Knock, knock. It's humanity.
The Philae lander in the middle of what might be an alien hive. (via ESA)
If you follow space news at all, you're probably aware that humanity has a doohickey called the Rosetta probe orbiting a comet, specifically comet 67P/Churyumov–Gerasimenko, or "67 Pikachu" for short (not really, I made that up). This was a pretty big deal, since people don't put space probes into orbit around a comet every day...or ever, since it was our first time. Even more groundbreaking, we shot an even smaller doohickey—the Philae lander—onto the comet's surface.
In space, no one can hear comets burp. (via ESA)
Since we've arrived, however, 67 Pikachu has constantly defied our expectations...and one explanation is that it harbors microscopic life. The most important part of this theory is that 67 Pikachu keeps spitting out geysers of gas and water. That's not unusual for a comet as it approaches the sun and heats up, but 67 Pikachu is doing it way too early. It should be too cold for that sort of activity right now, so scientists have speculated that some sort of microscopic life is generating the gas and turning part of the interior to liquid. Also, those geysers are chock-full of hydrocarbons and even amino acids—the materials of life.
67 Pikachu also has a different composition than scientists expected. In particular, it has a hard, black crust over an icy interior. This caused problems when the Philae lander's harpoons failed to stick to the surface as planned, sending the probe tumbling for over a kilometer on the comet's surface before falling into a crater. Some think the black crust could be the hydrocarbon byproducts of microscopic life. This is in part because, being a black surface, this material would heat up very quickly and evaporate when close to the sun—so, the thinking goes, something must be replenishing it. In other spots, there are smooth "seas" of what appear to be water that melted (whether because of internal heat or the sun's rays) and then re-froze.
Walk into the solar system like whatup, I'm a big rock(y lump of ice). (via ESA)
The idea that life, or at least the raw materials to allow it, arrived on Earth via comets is a fairly popular one. The idea that life may just be constantly zipping around the universe on such bodies is also not out of the mainstream. It should be noted that the two scientists advocating the "we're looking at life right now through Rosetta's cameras" are not quacks, but they are oddballs. Professor Chandra Wickramasinghe was involved with Rosetta's mission planning 15 years ago, although at the time the suggestion that the probe bring life-detecting equipment was "laughed out of the room." Now, he and Dr Max Wallis will be presenting their case for life on 67 Pikachu (again, I made that nickname up) at the Royal Astronomical Society's annual meeting in Wales this summer. So, slightly out of the mainstream, but still real scientists. Also, it should also be noted that over a long-enough timescale, mainstream scientists are almost always wrong.