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AnonTechie writes:

Way back in 2010, Gliese 581g made waves as "the Goldilocks planet". It was the first planet scientists found within the habitable zone - the region around a star where it's not too hot and not too cold for liquid water to endure on a planet's surface. But after the initial excitement about finding a planet that could potentially support life, some scientists started to seriously doubt whether Gliese 581g was really there, because the signal was weak. Despite all the debate, lots of astronomers listed Gliese 581g as the top spot to look for alien life. Now, new research says that Gliese 581g doesn't actually exist.

Astronomers can't actually see the planets in the Gliese 581 star system. Instead, they detected the planetary candidates by monitoring the star's light. As a planet orbits, its gravity tugs on the star and distorts the light coming off it, changing the wavelengths and thus the color of light that reaches telescopes here on Earth. (Here's a longer explanation of the radial velocity technique, if you're interested.) The problem is that the star also moves, and as it rotates its sunspots and other solar activity also distort the light coming off of it.