A Star’s Wobble Reveals a Hidden Planet

In the picture

1. 1Centre of mass: the shared point both bodies orbit, just off the star’s centre
2. 2Planet’s orbit: a wide circle around the centre of mass
3. 3Star’s orbit: a tiny wobble around the same point
4. 4Light from the star moving toward us: waves squeezed, shifted to blue
5. 5Light from the star moving away: waves stretched, shifted to red

Examples

• An ambulance siren sounds higher as it comes toward you and lower as it drives away.
• A hammer thrower spinning a heavy ball leans back and turns in a small circle of their own.

Quiz: A Star’s Wobble Reveals a Hidden Planet (Age 17)

1. 1

   A star is much heavier than the planet orbiting it. Where does their shared centre of mass lie?

   • Close to the planet, near its orbit
   • Exactly halfway between the star and the planet
   • Close to the star, just off its centre
2. 2

   An astronomer sees that a star’s light is shifted towards blue. What does that tell them about the star’s motion?

   • The star is not moving towards or away from us
   • The star is moving away from us
   • The star is moving towards us
3. 3

   Why does a star with a hidden planet wobble back and forth?

   • The star spins so fast that it shakes
   • The planet’s gravity tugs on the star, so both circle a shared centre of mass
   • Light reflected from the planet pushes the star

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.