He did not need the help of the earthly planet. Being the incarnation of Lord Visnu, Prthu Maharaja possessed the power of Sankarsana, which is explained by the scientists as the power of gravitation
SB Canto 4
You are very much puffed up with pride and have become almost insane. Presently you have assumed the form of a cow by your mystic powers. Nonetheless I shall cut you into small pieces like grain, and I will uphold the entire population by my personal mystic powers.
The earth informed King Pṛthu that if he destroyed her, he and his subjects would all fall down into the waters of the garbha ocean. King Pṛthu now replies to that point. Although the earth assumed the shape of a cow by her mystic powers in order to be saved from being killed by the King, the King was aware of this fact and would not hesitate to cut her to pieces, just like small bits of grain. As far as the destruction of the citizens is concerned, Mahārāja Pṛthu maintained that he could uphold everyone by his own mystic powers. He did not need the help of the earthly planet. Being the incarnation of Lord Viṣṇu, Pṛthu Mahārāja possessed the power of Saṅkarṣaṇa, which is explained by the scientists as the power of gravitation. The Supreme Personality of Godhead is holding millions of planets in space without any support; similarly, Pṛthu Mahārāja would not have had any difficulty supporting all his citizens and himself in space without the help of the planet earth. The Lord is known as Yogeśvara, master of all mystic powers. Consequently the planet earth was informed by the King that she need not worry about his standing without her help.