To the Editors:
In "Are Planetary Systems Filled to Capacity?" (September-October), Steven Soter mentions that we predicted apparently empty regions around certain stars contain planets that are too small to have been detected, thus far. He mentions our prediction of a planet in a "habitable zone" around the star 55 Cancri.
In the same January 20, 2005 Astrophysical Journal paper in which we made this prediction about 55 Cancri, we also predicted planets around the star HD 74156 and several others. Less that two weeks after Dr. Soter's article was published, a group in Texas announced the discovery of a planet around HD 74156 right in the region that we had predicted.
As Dr. Soter mentioned in his article, our planetary predictions are testable, and we're quite excited about the discovery. We hope that Dr. Soter is also pleased to learn that the hypothesis of "packed planetary systems" has received another piece of evidence.
University of Arizona
University of Colorado
Dr. Soter responds:
The new planet, discovered by Jacob Bean and his colleagues, has an orbit lying between those of two more-massive planets of the star HD 74156. The semi-major axis of the new planet's orbit is right on target with the stability prediction made by Drs. Raymond and Barnes. This appears to mark the first successful prediction of a planet since 1846, when Neptune was discovered in an orbit calculated independently by John Couch Adams and Urbain Le Verrier.