deuterium is an isotope of hydrogen containing a proton and neutron in its nucleus, while normal hydrogen has only a proton.
From that, they extracted the ratio of the number of deuterium atoms to the number of hydrogen atoms.
One chemical test involves measuring the relative amount of deuterium in water.
However, even that ginormous difference still shows about three times the amount of deuterium than we see on our planet.
1933, coined by U.S. chemist Harold C. Urey, with Modern Latin ending + Greek deuterion, neuter of deuterios "having second place," from deuteros "second," from duo (see two). So called because it is twice the mass of hydrogen.
deuterium deu·te·ri·um (dōō-tēr'ē-əm, dyōō-)
An isotope of hydrogen with one proton and one neutron in the nucleus having an atomic weight of 2.014. Also called heavy hydrogen, hydrogen-2.
An isotope of hydrogen whose nucleus has one proton and one neutron and whose atomic mass is 2. Deuterium is used widely as a tracer for analyzing chemical reactions, and it combines with oxygen to form heavy water. Also called heavy hydrogen. See Note at heavy water.