Neutrons in an atomic nucleus aren't all held equally tight. If you peel them off one by one, like leaves from a cabbage, the energy required changes each time. For isotopes with 149 particles in the nucleus (lanthanum, cerium, praseodymium), this energy has just been remeasured, and one number didn't match the previous value.
The work took place at Argonne National Laboratory, where rare isotopes are captured in a device called a Penning trap. It holds charged atoms in magnetic and electric fields like an invisible box. The nuclei are then set spinning, and from their rotation frequency, their mass is calculated with extraordinary precision.
The big surprise was lanthanum-149. Its mass turned out to be 221 kilo-electronvolts (a unit of energy related to mass) lighter than a recent Finnish measurement. As a result, the graph of two-neutron separation energy for the lanthanum isotope chain changed shape noticeably. Such graphs are like maps of nuclear strength: they show where neutrons are held especially tightly.
These data are crucial for understanding how heavy elements are born in space. Supernova explosions and neutron star collisions create a flood of neutrons that builds up nuclei. Accurate masses tell us which isotopes survive along the way and which fall apart. An error in mass of even a fraction of a percent can alter the entire picture of nucleosynthesis, just as a wrong price for one ingredient changes the recipe for the whole dish.
The Penning trap is so sensitive that it can hold a single ion and 'weigh' it to trillionths of a gram, as if you could feel a fly landing on a whale.
In Liu Cixin's novel 'The Three-Body Problem,' aliens manipulate physics at the level of elementary particles; precise nuclear masses are key to such manipulations, since all nuclear reactions depend on them.