Space photo of the week: The 1st image of an alien planet

What it is: exoplanet 2M1207b

Where it is: about 170 light-years away in the constellation Centaurus

When it was shared: Sept. 10, 2004

Why it's so special: A few decades ago, the idea that planets may exist around other stars was purely theoretical. But this image offered the first visual proof of a Jupiter-like planet orbiting a star in a distant solar system. The shot, released 20 years ago this week, shows an exoplanet called 2M1207b orbiting a brown dwarf, called 2M1207.

This is like nothing in our solar system. The brown dwarf star is right on the boundary between a star and a planet, with a mass that's about 42 times less than the sun and 25 times greater than Jupiter. The planet, 2M1207b, is five times more massive than Jupiter.

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"Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system," Gaël Chauvin, an astronomer at the European Southern Observatory (ESO) and leader of the team of astronomers who conducted the study, said when the image was released. "Instead, it must have formed the same way our sun formed, by a one-step gravitational collapse of a cloud of gas and dust."