RICHARD BIZLEY / SCIENCE PHOTO LIBRARY RICHARD BIZLEY / SCIENCE PHOTO LIBRARY
Habitable alien planet, artwork. This Earth-like planet is located near a globular star cluster (left). A planet's orbit and its size have to fit within a narrow range for it to have a surface temperature suitable for liquid water, and for it to retain an atmosphere. A globular star cluster, a dense spherical collection of stars, can contain over a million stars within a volume only a few hundred light years across. Globular star clusters formed early in the history of the universe, and their stars are thought to be among the oldest known. Many globular star clusters are far older than the galaxies they now orbit.
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