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Thursday, July 4, 2013 / Duncan Forgan

Exoplanet Transits and Star Shunters


If my last post on population synthesis modelling was a little dry for your tastes, I hope this SETI post will whet your whistle.  This particular post comes so late that other websites have already written about this work.  While this makes me look bad, it does mean that I will be using their very good graphics to illustrate my points.

Like so.

Like so. (via i09)

There has been a long tradition in SETI research in looking for evidence of large structures produced by alien civilisations, using whatever astrophysical data there is to hand.  The archetype is the Dyson sphere (or Dyson swarm), a spherical shell that completely encircles a star, and collects its entire energy output.  The Dyson sphere is a completely solid structure, which can double as a vast habitat many times the size of the Earth; the Dyson swarm is a group of satellites in a spherical configuration that simply collects energy.

Dyson spheres/swarms extract close to all energy from their host star, and depending on their efficiency, re-emit a fraction of this energy at longer wavelengths.  This makes a star like our Sun look more like a brown dwarf, albeit a gargantuan brown dwarf many many times the radius one would expect.  Several teams are searching catalogues of infrared surveys , such as WISE,  for anomalous brown dwarfs that could well be Dyson spheres in disguise.

I was interested in a variant of the Dyson sphere, which is referred to as a Class A Stellar Engine, or Shkadov thruster.  Instead of completely covering the star with a spherical shell, a partial spherical shell is built instead.  The interior of this partial shell is mirrored to reflect the star’s radiation.  Stars exist thanks to a balance between the force of gravity trying to collapse it, and radiation attempting to blow it apart.  Adding this Shkadov thruster disrupts this balance, and produces a net force away from the mirror.

If the mirror is placed in an orbit such that the radiation forces on it are balanced by the star’s gravitational attraction, the mirror stays in place, and the star, mirror (and any planets the star possesses) will begin to move together.  Advanced civilisations that can build Shkadov thrusters can now choose to shift their entire star system; either to avoid dangers such as approaching molecular clouds, or because conditions in another part of the Milky Way are more conducive to their way of life.

If the star system in question has an exoplanet we can detect via the transit method, and the mirror also comes in between the star and us, then the exoplanet transit curve will change, and in principle we will be able to measure this.

Artist's impression of the thruster obscuring the transit.

Artist’s impression of the thruster obscuring the transit. (via Discovery News)

Exoplanet transit curves get their crescent shape from a process called limb-darkening.  When astronomers look at the disc of a star on the sky, they find that the centre of the disc is brighter than the edges (the limbs).  There are two reasons for this: firstly, the edges of stars are cooler than the centre, and secondly, the edges are less dense.  A normal exoplanet transit curve will have a symmetric crescent shape – if part of the star is blocked thanks to the Shkadov thruster, then the limbs will be hidden, and part of the curve will not be limb-darkened, and the crescent shape will be lost.

The shape of a Shkadov transit, showing the limb-darkened and obscured sections of the curve.

The shape of a Shkadov transit, showing the limb-darkened and obscured sections of the curve.  (via a shameless screengrab of my talk this coming Friday)

I call these Shkadov transits, and in the paper I show how the curve can still yield the exoplanet parameters as well as information on the thruster itself.  Current observations (such as those made by the recently defunct Kepler Space Telescope) should be able to detect these asymmetries, although things like starspots can confuse the issue and make disentangling the contributions of planet and thruster difficult.  Even without the problems that come with noise, if we want to to fully unpack the curve and figure out what’s going on, we need to be able to combine the transit data with radial velocity (Doppler Wobble) observations, such as those being made by HARPS North right now.  This means that the thruster needs to be pushing on a bright star that is close by.  And this may not be likely!

Even if we’re quite optimistic about (a) how many stars have transiting planets we can see, (b) how frequently alien civilisations arise, and (c) how frequently alien civilisations build Shkadov thrusters, there is at most one visible thruster for every million stars in the Milky way.  At most!

We are only just reaching a thousand exoplanets, never mind a million, so the odds are currently against us.  But then, when has the odds ever stopped SETI researchers? Kepler has several thousand candidates in the pipeline, and we are due to see many more exoplanet transit missions on the ground and in the sky very soon.  Once this data goes public, SETI scientists can trawl the archives searching for these asymmetric transits.  The worst case scenario is that they find no thrusters, which would give upper limits on how many thrusters exist in the Milky Way.  In the best case, a Shkadov transit is detected, and the first evidence of extraterrestrial structures is found.  And we can get this for virtually no extra cost – exoplanet transit missions will always go on because astrophysicists want to understand planet formation, and the data will eventually be public.

All we need are SETI scientists to analyse this data, which may not be as straightforward as it sounds.  I’ll be on the Panel for the National Astronomy Meeting’s SETI session on the future of SETI science in the UK, and we’ll be addressing the issue of careers in SETI.  Feel free to come say hi!

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