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Although current sensitivity limits are such that true Solar System analogsremain challenging to detect, numerous planetary systems have been discoveredthat are very different from our own Solar System. The majority of systemsharbor a new class of planets, bodies that are typically several times moremassive than the Earth but that orbit their host stars well inside the orbit ofMercury. These planets frequently show evidence for large Hydrogen and Heliumenvelopes containing several percent of the planet's mass and display a largediversity in mean densities. Here we show that this wide range can be achievedby one or two late giant impacts, which are frequently needed to achievelong-term orbital stability in multiple planet systems once the gas disk hasdisappeared. We demonstrate using hydrodynamical simulations that a singlecollision between similarly sized exoplanets can easily reduce theenvelope-to-core-mass ratio by a factor of two and show that this leads to acorresponding increase in the observed mean density by factors of 2-3. Inaddition we investigate how envelope-mass-loss depends on envelope mass, planetradius, semi-major axis, and the mass distribution inside the envelope. Wepropose that a small number of giant impacts may be responsible for the largeobserved spread in mean densities, especially for multiple-planet systemscontaining planets with very different densities and which have not beensignificantly sculpted by photo evaporation.

Stealing the Gas: Giant Impacts and the Large Diversity in Exoplanet Densities