Gliese 581g is a planet that might exist outside our own Solar System. It's a little bit like Earth, and it's orbiting a star called Gliese 581.
Imagine you're standing outside on a sunny day, and you see a bird flying around in the sky. That bird might be compared to the planet Gliese 581g, and the Star it's orbiting is like the Sun.
One of the coolest things about Gliese 581g is that scientists think it might be able to support life! That's because it's located in a part of its solar system where the temperature is not too hot and not too cold. Just like how you have to be careful not to get too warm or too cold when you're outside playing.
Scientists haven't actually seen Gliese 581g with their own eyes, because it's really far away. But they have used special equipment to study its Star, and they can tell that something is orbiting around it.
So, while we can't say for sure that Gliese 581g exists yet, it's a really exciting idea that there could be a planet out there that's a lot like Earth and could even have other forms of life living on it!