The question why, if the Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical • Lenticular • Spiral • Irregular Click on Term to Read More is filled with intelligent and technological civilizations, they have not visited Earth? Fermi is supposed to have asked, “So? Where is everybody?” Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could colonize the entire Galaxy within a few million years (a very short time on Galactic time scales). The fact that aliens don’t seem to be walking our The term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More is taken to imply that there are no extraterrestrial anywhere among the vast tracts of the Galaxy. Many researchers consider this a radical conclusion to draw from such a simple observation. Many solutions to the paradox have been proposed, but in general, solutions to Fermi’s paradox come down to either (1) life is difficult to start and evolve (either hard for the process or hard to Meteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago. Click on Term to Read More the right conditions); or (2) advanced civilizations destroy themselves on short timescales.