(Image: A laser guide star cast on the night sky from the William Herschel Telescope at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands.)
When astronomers look for parts of the galaxy that could contain life, they generally search for elements like oxygen and carbon. But another element essential to life could be the key to finding systems in the Milky Way that have the right conditions for living organisms.
"Phosphorus is one of the six elements on which biology depends," Jane Greaves, an astronomer at Cardiff University in Wales, told Seniorhelpline in an email. "The others are carbon, hydrogen, nitrogen, oxygen and sulphur. Without phosphorus, there would be no (ATP), which is the molecule cells use to transfer energy."
Phosphorus is relatively rare in the universe, the rarest of the six elements required for life as we know it. It is created in trace amounts in some stars' natural evolution, but the majority of the universe's phosphorus is fused in supernovae. The element, atomic number 16, only accounts for about 0.0007 percent of all matter.
Greaves and fellow Cardiff astronomer Phil Cigan are presenting new research at the European Week of Astronomy and Space Science in Liverpool that compares the amount of phosphorus in the stellar dust of two supernova remnants— (Cas A) in the constellation Cassiopeia, and the in the constellation Taurus. The early results suggest that the Crab Nebula contains significantly less phosphorus than Cas A.
The discrepancy comes as a surprise, as computer models suggested the two collections of stellar dust, created by the same type of supernova, should contain similar amounts of phosphorus. Understanding this difference could help us understand how levels of this crucial element are distributed across the stars.
"Cas A and the Crab Nebula are Core Collapse Supernovae, where the middle of the star implodes and then rebounds very fast, expelling the new elements made," says Greaves. "My guess is that Cas A had more reactions that made phosphorus because the star was more massive or denser, but that's just a guess so far."
If unknown processes cause some stellar explosions to produce more phosphorus than others, then life could be isolated to phosphorus-rich areas of the galaxy. At this point, however, only Cas A and the Crab Nebula have been studied with telescope spectroscopy to determine their chemical compositions. "As far as I know, phosphorus has not been looked for in any other supernova, of any type," says Greaves.
The team stresses that their research is preliminary and uses limited data. Phosphorus was . Graves and Cigan only recently used the William Herschel Telescope in the Canary Islands to study the infrared spectrum of the Crab Nebula, measuring the proportion of phosphorus and iron to compare to that of Cas A. Observations of the Crab Nebula were somewhat hindered by cloudy weather, however, and follow up research is needed to confirm that it is indeed lacking in the element P.
Another possibility is that the age difference between the two clouds of cosmic dust could explain the different amounts of phosphorus. The Crab Nebula was created by a supernova seen and documented from Earth by Chinese astronomers almost a thousand years ago, while the light from the supernova that created Cas A is thought to have reached Earth about 300 years ago, though no one is known to have observed it.
"It is possible that with the older event, the Crab Nebula, that some phosphorus has disappeared from gas and [formed] into solid material, something we hope to learn more about at this scientific meeting," says Greaves.
After being ejected from supernovae, phosphorus gasses coalesce and are trapped in rocky objects. These rocky, icy, and metal bodies clump together further to create rocky planets, which is how most of the phosphorus made it into Earth. However, the phosphorus that was first used in cells to transfer energy, and spark reproductive life, likely came after the planet formed and had large bodies of water, as meteorites bearing phosphorous crashed into the wet parts of the world.
To find where else in the galaxy the spark of life could occur, the trick might be to look for planetary systems that came from phosphorus-rich areas. The upcoming 6.5-meter James Webb Space Telescope, designed for infrared astronomy, should be particularly suited to measuring phosphorus in supernova remnants—gasses that will ultimately form stars and planets.
"I'm very much looking forward to JWST, as this can potentially look for schreibersite  in discs around stars where new planets are forming, and it has a good wavelength range to look for this mineral we know occurs in meteorites," says Greaves.
With only two supernova remnants scanned for the element, and the capability to look for schreibersite in planetary systems coming online soon, the hunt for life-bearing phosphorus could just be getting started.