  Although current sensitivity limits are such that true Solar System analogs
remain challenging to detect, numerous planetary systems have been discovered
that are very different from our own Solar System. The majority of systems
harbor a new class of planets, bodies that are typically several times more
massive than the Earth but that orbit their host stars well inside the orbit of
Mercury. These planets frequently show evidence for large Hydrogen and Helium
envelopes containing several percent of the planet's mass and display a large
diversity in mean densities. Here we show that this wide range can be achieved
by one or two late giant impacts, which are frequently needed to achieve
long-term orbital stability in multiple planet systems once the gas disk has
disappeared. We demonstrate using hydrodynamical simulations that a single
collision between similarly sized exoplanets can easily reduce the
envelope-to-core-mass ratio by a factor of two and show that this leads to a
corresponding increase in the observed mean density by factors of 2-3. In
addition we investigate how envelope-mass-loss depends on envelope mass, planet
radius, semi-major axis, and the mass distribution inside the envelope. We
propose that a small number of giant impacts may be responsible for the large
observed spread in mean densities, especially for multiple-planet systems
containing planets with very different densities and which have not been
significantly sculpted by photo evaporation.
