  Having discovered that Earth-sized planets are common, we are now embarking
on a journey to determine if Earth-like planets are also common. Finding
Earth-like planets is one of the most compelling endeavors of the 21st century
- leading us toward finally answering the question: Are we alone? To achieve
this forward-looking goal, we must determine the masses of the planets; the
sizes of the planets, by themselves, are not sufficient for the determination
of the bulk and atmospheric compositions. Masses, coupled with the radii, are
crucial constraints on the bulk composition and interior structure of the
planets and the composition of their atmospheres, including the search for
biosignatures. Precision radial velocity is the most viable technique for
providing essential mass and orbit information for spectroscopy of other
Earths. The development of high quality precision radial velocity instruments
coupled to the building of the large telescope facilities like TMT and GMT or
space-based platforms like EarthFinder can enable very high spectral resolution
observations with extremely precise radial velocities on minute timescales to
allow for the modeling and removal of radial velocity jitter. Over the next
decade, the legacy of exoplanet astrophysics can be cemented firmly as part of
humankind's quest in finding the next Earth - but only if we can measure the
masses and orbits of Earth-sized planets in habitable zone orbits around
Sun-like stars.
