The formation of gas giant planets is assumed to be preceded by the emergence
of solid cores in the conventional sequential-accretion paradigm. This
hypothesis implies that the presence of earth-like planets can be inferred from
the detection of gas giants. A similar prediction cannot be made with the
gravitational instability (hereafter GI) model which assumes that gas giants
(hereafter giants) formed from the collapse of gas fragments analogous to their
host stars. We propose an observational test for the determination of the
dominant planet-formation channel. Based on the sequential-accretion (hereafter
SA) model, we identify several potential avenues which may lead to the prolific
formation of a population of close-in earth-mass (Mββ) planets
(hereafter close-in earths) around stars with 1) short-period or 2) solitary
eccentric giants and 3) systems which contain intermediate-period resonant
giants. In contrast, these close-in earths are not expected to form in systems
where giants originated rapidly through GI. As a specific example, we suggest
that the SA processes led to the formation of the 7.5 Mββ planet around
GJ 876 and predict that it may have an atmosphere and envelope rich in O2β
and liquid water. Assessments of the ubiquity of these planets will lead to 1)
the detection of the first habitable terrestrial planets, 2) the verification
of the dominant mode of planet formation, 3) an estimate of the fraction of
earth-harboring stars, and 4) modification of bio-marker signatures.Comment: 13 pages, 3 figures, Accepted for publication in ApJ