A rare isotope of helium gas that formed during the Big Bang is leaking out of the Earth’s core, providing a major clue as to how our planet first came into being. Known as helium-3, the primordial gas is predominantly found in solar nebulae, and its presence at the center of the Earth indicates that our world formed at the heart of the cosmic dust cloud from which the Solar System emerged.

While the majority of helium found on Earth exists as helium-4, small amounts of helium-3 have been detected in volcanic rocks known as ocean island basalts (OABs). Although most helium-3 comes from the very beginnings of universe, the isotope can also be produced by natural processes such as the radioactive decay of tritium, which is why scientists have never previously been able to ascertain the source of its presence on Earth.

This uncertainty feeds into debates around our planet's birth, as the existence of these OABs hints that the Earth may have formed inside a solar nebula while at the same time leaving room for doubt.

According to a new study in the journal Geochemistry, Geophysics, Geosystems, however, helium-3 has now been detected leaking from the Earth’s core, primarily along the mid-ocean ridge system. This discovery changes the landscape rather drastically, as high concentrations of the isotope within Earth’s core can only exist if the planet formed inside a thriving solar nebula, rather than at its fringes or during its waning phase.

“Each year, about 2 kg of the rare gas helium-3 escapes from Earth's interior,” write the researchers. According to study author Peter Olson, that’s “about enough to fill a balloon the size of your desk.”

“It’s a wonder of nature, and a clue for the history of the Earth, that there’s still a significant amount of this isotope in the interior of the Earth,” he says.

To calculate the amount of helium-3 within the planet’s core, the authors modeled Earth’s helium inventory during two key phases of its history. The first is defined as the “ingassing” phase, and marks the period during which the young Earth accumulated primordial gases as it formed.

This is then followed by the “degassing” phase, which began when a massive object collided with Earth around 4 billion years ago, scattering huge amounts of debris, which then recombined to form the Moon. The energy generated by this event would have re-melted the Earth’s mantle, allowing much of the helium-3 contained therein to escape.

However, researchers say the core would have survived the impact intact, holding on to much of its helium-3. “Our models of volatile exchange during Earth's formation and evolution implicate the metallic core as a leaky reservoir that supplies the rest of the Earth with helium-3,” they write in the study.

Based on the rate at which the gas has been leaking from deep within the planet over billions of years, they estimate that there are between 10 teragrams (10¹³ grams) to a petagram (10¹⁵ grams) of helium-3 within Earth’s core. Such a concentration could only exist if the planet formed inside the solar nebula that gave birth to our Sun.

The study authors now hope to confirm this theory by detecting other nebula-created gases leaking from the center of the Earth. According to Olson, the detection of gases like hydrogen escaping at the same rate as helium-3 would provide a “smoking gun” for the core as the source of these primordial materials. For now, however, he says “there are many more mysteries than certainties.”