Where on earth do heavy elements, such as gold and radioactive plutonium-244, come from? Inquiring minds want to know. Now they do: rare elements such as plutonium come from the mergers of binary neutron stars, says a Hebrew University of Jerusalem team.
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There were two riddles regarding very heavy elements like radioactive plutonium-244, which Earthling scientists have to "cook" themselves in nuclear reactors if they want to use any. Do these elements even exist in nature? And if they do, how are they created?
Anyway, science set out to seek its most stable isotope, plutonium-244, which has a half-life of about 80 million years (and is no friend to your body either).
Lo, some years ago, it was discovered that the early solar system did contain a lot of plutonium-244. Since that was before engineers rose from the dust and built nuclear reactors, clearly something astrophysical made the stuff in nature.
But today's planet has almost no plutonium-244: Recent measurements suggest that only very little amounts of the element reached us from outer space over the last 100 million years.
So we have a riddle. The early solar system had a lot of plutonium-244. It would have decayed by now anyway, but if it's made naturally, why didn't the solar system "get more?"
If it existed plentifully in the early solar system, it had to have been synthesized by astrophysical phenomena nearby (in galactic time scales and distances), since it decays very fast – 80 million years is an eyeblink in galactic terms.
The Hebrew University team reconciled these bewildering, contradictory observations thusly, as explained it its letter to the journal Nature Physics.
Radioactive plutonium (and other rare heavy elements, such as gold and uranium) originate in collisions of binary neutron stars. These extremely rare mergers would produce large amounts of heavy elements.
Evidently, says the team of Dr. Kenta Hotokezaka, Prof. Tsvi Piran and Prof. Michael Paul from the Racah Institute of Physics, such a merger happened near our Sol within less than a hundred million years before the solar system was born.
And thus we had a lot of plutonium-244 in the early Solar system. Since it hasn't happened nearby since, certainly not in the last 100 million years, which is just as well for other reasons, we have to rely on reactors to make more. And there you have it.
By the way, plutonium was named for the ex-planet Pluto, which has since been relegated into large lump of rockhood.