The Easy Way to Let Extraterrestrials Know We Exist

Apr 14, 2016

Photo credit: NASA

By Summer Ash

As I write these words, there are a total of 1,327 confirmed exoplanets that we have detected because they periodically block the light from their host star. Of those 1,327 exoplanets, only 38 orbit their stars in the habitable-zone (i.e., at distances that could allow for liquid water to exist on their surfaces). And of those 38 exoplanets, only 15 of them orbit long-lived, more stable stars like our Sun and its slightly cooler brethren. That’s only fifteen stars; fifteen habitable-zone, transiting planets around cool, stable stars. That’s where the search for intelligent life should begin according to Columbia astrophysicists David Kipping and Alex Teachey. Or at least the search for indirect signs of it.

We never actually see these planets directly. Instead, when a planet crosses in front of its star, we see the light from that star decrease by a measurable amount. How much it decreases, and how long, depends on the star system—and possibly its inhabitants.

NASA’s Kepler Mission is a space-based telescope dedicated to observing the light from over 150,000 stars in a patch of sky the size of your outstretched palm. (Fun fact: If we had 400 Keplers, we could observe the light from stars across the entire sky.) Kepler monitored each of these 150,000 stars for over four years looking for the tell-tale dip in light that could indicate the presence of a planet. That resulted in 150,000 light-curves, one per star.

In their new paper published last week in the Monthly Notices of the Royal Astronomical Society, Kipping and Teachey ask the question: What if another civilization in our galaxy had their own Kepler Mission? What would they see when they observed our star, the Sun?

They would see a tell-tale dip in the light from our Sun. This light curve would tell them there is a planet just under 13,000 kilometers in diameter orbiting a star 1,000,000 kilometers in diameter. And if they looked long enough (over one Earth year), they would see the signal repeat and be able to deduce how far we are from the Sun and the likely range of surface temperatures we experience.

What they would not be able to see with their Kepler is evidence of life on Earth. For that, they would need to detect the presence of certain “biosignatures” in our atmosphere such as water vapor, oxygen, or industrial pollutants such as ozone. We don’t yet have the technology needed to observe these fingerprints in the atmospheres of exoplanets, at least not until the James Webb Space Telescope launches in 2018. But if this hypothetical civilization did, then they might conclude that our planet plays host to advance life forms.

Would we want them to know this?


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