The sun doesn't move much, but it does lean slightly toward Jupiter as the planet revolves. Our alien astronomer 30 light-years away cannot see the planet, but maybe he can see the sun move a bit. That's exactly what Marcy has done. He developed a technique to catch stars leaning toward their planets as the planets revolve. This Doppler-shift phenomenon happens because the sound waves bunch up against each other like a compressed Slinky on the approach, and then spread out like an extended Slinky on the retreat.
Light waves do the same thing.
In late 1995, Marcy was scooped by Michel Mayor, a Swiss scientist who used the same technique of looking for the Doppler shift and found the first confirmed extrasolar planet, orbiting a star called 51 Pegasi in the constellation Pegasus. Nevertheless, Marcy's years of persistence collecting light and improving his instruments quickly set him ahead. He announced two planets in early 1996, giving him two out of three on the planet chart. Four years later, he still holds the lead with the same ratio and no chance of being overtaken. An orbiting planet was the simplest explanation for wobbling, and scientists like simplicity.