The search for alien intelligence has mostly centered on waiting to hear any interstellar radio signals, but we might now have a way to see aliens as well. We just need to look for mega-engineering projects on alien asteroid belts.
The idea is the brainchild of Duncan Forgan of the University of Edinburgh and Harvard-Smithsonian astrophysicist Martin Elvis. Asteroid belts like the one between Mars and Jupiter are the debris fields left over from failed planet formations, and the countless rocks that make them up are full of precious elements and minerals. Based on meteorite samples, we're already pretty certain that our own belt has abundant quantities of gold, platinum, iron, nickel, and silicon, amongst other valuable elements.
And we also know that our asteroid belt is far from unique. NASA's Spitzer Space Telescope has already detected a bunch of asteroid belts around other stars, including three nested belts around the star Epsilon Eridani. An advanced civilization - but perhaps only a thousand years more advanced than our own - could mine these asteroids as the raw material to build space colonies and solar power satellites, not to mention provide fresh resources vital for more earthly technologies like computers, cell phones, and high speed networks. They could even do their best Larry Niven impression and go ahead and build a Ringworld.
But could we actually detect a civilization mining its asteroid belt? Forgan and Elvis admit this would be very tricky, but not impossible. We might be able to pick up on an unusual chemical imbalance in the belt caused by the removal of certain desired elements. The alien miners would likely pull apart the larger asteroids in search of vital materials, which would throw off the size distribution of the asteroids. The mining operations might also create huge amounts of dust, which would throw off our temperature readings.
The problem is that any or all of these clues could also be created by natural forces, so just finding these signs wouldn't be proof of alien engineering. However, it would give us somewhere to look. Our exoplanet-hunting telescopes could investigate a solar system with an unusual asteroid belt for signs of a habitable planet, and SETI could sweep the area for potential telecommunication signals. It's a long shot - everything that involves hunting for alien life inevitably is - but for now asteroid engineering might just be our best chance at finding signs of extraterrestrial intelligence.