Theory is most of Earth's surface, not the core was delivered by oodles of meteorites. New problem finding is that the meteorite isotopes don't fit the surface isotopes here:-https://www.sciencedaily.com/releases/2016/09/160916110636.htm-"Planetary scientists have long believed that Earth formed from planetary objects similar to meteorites. Then, a decade ago, perplexing new measurements challenged that assumption by showing that Earth and its supposed "building blocks" actually contain significantly different isotopic compositions.-***-"'These recent measurements contribute to the growing evidence that the meteorites delivered to Earth provide an imperfect match to Earth's composition," said Richard Carlson, director of the Department of Terrestrial Magnetism at the Carnegie Institution for Science. Carlson was one of the scientists who found the compositional mismatch between meteorites and Earth 10 years ago. "This realization opens new views both to how Earth formed and to the bulk chemical composition of our home planet.'"-Abstract of the study:-A long-standing paradigm assumes that the chemical and isotopic compositions of many elements in the bulk silicate Earth are the same as in chondrites. However, the accessible Earth has a greater 142Nd/144Nd ratio than do chondrites. Because 142Nd is the decay product of the now-extinct 146Sm (which has a half-life of 103 million years), this 142Nd difference seems to require a higher-than-chondritic Sm/Nd ratio for the accessible Earth. This must have been acquired during global silicate differentiation within the first 30 million years of Solar System formation and implies the formation of a complementary 142Nd-depleted reservoir that either is hidden in the deep Earth6, or lost to space by impact erosion. Whether this complementary reservoir existed, and whether or not it has been lost from Earth, is a matter of debate, and has implications for determining the bulk composition of Earth, its heat content and structure, as well as for constraining the modes and timescales of its geodynamical evolution,. Here we show that, compared with chondrites, Earth's precursor bodies were enriched in neodymium that was produced by the slow neutron capture process (s-process) of nucleosynthesis. This s-process excess leads to higher 142Nd/144Nd ratios; after correction for this effect, the 142Nd/144Nd ratios of chondrites and the accessible Earth are indistinguishable within five parts per million. The 142Nd offset between the accessible silicate Earth and chondrites therefore reflects a higher proportion of s-process neodymium in the Earth, and not early differentiation processes. As such, our results obviate the need for hidden-reservoir or super-chondritic Earth models and imply a chondritic Sm/Nd ratio for the bulk Earth. Although chondrites formed at greater heliocentric distances and contain a different mix of presolar components than Earth, they nevertheless are suitable proxies for Earth's bulk chemical composition.-Comment: Was the Earth specially formed by a special nucleosynthesis as shown by the Neodymium isotopes? Again, how rare is the Earth, which has formed life?

We have no good idea why the Earth's inner core became solid:

https://www.livescience.com/61715-earth-inner-core-paradox.html?utm_source=ls-newslette...

"One day, about a billion years ago, Earth's inner core had a growth spurt. The molten ball of liquid metal at the center of our planet rapidly crystallized due to lowering temperatures, growing steadily outward until it reached the roughly 760-mile (1,220 kilometers) diameter to which it's thought to extend today.

"That's the conventional story of the inner core's creation, anyway. But according to a new paper published online this week in the journal Earth and Planetary Science Letters, that story is impossible.

"In the paper, the researchers argued that the standard model of how the Earth's core formed is missing a crucial detail about how metals crystallize: a mandatory, massive drop in temperature that would be extremely difficult to achieve at core pressures.

"Weirder still, the researchers said, once you account for this missing detail the science seems to suggest that Earth's inner core shouldn't exist at all.

"Everyone, ourselves included, seemed to be missing this big problem," study author Steven Hauck, a professor of Earth, Environmental and Planetary Sciences at Case Western Reserve University in Ohio, said in a statement. Namely, they were missing "that metals don't start crystallizing instantly unless something is there that lowers the energy barrier a lot."

"In chemistry, this extra energy is known as the nucleation barrier: the point at which a compound visibly changes its thermodynamic phase. Liquid water, for example, freezes into a solid at the familiar 32 degrees Fahrenheit (0 degrees Celsius). If you've ever made ice cubes at home though, you know that even water stored at its freezing point can take several hours to fully crystallize. To speed up the process, you need to either expose the water to significantly colder temperatures (this is called "supercooling") or expose it to an already-solid piece of ice to lower the nucleation barrier, reducing the amount of cooling required.

"Supercooling is easily achieved for a single ice cube, but for Earth's gigantic inner core, things get a little trickier, the researchers said.

"At the pressures of the core, it would have to cool 1,000 degrees Kelvin [726 degrees C or 1,340 degrees F] or more below the melting temperature in order to crystallize spontaneously from pure liquid," Hauck told Live Science. "And that's a lot of cooling, especially since at the moment, the scientific community thinks the Earth cools maybe about 100 degrees K per billion years."

"According to this model, "the inner core shouldn't exist at all, because it could not have been supercooled to that extent," study author Jim Van Orman, also a professor of Earth, Environmental and Planetary Sciences at Case Western, told Live Science. The molten inner core's nucleation barrier, he said, must have lowered some other way — but how?

"In their paper, the researchers proposed one possibility: Perhaps a massive nugget of solid metal alloy dropped from the mantle and plunged into the liquid core. Like an ice cube dropped into a glass of slowly freezing water, this solid chunk of metal could have lowered the core's nucleation barrier enough to kick-start a rapid crystallization.

"There's a big caveat, though: It would have to be a truly massive chunk of metal to work.
"In order to be released into the core and then make it all the way down to the center of the Earth without dissolving … this droplet would have to be on the order of about 10 km [6.2 miles] in radius," Van Orman said. That means a diameter about the length of the island of Manhattan.

***

"'We've talked about what ideas are implausible, and we've suggested an idea that's potentially plausible," Hauck said. "If it happened that way, it's possible that some signature of that event might be detectable through seismic studies. Studying the centermost part of the planet is about the hardest to access with these waves, so it'll take time.'"

Comment: Obviously we have no idea how it became solid. We have no idea abou other planet's insides except in our solar system and we have other planets with cores, but they are not studied so far. The Earth seems more and more unusual as we study it and try to study other planets, but the Earth supports life perhaps because it is so unusual.