Every other element was discovered here on Earth, often in a rock, a gas or a laboratory flask. Helium is the exception. It was first spotted in the light of the Sun, 150 million kilometres away, and for decades many chemists doubted it existed at all.

A yellow line in the sunlight

During a solar eclipse in August 1868, the French astronomer Pierre Janssen pointed a spectroscope at the Sun from India. A spectroscope splits light into its colours, and each element gives off its own set of sharp lines, like a fingerprint.

That year, Janssen and the English astronomer Norman Lockyer both worked out how to observe the Sun’s outer layers without waiting for an eclipse. Lockyer noticed a bright yellow line at a wavelength of about 587 nanometres. It sat close to the well-known yellow lines of sodium, but it didn’t match them, or any other known element.

A name with a mistake in it

Lockyer concluded that the line came from an element unknown on Earth, and named it with the chemist Edward Frankland after helios, the Greek word for the Sun. The ending -ium shows what they assumed: that it was a metal, like sodium or lithium. It is actually a noble gas, but the name stuck.

Many chemists were sceptical. An element that could only be seen in the sky was hard to accept, and a substance known only as one line in a spectrum could have been many things.

Found in a uranium mineral

The answer came in 1895. William Ramsay, who had recently helped to discover argon, treated a uranium mineral called cleveite with acid, hoping to find more argon in the gas it released. Instead he saw a bright yellow line, and he sent a sample to Lockyer and to William Crookes, who confirmed it was the line from the Sun.

In the same year, Per Teodor Cleve and Nils Abraham Langlet in Uppsala found helium in cleveite too, and were the first to measure its atomic mass with reasonable accuracy. The mineral itself had been named after Cleve.

Why it was in the rock

Finding helium in a uranium mineral was no coincidence, though nobody could explain it in 1895. When uranium and thorium decay, they emit alpha particles, and an alpha particle is a helium nucleus. Nearly all the helium on Earth was made this way, deep underground over billions of years, and some stays trapped in the same rock formations as natural gas.

That is how it was found in quantity. In 1903, a new gas well in Dexter, Kansas produced gas that wouldn’t burn. Two years later, chemists at the University of Kansas found that it contained about 2% helium.

Common in space, rare on Earth

Helium makes up about a quarter of the ordinary matter in the universe by mass, but it is scarce on Earth. It is so light that once it reaches the atmosphere it can escape into space, and it forms almost no compounds that could hold it in place. The helium we use in MRI scanners and balloons is a resource that took billions of years to build up, and it doesn’t come back.