The answer
Scientists do not say that the earth was part of the sun and separated from it, otherwise the sun will be missing this portion.
What scientists say is that earth is part of the solar system and not of the sun itself. It was part of the Nebula; that fiery mass which was no doubt luminous. This Nebula is what the Bible meant by saying on the first day "Then God said, let there be light, and there was light."
Earth was part of this mass and separated from it. The earth gradually cooled down until its surface became completely cool and on the third day became fit to grow
plants and trees on, using the light and heat radiating from the Nebula.
On the fourth day, from this mass God created, the sun, moon, stars, meteors and all other celestial bodies and regulated the interrelations and the movement of these bodies.
The sun, on the fourth day, remained as it is; a whole with the earth attached to it, but God set the relation between earth and sun, moon and other stars and planets through the astronomical laws.
Thrice-Blessed Pope Shenouda III of Blessed Memory