io9 has an intriguing story about a planet larger than Earth and as dense as lead
. (Not to mention being white hot).
A bit of digging turns up the paper that this story is based on
, made available last week. However, in the course of searching for it I found this paper by another team
, which whilst referencing the first paper (I assume they saw a draft) uses IR rather than optical measurements to come up with a diameter for 55 Cancri e that is half as much again as that derived in the other paper. That cuts the density dramatically for the same mass (which is fairly well-characterised) thus taking the planet from 'twice as dense as Mercury' to 'about as dense as Mars', which is a little less dramatic.
What both papers make clear, and which the io9 story doesn't, is that even if 55 Cancri e is as dense as lead, it isn't made of lead. Or comparably-dense silver or thorium, or even, alas, iron with a core of platinum. Rather, it's made of iron, but iron compressed to much greater than normal density by the sheer mass of the planet, some eight times that of Earth. Incidentally, a planet that close to its star couldn't be made of lead - at over 2,000 degrees, it would have boiled away...
So, be a little wary of dramatic science claims. It may be that someone has missed another paper, or just picked the one with the more dramatic conclusions.