Final answer:
The combination of Radial Velocity and Transit method allows astronomers to determine the mass, radius, and average density of exoplanets, providing insight into their composition.
Step-by-step explanation:
The combination of Radial Velocity and Transit method is a powerful tool for astronomers to determine the characteristics of exoplanets orbiting distant stars. Specifically, radial velocity measures the star’s motion induced by the gravitational pull of an orbiting planet, while the transit method detects the slight dimming of a star’s light when a planet passes in front of it.
By using these methods together, astronomers can calculate both the mass and radius of the planet, which in turn allows for the determination of the planet’s average density. This ability to derive density lets scientists make educated assumptions about a planet’s composition, for instance, whether it is a gas giant or a rocky terrestrial planet, like HD 209458b which was found to be a gas and liquid world.