Origin and Ubiquity of Short-Period Earth-like Planets: Evidence for the Sequential Accretion Theory of Planet Formation

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 model, which assumes that gas giants formed from the collapse of gas fragments analogous to their host stars. We propose an observational test for the determination of the dominant planetary formation channel. Based on the sequential accretion model, we identify several potential avenues that may lead to the prolific formation of a population of close-in Earth-mass (M_⊕) planets around stars with (1) short-period or (2) solitary eccentric giants and (3) systems that contain intermediate-period resonant giants. In contrast, these close-in Earths are not expected to form in systems where giants originated rapidly through gravitational instability. As a specific example, we suggest that sequential accretion processes led to the formation of the 7.5 M_⊕ planet around GJ 876 and predict that it may have an atmosphere and envelope rich in O₂ and liquid water. Assessments of the ubiquity of these planets will lead to (1) the detection of the first habitable terrestrial planets, (2) verification of the dominant mode of planet formation, (3) an estimate of the fraction of stars harboring Earth-like planets, and (4) modification of biomarker signatures.