And not just define but be able to detect the essential components of our definition. I find sentences like the above quoted from here, to be annoying because from what I understand it is misleading and trivialises science for the general populace who on the whole have no conception of distances as it is, never mind about the physics of planets. I do not think that any reasonable definition of an Earth like planet would qualify for the one referred to in the quote. The article even mentions a three essential characteristics none of which were detected. In fact the planet is not even known to be solid. As one of the essential characteristics for an Earth like planet must surely be protection from deadly cosmic rays, a liquid metal core would seem to be a must have item. Therefore while we can detect a slight dimming as a planet transits a relatively nearby star, would it ever be possible in the foreseeable future for us to be able to detect whether one of these transit events that are hundreds or thousands of light years away, has a liquid metal core? It doesn't seem like it to me but I could be wrong. and this quote here... That is also not true. How has it been shown?