Aliens Can't See Us - What a Relief!

Put yourself in the place of an alien scanning the universe for exoplanets and maybe even intelligent life.

Would they see Earth?

That’s a question that researchers have been trying to answer. But would the aliens even be interested?

An alien species might after all have entirely different life-chemistries to us, perhaps silicon-based (ours is based on carbon) and so what would be a good Earth-like exoplanet for us could be highly toxic to them. So, they might pass us over.

But let’s assume for now that Planet Earth would be of interest to them.

How might they be looking?

Transit technique

There are number of techniques, but the most successful of these has been the transit technique.

This method has made about 75% of all the exoplanet discoveries so far. It involves watching for the periodic dimming of a star’s light as a planet passes between the star and an observer on Earth.

But think about it. You can only observe the planet as it passes the star. So, you have to be looking in the right place at the right time. That’s a big ask even using automated scanning and differential image processing.

But there is now an alternative.

Photometric micro-lensing

Photometric microlensing utilises the gravitational lens effect that occurs when one star passes in front of another, temporarily magnifying the light from the more distant, “source” star.

If the nearer of the two stars has a planet orbiting it, this can further perturb the light path. This can result in characteristic spikes in the observed light intensity and spectrum.

One plus is that this technique works at ‘long distance’ (yes, it’s all relative isn’t it) and so increases the potential observational sample size.

Earlier exoplanet detection techniques have typically only yielded planets up to one kiloparsec (about 3261 light–years) away from Earth.

Up to now, most of the 130 exoplanets detected using microlensing are up to seven times that distance from Earth. To put that number in context, we have currently identified more than 5,000 exoplanets using all methods. (NASA)

Back to the aliens

According to Physics World:

It has long been considered that those locations from which Earth could be detectable via the transit method are themselves good candidates for targeted SETI searches — following a game theoretic, “Schelling Point” cooperation strategy for two parties looking for each other who have no means of communicating. Applying the same logic to the microlensing technique has, therefore, the potential to identify new and more distant targets for the search for extraterrestrial intelligence.

That sent me off on another fascinating mission to explore the Schelling Point, somewhere I had not been before:

In game theory, a focal point (or Schelling point) is a solution that people tend to choose by default in the absence of communication. The concept was introduced by the American economist Thomas Schelling in his book The Strategy of Conflict (1960). Schelling states that “(p)eople can often concert their intentions or expectations with others if each knows that the other is trying to do the same” in a cooperative situation, so their action would converge on a focal point which has some kind of prominence compared with the environment. However, the conspicuousness of the focal point depends on time, place and people themselves. It may not be a definite solution. — Wikipedia

The nub of this is that if we assume that an alien race is trying to do the same as us and use microlensing, then how likely are they to ‘see’ Earth?

Astronomer Eamonn Kerins of the University of Manchester and his colleagues did just that, first defining an ‘EMZ’.

“[We] dub the regions of our galaxy from which Earth’s photometric microlensing signal is most readily observable as the ‘Earth microlensing zone’ (EMZ),” the researchers explain, adding: “The EMZ can be thought of as the microlensing analogue of the Earth Transit Zone (ETZ) from where observers see Earth transit the Sun”. (Physics World ibid.)

The sample

The researchers analysed the second data release (DR2), which includes information on more than 1.1 billion stars from the European Space Agency’s Gaia telescope. The researchers then calculated those areas from where Earth’s microlensing signature (with the Sun as the source star) would be visible.

What did they conclude?

The total Earth discovery rate is only 14.7 observers per year across the entire sky, they estimated.

That is very small.

In an interview with Physics World, Kerins said that “Earth would be a challenging target, in part because it is rather too close to the Sun to give a strong lensing signal for most potential observers”.

“Furthermore” he continued, “our location 27,000 light-years from the [Milky Way’s] galactic centre is something of a blind spot for any observer using microlensing.”

There are areas of the Milky Way where there might be better chances of our discovery using source stars other than the Sun behind us:

The highest chance of detecting Earth is towards the Milky Way’s Orion–Cygnus arm, in the galactic plane. There, Earth’s microlensing probability and discovery rate values are 3.28×10−10 and 2.35×10−2 observer per year per square degree respectively. (Physics World ibid.)

Advanced?

We are an arrogant race to think of ourselves as an advanced civilization, when we have still not cracked the peaceful nuclear fusion power problem. And while we still kill each other en masse.

There may well be a whole host of intergalactic communication techniques that we know nothing about.

As Arthur C. Clarke wrote:

Any sufficiently advanced technology is indistinguishable from magic.

Entertainment

Both Star Trek and The X-Files have recorded episodes in which silicon-based life is a feature.

X-Files:

And after that I’ll just leave you with the Fermi Paradox:

The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high a priori likelihood of its existence, and by extension of obtaining such evidence.

But the aliens are here, Enrico (wherever you are right now), it’s just that we cannot see them, unless we have the equipment.

And silicon based life? I did once feel some. Outstanding! They certainly had a life of their own. Then I realised they were silicone. There’s a difference.

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Canonical link: This story was originally published in Medium on 27 November 2022 [modified]