“Tandem” planet formation, a new theory of planet formation, is described. A steady-state, 1-D model of the accretion disk of a protostar is obtained taking into account magneto-rotational instability (MRI) and porous aggregation of solid particles. The disk is found to be divided into an outer turbulent region, a MRI suppressed region, and an inner turbulent region. The outer turbulent region is fully turbulent because of MRI. However, MRI is suppressed around the midplane of the gas disk, and a quiet area without turbulence appears at rout ( 8-60 AU) from the central star, because the degree of ionization of gas becomes sufficiently low. The disk becomes fully turbulent again at rin ( 0.2-1 AU), because the midplane temperature becomes sufficiently high (> 1000 K). Planetesimals are formed through gravitational instability at both outer and inner MRI fronts. At the outer MRI front, icy particles grow through low-velocity collisions into porous aggregates with low densities (down to ≃ 105 g cm−3). They eventually undergo gravitational instability to form icy planetesimals. Rocky particles, on the other hand, accumulate at the inner MRI front due to the local maximum in gas pressure. They undergo gravitational instability in a sub-disk of pebbles to form rocky planetesimals. They are likely to be volatile-free because of the high temperature (> 1000 K) at this formation site. This is consistent with a model in which the Earth was initially formed as a completely volatile-free planet. Water and other volatile elements came later through the accretion of icy particles with occasional scatterings in the outer regions after solidification of the planet surface. Our new proposed tandem planet formation regime shows that planetesimals are formed at two distinct sites. The former is likely to be the source of outer gas giants and the latter of inner rocky planets. The tandem regime also explains the gap in the distribution of solid components (2-4 AU), and therefore a relatively small Mars and a very small mass in the main asteroid belt. This tandem regime is found not to take place when the vertical magnetic field of the disk is five times weaker than that assumed, because the outer MRI front shifts outward beyond 100 AU. Such a “dispersed planetary formation” regime may explain high eccentricity planets, which are detected in exosolar planetary systems. On the other hand, when the ionization rate due to galactic cosmic-rays is 100 times larger than that of the present value, the outer MRI front shifts down to the inside of the water sublimation zone. Such a “single star formation” regime might explain super Earths or hot Jupiters, because almost all of the rock components in the disk transported to the inner MRI front contribute to planetary formation.