Earth's Doppelgänger

I've always fantasized about this idea, but never knew whether it was true or not.

The idea of a twin planetary system orbiting the sun has been part of science fiction for decades. At one time, people thought that Mars was like Earth, but Mars is in an orbit further out than the planet Earth. What I'm interested in is multiple planets in the same orbit. For a long time, this was considered impossible, but it turns out that it is possible.

Recently, the Kepler Space Telescope has discovered a planetary system in which two planets share the same orbit around a star. This system is labeled KOI-730, and it consists of four planets, two of which are in the same orbit with a period of 9.8 days.

This phenomenon is called a co-orbiting configuration in which two or more planets orbit a star at the same distance or two moons orbit a planet under the same conditions. In many cases the planets or moons occupy the Lagrange points, especially L4 and L5. These two points are separated by 120 degrees. I discussed this idea already.

Science fiction has always proposed that there was another habitable planet in our solar system, one that developed an intelligent race that coveted our Earth. Is such a thing possible?

For a long time this was neither proved nor disproved. The reason is that we couldn't see a planet on the other side of the sun. The sun is too large and too bright. Thus, it was only a speculation that there was a twin Earth. This argument was settled with the launch of probes that could see the side of the sun that's turned away from us. The most important of these are two STEREO (Solar Terrestrial Relations Observatory) spacecraft. These were launched October 26, 2006 and are separated 180 degrees on February 6, 2011, which allowed them to view the entire sun.

In any event, there have been enough spacecraft sent out there that if there were another Earth it would have been seen by now.
The thing to keep in mind here is that it is possible there were two planets in the same orbit around our sun in the distant past, perhaps not long after planets formed. One of these was the primordial Earth and the other was a Mars-sized planet that eventually caught up to the Earth and collided with it, forming the moon.

So, the answer is that there is no twin Earth system in our solar system. However, there are several twin planet systems like this out there in our galaxy. One has already been discovered.
I like the idea and used it in my story 'Space Chronicles, the Beginning'.

Thanks for reading.