“Without phosphorus there would be no thought,” observed Jacob Moleschott a 19th-century physiologist and philosopher. Phosphorus is one of the six main elements that make up the human body and an essential element for life as we know it for the creation of DNA, cell membranes, bones and teeth in humans, and even the microbiome of ocean-dwelling plankton–as a possible signature in the search for life in the cosmos.
Unlike hydrogen, oxygen, carbon, nitrogen and calcium, phosphorus is rare. And it is even more scarce elsewhere in the Solar System. The source of its arrival on early Earth remains a mystery.
Key Element in the Origin of Life Enigma
“Life appeared on Earth about 4 billion years ago, but we still do not know the processes that made it possible,” said astronomer Víctor Rivilla, the lead author of a study published in January 2020 based on results from the Atacama Large Millimeter/Submillimeter Array (ALMA) and from the ROSINA instrument on board Rosetta, showing that phosphorus, present in our DNA and cell membranes, is a key element in the origin-of-life puzzle.
“Phosphorus is one of the key elements in biology,” says Matthew Pasek, an astrobiologist and geochemist at the University of South Florida about research suggesting that phosphorus condensed inside asteroids in the outer Solar System before moving back towards the sun, where some of it ended up on Earth. Phosphorus is mainly found in solid form, whereas elements like hydrogen, oxygen and nitrogen are often found as a gas.
The Mystery Deepens
Now, a team of scientists has deepened the mystery of the element’s origins by discovering 15 “chemically peculiar stars” in the Milky Way that show “very high phosphorus abundances” that cannot be explained by any existing theories, according to a study published in Nature Communications.
Astronomer Thomas Masseron, a postdoctoral fellow at the Instituto de Astrofísica de Canarias (IAC) and his colleagues outlined several models to explain these phosphorus-rich (P-rich) stars, which may be similar to the long-dead stars that seeded our own solar system with this life-giving element.
“I have been wondering for years about the nucleosynthetic origin of this particular atom, in particular because Galactic chemical evolution under predicts the phosphorus we observe in the Milky Way (especially at low metallicity), suggesting that a source of phosphorus is neglected in the models or even possibly unknown,” says Masseron.
A New Type of Object –”P Rich Stars”
“These results show that not only are we dealing with a new type of objects, but that their discovery opens the way for the exploration of new physical mechanisms and nuclear reactions which occur in stellar interiors” explains Masseron, the leader of the project.
“It could be an important clue about the origin of the phosphorus, which is a fundamental component of life”, says Aníbal García-Hernández, IAC researcher, who is the second author of the article.
“This spectrum allowed us to obtain the chemical abundances of further elements in these stars which are peculiar and rich in phosphorus, and to rule out definitively any known stellar candidate which could explain the stars which are rich in this elements”, indicates Olga Zamora, a co-author of the article, and an IAC support astronomer.
Image of the main conclusion of the study. What is the stellar source for phosphorus, element so important for life? (Gabriel Pérez Díaz, SMM –IAC).
Born from Ashes of a Prior Generation of Massive Stars
The most likely origin story is that the stars were born from the ashes of a previous generation of massive stars that, for some reason, overproduced phosphorus in their lifetimes. This would require the progenitor stars to go through “a quite specific and peculiar nucleosynthesis,” the team said, referring to the process that goes on in the interior of stars to create new elements.
These massive stars would have spewed their phosphorus reserves across space when they exploded into supernova, thereby giving rise to a new brood of P-rich stars. However, computer models of this scenario suggest that it should also lead to carbon, sodium, and sulfur enrichment in the next generation of stars. The P-rich stars observed by the team, in contrast, do not show this pattern.
P-Rich Stellar Theft?
It’s also possible that the P-rich stars stole phosphorus from a companion star orbiting close to them, or that they fully merged with another star. Collisions between neutron stars, a class of dead, collapsed stars, provide another potential way to create phosphorus-enriched clouds of gas and debris. However, all of these explanations are contradicted in one way or another by the actual observations.
“The paper is all about exploring all possibilities and ruling out all of them,” said Masseron. “Basically, the answer is we don’t know.” The team is planning to study the stars in different wavelengths of light to learn more about their chemical fingerprints. They also hope to spot more examples of these outlier stars in order to see if they belong to a broader family of stars with equally inexplicable histories.
A New Physics to Find Their Origins?
“Beyond the additional observations we are going to make, there will be another important aspect regarding collaboration with stellar interior and evolution experts, who now need to develop new models and possibly implement new physics to try to reproduce what we have observed, and thus attempt to find the origin of those stars,” Masseron concluded.
In addition, the IAC team could obtain the optical spectrum of the most brilliant of the phosphorus stars with the Echelle spectrograph (FIES) on the Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory (Garafía, La Palma).
Image credit: NASA, early massive stars