Discovering The Origins Of Our Planet Part 2
To date no truly Earth-like planet has yet been found, with most of the rocky planets being at least two to three times the size of our Earth, but even at those scales it is theoretically possible to find a somewhat Earth-like living ecosystem.
Optimistically speaking, it's only a matter of time until we can go out and take photos of neighboring planetary systems. Right now we're limited to determining the presence of somewhat Earth-like planets by gravitational inference and other mostly indirect means. The day is coming, researchers assure, when planet formation will be an observational science, and not merely theoretical. Perhaps then the telling details from space will help us decipher our own planet's story.
For now, though, we're left with this: Scientists think that the Earth was about 90 percent of its current size within 50 million years of starting out as space dust. In this final phase of the proto-Earth, scientists theorize that a massive object, perhaps as big as the planet Mars, smashed into Earth. Although there is some quibbling about the mass of the object and the number of times it collided with Earth, most scientists agree that it was, by far, the most traumatic event in the formation of the Earth.
This huge impact knocked off a sizable chunk of our planet, sending the debris into space, where it glommed together to form our moon. The moon, a molten mass in its early stages, is thought to be about 4.5 billion years old.
Because the conditions on the early Earth were so violent and extreme, life couldn't begin to get a foothold for at least 500 million to a billion years. The planet was probably cool enough for oceans to form and to be intermittently habitable about 4 billion years ago, but until 3.7 billion years ago it still was being strafed by massive space junk capable of boiling the seas. Earth probably wasn't habitable on a continual basis until about 3.5 billion years ago.
To Build a Solar System
Most research models of the formation of the inner solar system begin with about 1,000,000,000,000 planetesimals (really), which is enough material to make planet embryos in a zone between the Earth and the sun. The supercomputer models distribute them throughout the inner solar system, then instruct all the pseudo-particles to obey physical laws. In short order, the planetesimals smash together and eventually grow to Mars-sized embryos; then the embryos collide and finally accumulate into the terrestrial planets: Mercury, Venus, Earth and Mars. When all is said and done, the computer models end up with a planetary system that in many ways resembles our own.
In other ways, it doesn't resemble our solar system at all. Sometimes the biggest planet is the closest to the sun; sometimes it's the farthest. As we're finding through our discovery of planets circling other stars that's the way it really is: There is a great deal of randomness out there. Sometimes the end is happily ever after; sometimes not. Let's just say that we're exceptionally lucky to have evolved on a fairly stable planet which is relatively free from any nearby asteroid fields which would smash us into smithereens at regular intervals!