system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How we know that there isn't another planet in the exact same spot as Earth on the other side of the Sun. Like 6 Months behind?
gpt
Pick up a bucket of water and spin around, holding it out at arms length.  Notice how you're wobbling as you spin?

Now, take two buckets of water, one in each hand, and spin around.  They are balancing each other, and you are no longer wobbling.

The sun is "wobbling" because of all of the planets that circle it.  Scientists are able to measure the amount of wobble of the sun, and knowing the mass of each planet, can calculate that the Earth is also causing the Sun to wobble.  If it weren't, it would be because there is  an equal mass on the other side of the sun.  However, it is, so at the very least, there is no equal mass there.