These planets are the first four from the sun: Mercury, Venus, Earth and Mars. They’re mostly made of rock and iron – whose particles don’t readily stick together.
They could have been sticky enough if they had a coating of snow and organic goo, Hubbard says. But despite all Earth’s oceans and carbon-based life, our planet has too little water or carbon to support this explanation.
Now Hubbard has suggested an intriguing solution to Earth’s difficult birth. In 1936, an infant star began to brighten, eventually shining over 100 times more brightly than it did originally. Now named FU Orionis, this star has stayed bright ever since. And several other stellar youngsters have done the same thing.