Cosmic Rulers
Planetary Planarity
Radial Velocity Technique
Target: Gamma Cephei
Other Targets
Tools of the Trade

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 67 years.

Gamma Ceph position in the sky

The primary star is about 1.4 times as massive as the Sun and almost five times wider. It’s also farther along in its life cycle. It has consumed the hydrogen in its core and now is 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. The average distance between the two stars is about 20 times the distance from Earth to the Sun, although their orbit is elongated.

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.

Bruce Campbell and Gordon Walker first reported this wobble in 1988, making Gamma Cephei’s planet (Gamma Cephei Ab) the first to be discovered with this technique. (The astronomers later questioned their own discovery, attributing the observations to changes in the star itself. McDonald Observatory astronomers confirmed the planet’s existence in 2003).

The planet is at least 1.85 times as massive as Jupiter, which is the largest planet in our own solar system. It orbits the star every 2.47 years, at twice the distance from Earth to the Sun.

Gamma Cephei

Artist’s concept of the known planet in the Gamma Cephei system, with the two stars in the background [Tim Jones/McDonald Observatory]

‘Messy’ Science

While radial velocity is a powerful technique for discovering planets, 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. Without that information, which adds a third dimension to the system, it’s impossible to determine a precise mass for the planet or to tell whether the stars and planets in the system lie in the same plane.

Texas astronomers Fritz Benedict, Barbara MacArthur, and their colleagues used Hubble Space Telescope’s Fine Guidance Sensors (FGS) to 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 crisp pinpoint 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. This motion reveals the orientation of the system and the mass of the companions.

Yet it is a complicated measurement to make. As Earth orbits the Sun, the entire system appears to shift position against the background of more-distant stars by a tiny bit. The Gamma Cephei system consists of two stars in a mutual orbit, with a planet orbiting one of the stars. That star also pulses in and out a bit, further complicating the observations.

The Benedict team accumulated about two years of data with Hubble, in 2008-10, which was a bit less than one full orbital period for the planet. Without a full orbit of Hubble observations, the team has had to supplement its data set with observations made by several ground-based telescopes over the last few decades, which are less precise than the Hubble measurements.

That has not provided enough good data points to accurately establish the system’s orientation and the mass of the planet.

“If the orbit isn’t properly established, you end up with apparent nonsense,” says Benedict. “It says that the system should dissolve in 10,000 years or less, and the planet gets ejected -- it goes bye-bye. But that means that we would be looking at the system at a unique moment in its history. So we back to the drawing board. If we assume that everything is in the same plane, then we get a mass for the planet that’s about twice the mass of Jupiter. But it’s unsatisfying to assume the answer you were trying to get.”

So the exact configuration of Gamma Cephei remains in limbo as the astronomers continue to analyze the observations from Hubble and other telescopes. “Science is messy,” says Benedict. “But that’s part of the process.”

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