Discovering The Origins Of Our Planet
When scientists tell the story about the birth of the Earth, they might as well start with the phrase "once upon a time ..." Because although they know certain things about the big picture, the details are still fairy tales.
Brilliant minds are hard at work, pondering how a literal grain of space dust eventually became our home. But frankly, the trail is cold. Their chances of getting the story right are small, according to the most brilliant minds who have devoted their careers to the subject. Even today, we can't yet really say for sure what happened right here to get life started.
Perhaps some things are unknowable. While scientists can determine our solar system's age: 4.5 billion years, give or take a few million, from measuring radioactive isotopes in certain meteorites, they have no hard evidence of what really went on during our own planet's infancy.
The actual sequence of events that led to the formation of the Earth is chaotic. The oldest rocks on Earth themselves are significantly younger than the beginnings of the planet. There's about a half-billion-year gap between the oldest solid object ever identified on Earth and when we think the Earth was formed. That's a huge blind spot from what were, quite literally, Earth's formative years.
A "Respectable" Science
Unlike scientists studying every other era in our planet's history, those working on Earth's origins toil almost exclusively with theory. It has never been a major field of science, though it definitely is becoming one right now. The ultimate historical question of how the Earth was formed simply was not what a respectable scientist devoted full time to. Star formation was a more honorable thing to study. Planet formation was more like a hobby; something to be fiddled with in old age.
The Earth's story, as scientists now tell it, begins back when the sun was in its infancy. Recycled star dust and gas spun around the sun in the form of a flattened disk. After perhaps 100,000 years of debris randomly bumping into one another and sticking, some of the dust grains managed to grow into strong and solid objects. By means of a mysterious process (which is still poorly understood), the objects eventually glommed together to form "planetesimals" of about 100 meters in size, at which point their gravitational force began pulling them into each other. They collided. While some fragmented, the big ones became bigger, using their expanded gravitational pull to capture nearby space stuff.
This process known as collisional accumulation, went on for a span of about 100 million years, until the Earth was fully formed (although far different from the planet we call home today).
The Search For Other "Earths"
The news reports have been filled in the past year or two with discoveries of planets circling faraway stars. Most of these planets are gas giants along the line of Jupiter or Saturn, but as sensing techniques are refined, more and more "rocky" planets are being discovered in the "Earth-like zone" where the heat from the star is not too high or too low and various other aspects favor the development of the type of life that we find right here on this Earth.