Question

You observe a star very similar to our own Sun in size and mass. This star moves very slightly back and forth in the sky once every 4 months. You attribute this motion to the effect of an orbiting planet. What can you conclude about the orbiting planet from the above information?

Answer 1

I can conclde the planet must be closer to the star than Earth is to the Sun.

Step-by-step explanation:

The gravitational effect of the star forces the planets into an orbit around it, but gravity works both ways and the gravitational effect of the planet will be pulling on the star too.

Since the star moves back and forth in the sky once every four months we can deduce that this planets influences is greater that that of The Earth and the Sun. Therefore, the logical conclusion is that this planet has a shorter orbit, in other words it is closer to the Sun.