  The high resolution non-dispersive spectroscopy and unprecedented sensitivity
of Athena+ will revolutionize solar system observing: the origin of the ions
producing Jupiter's X-ray aurorae via charge exchange will be conclusively
established, as well as their dynamics, giving clues to their acceleration
mechanisms. X-ray aurorae on Saturn will be searched for to a depth
unattainable by current Earth-bound observatories. The X-ray Integral Field
Unit of Athena+ will map Mars' expanding exosphere, which has a line-rich solar
wind charge exchange spectrum, under differing solar wind conditions and
through the seasons; relating Mars' X-ray emission to its atmospheric loss will
have significant impact also on the study of exoplanet atmospheres. Spectral
mapping of cometary comae, which are spectacular X-ray sources with extremely
line-rich spectra, will probe solar wind composition and speed at varying
distances from the Sun. Athena+ will provide unique contributions also to
exoplanetary astrophysics. Athena+ will pioneer the study of
ingress/eclipse/egress effects during planetary orbits of hot-Jupiters, and
will confirm/improve the evidence of Star-Planet Interactions (SPI) in a wider
sample of planetary systems. Finally Athena+ will drastically improve the
knowledge of the X-ray incident radiation on exoplanets, a key element for
understanding the effects of atmospheric mass loss and of the chemical and
physical evolution of planet atmospheres, particularly relevant in the case of
young systems.
