Target: Gamma Cephei

Gamma Cephei, which is 45 light-years away, is one of the brightest stars in the constellation Cepheus, the king. It is a binary system, with two stars orbiting each other once every few decades.

The primary star is larger and more massive than the Sun, and farther along in its life cycle. It has used up the hydrogen in its core, and is now beginning to “burn” the helium that it created through billions of years of nuclear fusion. As a result, it is puffing up to form a red giant.

Gamma Cephei’s other star is a red dwarf, which is far smaller, cooler, and less massive than the Sun. Because of its low mass, it will continue to shine for billions of years after its heftier companion expires.

A planet orbits the larger star.

Astronomers discovered it with a technique known as radial velocity. As it orbits the star, the planet’s gravity pulls at the star. Astronomers measure this pull as a back-and-forth “wobble” in the star’s light. Precise measurements of the wobble show that the planet is at least 1.6 times as massive as Jupiter, which is the largest planet in our own solar system, although it could be up to 16 times Jupiter’s mass. It orbits the star every 2.47 years, at twice the distance from Earth to the Sun.

While radial velocity is a powerful technique for discovering planets and learning some basic information about them, it doesn’t reveal the system’s orientation with respect to Earth. We might see the system edge-on, face-on, or at some other angle. And without that information, which adds a third dimension to the system, it’s impossible to tell whether the stars and planets in a system all lie in the same plane.

Hubble Space Telescope’s Fine Guidance Sensors (FGS) add this third dimension through a technique called astrometry, which measures the precise positions of astronomical objects.

Above Earth’s obscuring atmosphere, the sensors see each star as a tiny pinpoint of light instead of the smudged blob that ground-based telescopes see. That allows the astronomers to track a star’s position as it is pulled by the gravitational attraction of its companion stars and planets. The shape and size of this motion reveals the orientation of the system and the mass of the companions.

With the precise maps of these four planetary systems, astronomers will have more examples to plug in to their models of how planetary systems form and evolve. These systems, plus others that astronomers will study in the years ahead, will tell us if the models are correct or if astronomers need to tweak their ideas of how planets are born.

Other Targets: Mu Arae, HD 128311, and HD 202206