Re: How long would it take to get to another solar system?

Erin:
     That is an interesting question indeed.  I like the fact that you are 
interested in creating a scenario of plausibility in your science fiction 
writing, but certainly don't let the accomplishments to date sway you from 
dreaming up completely new technologies and propulsion systems.  I will 
tell you about a few ideas concerning interstellar travel and then I will 
tell you about semi-realistic technologies to approach that, and then some 
partially theoretical, approaches.
     First of all, one would have to know where the "Earth-like" planet to 
which you were travelling was.  It matters to some point if it is located 
at a relatively close star like Alpha Centauri (4.5 light years), or a 
more distant star, such as Capella (In the constellation Taurus).  One has 
to appreciate the massive distances involved in interstellar travel too.  
The distance to the nearest star (Not the Sun) is about five light years, 
which is orders of magnitude further than Pluto, which we have yet to send 
a spacecraft to.  Assuming you sent a probe from Earth today, using 
standard methods of acceleration, getting it to a speed somewhere around 
30,000 miles an hour, it would take about 150,000 years to reach the 
nearest star.  So, obviously, liquid (rocket) propulsion is not going to 
be an effective, nor plausible means of stellar travel.  As for the fuel 
supply, it is certainly not practical to carry the amounts of hydrogen and 
oxygen necessary to provide the propulsion, and at some point, it is rate 
limiting with regard to most of your power is necessary to accelerate your 
fuel supply.  So, let’s discuss other technologies.
     A technology that NASA is currently employing that is more efficient 
than rocket propulsion, but not suitable for short term duration flights 
is ion drive.  The process is essentially like spraying charged gas atoms 
out the back of the craft like a rocket, but they provide very little 
push, about the force of a piece of paper on your hand, but the efficiency 
is much higher and the engines run continuously, so the effect is 
cumulative, so in theory it is plausible to carry large stocks of (Xenon) 
in this case, and presumably get to another solar system, perhaps an order 
of magnitude or two quicker than standard propulsion...   
     There are two other technologies being explored for propulsion, and 
both of these utilize ejection of some matter, (energy) from the craft and 
utilize Newton's 3rd law to propel the craft.  They are anti-matter drives 
and nuclear drives.  
      A nuclear drive works much like an ion drive, except the material 
being ejected is neutrons (Which have no charge), and are produced in 
massive quantities from nuclear reactions.  Another variation is the use 
of a nuclear drive as a small controlled nuclear explosion, with the force 
of the blast directed towards space.  I know little of antimatter drives, 
which is why I included an URL for you to browse, but essentially 
antimatter (antiprotons) when they encounter matter (normal protons, 
hydrogen), they annihilate each other with the release of large amounts of 
energy, so antimatter is a very efficient(energy dense)method of storing 
fuel.  
     There are certainly physical limits to the velocities one can attain 
with these physical drives, and to the limit of the technology is 
relativity.  Simply put, that ubiquitous equation E=MC^2, means that as 
you go faster and faster (apply more energy E), the equation has to 
balance since C, the speed of light is constant, so the mass M has to 
increase...  Basically, it says that as you approach the speed of light 
(Obviously really fast, about 3 x 10^8 meters per second) your mass 
approaches infinity.  With this in mind, any of these technologies are 
limited in upper speed, by various things, and mostly my physics.  To make 
them 'practical' a suspended animation of people involved would have to be 
undertaken, or a community could live on the spacecraft for multiple 
generations and the great...great...great... grandchildren could visit the 
surface of a planet in a new solar system.  
     There are 'warp' technologies too, that people have conceived of, and 
there are lots of references that illustrate this much better than I and I 
will list them at the end, but the basic concept is that if you compress 
space-time in front of you and expand it behind you, you will travel these 
large distances in short periods of time relative to you... I do not have 
the space here to explain curved space, strings, or space time, but again, 
some of the references  at the end of this should help.  
     Essentially, this massive e-mail is telling you that we have only 
ourselves landed a human on another object a few times, and it was only 
367,000 km away (The Moon).  So, anything of which you can conceive is 
great, let your imagination race.  Great question, there are tons of 
possibilities.  

-Matt-
 http://www.nasa.gov http://antimatter.phys.psu.edu/

The Physics Of Star Trek, Lawrence M. Krauss.
A Brief History Of Time, Stephen Hawking.
At Home In The Universe, Stuart Kauffman.
The Edge Of The Unknown, James Trefil
Gravity, George Gamow, Scientific American March 1961, pp.94
Time And The Space Traveller, George Allen & Unwin, (1971)