Did you know astronomers have found water on planets in other solar systems and that they'd found extra solar planets, over 400 as a matter of fact?
Oceans Appear Blue
The planets are so distant, even with a telescope they must be too small to see barely a little point of light. It's that light that's important. You see, every molecule, including water, absorbs certain wavelengths or colors of light and reflects others.
It is similar to how Earth's oceans look blue from space but other things like methane can appear blue too.
How Do You Tell If A Planet Has Water?
When the planet passes between the star it orbits and our telescopes, starlight passes through the planet's atmosphere.
Telescope spectrographs break down the light into individual wavelengths like a prism separates light into a rainbow of color. Only one particular arrangement of wavelengths identifies water molecules.
Isn't It Difficult To Collect This Light?
Yes. When scientists use Earth based telescopes, the process is difficult because the Earth's atmosphere interferes. The Hubble and Spitzer telescopes are in space and don't have that problem, but they are small telescopes and collect less light. Fortunately, new techniques of calibrating the telescopes the have made it possible to find water worlds.
Planet With Water!
A planet known as HD 189733b orbiting a star in the constellation Vulpecula the Fox has water in its atmosphere.
So, We Could Visit There Someday?
HD is a gas giant like Jupiter that orbits very close to its sun. It's not the sort of planet you'd want to visit. But someday, I'm sure we'll locate an Earth like planet with water. New discoveries are made every day.