NASA Science
Radial velocity
Finding planets by the wobble they induce in their star's motion.
A planet does not orbit its star so much as the two orbit their shared centre of mass, which means the star moves in a small circle too. That motion shifts the star's spectrum: fractionally blue as it comes toward us, red as it goes away. Jupiter makes the Sun move at about 12 metres per second; Earth manages nine centimetres per second. Instruments like HARPS at La Silla and ESPRESSO at Paranal are built to measure exactly this, and the technique gives you the planet's mass — the number the transit method cannot supply. Get both for the same planet and you have its density, which is the difference between a rock and a gas ball.
Where it has come up lately
Related terms
Transit method
Finding planets by watching a star dim slightly as one passes in front.
Habitable zone
The orbital band where a planet could hold liquid water — no more than that.
Sub-Neptune
The most common kind of planet in the galaxy, and there is not one in our solar system.
Hot Jupiter
A gas giant orbiting closer to its star than Mercury does to the Sun.
Questions
What is Radial velocity?
Finding planets by the wobble they induce in their star's motion.
Why does it matter?
Instruments like HARPS at La Silla and ESPRESSO at Paranal are built to measure exactly this, and the technique gives you the planet's mass — the number the transit method cannot supply. Get both for the same planet and you have its density, which is the difference between a rock and a gas ball.