Illustration of a supernova explosion, created on July 19, 2015. (Illustration by Tobias … [+]
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Phosphorus, the vastly underappreciated 15th element on the periodic table, is essential to all life as we know it.
Phosphorus is the structural backbone of the phosphate nucleotides in DNA and RNA, Natalie Hinkel, a planetary astrophysicist at Louisiana State University in Baton Rouge, told me via email. But phosphorus is also the energy currency in nearly all metabolism, like adenosine triphosphate (ATP) which moves proteins and lipids in and out of cells, she said.
Because it’s so highly reactive, phosphorus is never found as a free element on Earth, but instead is always bound up in some kind of mineral or molecular compound.
Above all, it’s crucial for the formation of cell membranes, bones and teeth in people and animals, as well as microscopic ocean plankton. Trouble is, astrophysicists are still debating exactly how and where it forms within our own Milky Way Galaxy.
Galactic chemical evolution models of the Milky Way historically underproduce phosphorus compared to observations, Marco Pignatari, an astrophysicist at Hungary’s Konkoly Observatory in Budapest, told me via email. So, we know that the problem is in the stellar yields used to simulate the chemical evolution of our galaxy, he said.
To date, less than 300 stars have been found to contain phosphorus in any quantity.
Phosphorus stars are predominantly found in the Milky Way’s inner galactic halo and thick disk, indicating that their distribution is not location-specific, Maren Brauner, a PhD student at Spain’s Instituto de AstrofÃsica de Canarias and the Universidad de La Laguna, told me via email. This suggests that the process responsible for their enrichment in phosphorus and other elements is likely independent of their position in the galaxy, she said.
An Explosive Beginning
Phosphorous is created by nuclear burning in stars at least ten times more massive than the sun that end their lives in a core collapse supernova explosion, Maria Lugaro, an astronomer at Hungary’s Konkoly Observatory in Budapest, told me via email. The star’s iron-rich core collapses because iron cannot generate energy and the external material is ejected into the interstellar medium from which new stars form, she said.
Our Sun Has An Abundance Of Phosphorus
The sun has a relatively high amount of phosphorus when compared to nearby stars, said Hinkel. On rocky planets that form around host stars with substantially less phosphorus, the strong shifting of phosphorus into planetary cores could rule out the potential for life on that planet’s surface, she said.
As a result, it’s important to have a better handle on the prevalence of phosphorous around other nearby solar type stars.
These stars are particularly intriguing because they are not expected to produce phosphorus themselves due to their low mass, said Brauner. This implies that we are searching for a progenitor or polluter that somehow produced the high amounts of phosphorus imprinted in these stars, she said.
Phosphorus is still very difficult to detect. That’s partly because its spectra in starlight cannot be seen in the optical wavelength, but only in the near-infrared or the ultraviolet.
Its abundance is relatively low, so to be seen, its signature requires observations at very high resolution, which are rare in this part of the light spectrum, said Brauner.
What’s most puzzling about the amount of phosphorus in our galaxy?
Models slightly underestimate the observed amounts of phosphorus, even in normal stars, said Brauner. These phosphorus-rich stars suggest the presence of an unknown source of phosphorus in the Milky Way or that the assumptions regarding the nucleosynthesis of phosphorus need to be revised, she said.
But fortunately, phosphorus will likely play a major role in the hunt for life beyond the solar system. As Hinkel and colleagues noted in a 2020 paper appearing The Astrophysical Journal Letters, if there are stars with practically insignificant amounts of phosphorus, then their planets are likely inhospitable for life; perhaps to the extent that we could rule out the possibility of life altogether on the planet’s surface.
The Bottom Line?
We don’t currently have the technology to directly measure the interior or surface composition of a small, earth-sized planet outside our Solar system, said Hinkel. But stars and planets are formed at the same time and from the same materials, so we can measure the elemental abundances for the star and use that as a direct proxy for the planet’s composition, she said.
