  A successful solar system model must reproduce the four terrestrial planets.
Here, we focus on 1) the likelihood of forming Mercury and the four terrestrial
planets in the same system (a 4-P system); 2) the orbital properties and masses
of each terrestrial planet; and 3) the timing of Earth's last giant impact and
the mass accreted by our planet thereafter. Addressing these constraints, we
performed 450 N-body simulations of terrestrial planet formation based on
narrow protoplanetary disks with mass confined to 0.7-1.0 au. We identified 164
analogue systems, but only 24 systems contained Mercury analogues, and eight
systems were 4-P ones. We found that narrow disks containing a small number of
embryos with individual masses comparable to that of Mars and the giant planets
on their current orbits yielded the best prospects for satisfying those
constraints. However, serious shortcomings remain. The formation of Mercury
analogues and 4-P systems was too inefficient (5% and 2%, respectively), and
most Venus-to-Earth analogue mass ratios were incorrect. Mercury and Venus
analogues also formed too close to each other (~0.15-0.21 au) compared to
reality (0.34 au). Similarly, the mutual distances between the Venus and Earth
analogues were greater than those observed (0.34 vs. 0.28 au). Furthermore, the
Venus-Earth pair was not reproduced in orbital-mass space statistically.
Overall, our results suggest serious problems with using narrow disks to
explain the inner solar system. In particular, the formation of Mercury remains
an outstanding problem for terrestrial planet formation models.
