of having planets orbiting it: more giant planets form around stars of higher metallicity. The kinetic energy of an expanding supernova remnant can trigger star formation by compressing nearby, dense molecular clouds in space. The increase in turbulent pressure can also prevent star formation if the cloud is unable to lose the excess energy. Evidence from daughter products of short-lived radioactive isotopes shows that a nearby supernova helped determine the composition of the Solar System 4.5 billion years ago, and may even have triggered the formation of this system. On 1 June 2020, astronomers reported narrowing down the source of Fast Radio Bursts (FRBs), which may now plausibly include "compact-object mergers and magnetars arising from normal core collapse supernovae". Cosmic rays Supernova remnants are thought to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production has only been found in a small number of remnants. Gamma rays from pion-decay have been detected from the supernova remnants IC 443 and W44. These are produced when accelerated protons from the remnant impact on interstellar material. Gravitational waves Supernovae are potentially strong galactic sources of gravitational waves, but none have so far been detected. The only gravitational wave events so far detected are from mergers of black holes and neutron stars, probable remnants of supernovae. Effect on Earth A near-Earth supernova is a supernova close enough to the Earth to have noticeable effects on its biosphere. Depending upon the type and energy of the supernova, it could be as far as 3000 light-years away. In 1996 it was theorised that traces of past supernovae might be detectable on Earth in the form of metal isotope signatures in rock strata. Iron-60 enrichment was later reported in deep-sea rock of the Pacific Ocean. In 2009, elevated levels of nitrate ions were found in Antarctic ice, which coincided with the 1006 and 1054 supernovae. Gamma rays from these supernovae could have boosted atmospheric levels of nitrogen oxides, which became trapped in the ice. Type Ia supernovae are thought to be potentially the most dangerous if they occur close enough to the Earth. Because these supernovae arise from dim, common white dwarf stars in binary systems, it is likely that a supernova that can affect the Earth will occur unpredictably and in a star system that is not well studied. The closest known candidate is IK Pegasi, about 150 light-years