The planet orbits a dim red dwarf star called Gliese 581, and seems to be at the right distance from the star to maintain liquid water on its surface. That, of course, makes alien-philes wonder if Gliese 581g also hosts life. And that makes people want to go check.
Announcing the find on NBC Nightly News, Brian Williams said: "They say it's about 20 light years away, but that's practically nothing in astronomy terms." And he declared at the end of the segment: "It's just nice to know that if we screw this place up badly enough there is some place we can all go."
So they get a couple of folks to do the math, one with a sock-puppet avatar well past its sell-by date, and the answer is either 180,000 years, or three million, depending.
This should be no surprise to those who have already allowed Stross to bum them out about this with his pesky arithmetic. Only marginally related, I've been really enjoying his series on "books I will not write" because of hilarious little asides like
Writing a space opera with FTL means accepting causality violation. And accepting causality violation means computing with closed timelike curves or, in simpler terms, really strong deterministic solutions to P=NP, and then some. Procedural AI hops out of the FTL hat like a demented magician's rabbit and the singularity takes a shit all over your neatly designed Napoleonics-in-Spaaaaaace boardgame table.