This is called an hypothesis which is quite a bit lower than a scientific theory. However it makes sense.

hypothesis: An educated guess, based on observation. Usually, a hypothesis can be supported or refuted through experimentation or more observation.

theory: One definition of a theory is to say it's an accepted hypothesis.

Theory according to the National Academy of Sciences: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses. The contention that evolution should be taught as a "theory, not as a fact" confuses the common use of these words with the scientific use. In science, theories do not turn into facts through the accumulation of evidence. Rather, theories are the end points of science. They are understandings that develop from extensive observation, experimentation, and creative reflection. They incorporate a large body of scientific facts, laws, tested hypotheses, and logical inferences. In this sense, evolution is one of the strongest and most useful scientific theories we have.

Earth might have been spared from a collision with Mars or Venus by the process in which a fifth giant world was ejected from our solar system.

Please read it. It's interesting.

This is a quite a bit more than an idea. Much research went into it. While it may not be true it's a reasonable idea to explain an unsolved problem.

What can be called true is a scientific theory, especially an extremely strong theory like evolution.

What's unfortunate is most non-scientists confuse the two different definitions of theory. The definition that is NOT used by science is theory = wild guess, but that's the definition science deniers like to use to justify their childish belief in their favorite alternative to scientific facts like evolution. They prefer to invoke their fairy's magic wand for everything. I suppose they have to do this to justify the magical fantasy land they live in. It's too bad for them they have to be stupid and/or dishonest to live in their Dark Ages.