Answer to Question 1We have witnessed the effects of extrasolar planets on their parent stars, both in the periodic wobble of the parent star caused by the gravitational tug of the planet, and in the periodic dimming of light coming from the star as the planet passes in front of it. As of this writing, over 400 planets in 324 star systems have been discovered. For an up-to-date count, go to NASA's Planet Quest New Worlds Atlas Search, which can be found here:
http://planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm.
Or, try the Extrasolar Planets Encyclopedia at:
http://exoplanet.eu/Answer to Question 2We can estimate Ng, the number of stars in a galaxy; fp, the fraction of stars that have planets; and ne, the number of Earthlike planets per planetary system. Currently, fl, the fraction of habitable planets on which life evolves; fi, the probability that life will evolve to a steady state; fc, the probability that intelligent life will develop the capacity to communicate over long distances; and fL, the fraction of a planet's lifetime during which it supports a technological civilization, are not well constrained.
Much of the dark space between stars, as seen in the sky of the image above, is due to the human eye
The same star field photographed with (a) a ground-based telescope, which is subject to poor seeing
Classifying the Spectra of Stars
Stars and the Interstellar Medium
Selected Pediatric Drugs for Which Specific Indications and Dosage Guidelines Exist
Orbital Motion of Earth and Other Planets