10¹⁸ meters

More at 1 exameter

I have stressed the vast distances between stars and tried to convey the enormous timespans involved in potential travel to other stellar systems. For example, just to send a message to our nearest neighbor and receive a reply requires nearly 9 years. Interstellar travel at speeds approaching light speed introduce the effects of time dilation -- the traveler ages slower than the people left behind.

What are the chances for life on other worlds? How many civilizations exist in our galaxy willing and able to communicate with us? Are we alone? These questions were debated in earnest at the beginning of the space age. In 1961 Frank Drake quantified the factors that could provide an answer -- or least a basis for further debate. The Drake Equation consists of seven terms that when multiplied together define N, the number of communicating civilizations in the galaxy:

N = R* × f_P × n_E × f_L × f_I × f_C × L

R*	Rate of creation of suitable stars per year
f_P	The fraction of those stars with planets
n_E	The number of planets per star that are capable of sustaining life
f_L	The fraction of those planets on which life does evolve
f_I	The fraction of those planets on which intelligent life evolves
f_C	The fraction of those planets which develop interstellar communication
L	The length of time the civilization maintains communication

There are a number of sites that discuss these factors in more detail and allow you to play with your own set of numbers. A very thorough review of this topic is titled "Beyond the Drake Equation".