Is there really a mysterious new planet out there? NASA is still skeptical.

Yesterday, a team of Caltech astronomers published evidence suggesting there's a massive undiscovered planet floating in the solar system beyond Neptune.

The astronomers didn't claim to have observed the planet directly. Instead, they've found evidence of its gravitational pull. Their paper argues that the only thing that could influence the odd orbits of objects way out in the solar system is an as-yet-undiscovered massive planet.

The news proved irresistible on the internet and social media. Planet fans still bummed about the demotion of Pluto to dwarf planet could rejoice (or feel scorned on behalf of Pluto). And what's more exciting than an unknown, undiscovered world?

But today NASA has stepped in to say, in effect, "Cool your jets."

Jim Green, NASA's director of planetary science, has posted a video response to the blockbuster findings. In it, he takes pains to stress that these results have not yet been verified by direct observations.

This is just a "theory," he says. It serves "to stimulate ideas and conversation." There's a "healthy debate" going on, he says.

It's not, however, the detection of a new planet. It's too early to say with certainty there's a so-called Planet X out there. What we're really seeing is an early prediction based on modeling from limited observations.

The Caltech team didn't say they had absolute proof, only that they could come up with no other possible explanation for the odd orbits. Maybe NASA just wants to remind the world that it's the final word on planetary exploration.

"If Planet X is out there, we'll find it together," Green says.

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