Nature abhors a vacuum, it is said. And when it comes to solar systems, the adage appears to be true.

That's the conclusion of astronomers reporting a successful test of the "the packed planetary systems" hypothesis. The hypothesis asserts that wherever a planet can exist in a stable orbit, it will. In solar systems with multiple planets, nature will leave no gap unfilled.

Three years ago, Rory Barnes, now at the University of Arizona in Tucson, and Thomas Quinn at the University of Washington in Seattle, both in the US, noticed that while most newly discovered solar systems obey this rule, there was at least one puzzling exception.

The planetary system around the star HD74156 in the constellation Hydra had a large gap between the orbits of its two giant planets, which are both more massive than Jupiter.

Barnes and Quinn calculated that a region of stability existed in the gap and published a prediction that a third, Saturn-sized planet would be found there. Unaware of the prediction, a team led by Barbara McArthur at McDonald Observatory in Texas, US, recently announced the discovery of a planet of the appropriate size within the apparent gap.

Barnes highlighted the successful prediction this week at the annual meeting of the American Astronomical Society in Austin, Texas, and went on to make further predictions about where planets should turn up in other multiple systems.

Efficient process

The predictions include a new planet for the star 55 Cancri, where a fifth planet was recently added in a region of stability among four previously discovered worlds. However, says Barnes, the new planet is small and the region of stability it occupies is large enough to accommodate still more undiscovered planets.

Barnes also predicted a new planet around the star HD38529 in Orion. There, a large gap separates a previously discovered planet and an object called a brown dwarf, which is more massive than a planet but less massive than a star.

"This field is littered with predictions that turn out to be wrong," said Sara Seager of MIT in Cambridge, US, congratulating Barnes on his work. But she also points out that this is not the first time astronomers have successfully predicted the existence of an exoplanet. They did it once before, finding a large planet around a star that was rich in heavy elements and therefore appeared to be a good candidate for hosting a solar system.

However, the case of HD74156 marks the first time an exoplanet has been predicted based on the dynamics of its solar system.

According to Barnes, the successful prediction suggests astronomers can be optimistic about how many planets are likely to exist in the galaxy because planet formation appears to be an efficient process.

Solar systems may begin with more planets than can co-exist comfortably. Then, they may eject unstable planets until a densely packed but stable system remains. In the future, Barnes hopes to model the formation of packed systems.

Meanwhile, he says, observers should feel encouraged that there are discoveries waiting to be made among multiple planetary systems. McArthur agrees: "Multiple planetary systems are the exciting ones because that's where you can test theory."

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