RADIAL VELOCITY
Watching for Wobble
1091 planets discovered
Orbiting planets cause stars to wobble in space, changing the color of the light astronomers observe
Size rules
One way to make sense of the gravitational interaction between a planet and a star is to imagine a game of tug-of-war. On one side, you have the star - a massive object with a really powerful gravitational field.
On the other side, you have the planet, much smaller, with a whole lot less gravity.
On the other side, you have the planet, much smaller, with a whole lot less gravity.
But even though the planet is small, it still has some gravitational force. It still has an effect on its host star, even if that effect is much less pronounced than the one the star has on the planet.
- but two can play at the gravity game
Take a look at the above animation. At first glance, things look normal. There's a big star and a small planet, and the small planet orbits the big star. You've probably seen this many times.
But check out the star. See how it's moving a little bit, too? The effect is exaggerated for this animation, but that's what actually happens in space. The planet's gravity causes the star to 'wobble'around a little bit.
As you might imagine, the bigger the planet, the bigger the effect it has on its star. Small planets, like Earth, make their stars only wobble a tiny bit. Bigger planets, like Jupiter, have a much stronger effect.
A star's 'wobble' can tell us if a star has planets, how many there are, and how big they are.
Digging into the Doppler data
Wobbling stars are great for finding exoplanets, but how do we see the wobbling stars?
The method used is one called 'Doppler shift'. It's named after the physicist who figured it out about 150 years ago.
Energy - sound, radio waves, heat, and light - moves in waves. Like the waves you see in the animation above.
Those waves can be stretched and squeezed, based on the movement of the object that's producing them.
You may not know it, but you've probably experienced the Doppler effect before. Have you ever noticed how the sound of an ambulance passing you on the street gets higher in pitch as it gets close to you, and then lower in pitch as it speeds away?
The reason is because when an object that emits energy (like an ambulance speaker or a massive, burning star) moves closer to you, the waves bunch up and squish together. And when the object is moving away, the waves stretch out.
Those changes in the wavelength change how we perceive the energy that we're seeing or listening to. As sound waves scrunch together, they sound higher in pitch. And when visible light waves scrunch together, they look more blue in color.
When sound waves stretch out, they sound lower in pitch. And when visible light waves stretch out, they make an object look more reddish.
This change in color is called 'redshift', and scientists can use it to see if an object in the sky is moving towards us or farther away.
Putting it all together
​You can see this method working in the animation above. The planet causes the star to wobble around in its orbit, and as the planet moves to and fro, the light waves compress together and then stretch out, changing the color of the light we see.
Super successful
​You can see this method working in the animation above. The planet causes the star to wobble around in its orbit, and as the planet moves to and fro, the light waves compress together and then stretch out, changing the color of the light we see.
Lots of astronomers and telescopes around the world use this method to discover exoplanets, but two notable observatories where this work happens are the Keck Telescopes in Hawaii and the La Silla Observatory in Chile.