The Invisible Majority? Evolution and Detection of Outer Planetary Systems without Gas Giants

Observations suggest that only a minority (25-50%) of stellar systems contain gas giants. Hence the majority (50-75%) of stellar system do not. This report is the first significant study of these non-gas-giant systems that I am aware of.

The Invisible Majority? Evolution and Detection of Outer Planetary Systems without Gas Giants

5.2. Major conclusions and implications

In the vast majority (169/180) of our primary runs, and across all initial conditions we investigate, an oligarch migrates interior to the ice line, settling to between 25% and 60% of the ice line distance in about 10 Myr and growing into a planet with a median mass of 0.23 Mice. We call this object the innermost migrated planet, or IMP. In 123 of the 169 primary runs with an IMP, the IMP is the most massive planet. IMPs are clearly distinguishable from the other planets (Figures 7 and 8 ). The migration is a result of exchange of angular momentum between the IMP and the other, exterior oligarchs: 5 of the 11 runs that did not produce an IMP contain only a single planet at 5 Gyr. The migration is significant because, unlike with gas giants, the planets in our simulations have low masses compared to the residual disk mass. The existence of mass in the inner system only slightly affects the final position and mass of IMP, but the converse is not true (see below). IMPs may be the visible representatives of an otherwise “invisible” majority: The common occurrence, relatively high mass, and small semimajor axis of IMPs make them eminently detectable by microlensing, transits (with Kepler), and astrometry (with SIM-Lite), but not yet by current Doppler capabilities.

Ground-based microlensing is currently capable of detecting planets as small as ∼ 3 M⊕ at separations of 1.5-3 AU, and indeed several planets with (uncertain) masses between a few M⊕ and one or two Neptunes have been found in this distance range (Beaulieu et al. 2006; Gould et al. 2006; Bennett et al. 2008; Sumi et al. 2010). These few detections may represent only the tip of the IMP-berg: all available constraints on the frequency of gas-giant and lower-mass planets from current radial velocity and microlensing surveys are consistent with the scenario that the minority of stars host gas giants, and that at least ∼ 60% of stars host systems such as those we have simulated (Gould et al. 2006; Sumi et al. 2010). Future microlensing surveys will provide a definitive statistical measurement or upper limit on the frequency of systems like those predicted here. If 60% of stars indeed host systems similar to those we simulate, we estimate that next-generation ground-based microlensing surveys (Gould et al. 2007; Gould 2008; Gaudi et al. 2009) will detect ∼ 26 planets per year, including a handful of multiple-planet systems. A space-based microlensing survey would be sensitive to essentially all of the planets we have simulated (Bennett & Rhie 2002; Bennett et al. 2009; Beaulieu et al. 2010).

Assuming an ice-rock composition, all IMPs predicted here would produce a transit sufficiently deep to be detected by Kepler. 83% have periods less than the spacecraft’s 3.5 yr mission. If IMPs are present around 60% of solar-type stars, we predict that Kepler will detect ∼129 of them with two or more transits. Observations of additional transits in high cadence (1 min resolution) mode in an extended Kepler mission could reveal additional, exterior planets through the variation of the timing of transits. Direct calculations show variations of 20-90 min in our predicted systems. Finally, SIM-Lite should be able to detect 96% of IMPs.

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Intriguingly, inner systems of two dominant planets are not stable in our scenario. In all simulations with Earth and Venus analogs, the two bodies collided, forming a single body at ∼ 0.8 AU. No contradiction with the solar system is engendered because it contains giant planets. Such a conglomerate would induce a barycenter motion of 0.2 m s−1 (260 d period) which may be detectable by future ultra-high precision Doppler monitoring (Pepe & Lovis 2008). On the other hand, if planet formation in the inner system has progressed only to the giant impact (oligarch-dominated) phase by the time the IMP migrates inward, the IMP will clear most of this mass, leaving only small (< 0.3 M⊕) bodies.

If disruption of the inner system does occur, the IMP is left as the only detectable planet near, but exterior to, the nominal habitable zone of an Earth “twin” (0.95-1.37 AU) (Kasting et al. 1993). However, it is expected that surface temperatures will be higher on more massive planets with thicker atmospheres, such as could be the case for the IMP. Given that IMPs have large quantities of water, it then follows that IMP-like planets could be the most numerous type of habitable planet in the universe.