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PSR B1257+12 c (ex PSR B1257+12 B) is an extrasolar planet (approximately 980 light-years away) in the constellation of Virgo (the Virgin). PSR B1257+12B was the first planet ever discovered outside the Solar System, and is one of three objects known to be orbiting the pulsar PSR B1257+12, which it circles at a distance of 0.36 AU with an orbital period of approximately 66 days. The planet is over four times as massive as the Earth. Because planet B and planet C have very similar masses (as well as orbiting close to each other), they cause measurable perturbations in each other's orbits. As expected, perturbations were detected confirming that the planets were real. Accurate masses of the two planets, as well as their inclinations, were measured by calculating how much the planets interfere with each other.
The planets of PSR B1257+12 are designated from A to D (ordered by increasing distance). The reason that these planets have different naming conventions from other extrasolar planets is because the naming conventions differed at the time of their discovery. Being the first ever exoplanets discovered, and being discovered around a pulsar, the planets were given the uppercase letters "B" and "C." When a third planet was later discovered around the system (in a closer orbit than the other two), the name "A" was used.
- Wolszczan, A., Frail, D. (1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature 355 (6356): 145–147. DOI:10.1038/355145a0.