A Transit Signature for SETI?

David Kipping and Alex Teachey have a new paper out on the possibility of ‘cloaking’ a planetary signature. The researchers, both at Columbia University, make the case that any civilization anxious to conceal its existence — for whatever reason — would surely become aware that all stars lying along its ecliptic plane would see transits of the home world, just as its own scientists pursued transit studies of planets around other stars. And it turns out there are ways to make sure this signal is masked by adjusting the shape of the planet’s transit light curves.

Now this is a fascinating scenario as presented by the head of the Hunt for Exomoons with Kepler, whose business it is to know about the slightest of variations in light curves because they may contain information about exomoons or rings. Thus Kipping is a natural to look into the artificial manipulation of light curves, a study with definite SETI implications. Because methods like these work in two directions — a civilization that does want to communicate could also alter its light curve in ways that would be unambiguously artificial. Today I want to focus on the latter idea, reserving cloaking methods for tomorrow’s post.

This veers into the METI debate, but that’s not my purpose. Messaging to Extraterrestrial Intelligence is highly controversial, triggering arguments in these pages for the last decade. But let’s hold METI at one remove. The paper, delightfully titled “A Cloaking Device for Transiting Planets,” allows us to imagine how the manipulation of transit signatures could change a distant planet’s visibility. We may well decide not to brighten the Earth’s visibility through intentional transmissions, while understanding that an extraterrestrial culture might choose differently. Knowing what is possible by way of cloaking or enhancing a planetary signature, then, gives us plenty of food for thought for SETI as we consider what might turn up in our data.

Modifying the Light Curve

Here’s the notion in a nutshell, as drawn from the paper.

Image: Figure 1. Illustration (not to scale) of the transit cloaking/broadcasting device. A laser beam (orange) is fired from the night side of inhabited planet (blue) towards a target star whilst the planet appears to transit the star, as seen from the receiver. In the case of the Earth, the planet could be cloaked by generating an inverse transit-like signal of peak power 60 MW. Credit: Kipping and Teachey.

Controlled laser emissions are the key, effectively distorting the shape of a transit light curve. Just how this could be done is something we’ll discuss in the next post. But to begin, let’s think about how visible we are to any advanced civilization studying us. Forget about our radio and television signature, which is infinitesimal, and focus instead on the things we are doing right now to study other stars. I often write about transmission spectroscopy, which is how we search for the constituent molecules in a planetary atmosphere. The transiting planet, as it moves in front of its host star (as seen by our instruments) is bathed in the star’s light, its atmosphere providing a particular ‘overlay’ to the star’s spectrum.

We’ll use the method to look for biosignatures as we refine it for smaller and smaller worlds, but we’ve already seen how successfully we can examine the atmospheres of ‘hot Jupiters’ like HD 189733b. A sufficiently capable civilization able to see our world transiting should be able to pick up the cluster of biosignatures that would identify the Earth as a living world. There have been proposals to look for atmospheric pollutants produced by industrial activity as a part of future SETI practice, and such activity could indeed become visible, though the idea of widespread pollution lasting for millennia seems a stretch. Understanding the damage it caused, surely the culture in question would either solve its pollution problems or else succumb to them.

Let’s not forget the recent flurry of interest in the unusual star KIC 8462852. Here we’re looking at what could well be a natural phenomenon in the form of clouds of comets, but could possibly be evidence of artificial megastructures throwing highly distinctive light curves. We have much work ahead on KIC 8462852, so I only bring it up to suggest that there are many ways an intelligent species might make itself visible whether its intent was to do so or not.

Turning Transits into ‘Broadcasts’

Scientists as diverse as Ronald Bracewell, Richard Carrigan, Michael Papagiannis and Robert Freitas have studied the possibilities of such detections, with Carrigan most prominently identified with the search for Dyson spheres, swarms of energy collectors that surround a star to feed its colossal energies to a growing Type II civilization. And back in 2005, Luc Arnold (now at Aix Marseille Université) suggested that a civilization wanting to be known could resort to deliberate signaling by building a particular kind of geometric megastructure, one that when viewed in a transit would all but shout, through its distinctive light curve, that it was artificial.

Kipping and Teachey are, as you would imagine, well aware of Arnold’s work, and argue that lasers offer far more practical methods. From the paper:

Whilst any number of artificial transit profiles can be created with lasers, one ideally seeks a profile which is both energy efficient and unambiguously artificial. Producing upward spikes in-transit might seem like an obvious suggestion, but star spot crossings produce these forms with complex and information rich signatures (e.g. see Beky et al. 2014). Here, we argue that cloaking the ingress/egress of a transit, but leaving the main transit undistorted, would be a highly effective strategy since no known natural phenomenon is likely to produce such an effect.

Image: Figure 6 from the paper, highlighting broadcasting rather than cloaking. Top: The power profile of a laser array designed to broadcast the Earth. An array of lasers producing a peak power of ∼ 20 MW for approximately 15 minutes nullifies the transit ingress and egress. Bottom: The resulting light curves as viewed by different broadband optical photometers. The observed impact parameter would be complex infinity, for which a normal light curve fit would be unable to explain and thus indicating the presence of artificial transit manipulation.

A monochromatic laser emission could be spotted in a transit survey (and could be searched for in Kepler data), with follow-up spectroscopy confirming the artificial nature of the transmission. Kipping and Teachey also note what James and Dominic Benford recently pointed out in their paper on SETI efforts at KIC 8462852 — a beamed signal of whatever kind could carry information, so that in addition to identifying the presence of a technological culture, the beam could attempt more detailed communication (see SETI: Power Beaming in Context).

I’ve focused in on broadcasting via transit light curve alteration because, as the paper argues, it is the most efficient use of these techniques as compared to cloaking, which I’ll describe tomorrow. I’m trying to stay out of the METI weeds here — this is not an argument in favor of identifying the Earth to ETI. Rather, it is an argument that other civilizations may use such methods, and thus this paper shows us a signature we should add to our catalog.

And it has this further implication: If a civilization did choose to broadcast its existence through these methods, it would probably choose the shortest period planet in its solar system to carry the message. The choice is obvious, as the paper notes, for this produces “a higher duty cycle of distorted events,” making the detection all the more obvious. “We therefore suggest that any survey in archival data should not be limited to rocky planets in the habitable-zone of their host star.” An excellent reminder not to succumb to easy assumptions!

Tomorrow I’ll return to the Kipping and Teachey paper with a look at cloaking possibilities. The paper is “A Cloaking Device for Transiting Planets,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).