Assalamu alaikum wa rehmatullahi wa barakatuh
It is Allah who has created seven heavens and of the earth, the like of them. [His] command descends among them so you may know that Allah is over all things competent and that Allah has encompassed all things in knowledge.(65:12)
The word Earth is singular,not plural.So it is not about seven Earths but Seven of the Earth,just like seven of the heavens.
And We have created above you seven layered heavens, and never have We been of [Our] creation unaware.(23:17)
Seven Layers of Heavens.It could be either seven Layers of Atmosphere,the sky above.or the seven layers of Universe,everything beyond the Earth.
Though the seven are not discovered,but the layers of universe are a new discovery in cosmology.