By Will Dunham
WASHINGTON (Reuters) - They are the most common type of planet observed in our Milky Way galaxy - two to three times the diameter of Earth but smaller than Neptune, and orbiting closer to their stars than our solar system's innermost planet Mercury does to the sun.
Called "sub-Neptunes," they are absent from our solar system and their fundamental nature has remained a puzzle. But the discovery announced on Wednesday of six of them in synchronized orbits around a star about 20% smaller in mass than the sun is giving astronomers hope that an answer could come soon.
The researchers determined that the six planets were in a rare condition called orbital resonance, with their synchronized orbits around the star apparently unchanged since they formed about 4 billion years ago. That indicates no chaotic event like a giant impact event has perturbed their orbits.
"The resonance aspect is really interesting - partly the mathematical beauty of it," said astronomer Hugh Osborn of the University of Bern in Switzerland, one of the authors of the research published in the journal Nature.