How do we know that planets exist around other stars? How many ...

[penguins]

How do we know that planets exist around other stars? How many such planets have been detected to date?
 
  What will be an ideal response?

Question 2

What factors in the Drake equation are we capable of estimating today?
 
  What will be an ideal response?

[juiceman1987]

Answer to Question 1

We 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 2

We 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.