JOE TUCCIARONE / SCIENCE PHOTO LIBRARY JOE TUCCIARONE / SCIENCE PHOTO LIBRARY
Halo planets. Artwork showing the sky of a planet in the outer halo of a galaxy (seen from far left to upper right). The planet's moon is seen at centre left. This might be part of the planetary system of a brown dwarf, an object that forms like a star but does not emit light because it is too small to sustain nuclear fusion. Concentrations of dim objects such as brown dwarfs in galactic halos are one candidate for "dark matter", mass that is predicted by theory but cannot be observed. Such objects are termed MACHOs (Massive Astronomical Compact Halo Objects).
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