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C > C - Pg. 230

230 gloSSary wobble in the star's position, so astrometry can be used to find extrasolar planets. astronomical unit (aU): The average distance between the Sun and the Earth, roughly about 150 million kilometers, or 93 million miles. It is a useful unit for discussing distances within a planetary system. Mercury is roughly 0.4 AU from the Sun, Jupiter is at 5 AU, and Neptune at 30 AU. Binary star: Two stars that orbit each other. Binary stars are common, and in most cases, the pair is believed to have been born together out of the same natal cloud, which fragmented as it contracted. Biosignature: A substance, often a molecule, indicating the existence of living organisms. Also known as a biomarker. Brown dwarf: An object not massive enough to become a star, because its core temperature never gets high enough to