Everything that we care about, everything that we understand and everything that we perceive is just on one of the eight planets orbiting one of the hundreds of billions of stars in the Milky Way Galaxy which is one of the hundreds of billions of galaxies in the observable universe. Perhaps, somewhere in a galaxy far far away there is an alien being looking at the Milky Way where we can be found. What will they see?
That is exactly the question that researchers asked themselves when they reconstructed how alien astronomers observing the Milky Way from far away would find if they analysed the galaxy’s chemical composition. The findings of the study published in Nature Astronomy indicate that our home galaxy is “unusual, but not unique.”
Flipping the telescope
To understand how aliens will see our galaxy, it would perhaps be prudent to first understand how we see distant galaxies. Our telescope observations show us a galaxy’s shape and its “spectrum,” which is the sum of the parts that make up the light from the galaxy.
Our astronomers here on Earth have developed some clever methods to deduce the properties of a faraway galaxy by observing the little light that we get from it. So, we can start by assuming that a sufficiently advanced alien civilisation will have similarly sophisticated methods to look at the Milky Way.
“Finding ways to compare our home galaxy with more distant galaxies is what we need if we want to know whether the Milky Way is special or not. This has been an open question since astronomers realiszed a hundred years ago that the Milky Way is not the only galaxy in the universe,” lead author Jianhui Lian, in a press statement.
Local cosmology is the first step
To understand what alien astronomers can deduce when they turn their telescopes towards the Milky Way, the researchers first considered the chemical composition of the stars around us. The close ones that we can see are mostly made of hydrogen and helium but that is not all. They also contain bits of other elements heavier than helium. In astronomy, these elements are called “metals.” This nomenclature does not work for ordinary chemistry, of course.
According to the Max Planck Institute, some of these metals are produced inside stars and flung into space when they explode while others are produced in the outer layers of bloated giant stars and left to drift in space from there.
An important trend can be deduced from this. The interstellar medium—the low-density mix of gas and dust that fills the space between time—will see an increase in the concentration of metals over time. This means that stars that were born earlier will contain fewer metals while those born later will contain more.
Thanks to this, you can figure out which regions of the interstellar medium had stars forming earlier and which regions had stars forming earlier.
Switching to an alien perspective
With the current state of technology, the Milky Way is the only spiral galaxy where astronomers can directly conduct a large-scale survey of individual stars. This includes measuring their positions within our galaxy and through their spectra, their metal content and other physical properties.
So now the question is, how would the abundance of metals detected in a region by astronomers change based on the distance of the region from the centre?
Some regions will have more dust while some other regions will have less. So, in some directions, there will be more dust between the observer and distant stars. This will attenuate some of the starlight and hide some of the dimmest stars altogether. The astronomers had to combine observational data with what they know about dust and about the properties of stars to reconstruct the real distribution of stars in the galaxy
‘Belt’ of high metallicity
If the alien astronomers track the average metal content of stars from the center of the galaxy outwards, it will increase, reaching a metal content close to that of our Sun at a distance of about 23,000 light-years away from the centre. At larger distances, the average metal content goes down again, reaching about one-third of the solar value at around 50,000 light-years away.
Unusual compared to other galaxies
The researchers compared the results of analysing the Milky Way with the properties of 455 galaxies similar to Milky Way—321 from the MaNGA survey and 134 from the TNG50 survey. Only 11 per cent of the galaxies in the TNG50 sample and about 1 per cent of the galaxies in the MaNGA sample showed a make-up similar to what would be seen from our galaxies.
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