Most planets around suns in the life giving range tend to be gas giants, or so it is believed from all the discoveries so far. And yet, our solar system only has rocky planets in this region.
So, lets suppose our solar system is an exception rather than the rule, which leads us to the conclusion that planet Earth was a gas giant. Does it get us anywhere to think that way?
Firstly a mechanism is required to turn a gas giant into a rocky planet.
So, the sun had a nova event, an extreme shedding of material - coronal mass ejections gone mad, if you like. This might have happened suddenly or over a longer period. But not too long. Perhaps it was just a violent episode in the Sun's history, or perhaps Oliver Manuel's theory has a lot to say about the development of the solar system.
Such an event might have been strong enough to overcome the Earth's magnetic field, or perhaps the magnetic field was in a lull, and the nova material could have stripped much of the gas giant atmosphere from the Earth.
So, then we have a smaller Earth and a correspondingly larger satellite, the Moon. Now a double planet system rather than a gas giant and satellite. So, there we have an explanation for the anomalous size of the Moon in comparison to the Earth. And we have an explanation for the disparity between exoplanet data and our solar system.
Going further, the loss of pressure due to less atmosphere led to the expansion of the rocky core, leading to a resultant crustal expansion and the development of oceans - a new chemistry deep in the earth due to change in pressure and a resulting product of water to fill crustal spread. Also, coronal mass ejections have much hydrogen and oxygen in them and water may have come from the sky also in the form of small icy comets. water may have already been in the atmosphere also. So, we have an explanation for the state of the Earth's crust and for the development of mountains and continental shelves and oceans.
We could even hazard a guess as to when this happened - just prior to the Cambrian explosion. Life gets a chance to develop from primitive pre-existing forms that were born in the gas giant Earth, and this life gets the chance to modulate the atmosphere and recycle oxygen for respiration. An explanation for the mysterious Cambrian explosion. Perhaps also an explanation for the transition from a reducing to an oxidising atmosphere.
Over time the moon slows down the Earth. A factor in the change in species?
Other events may have happened around this time along with this. The destruction of a planet between Earth and Jupiter. Venus moving away from Earth (possibly once a satellite of a gas giant Earth - or a gas giant in its own right with a satellite Mercury - but subject to more devastation and different chemistry. Of course other theories exist and thats fine.
Other aspects: Gas giants often have mega-lightning. Possible carving of landmasses by megalightning on gas giant Earth - the scars of which exist to this day. Megalightning in a reducing atmosphere may also have encouraged the formation of the building blocks of life, such as witnessed in the Urey experiment on a small scale.
The scenario tries to marry different theories into a synthesis. A just so story, but one i happen to like.