In my book, I support the idea that planets are born out of storms, either in the atmosphere of stars and large planets, or in the violent whirlwind of collapsing plasma that are the supernovas that sometimes light up galaxies.
In either case, we have a situation in which the densest part of the newly formed object will be away from the center. Just like storms have an eye at their center, planets, stars and moons harbor a hollow inside of them.
This theory was partially confirmed by NASA in the spring of 2017. Measurements taken by the space craft Juno revealed that Jupiter has a large and diffuse core.
This full-disc image of Jupiter was taken on 21 April 2014 with Hubble’s Wide Field Camera 3 (WFC3)
By NASA, ESA, and A. Simon (Goddard Space Flight Center)
http://www.spacetelescope.org/images/heic1410a/ or http://hubblesite.org/newscenter/archive/releases/2014/24/image/b/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=32799232
Jupiter’s core was measured to extend up to half of Jupiter’s 70.000-kilometer radius. Out of the planet’s total mass, which is 318 times that of Earth, the core was measured to be no more than 7 to 25 Earth masses. That is a whole lot less than the 40 Earth masses that would be required to make Jupiter’s core as dense as its outer layers.
To say that something has a large and diffuse core is of course an euphemism for hollow. What NASA has discovered is that Jupiter, and possibly all stars, planets and moons, are hollow.