In Bohr's model, the planet model, it was thought that electrons were orbiting a nucleus just as planets are orbiting the sun. Over time, this model was proved to be inconsistent with the further findings, especially with the fact that electrons exhibited wave properties.
This lead to defining orbitals in terms of quantum mechanics. An orbital is defined as a space in which the probability of finding an electron is 95 %. Each allowed orbital has a precisely known energy, this is why we can calculate the energy needed to promote an electron to a higher orbital, as we know the difference in energies between the orbitals.
Pierre and Marie Curie were awarded the 1903 Nobel Prize in Physics for their joint research in radioactivity. They proved that radioactivity does not depend on the arrangement of molecules in atoms, but that radioactivity originates from the atoms themselves. They also introduced two new elements: radium and polonium.