Introduction
Cosmic Rulers
Planetary Planarity
Radial Velocity Technique
Target: Gamma Cephei
Other Targets
Tools of the Trade
Fritz Benedict
Audio Feature with Fritz Benedict

Solar System Architecture

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Planetary Planarity

Do other planetary systems have the same architecture as our own?

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Careful measurements of a star’s position can reveal motion as the result of the gravitational pull of an unseen companion star or planet. These measurements can reveal whether we see the system from above or below, like a bullseye (top); edge on, so the star simply shifts from side to side (center); or at some in-between angle (bottom). When combined with other data, these observations can reveal the mass of the unseen companion. [Tim Jones]

Astronomers have pretty good ideas of how planetary systems take shape. The basic outline says that as a giant cloud of interstellar gas and dust collapses under its own gravitational pull, it begins to spin. Most of the material congregates in the center of the cloud to form a star, but some of it forms a broad, flat disk that rotates around the star. Tiny dust grains in this disk stick together to build larger and larger bodies, which in turn stick together to form planets.

The problem is that until recently, there was only one known system for testing this model: our own solar system. But since the early 1990s, astronomers have discovered possible planets orbiting more than 400 other stars. With such a large sample, they can test their models more thoroughly.

A team led by University of Texas astronomer Fritz Benedict is providing important information for evaluating the models by plotting the way several other planetary systems are laid out. Their research includes new observations of the systems with Hubble Space Telescope and telescopes on the ground, as well as archives of observations made by other teams over the last decade or longer.

In particular, Benedict and his colleagues are trying to find out if the planets in these systems all orbit their parent stars in the same plane, like marbles rolling along on a flat tabletop, as is the case in our solar system.

The astronomers are targeting four stars with suspected planetary companions.

They use HST’s Fine Guidance Sensors, which help keep the telescope stable, to measure tiny motions in each star’s position in the sky, which are caused by the gravitational tug of the star’s planets. Precise measurements of these motions reveal the shape of each planet’s orbit, and how the system is tilted as seen from Earth — whether we see it face on, from the side, or some angle in between.

They combine those observations with others made by ground-based telescopes to yield a three-dimensional view of the systems, which reveals whether their planets orbit in the same plane or are scattered in different orientations.

Artist’s concept of Gamma Cephei’s planet

Artist’s concept of Gamma Cephei's planet, with the primary star at right and the fainter secondary star at left. [Tim Jones/McDonald Observatory]

The list of target star systems includes Gamma Cephei, Mu Arae, HD 128311, and HD 202206.

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. More