tokens: small orbital eccentricities () of the terrestrial planets. Here, we propose that the observed wide gaps in some debris disks may have been cleared by the secular resonances of a known planetary candidate (in Fomalhaut) or yet to be detected (around Vega and other systems) isolated or multiple gas giants or super Neptunes, as the secular resonances sweep across this region during the depletion of the gas disk. Analogous to the asteroids, excited eccentricities of the residual planetesimals in the region between the inner disk and outer ring are mainly damped by the hydrodynamic drag of the diminishing disk gas. In addition to the extended gap, this scenario offers a potentially exciting prospect of finding signatures of ongoing planetary assemblage in debris disks around stars with an age comparable to the formation epoch of terrestrial planets in the solar system (Su et al. 2009 ). Since not a single planet with a mass in excess of (an observationally inferred upper limit) has been found around Vega and Fomalhaut, we explore here the possibility that the extensive gaps are produced by a sweeping secular resonance of a lone-planet with a modest mass and eccentricity in the debris disks around these and other similar systems. Although systems of multiple Neptune-mass planets may also lead to a similar effect, we opt the lone-planet scenario as an idealized simplest possibility. This paper is organized as follows. In Section 2 , we briefly introduce the lone-planet possibility in the Vega system, including detailed physical models to mimic the dust-free gap opening process by the lone-planet scenario in the Vega system, develop a rough method to constrain planet candidates based on observed features, and provide predictions for future planet detection. In Section 3 , we further extend our model to Vega-like system, Fomalhaut, inferring the properties of the prospective planet and compare these with a planet candidate, Fomalhaut b. A summary of our results and an extensive discussion on the other two-belt debris disks are presented in Section 4 . 2. A Lone-planet Scenario for the Vega System 2.1. Dynamical Causes for Wide Dust-free Gaps in Debris Disks Infrared observations indicate that Vega’s debris disk contains a large gap that spans from ∼15 au to ∼110 au (Su et al. 2013 ). The relatively large size of the gap in Vega’s debris disk can be used to constrain the orbital configuration of Vega’s (hypothetical) planetary system. In our solar system, the four gas giants cleared most of the debris between the outer asteroid belt ∼3 au and the inner Kuiper Belt ∼35 au. Similarly, in the young HR 8799 system, (at least) four giant planets have been detected (Marois et al. 2008 , 2010 ) and they are believed to be the chief culprits for the large separation between the warm (∼6–15 au) and cold (∼90–300 au) debris disks (Su et al. 2009 ). Therefore, a multiple-planet configuration is a preferred scenario in the Vega and Vega-like systems, which has a
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