the first detections of exoplanets around "normal" solar-like stars, were found in orbit around a millisecond pulsar, PSR B1257+12. These planets remained, for many years, the only Earth-mass objects known outside of the Solar System. One of them, PSR B1257+12 D, has an even smaller mass, comparable to that of the Moon, and is still today the smallest-mass object known beyond the Solar System. Pulsar rotational speed limits The first millisecond pulsar, PSR B1937+21, was discovered in 1982 by Backer et al. Spinning roughly 641 times per second, it remains the second fastest-spinning millisecond pulsar of the approximately 200 that have been discovered. Pulsar PSR J1748-2446ad, discovered in 2004, is, as of 2023, the fastest-spinning pulsar currently known, spinning 716 times per second. Current theories of neutron star structure and evolution predict that pulsars would break apart if they spun at a rate of c. 1500 rotations per second or more, and that at a rate of above about 1000 rotations per second they would lose energy by gravitational radiation faster than the accretion process would accelerate them. In early 2007 data from the Rossi X-ray Timing Explorer and INTEGRAL spacecraft discovered a neutron star XTE J1739-285 rotating at 1122 Hz. The result is not statistically significant, with a significance level of only 3 sigma. While it is an interesting candidate for further observations, current results are inconclusive. Still, it is believed that gravitational radiation plays a role in slowing the rate of rotation. One X-ray pulsar that spins at 599 revolutions per second, IGR J00291+5934, is a prime candidate for helping detect such waves in the future (most such X-ray pulsars only spin at around 300 rotations per second). References External links "How Millisecond Pulsars Spin So Fast". Universe Today. "Fast-Spinning Star Could Test Gravitational Waves". New Scientist. "Astronomical whirling dervishes hide their age well". Astronomy Now. Audio: Cain/Gay - Pulsars Astronomy Cast - Nov 2009. Millisecond pulsar