  We study the possibility that hot Jupiters are formed through the secular
gravitational interactions between two planets in eccentric orbits with
relatively low mutual inclinations ($\lesssim20^\circ$) and friction due to
tides raised on the planet by the host star. We term this migration mechanism
Coplanar High-eccentricity Migration because, like disk migration, it allows
for migration to occur on the same plane in which the planets formed. Coplanar
High-eccentricity Migration can operate from the following typical initial
configurations: (i) inner planet in a circular orbit and the outer planet with
an eccentricity $\gtrsim0.67$ for $m_{\rm in}/m_{\rm out}(a_{\rm in}/a_{\rm
out})^{1/2}\lesssim0.3$; (ii) two eccentric ($\gtrsim0.5$) orbits for $m_{\rm
in}/m_{\rm out}(a_{\rm in}/a_{\rm out})^{1/2}\lesssim0.16$. A population
synthesis study of hierarchical systems of two giant planets using the observed
eccentricity distribution of giant planets shows that Coplanar
High-eccentricity Migration produces hot Jupiters with low stellar obliquities
($\lesssim30^\circ$), with a semi-major axis distribution that matches the
observations, and at a rate that can account for their observed occurrence. A
different mechanism is needed to create large obliquity hot Jupiters, either a
different migration channel or a mechanism that tilts the star or the
proto-planetary disk. Coplanar High-eccentricity Migration predicts that hot
Jupiters should have distant ($a\gtrsim5$ AU) and massive (most likely
$\sim1-3$ more massive than the hot Jupiter) companions with relatively low
mutual inclinations ($\lesssim 20^\circ$) and moderately high eccentricities
($e\sim0.2-0.5$)
