Pluto the dwarf planet pursues its orbit almost fifty times further away from the sun than the earth. Its atmosphere is thin, as cold as stone and appears to have some kind of reverse greenhouse effect, making it even colder than you would expect. The atmosphere consists mainly of nitrogen and methane; planetary researchers are particularly interested in the nitrogen part.

The nitrogen in the air you are now breathing consists mainly of molecules of two nitrogen atoms, each with seven neutrons and seven protons. Seven and seven makes fourteen and chemists abbreviate the previous sentence to 14N2.

Here on earth, approximately 0.4% of all nitrogen has one extra neutron: 15N14N. Does it occur on Pluto too? Scientists would love to know the answer to that question because they could learn more about how atmospheres are created and function on other far-flung lumps. The problem is that Pluto is so far away, you can’t just pop round to count neutrons.

A group of scientists, including Leiden’s Alan Heays, are looking ahead in the planetary science journal Icarus. A small NASA craft is on its way to Pluto and is expected to arrive in mid-2015. The craft will take measurements and in the article, Heays and his colleagues have produced calculations that will reveal the proportions of the two types of nitrogen from the measurements. The trick should also work on planets outside our solar system if the measurements are sufficiently accurate.

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