Where planets are born may set their chances of hosting life


Phosphorus is a vital element for life, but a new study of the Crab Nebula suggests it may not be evenly distributed around the galaxy. Vhairi Mackintosh reports.


The Crab Nebula displays a puzzling shortage of phosphorus, one of the elements necessary for life on Earth.
The Crab Nebula displays a puzzling shortage of phosphorus, one of the elements necessary for life on Earth.
NASA / Hubble

When it comes to searching for life, we’ve usually assumed that all parts of our galaxy are equally likely contenders.

Maybe not, according to Cardiff University astronomers Jane Greaves and Phil Cigan, who presented some new findings at the European Week of Astronomy and Space Science (EWASS) in Liverpool last week.

The astronomers found that the Crab Nebula, a giant cloud of dust and gas left over from a supernova that was observed by Chinese astronomers in the year 1054, is very low in phosphorus – a vital component in DNA. This suggests planetary systems that form in the region will have a hard time supporting life.

Phosphorus – along with hydrogen, carbon, nitrogen, oxygen and sulfur – is one of six elements essential for life on Earth. It is produced when massive stars, more than eight times the size of the Sun, explode as supernovae. The phosphorus and other stellar shrapnel are ejected and disperse out into space to form a supernova remnant.

It was only in 2013 that phosphorus was directly detected in a supernova remnant. In that remnant, Cassiopeia A, researchers found the ratio of phosphorus to iron was as much as 100 times the average ratio found throughout the Milky Way.

However, the amount of phosphorus measured in Cassiopeia A was at odds with the predictions of some computer models. And that got Greaves thinking: “I wondered what the implications were for life on other planets if unpredictable amounts of P [phosphorus] are spat out into space and later used in the construction of new planets.”

So the Cardiff team used the UK’s William Herschel Telescope to observe infrared light from phosphorus and iron in the Crab Nebula. They found that the Crab Nebula contains far less phosphorus than Cassiopeia A, although the exact abundances are still to be measured.

The comparison suggests that the balance of elements in material ejected into space during supernovae can vary dramatically. Although it is still uncertain exactly how planets in the cosmos get their phosphorus, if supernovae are responsible, “the route to carrying phosphorus into new-born planets looks rather precarious,” remarks Greaves.

Lucky regions of the cosmos “could potentially ‘hit the jackpot’ and be showered with plentiful phosphorus,” says Cigan. On the other hand, planets born near phosphorus-poor supernova might struggle. Cigan cautions, however, that more observations are needed to ensure that their preliminary results are representative of the Crab Nebula as a whole. The team also hope to extend their phosphorus inventory across the galaxy by studying other supernova remnants.

Cigan is pleased with the study’s reception at the meeting last week: “They said [the findings were] very cool.”

Vhairi Mackintosh is a scientific educator, writer and researcher based in Melbourne who holds a PhD in earth sciences.
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