims that Jupiter was part of a binary system. The nearest multiple star system of any note is Sirius which is definitely a binary system and thought to be a trinary system.

The solar system has one star and always had only one.

A star is a big thing where the heck could another one go?

I'm not exactly sure to be honest.

I'm answering cos I felt I should correct someone above me. Alpha Centauri is a multiple star system (Trinary in fact, with Alpha Centauri A & B and Proxima Centauri). Sirius is 8.6ly away so close and a significant star though.

I don't know what all the hassle is all about.

Such a simple question. So easy to resolve.

Grab the nearest Time Machine you can get hold of, go back in time say x billion years ago, and then look out the window.

If you see 2 suns there, then you're right.

If not, you're dead wrong.

Aref