Strontium-87 (meaning 87 neutrons) is the isotope that can take us backwards through time to when Earth got its water. Tissot and his team found that the grains of stardust in the meteorite contain this isotope, which is a by-product of rubidium-87 decay. The amount of both these isotopes in the meteorite could tell us whether Earth formed as a dry planet or once had more water it eventually lost. “There is a lot of rubidium-87 in the solar system because it is produced by other nucleosynthetic processes, which are the main contributors to the Solar system’s composition,” Tissot told SYFY WIRE.