Planets orbiting a pulsar (upper left), artwork. A pulsar, a rotating neutron star, is the collapsed, super-dense core of a massive star that has blown off its outer layers in a supernova. It rotates extremely rapidly, emitting energetic X-ray and radio pulses that cause the surrounding gas and dust to glow along the pulsar's magnetic field lines. The radiation has also caused aurorae in the northern polar region of the foreground planet (circular glows, lower right). This system is based on the pulsar planet system discovered by Aleksander Wolszczan in 1992 around pulsar PSR B1257+12. These were the first extrasolar planets ever discovered, and still (as of 2006) the only ones found orbiting a dead star.