Would the solar wind knock a planet off its orbit if the planet didn't have a magnetic field?

Nope! I think people give far too much credit to what magnetic fields can do that such fields deserve. The magnetic field of the earth does divert solar wind particles away from the earth, but both this field and the wind are currently too weak to even affect a tiny Space Shuttle. The density of the wind is only about 5 atoms per cubic centimeter! It constitutes a far better vacuum than what we can typically generate under laboratory conditions ( typically 10,000 or more atoms/cc).

There is only one time when the wind was far more significant, and that was when the solar system was less than about 10 million years old. The protoplanetary nebula was dense enough to provide friction against the planets...even jupiter-sized...that had just formed, and perhaps many planets formed and 'slid' into the sun. The solar wind was far denser...by many billions of times than it is today, so at THAT time, planetary motion was regulated and controlled both by solar wind and nebular gas interactions.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the