The Earth is missing 3T tons of mass

This explains why the assembled Earth weighs slightly less than the sum of its constituent atoms: binding energy. Using the familiar nuclear example, splitting a U-235 nucleus into fission products conserves protons and neutrons yet shows a measurable mass deficit (one example gives a start total of 236.0526 daltons and an end total of 235.8673 daltons, a loss of 0.1853 daltons). That deficit arises because assembling nucleons into a nucleus lowers the system's energy; mass and energy are equivalent (E=mc^2), so the bound system has less mass than the loose particles. The write-up cites the full difference between free nucleons and an assembled U-235 atom as about 1.9151 daltons, corresponding to a binding energy of roughly 2.858×10^-10 joules per atom.

The same principle applies to gravity: assembling a planet releases gravitational binding energy, reducing the total mass compared with the sum of separated material. Earth’s gravitational binding energy is given as 2.49×10^32 joules, which corresponds to roughly 3 trillion metric tons of “missing” mass - about one billionth of the planet’s mass, so negligible for most purposes. By contrast, much denser objects such as neutron stars can lose a substantial fraction of mass to binding energy (order 10-15%), so the effect becomes astrophysically significant only in extreme gravity.