The solar system is awash in X-ray photons produced by solar coronal emission, solar wind auroral precipitation, and by charge exchange between the hot (~106 oK) solar wind and cold (101-103 oK) planetary, cometary, and instreaming ISM neutrals. Measurement of X-ray emission lines can thus provide a unique probe of the physics of the Sun, planets, comets, and heliosphere in our solar system, and in the nearby interstellar medium (ISM). The proposed Line Emission Mapper (LEM) Probe will provide ~1 eV spectral resolution from 0.2 -2.0 keV to explore this science. For example, at 1 eV resolution, individual charge exchange lines produced in the tenuous outer atmosphere of Mars, the Moon, and comets can be isolated. These lines identify, in great detail, the charge state and velocity broadening of the emitting highly stripped ions, providing clues to the impinging solar wind and secondary ion particle populations, their velocities, and the underlying physical interaction mechanisms (Lisse+ 2001, 2005; Bodewits+ 2007; Dennerl+ 2006, 2012; Collier+ 2014). Further, through comparison of the line ratios, it may be possible to track the neutral populations that are lost from the planetary atmosphere or comet. Mars, Venus and the Moon are also excellent X-ray scatterers, providing important optically thick "surface" compositional information (Dennerl+ 2002, 2008; Dong+ 2016, Gloudemans+ 2021). At 1 eV resolution, Jupiter's magnetosphere, driven by its enormous ExB fields and 10 hr planetary rotation, is a fundamental atomic and plasma physics laboratory (McEntee+ 2023). At many of these targets, LEM data will be complemented by contemporaneous in situ data from orbital spacecraft. This enables the opportunity to tie the drivers to their X-ray signatures, offering irreplaceable ground-truths for more exotic astrophysical environments that cannot be visited in-situ. The outer solar system, e.g. Uranus and Pluto, is just beginning to be detected by the current X-ray observatories with handful of photons (Chandra and XMM, Lisse+ 2017, Dunn+ 2021); LEM will go 10x deeper on these systems, allowing determination of the operant driving emission mechanisms. On a larger scale, LEM measurement of spectral lines common to heliospheric SWCX and the local hot bubble (Koutroumpa+ 2009, 2012, 2017), but with different ratios and Doppler shifts that allow to distinguish the source region, will reveal both the large-scale structure of the heliosphere and that of the VLISM.