Illustration of a supernova explosion, created on July 19, 2015. (Illustration by Tobias … [+]
Phosphorus, greatly underestimated 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 over email. But phosphorus is also the currency of energy in almost all metabolism, like adenosine triphosphate (ATP) which moves proteins and lipids in and out of cells, she said.
Because it is so reactive, phosphorus is never found as a free element on Earth, but instead is always bound in some kind of mineral or molecular compound.
Above all, it is essential for the formation of cell membranes, bones and teeth in humans and animals, as well as microscopic ocean plankton. The problem is that astrophysicists are still debating exactly how and where it forms within our own Milky Way galaxy.
Models of the Milky Way’s galactic chemical evolution historically produce no phosphorus compared to observations, Marco Pignatari, an astrophysicist at Hungary’s Konkoly Observatory in Budapest, told me via email. So we know the problem is in the stellar yields used to simulate the chemical evolution of our galaxy, he said.
To date, fewer than 300 stars have been found to contain phosphorus in any quantity.
Phosphorus stars are mostly found in the Milky Way’s inner galactic halo and thick disk, indicating that their distribution is not location-specific, Maren Brauner, a doctoral student at the Instituto de Astrofísica de Canarias in of Spain and Universidad de La Laguna. . This suggests that the process responsible for their enrichment in phosphorus and other elements is likely to be independent of their position in the galaxy, she said.
An explosive start
Phosphorus is created by nuclear burning in stars at least ten times more massive than the sun that end their lives in a nuclear-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 the iron can’t generate energy, and the outer material is thrown 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 compared to nearby stars, Hinkel said. In rocky planets that form around host stars with substantially less phosphorus, the strong displacement of phosphorus in planetary cores can rule out the potential for life on that planet’s surface, she said.
As a result, it is important to get a better handle on the prevalence of phosphorus 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, Brauner said. This implies that we are looking for a progenitor or contaminant that somehow produced the high amounts of phosphorus embedded in these stars, she said.
Phosphorus is still very difficult to detect. This is partly because its spectrum in starlight cannot be seen at optical wavelengths, but only in the near-infrared or ultraviolet.
Its abundance is relatively low, so to see its signature requires very high-resolution observations, which are rare in this part of the light spectrum, Brauner said.
What is strangest about the amount of phosphorus in our galaxy?
The models slightly underestimate the observed amounts of phosphorus, even in normal stars, Brauner said. These phosphorus-rich stars suggest the presence of an unknown source of phosphorus in the Milky Way or that assumptions about phosphorus nucleosynthesis need to be revised, she said.
But fortunately, phosphorus is likely to play a major role in the search for life beyond the solar system. As Hinkel and colleagues noted in a 2020 article that appeared The Astrophysical Journal Lettersif there are stars with practically negligible amounts of phosphorus, then their planets are likely to be inhospitable to life; perhaps to the extent that we could rule out the possibility of life on the planet’s surface.
The conclusion?
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, Hinkel said. But stars and planets formed at the same time and from the same materials, so we can measure the elemental abundance for the star and use that as a direct proxy for the planet’s composition, she said.
