Mirror Earth is about one of the most exciting research topics in astrophysics at the moment, planets that exist beyond our solar system (exoplanets). It was only in the 1990s that scientists were able to detect such planets, and they've been overturning our expectations about the universe ever since. Before exoplanets, scientists assumed that planets near their stars must be small and rocky, with large gaseous planets further away (such as Jupiter and Saturn). But the first exoplanets discovered were many times larger than Jupiter and many times closer to their stars than Mercury is to our sun. Many have strange orbits and don't behave in the way that we expect planets to at all. Theoretically, these planets just shouldn't exist. But they do.Of course, discovering exoplanets (and there are many known ones now) is only the first step for scientists who hope to find a 'mirror' earth, one that could meet all the conditions for life. This is really the meat of the book, as Lemonick discusses attempts to isolate and study planets that could possibly be habitable. He takes a Bill Bryson-like approach of mixing the science with information about the scientists and the process of discovery, and this is largely successful. I spent the first half of the book in awe of the perseverance and ingenuity of scientists who have been able to identify invisible planets around stars that are mere sparkling dots in the sky. It's not simply a case of looking into the sky with a telescope, it's about the slightest movement of a star due to the planet's gravity and the tiniest possible blurring. It's technical stuff.Things get even more interesting as the discussion moves on to questioning our assumptions about life. Most of the book and indeed, most of recent scientific history, is taken up with the quest to find planets like our own because scientists have assumed that life can only exist in similar conditions. But in the final chapter, Lemonick considers other possible planetary systems from the realm of theoretical physics. It's possible there are carbon-based planets with cores of pure diamond and "diamond continents sloping down to seas of tar." What kind of life might such a planet harbour? It seems physicists are questioning their basic ideas about life and I can't wait to see what research comes out of this in the future.I really enjoyed Mirror Earth, but it wasn't perfect. Whilst it was interesting to read about the actual scientists and their quests to discover planets (especially their funding issues), some of these sections went on for too long. Had I not studied astrophysics at an introductory level, I would have struggled to understand the techniques the physicists used, although I could have kept up with the theories and discoveries easily. But for an "awe and wonder" book, it's hard to beat. The universe is more strange and diverse than we ever thought, and we're only scratching the surface of all the different possibilities.