How can transit and radial velocity observations be combined to measure an extrasolar planet’s density?

Combining transit and radial velocity observations allows astronomers to determine the physical nature of an extrasolar planet by calculating its density.12

The process works as follows:

• Transit Observations: When a planet passes between its host star and a telescope, it causes a periodic drop in the star's brightness. This transit data reveals the planet's orbital period and, crucially, its physical size (radius).12
• Radial Velocity Observations: This method measures the periodic "wobble" or motion of a host star caused by the gravitational tug of an orbiting planet. While radial velocity data alone typically provides only a minimum mass for the planet, it yields a precise mass when the orbital inclination is known—which is provided by the transit geometry.32

By combining the radius derived from the transit with the mass derived from radial velocity, astronomers can calculate the planet's density. This density measurement is essential for determining the planet's composition, allowing scientists to distinguish between rocky bodies, like Earth, and gas giants, like Jupiter.24