'Alien' plutonium that crashed to Earth millions of years ago has been discovered on the ocean floor by scientists.

The finding of the rare radioactive plutonium isotope has led experts to question how it originated on Earth.

The dangerous element known as plutonium-244 – discovered nearly 5,000ft below the Pacific Ocean in the ocean crust – is the result of "violent cosmic events" that occurred in deep space millions of years ago.

Plutonium-244's presence on Earth suggests that an exceptionally violent event happened billions of years ago, such as neutron stars colliding, may have caused it to appear on our planet.

The study's lead author, the Australian National University's Anton Wallner, said: "The story is complicated – possibly this plutonium-244 was produced in supernova explosions or it could be left over from a much older, but even more spectacular event such as a neutron star detonation."

The element iron-60 was also discovered, according to the experts' findings.

Dating of the two elements confirmed that plutonium-244 and iron-60, both of which exists when the Earth formed more than four billion years ago.

Dr Wallner added: Our data could be the first evidence that supernovae do indeed produce plutonium-244.

Iron-60 has a half-life of approximately 2.5 million years before it decays and turns into the valuable commodity, nickel.

On the other hand, plutonium-244 has a half-life of approximately 80 million years.

This extended half-life is 'long enough for most of it to remain from events of the past few million years but short enough that none is left from the time the Solar System was created,' the study's co-author, Dr. Michael Hotchkis, Principal Research Scientist at ANSTO’s Centre for Accelerator Science, added.

The findings were published in the research journal Science.

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