Gliese 581 d orbits the star Gliese 581 about 20 light-years away in the constellation of Libra, and is one of only a few planets discovered outside our solar system that's comparable to the size of Earth and lies within the habitable zone where liquid water and, therefore life, could exist. It is the first serious candidate for being an "ocean planet," with the possibility that its surface is covered with deep bodies of water. In fact, a high-power radio transmission has been sent by enthusiasts hoping that advanced alien life may exist there. The message will arrive in 2029. If there are aliens who wish to respond, a response could be received by 2049.