The case of the missing phosphorus: Astronomers find life ingredient at galaxy's edges
The discovery of phosphorus in a molecular cloud at the edge of the Milky Way galaxy challenges current views of how the element originates and extends the galactic habitable zone.
Astronomers at the University of Arizona have discovered excephosphorus, a critical ingredient for life as we know it, in an unexpected location: the outskirts of the Milky Way galaxy. According to conventional wisdom, the element is produced by fusion processes inside very massive stars, which are not believed to exist in the outer reaches of the Milky Way.
Other, less heavy elements necessary for life, such as carbon, oxygen and nitrogen, can form in lower-mass stars, which are much more abundant. When those stars reach the end of their life, they release those elements relatively calmly into the interstellar medium.
"But to make phosphorus, you need some kind of violent event," explained Lucy Ziurys, Regents Professor of chemistry and biochemistry and astronomy and astronomer at Steward Observatory. "It is thought that phosphorus is created in supernova explosions, and for that, you need a star that has at least 20 times the mass of the sun."
When a star goes supernova, it spills its innards into the surrounding space, including the elements that formed the building blocks for life in our solar system.
"In other words, if you're going to have life, you better be near a supernova, if that's indeed the only source where phosphorus is created," Ziurys said.
The study, published in Nature, calls into question the conventional wisdom that nature's only way of producing phosphorus is through supernova explosions.
"The phosphorus we detected is at the edge of the galaxy, where it shouldn't be," said Lilia Koelemay, a doctoral student studying chemistry and the first author of the paper. "And so that means that there has to be some other way phosphorus is created."
One such mechanism that had been proposed in the past invokes "galactic fountains" that lift phosphorus from the inner reaches of the Milky Way far above the galaxy's plane and shower it back onto the disk farther out. However, evidence of such fountains remains scarce, and even if they exist, Koelemay says it would be unlikely they reach farther than a kiloparsec or so, or about 3,260 light-years.
"Even then it would take so long for the material to fall back into the galaxy that it probably wouldn't have formed the molecules that we were seeing," she added.
So, if supernovae can't account for phosphorus at the edges of the galaxy, how did it get there?
According to one theory, Ziurys said, low and intermediate mass stars may generate excess neutrons by stripping them off of carbon atoms during the end of their life cycle and accumulating them in pockets between their hydrogen-burning and their helium-burning shells. Adding those neutrons onto silicon atoms would result in phosphorus.
"This has been postulated in theory, and so it could possibly explain another source of phosphorus in addition to supernovae, and I think we have good evidence supporting this now," Ziurys said.
Koelemay and her co-author, chemistry doctoral student Katherine Gold, used the 12-meter radio telescope of the Arizona Radio Observatory on Kitt Peak and a 30-meter radio telescope near Granada, Spain, operated by the Institute for Radio Astronomy in the Millimeter Range, or IRAM.
Their observations detected the telltale signatures of phosphorus – specifically phosphorus monoxide and phosphorus nitride – in a molecular cloud named WB89-621. Located nearly 74,000 light-years from the center of the Milky Way, the discovery extends the presence of phosphorus almost twice as far out as where it was known to exist. Because matter becomes sparser the farther one ventures out from the galaxy's center, the outer reaches simply don't have enough mass to support the formation of stars big enough to end their lives as supernovae.
The project began with a homework assignment: Koelemay and Gold took a course in astrochemistry that Ziurys teaches, and she encouraged them to scan the cloud for phosphorus-containing molecules, admitting that it might be a long shot. As part of the assignment, the students wrote a proposal to ask for observing time on the IRAM telescope, which is very rarely granted to institutions outside IRAM's European members. Their request was granted, and both traveled to Spain to be trained on the instrument.
"I guess our homework assignment was good enough for them," Koelemay said.
The discovery of phosphorus has direct implications for the search for planets around other stars that may be Earth-like and capable of sustaining life, the researchers say, pointing to the so-called NCHOPS elements, which make up the critical ingredients of life on Earth: nitrogen, carbon, hydrogen, oxygen, phosphorus and sulfur.
"For a planet to be habitable to life as we know it, you have to have all the NCHOPS elements, and their presence defines the galactic habitable zone," Ziurys says. "With our discovery of phosphorus, all of them have now been found at the edge of the galaxy, which extends the habitable zone all the way out to the galactic outskirts."
Phosphorus in particular is important in the search for Earth-like planets, according to Gold, because minerals containing phosphorus are important for Earth-like planets with solid surfaces.
"Exoplanets in the outer galaxy have not been fully considered in the search for life due to an assumed dearth of phosphorus," Gold said. "We hope that the detection of phosphorus at the edge of the galaxy may motivate studies of distant exoplanets."
According to Koelemay, phosphorus is an "amazing" element. "It is so important in biological molecules, yet we really don't know very much about it," she said.
Next, the team plans to scan other molecular clouds in the far reaches of the Milky Way for phosphorus. Finding it elsewhere would bolster the case that the textbook story of how phosphorus finds its way from star factories onto the surfaces of planets and into the backbone of a universal molecule defining life on Earth, may need updating.
"Now that we have trained at the IRAM telescope, we can observe remotely," said Gold, explaining that she has already started observing on a few other projects. "I'm confident that there will be more papers coming from those projects."
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