"Planetary embryos" refers to the early stages of formation of planets. These are smaller celestial bodies, similar to embryos, that eventually grow and merge together to form planets. Full definition
The finding provides more information on the early days of our solar system about 4.4 billion years ago, when the zone near the sun had several planetary embryos. (livescience.com)
Many planetary embryos were Mars - sized bodies, such as the one that collided with Earth to give rise to the Moon. (sciencedaily.com)
His team found that by tuning the model so that the pebble formation process takes a bit longer, there is more time for the large planetary embryos to interact with each other gravitationally. (sciencemag.org)