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?
Kipping and Teachey leave that question aside, but they do explore how we could hide or broadcast our presence. And they argue that other civilizations could already be doing this. Given the high probability of planets with life in our galaxy, this is one possible reason we haven’t encountered it yet. Maybe other civilizations don’t want to be found.
Using the example of a hypothetical planet the same size as Earth and orbiting a star like our Sun, Kipping and Teachey show how a technologically advanced civilization on this planet could alter the light-curve that other (hypothetical) Keplers would observe.
To escape detection, Kipping and Teachey propose a cloaking method in which residents of this planet could potentially counter-act the dip in the observed light curve by using a system of lasers. That may sound fantastical (and complicated), but it turns out to be more feasible than you would think.
To cancel the light blocked by an Earth-like planet passing in front of a Sun-like star, it would only take a 30 megawatt (MW) laser pointed at us for 10 hours in order to fool Kepler into thinking the planet didn’t exist. Thirty megawatts is roughly equal to what 6,500 American homes use in a year (compared to more than 100 billion homes nationwide). It’s also equivalent to the amount of power the solar arrays of the International Space Station collect in a year. This hypothetical civilization would be well aware of the all the potentially habitable planets nearby that lie close to the ecliptic plane of its star system. Thus, they could easily determine when and where to point their laser.
Alternatively, instead of hiding the entire planet, the residents could hide just the signs of their existence. Using lasers at specific wavelengths, they could counter the absorption features of oxygen and ozone in their atmosphere (these act as biosignatures) and we’d never be the wiser.
But what if another civilization wanted to be found? What if they wanted to send a message across the stars to let us know they were out there? In a way, we’ve already done this ourselves with the Search for Extraterrestrial Intelligence (SETI) and the Pioneer and Voyager spacecraft, but not in as targeted a fashion as Kipping and Teachey suggest.
If residents of a transiting Earth-like planet really wanted to be noticed, they could alter their light-curve in a different way. Instead of using lasers to remove the drop in light as they cross in front of their star, they could use lasers to enhance their transit signature in a way that is both “energy efficient and unambiguously artificial.” Kipping and Teachey propose adapting the cloaking method described previously, but only at the beginning and end of the transit, thereby creating a dip in the light that no planetary model would fit—meaning it would have been altered on artificially. To do this would require orders of magnitude less power than for the full cloaking.
Perhaps what’s most interesting about Kipping and Teachey’s calculations is that this is well within our technological reach. So it’s not that great a leap to hypothesize that another civilization on an Earth-like planet could be doing it already. And that’s where things get really exciting.
As I mentioned earlier, Kepler collected over 150,000 light curves, but Kepler was not the only project searching for transiting exoplanets. There’s also SuperWASP, CoRoT, HATNet, XO, and TrES to name a few and there are also new projects coming online in the next few years such as NGST, TESS, and PLATO. To date, over 21 million light curves have been amassed and while a significant fraction of them have been analyzed for planetary transit signatures, to my knowledge, none have been analyzed for artificial signatures, as suggested by Kipping and Teachey. In fact, they are actively applying for funding to do just that.
The amazing thing about astronomy is that we can’t go out and manipulate the objects we want to study, yet we can deduce so much about them just from the smallest fractions of light that reach us. What if other planets are broadcasting their existence to us? There’s a nonzero possibility that we’ve already gotten the message, but we just didn’t recognize it for what it was. It gives a whole new meaning to the old phrase “hello world.”