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The resonance discovered in the star HD 202206 system is not unique, and it isn’t true that “[t]his particular synchrony has never been seen before in a planetary system.” Resonances abound in the solar system. The orbital periods of the Galilean satellites of Jupiter, many asteroids’ orbits, Jupiter’s orbit, and Pluto’s orbit with Neptune are some examples. Resonances also exist in numerous extrasolar systems discovered over the past 10 years. Also, the extrasolar system around HD 37124 that consists of two planets with periods of 155 and 31 days shows a 5:1 mean motion resonance, so even the particular commensurability seen for HD 202206 is not new.

Derek Wallentinsen
San Pedro, Calif.

There are clearly other synchronies in our solar system. However, researchers say that the 5:1 synchrony is new and doesn’t describe the planets of HD 37124, which have approximate periods of 150 days and 1,500 days. The word particular was intended to describe only the 5:1 synchrony, which has also been suggested as one of several ratios for two planets in another system, HD 160691 .—R. Cowen