In short, we thought it had a period of 2.8 days and a minimum mass of 14 Mearth. This is from radial velocity measurements. The paper from last year, however, made the case that this period may be due to aliasing in the data. If the planet has a period closer to 0.7 days, it could appear to have a 2.8 day period in radial velocity observations. And if its period is 0.7 days, then there’s a really good chance that it could transit its star.
Which, it turns out it does. These new observations confirm it has a 0.74 day orbit, and that it’s mass is much lower: 8.5 Mearth. But because it’s transiting, we can much more accurately determine its radius: 1.63 Rearth. This gives it an average density of 11 g cm^-3. For comparison, that makes it twice as dense as the Earth or Mercury. For further comparison, that iron meteorite we had at our Physics Fair table (just to the left of Veselin’s laptop in the picture) has a density of roughly 7.5 g cm^-3 and weighs 24.5 lbs. It was roughly the size of a large dog’s head. If it were a chunk of 55 Cnc e, then it would weigh 36 lbs., roughly a third heavier.