Much of what makes science work is following up. After the discoveries are made, more research is needed to fill in the details. And the launch window for a spacecraft that’s all about the follow-up opens next week.

CHEOPS is a European mission that will study exoplanets — planets that orbit stars other than the Sun. It won’t hunt for new planets. Instead, it’ll make detailed observations of star systems with known planets.

CHEOPS will look for the planets to pass in front of their parent stars. Such a passage is called a transit. It causes a tiny “dip” in a star’s light. How much the star dims, and how long the transit lasts, reveals the size of the planet. Scientists especially want to study worlds that are between the size of Earth and the size of Neptune, the Sun’s fourth-largest planet.

The size will be combined with measurements of the planet’s mass made from the ground. The combination will tell astronomers how dense a planet is. And that will give them an idea of the planet’s composition. Less-dense planets are made mainly of gas, while heavy planets are made mainly of rock.

CHEOPS will keep an eye on target stars between transits as well. That’ll allow it to see how much starlight a planet reflects. From that, astronomers can deduce whether the planet has an atmosphere, and whether the atmosphere is topped by clouds.

It’s possible that CHEOPS will find more planets orbiting its target stars — giving it a chance to follow up on its own discoveries.

Script by Damond Benningfield