  Planets in the mass range from 2 to 15 MEarth are very diverse. Some of them
have low densities, while others are very dense. By measuring the masses and
radii, the mean densities, structure, and composition of the planets are
constrained. These parameters also give us important information about their
formation and evolution, and about possible processes for atmospheric loss.We
determined the masses, radii, and mean densities for the two transiting planets
orbiting K2-106. The inner planet has an ultra-short period of 0.57 days. The
period of the outer planet is 13.3 days.Although the two planets have similar
masses, their densities are very different. For K2-106b we derive
Mb=8.36-0.94+0.96 MEarh, Rb=1.52+/-0.16 REarth, and a high density of
13.1-3.6+5.4 gcm-3. For K2-106c, we find Mc=5.8-3.0+3.3 MEarth,
Rc=2.50-0.26+0.27 REarth and a relatively low density of 2.0-1.1+1.6
gcm-3.Since the system contains two planets of almost the same mass, but
different distances from the host star, it is an excellent laboratory to study
atmospheric escape. In agreement with the theory of atmospheric-loss processes,
it is likely that the outer planet has a hydrogen-dominated atmosphere. The
mass and radius of the inner planet is in agreement with theoretical models
predicting an iron core containing 80+20-30% of its mass. Such a high metal
content is surprising, particularly given that the star has an ordinary (solar)
metal abundance. We discuss various possible formation scenarios for this
unusual planet.
