LYNETTE COOK / SCIENCE PHOTO LIBRARY LYNETTE COOK / SCIENCE PHOTO LIBRARY
Alien planets. Artwork of two planets around the red dwarf star Gliese 876 (upper right). They are seen from a hypothetical moon of the larger planet (Gliese 876b). Gl876b (upper left) orbits in 61 days. The smaller planet (Gl876c) orbits in just over 30 days. This orbital system is very stable, with both orbital paths reinforcing the stability of the system (resonance). Gl876b was discovered in 1998 but its orbit hid that of Gl876c, which was not found until 2001. Gl876b has a mass 600 times that of Earth and orbits some 31.5 million kilometres (km) from the star. Gl876c has a mass 180 times that of Earth and an orbital radius of 19.5 million km.
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