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UA scientists find life ingredient phosphorus in galaxy's outskirts

By Nicholas Gerbis
Published: Friday, December 1, 2023 - 10:26am
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A handful of elements are considered vital for life.

Now University of Arizona researchers have found one of them — phosphorus — in a surprising location: the far fringes of our Milky Way galaxy.

The research, which used the 12-meter radio telescope of the Arizona Radio Observatory on Kitt Peak and a 30-meter radio telescope near Granada, Spain, appears in the journal Nature.

Phosphorus is generally thought to form in the furnaces of massive stars, which release the element into space when they explode as supernovae.

But no one has ever found those types of stars out in the boondocks 75,000 light years from the galaxy’s center — more than two-and-a-half times as far out as our solar system.

But that’s where Lucy Ziurys and her team found phosphorus, an element essential for the creation of biological macromolecules like DNA, the functioning of cell membranes and the powering of plant energy cycles.

“By finding phosphorus way out at the outskirts of the galaxy, we've kind of extended where the galactic habitable zone might be,” said Ziurys, who is a Regents professor and astronomer at Steward Observatory.

As the name implies, the “galactic habitable zone” refers to neighborhoods in the galaxy that might have planets that support life. Typically, qualifying as a member of the zone means having the six “NCHOPS” elements vital to life on Earth: nitrogen, carbon, hydrogen, oxygen, phosphorous and sulfur.

“Five of them had been found in molecular form in these edge clouds,” said Ziurys. “And so the last one that hadn't been found was phosphorus. And you need phosphorus for living systems, as far as we know, and you need phosphorus to help form the core of rocky planets such as Earth.”

The problem is, now that they’ve found it, how do they explain it?

“This has led us not only to an interesting discovery about habitability in our galaxy, but also raises questions about the origin of elemental phosphorus in stars,” said Ziurys.

The best explanation so far involves a theory that says phosphorus might also form in more common, lower-mass stars nearing their final phase.

Such stars, known as asymptotic giant branch (AGB) stars, can convert silicon to phosphorus by creating carbon 13 in a certain layer, or shell, surrounding the core. The C13 then falls apart and generates the excess neutrons needed.

“Their atmospheres are very convective,” said Ziurys. “And so they mix material produced in the shells to the surface.”

If the AGBs are of the thermal-pulsing variety, they can then eject the phosphorus into space as part of their normal mass-shedding process.

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