But now they have a picture.
Images taken by the Very Large Telescope in the South American country of Chile show two separate objects moving together through space. One is a sun, and the other is an enormous planet, five times bigger than Jupiter.
The speck, named 2M1207b, was discovered last year. But astronomers couldn't prove it was a planet and not some other distant object.
"Our new images show convincingly that this really is a planet," said Gael Chauvin, an astronomer at the European Southern Observatory who lead the research team.
Planets outside our solar system, called exoplanets, are too tiny and too far away to be seen even with the most powerful telescopes. Scientists have had to figure out their existence from looking at their suns. For instance, when a planet passes in front of its sun, the sunlight dims.
Life in Hydra
So what's it like on planet 2M1207b? Measurements show water in the atmosphere, which means that it is not boiling hot. If you stood on its surface, the sun would appear much smaller than Earth's sun. That's because the planet orbits its sun at a great distancetwice as far as Neptune is from our sun. It's also because the sun is a brown dwarf, which is pretty little for a star. They're so puny that they can be hard to tell apart from planets.
The planet's discoverers will publish a paper about their find in an upcoming issue of Astronomy and Astrophysics.