  We discuss the current state of knowledge of terrestrial planet formation
from the aspects of different planet formation models and isotopic data from
182Hf-182W, U-Pb, lithophile-siderophile elements, 48Ca/44Ca isotope samples
from planetary building blocks, 36Ar/38Ar, 20Ne/22Ne, 36Ar/22Ne isotope ratios
in Venus' and Earth's atmospheres, the expected solar 3He abundance in Earth's
deep mantle and Earth's D/H sea water ratios that shed light on the accretion
time of the early protoplanets. Accretion scenarios that can explain the
different isotope ratios, including a Moon-forming event after ca. 50 Myr,
support the theory that the bulk of Earth's mass (>80%) most likely accreted
within 10-30 Myr. From a combined analysis of the before mentioned isotopes,
one finds that proto-Earth accreted 0.5-0.6 MEarth within the first ~4-5 Myr,
the approximate lifetime of the protoplanetary disk. For Venus, the available
atmospheric noble gas data are too uncertain for constraining the planet's
accretion scenario accurately. However, from the available Ar and Ne isotope
measurements, one finds that proto-Venus could have grown to 0.85-1.0 MVenus
before the disk dissipated. Classical terrestrial planet formation models have
struggled to grow large planetary embryos quickly from the tiniest materials
within the typical lifetime of protoplanetary disks. Pebble accretion could
solve this long-standing time scale controversy. Pebble accretion and streaming
instabilities produce large planetesimals that grow into Mars-sized and larger
planetary embryos during this early accretion phase. The later stage of
accretion can be explained well with the Grand-Tack, annulus or depleted disk
models. The relative roles of pebble accretion and planetesimal accretion/giant
impacts are poorly understood and should be investigated with N-body
simulations that include pebbles and multiple protoplanets.
