Phosphorus - a key ingredient of life as we know it - is thought to be relatively rare in space. But now, astronomers have detected surprising amounts of phosphorus at the edge of the Milky Way, suggesting life may be more common in the universe. Life on Earth requires six key elements: nitrogen, carbon, hydrogen, oxygen, phosphorus and sulfur (NCHOPS). Most of these elements are relatively easy to obtain because ordinary low-mass stars are blown away into space when they end their lives. Phosphorus, on the other hand, is much rarer and is therefore generally thought to be the limiting factor for life in the universe.

"To make phosphorus, you have to go through some sort of violent event," said Lucy Ziurys, corresponding author of the study. "It's thought that phosphorus is produced in supernova explosions, and for that you need a star that's at least 20 times the mass of the sun. In other words, if you're going to have life, you'd better be near a supernova, if that's really going to be the only source of phosphorus in the universe."

But in new research, astronomers have detected phosphorus where it "shouldn't be" there, suggesting that unknown mechanisms for making phosphorus may be more abundant than we thought.

The research team used the Arizona Radio Observatory and the radio telescope of IRAM in Spain to observe a molecular cloud named WB89-621. Excitingly, they detected clear signs of phosphorus monoxide and phosphorus nitride.

The cloud is about 74,000 light-years away from the center of the Milky Way, almost twice as far away as the area where phosphorus was previously found. In the outer reaches of the Milky Way, there isn't enough material to form massive stars, which produce phosphorus when they die.

So, how does it get there? Previously proposed mechanisms include a "galactic fountain" that leaks phosphorus from the center and sprays it across the disk. But there's little evidence to support this, and even if such a fountain existed, it's not expected to eject any farther than about 3,000 light-years. Instead, the team believes that low- and intermediate-mass stars can create phosphorus by stripping neutrons from carbon atoms and adding them to silicon atoms.

"This is theoretically hypothetical, so it could potentially explain another source of phosphorus besides supernovae, and I think we now have good evidence to support that," Ziewurs said. Other research groups have also found evidence of phosphorus-rich stars that could also be a source of phosphorus. This astrochemical discovery could have major implications for alien life. It has been thought that phosphorus's relative rarity would put a hard limit on the prevalence of life across the universe, but if phosphorus is found throughout the galaxy, perhaps we're ruling out promising planets too soon."

For a planet to be habitable for life as we know it, it must have all of the NCHOPS elements, and the presence of these elements determines the habitable zone of the Milky Way. With our discovery of phosphorus, all phosphorus elements are now found at the edge of the Milky Way, which extends the habitable zone all the way to the outer reaches of the Milky Way.

The research was published in the journal Nature.