Nearly two years after spotting Gliese 581g, the celebrated "first potentially habitable" alien world, the planet's discoverers continue to fight for its existence.

The discovery of Gliese 581g made headlines around the world in September 2010, because the planet was said to orbit in the middle of its star's "habitable zone" — that just-right range of distances where liquid water, and perhaps life as we know it, could exist.

Just a few weeks later, however, another prominent research team began casting doubt on the find, saying the alien planet didn't show up in their observations. This group, led by Michel Mayor of the Geneva Observatory in Switzerland, had found the previously known four planets in the Gliese 581 system.

But in a new study that will be published Aug. 1, 581g's discoverers examine the Swiss team's since-expanded data set and take issue with their conclusions, saying that the evidence supports the planet's existence after all.

The data and analyses "point to there being at least one other planet beyond the confirmed 4, a 5th planet, with a period in the 26-39-day regime," lead author Steve Vogt, an astronomer at the University of California, Santa Cruz, told SPACE.com via email. [The Strangest Alien Planets]

"This 5th planet would have a mass of only 2-3 [times that of] Earth, and would orbit pretty much squarely in the star's habitable zone," he added.