There is a theory that our Sun was not born alone 4.5 Billion years ago. Some believe our Sun has a twin called Nemesis, a brown dwarf star and therefore as it is brown, it is hard to spot and hasn't been discovered yet. The first brown dwarf star that's been discovered was Teide 1 in the constellation of Taurus in 1989. Ref Wiki.
According to analysis by University of California at Berkeley, our Sun and all other stars of its type and size was born in pairs or more. This poses a question, where is our twin star? Ref: Berkeley
The theory of Nemesis isn't new, the theory has been around for a while. Scientists studying the Perseus stellar nursery looked at the stars and ran a number of statistical models. They found that stars like our Sun always formed in pairs or more. Over time, the stars would either break free or get closer to one another. The stars would form at a distance of about 500 A.U. An A.U. is the distance between the Earth and the Sun. 500 AU is also equivalent to seventeen times the distance from Earth and the Neptune, the furthest planet so far discovered planet, note, Pluto is not classed as a planet anymore. They predict that we would never be able to fully identify which star is our twin. Ref: Engadget
The Nemesis star could have been unstable when it was created and went supernova straightaway. A star when it is born will have a huge amount of hydrogen which through nuclear fusion will turn it into helium. When the hydrogen has run out, it will use helium as a fuel until all that has run out. It will carry on until it has turned everything to iron and will not be able to go on further therefore go supernova.
Elements higher than iron such as gold and uranium, they can only be created in a supernova explosion. Therefore the elements higher than iron would have been created by a supernova explosion. Therefore, it could well be that our twin star has died already and the planets such as Earth is the remnants of the death of the twin star? Science Learn
Nemesis is the name given to a hypothetical twin star of the Sun. Nemesis is believed to be in orbit in the Milky way that brings it close to the Sun every 26 million years. This coincides with roughly mass extinctions that have happened over the last 500 million years. It is believed that Nemesis could be responsible for the death of the dinosaurs.
The name Nemesis comes from the mythological goddess of divine retribution. Nemesis is also used to define someone that someone is not able to overcome, an opponent, an enemy. Ref: Dictionary.
In terms of the dinosaurs', it is believed that the gravitational power of the brown dwarf star forced a comet or asteroid in the Oort Cloud to break free and head towards Earth. The object crash landed in the Yucatan Peninsula in modern day Mexico causing the extinction of the dinosaurs and therefore heralding the future age of the mammals such as us.
Nemesis could also explain the strange and wide orbit of Sedna, which at the time of its discovery in 2003 was the furthest object discovered. The V774104 is as of 2015, the furthest object from Earth in the solar system. It is 103 times the distance between Earth and the Sun with Sedna being 96 times the distance. The theorized Nemesis would be located at roughly about 1.5 light years outside our solar system. The currently recognised nearest star is Proxima Centauri which is roughly 4 light years away. Proxima Centauri is in a group of stars that also contains Rigil Kentaurus which is better known as Alpha Centauri. Business Insider
If our Sun did have a twin, where is it. Jupiter is sometimes referred to as a failed star because it doesn't have sufficient mass in order to start nuclear fusion. It is known that the smallest star so far discovered in our Galaxy is indeed smaller than Jupiter, that star is referred to as EBLM J0555-57Ab in the constellation of Pictor. The star was discovered by researchers at the University of Cambridge looking for exoplanets. The star is just slightly smaller than Saturn which is in turn smaller than Jupiter. Likewise, all the other gas giants could be classed as failed stars because they are gas giants but don't have sufficient mass to start nuclear fusion.
Another theory for the comets rather than a star is Tyche, a gas giant planet that is hidden in the Oort Cloud. Tyche was hypothesized in 1999 to explain why comets tend to cluster rather than disperse randomly. The Oort Cloud hasn't been even been proven yet therefore Tyche is nowhere near being proven either. Owlcation
A little closer to home is the theory of a ninth planet. Pluto had been known as the ninth planet since its discovery until it was downgraded to dwarf planet by the IAU. Professor Michael Brown and assistant professor Konstantin Batygin believe there could be a new ninth planet out there based on orbits of Trans-Neptunian Objects (TNOs) which are objects in space that orbit further out from the Sun than the planet Neptune. Ref: CalTech.
The picture is based on similar pictures done by Caltech. The green circle extending to the right is the hypothesized orbit of the ninth planet. The red orbit to the right is 2013FT28, one of the few objects whose orbit is different to all other TNOs such as Sedna.