The Real-Life Search for Aliens

Absorption spectroscopy, from Coel Hellier's blog post

Diagram explaining absorption spectroscopy, from Coel Hellier’s blog post

In a previous series of posts, I presented a simple analysis of our chances of ever contacting aliens. The foundation of my argument was that any planet that does harbor intelligent life will almost certainly be too far away, not just for us to reach by spaceship, but even for us to contact with any type of signal. Thus, although the universe may be teeming with life, we’re never going to see any of it (except what’s right here on Earth).

Today, I ran across this blog post by Coel Hellier, an actual scientist who searches for the signatures of extraterrestrial life (though I think that’s not the only goal of his work). In his blog, he does a great job of explaining in layman’s terms how real-life searches are actually carried out, from identifying stars that have planets orbiting them to analyzing the chemical composition of those planets’ atmospheres. I encourage you to read his original article, but I’ll give a brief summary of it below in case you’re too lazy to click a mouse.

  • We can identify a star that has a planet orbiting it by the periodic dimming of the star as the planet passes between us and the star, blocking part of the star’s light.
  • The planet’s size is determined from how much of the star’s light it blocks.
  • Its mass is determined by measuring the redshift of the star’s light when the planet is behind the star (pulling the star away from us) or the blueshift of the star’s light when the planet is in front of the star (pulling it toward us).
  • From size and mass, we get density, and from density, we get a rough idea of what chemical elements are most abundant on the planet.
  • We can even determine what molecules are present in large quantities in a planet’s atmosphere by looking at the fringe of light that passes through the planet’s atmosphere as the planet passes in front of the star. Different molecules will absorb light from different parts of the spectrum of the star’s light, so all we have to do is compare unimpeded light from the star to light that passed through the planet’s atmosphere to determine what types of molecules are in the planet’s atmosphere. This is called spectroscopy.

Coel writes that, using spectroscopy,

we are beginning to be able to detect the atmospheres of extra-solar planets, despite them being hundreds of light-years away. If we can detect molecules in the atmospheres of exoplanets then, in principle, we might detect “biomarker” molecules that indicate organic activity (such as free oxygen). Thus it is realistic that, within a couple of decades, we will have found other Earth-like planets that we know to bear life.

The beauty of this approach is that with an array of sensitive telescopes that can record light from stars all over the sky, we will be able to use automated analysis software that will process the data from thousands or millions of stars in a relatively short period of time. Thus, it might actually be possible to verify the presence of life on other planets in a relatively short time frame. Exciting!

Having said all of that, it is still the case that even if we do find planets that harbor life, they will almost certainly be hundreds if not thousands of light-years away from us. We can try sending them a signal, but even on the off-chance that the life there is advanced enough to detect and decode our signal, it would be hundreds or thousands of years before their reply ever reached us.

Thus, I maintain for now my pessimistic conclusion that we’ll never contact aliens.

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