Six New Planets: Mini-Neptunes Found Around Sunlike Star
"This is the most closely packed known planetary system," expert says.
Five of the new planets are closer to their parent star than Mercury is to the sun. The sixth world lies farther out, within a region that would fit inside the orbit of Venus.
"This is the most closely packed known planetary system," said study co-author Jonathan Fortney, an astronomer with the Lick Observatory at the University of California, Santa Cruz.
(Related: "New Planet Found; Star's Fourth World Stumps Astronomers.")
The planets are relatively small—ranging from roughly 2 to 4.5 times Earth's radius—but they are also surprisingly lightweight, indicating they are made mostly of gases. Based on their densities, four of the planets appear to have thick atmospheres of hydrogen and helium.
The two planets closest to the star have higher densities,
