We may be more alone in the Universe than we realize. According to Cardiff University astronomers Jane Greaves and Phil Cigan, the chemical element phosphorus is abundant on Earth, but may be very rare outside of our Solar System. Because phosphorus is essential to life, it's rarity may mean that life may be equally rare.
One of the pillars of the search for extraterrestrial life is that there is nothing exceptional about the Earth or our Solar System. The assumption is that planets like our Earth, stars like our Sun, and galaxies like our Milky Way are commonplace. Therefore, the conditions that gave rise to life here must also be equally commonplace, so by extension life must also be.
The problem is that the origin and evolution of life are very complex processes that are highly dependent on the presence of various elements and conditions. If one of these elements turns out to be extremely rare, then the spontaneous emergence of life becomes extremely difficult. One example of this is phosphorus, which is relatively common on Earth, but may be the result of an uncommon event.
"Phosphorus is one of just six chemical elements on which Earth organisms depend, and it is crucial to the compound Adenosine TriPhosphate (ATP), which cells use to store and transfer energy," says Greaves. "Astronomers have just started to pay attention to the cosmic origins of phosphorus and found quite a few surprises. In particular, [phosphorus] is created in supernovae – the explosions of massive stars – but the amounts seen so far don't match our computer models. I wondered what the implications were for life on other planets if unpredictable amounts of [phosphorus] are spat out into space and later used in the construction of new planets."
To learn more about how phosphorus is produced, in November 2017 the Cardiff scientists turned to Britain's William Herschel Telescope at La Palma in the Canary islands. They used the giant infrared instrument to look at the spectra of phosphorus and iron in the Crab Nebula. Some 6,500 light years away in the constellation of Taurus, the Crab Nebula was formed by a supernova explosion that was recorded by Chinese astronomers in 1054 CE. The idea was that by comparing the spectra of the nebula with that of another supernova remnant, the astronomers could get a better understanding of the origin and distribution of phosphorus.
"This is only the second such study of phosphorus that has been made," says Cigan. "The first looked at the Cassiopeia A (Cas A) supernova remnant, and so we are able to compare two different stellar explosions and see if they ejected different proportions of phosphorus and iron. The first element supports life, while the second is a major part of our planet's core.
"These are our preliminary results, which we extracted only in the last couple of weeks. But at least for the parts of the Crab Nebula we were able to observe so far, there seems to be much less phosphorus than in Cas A. The two explosions seem to differ from each other, perhaps because Cas A results from the explosion of a rare super-massive star. We've just asked for more telescope time to go back and check, in case we've missed some phosphorus-rich regions in the Crab Nebula."
According to the team, the comparison indicates that phosphorus may be unevenly distributed in the galaxy, as the element is created by particular types of supernovae and brought to forming planets in meteorites or other objects. This could mean that life arising on Earth could have been dependent on the Solar System forming in the lucky proximity of a phosphorus-spewing supernova. However, other planets might not have been so lucky. To find out if this is the case, the Cardiff team plans to look at other supernovae remnants to see if a similar phosphorus shortage can be seen.
"The route to carrying phosphorus into new-born planets looks rather precarious," says Greaves. "We already think that only a few phosphorus-bearing minerals that came to the Earth – probably in meteorites – were reactive enough to get involved in making proto-biomolecules.
"If phosphorus is sourced from supernovae, and then travels across space in meteoritic rocks, I'm wondering if a young planet could find itself lacking in reactive phosphorus because of where it was born? That is, it started off near the wrong kind of supernova? In that case, life might really struggle to get started out of phosphorus-poor chemistry, on another world otherwise similar to our own."
The research will be presented at the European Week of Astronomy and Space Science in Liverpool.
