Any answer to this question will depend on how much precision you’re requiring. A perfect straight line is an infinitesimally small target, so you’ll never actually hit it. Something “close enough” to a straight line, you might get, but how close is “close enough”?
I imagine “close enough” would be that you could draw a line that passes through some part of each planetary body, as viewed “from above” (I do not believe that there is an actual straight line path that intersects all of the orbits, but I could be wrong). Of course, determining what constitutes the “body” of the outer giant planets (if excluding the atmosphere) could be fraught.
I would think, if a protoplanet began accumulating primordial gas and dust from the early solar system, the protoplanet would essentially encounter friction as it orbited, slowing down, and, thus, slowly spiral inward towards the Sun, eventually stopping at a stable orbit when the orbital debris had been cleared away from the planet’s trajectory.
Actually it is the opposite; planets tend to form nearer the star where the protoplanetary disk is most dense and migrate outward though a combination of near-collision momentum transfers and the protoplanetary medium being pushed outward by solar wind and heating. The rings or arcs that planets form from will generally be moving faster than the body itself as they accrete, transferring momentum to the body which provides a gentle impulse to move outward. Of course, there are also interactions between bodies that throw planets back inward (which probably explains why the Moon-sized Mercury is so close to the Sun, and there are certainly radical dynamics that can create the wide diversity we see in exoplanetary systems whose mechanisms we can only begin to guess at.
Stranger
Orbits work the opposite way. Objects in lower orbits move faster, in higher orbits they move slower. There is the strange case of Saturn’s moons Janus and Epimethus, which almost move in the same orbit. The moon in the lower orbit moves faster, slowly catching up to the moon in the higher orbit. When they get close, the lower moon pulls the upper moon down while the upper moon pulls the lower moon up. They swap positions and the new lower moon advances away from the new upper moon until about 4 years or so later, it catches up and they trade places again.
Yes; for example the Moon is slowly moving away from the Earth a few centimeters a year.
That’s the counter-intuitive nature of orbital mechanics: when an orbiting spacecraft fires its engines to move faster within its orbit, it spirals out to a higher but slower orbital velocity.
Firing its engines to slow down, the craft spirals down into a low, fast orbit.
To speed up, slow down. To slow down, speed up.
This is why you just can’t fly straight to the International Space Station: you must plot a course full of strange zig-zags which could take days to fly.
Why does it take so long to rendezvous with the ISS? – Orion blog