Would Centaurians know we exist?

In which we ask the question: If we were denizens of the Alpha Centauri system, and we were looking for extra-terrestrial life from there, using the technology that we have here today, would we be able discover Earth, and/or human life?


To borrow from the brilliant Karl Popper, as long as the existence of extraterrestrial life or intelligence is not falsifiable (meaning you would have to check every exoplanet around every star), you could continue the search for ET forever. I’m not sure that this is in line with the scientific method; ideally you’d be looking objectively at what’s out there and trying to understand what you see from that standpoint.

Radial velocity

Let’s start by pondering our methods for exoplanet detection. One of our most successful is the radial velocity method. It’s not an accident that the first planets we have found this way, are predominantly large planets, in tight orbits around relatively small stars. These simply the easiest planets to find. Unfortunately for the general public, this gives the impression that most planets are very large, in tight orbits around relatively small stars!

On the other hand, our island Earth orbits the sun at 1.0 AU. Its orbit causes the sun to deviate by a mere 0.09 m/s, or 9 centimeters per second. This leads me to wonder what our current best precision is? Only about 1.0 m/s. That’s right, we would need to increase precision by about 100 times in order to detect Earth from Alpha Centauri using the methods we have right now.

Notice that I didn’t say “see” the Earth. That requires a set of optics that we don’t have in orbit yet. Perhaps the James Webb space telescope will come close to that. It will be launched in a few years.

Transit photometry

Using transit photometry, another way of detecting exoplanets, the actual size (rather than just its mass) can be estimated. But for a planet orbiting a Sun-sized star at 1.0 AU, the probability of a random alignment producing a transit is 0.47%. In other words, since the exoplanet has to be lined up just right, the chances of detecting a random planet around any particular star is about one in one hundred (1:100).

Radio emission (SETI)

If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth?

Many times, we hear that our radio transmissions, which date back many decades, are flying out to outer space at the speed of light. That our broadcasts from 40 years ago are reaching stars that are 40 light-years away. Might xenomorphs be puzzling over TV broadcasts of I Love Lucy or Adolph Hitler’s early radio broadcasts?

That’s partially true. However, there are some problems. Firstly, the vast majority of our radio signals are very weak and are – in effect – aimed at local terrestrial receivers rather than turned up to the stars.

Secondly, the strength of the signal diminishes as the inverse square of the distance. In other words, it would drop off extremely quickly. Even from Alpha Centauri, we would probably not be able to detect radio transmissions from Earth.

Mix that in with these facts: our scientists, possibly frightened by too many viewings of Ridley Scott’s movie Alien, are extremely reluctant to beam radio messages deliberately into space. And surely civilizations on other planets have equally cautious scientists!

Also, when we do think that we have received a signal from outer space, we wait for it to be repeated. This is for scientific verification. But the one signal that we have deliberately beamed into space (the fools!) was NOT repeated. So again, we can imagine alien scientists, possibly detecting one of our signals, and waiting for it to repeat, so they can verify it.

Long story short (or perhaps it’s too late for that) we are not ready to say “yes” or “no” about whether we have alien companions out there in the universe. Baby steps, we are still making baby steps.


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