Friday, 30 October 2015

Another hint that the Kepler system KIC8462852‬ is probably not harbouring an advanced alien civilisation.


KIC8462852‬ ( Boyajian et al. 2015 ) has created quite some excitement as perhaps the first example of an alien megastructure eclipsing what appears to be a fairly normal, main sequence star. In particular, Wright et al. have suggested that the light curve is consistent with a Dyson “swarm” (see image left for my poor attempt at how that might actually appear).

For the Dyson swarm scenario, the remarkable depth of the obscuration in the light curves of KIC8462852‬ (dips in the stellar luminosity in excess of 20% are observed) suggests a scale of astro-engineering that is best associated with something approaching a Kardashev Type 2 civilisation - in other words, a civilisation with energy requirements of order 400E24 Watts or 400 YW (yottawatt) - i.e. around 14 orders of magnitude greater than the current total energy consumption by our own species on planet Earth).

Waste (leakage) Radio emission form a Type II civilisation

If one assumes that advanced Type II civilisations use radio waves to communicate across their planetary system (and why wouldn't they - radio waves are cheap to generate, diffract around and sometimes through solid structures, are immune to scattering and absorption by dust and are harmless when permeating the local environment of complex biological lifeforms), then it becomes interesting to consider what level of broadband artificial radio emission we might expect from such an advanced civilisation, and whether this can be detected by the current or next generation of radio telescopes. Note that this is a rather different approach from convention SETI radio searches currently underway, since the goal there is to detect artificial narrow-band carrier wave signals that are a clear indicator of intelligent life.

I estimate that our own civilisation (Kardashev Type ~ 0.7 - note that it's a logarithmic scale...) currently emits an equivalent isotropically radiated power (EIRP) of at least 50 MW at frequencies < 2000 MHz. This figure is likely an underestimate, and is based only on the collective output of GSM base stations located around the globe (there are 750000 of these in the USA alone with average output power levels of ~50 Watts). These antennas provide relatively broad coverage, sweeping across the horizon in all directions. I have excluded the billions of handheld devices worn outside in the open air (these deliver average powers of << 1 Watt) and relatively rare but powerful radar systems (the average output of an airport radar system is a few hundred Watts but there are probably only a few hundred systems in the world). Other broad beam transmitters such as satellite communication systems, deep space networks, radio and tv transmitters, ionospheric radar transmitters, power-lines and other likely absorbed indoor radio devices (house-hold wifi routers, laptops etc) are also excluded from this estimate. While almost all of the available radio spectrum from 1-2000 MHz is occupied by various radio communication systems, it should be noted that the GSM base stations are currently limited to a few main operational frequency bands e.g. 930-960 MHz and 1805-1880 MHz.

Level of waste radio emission expected from KIC8462852

In this back-of-an-envelope analysis, we make two assumptions: (i) the waste radio emission generated by a communicating civilisation scales linearly with its total energy consumption and (ii) the radio emission is fairly evenly distributed across the available radio spectrum. Assumption (i) predicts a total isotropic waste radio emission output from a Type 2 civilisation associated with KIC8462852‬ of ~ 50E20 Watts or 2.5E12 Watts/Hz (10-2000 MHz). Since KIC8462852 is located at a distance of ~ 450 pc or ~ 1.4E19 metres, this output translates to a flux density of 0.1 Jansky, a level of output that is readily detectable by most radio telescopes today with integration times of only a few seconds. Note that as a main-sequence star, any natural emission from KIC8462852 is essentially undetectable at these sensitivity levels.

Radio emission observed from KIC8462852 via shallow, existing radio surveys

Relatively shallow radio surveys (e.g. NVSS and WENSS) detect no radio emission at the position of KIC8462852 (see images below: first the WSRT WENNS 326 MHz image, followed by VLA NVSS 1400 MHz image - the green cross hairs indicate the position of the KIC8462852). NVSS reaches 1-sigma r.m.s. noise levels better than 0.0025 Jy i.e. about 40 times better than the 0.1 Jy emission that we might predict from KIC8462852‬. With a resolution of 45 arcseconds, NVSS is unlikely to resolve any radio emission associated with KIC8462852‬, even if this is distributed on scales associated with a space-faring civilisation active within an extended planetary system. Naturally, any conclusions based on this analysis depend crucially on the validity of assumption (i). However, much deeper integrations by the JVLA and LOFAR can place far better limits (x1000) on any broadband radio emission that might be associated with KIC8462852, and are to be greatly encouraged (together with conventional SETI searches too). Finally, it should be noted that the full SKA and other next generation telescopes such as the NG VLA will go 50-100x deeper than current facilities, permitting interesting limits to placed on nearby stellar systems within a few hundred light years from the Sun.