Then again, it could also mean there are only eight planets, depending on your point of view.

What the discoverers are calling 2001 KX76 might be one of the largest "Kuiper Belt Objects" or KBO's, found in the what is essentially a second asteroid belt beyond the orbit of Neptune. Initial reports give 2001 KX76 a diameter of 900 to 1200 kilometers — roughly the size of Pluto's moon, Charon. Pluto itself, it should be noted, is smaller than our own moon.

Because KBOs are believed to have very elongated orbits around the sun they spend a lot of time on dark, centuries-long excursions into deep space. That makes them very hard to find, said astronomer Robert Millis, director of the Lowell Observatory, which was involved in the discovery.

Astronomers at the Lowell Observatory have teamed up with colleagues from MIT and the Large Binocular Telescope Observatory to hunt for KBOs on the less remote parts of their orbits.

"There are certainly lots of (KBOs) in distant parts of their orbits now and we can't detect them," said Millis.

The possibility that 2001 KX76 has big brothers and sisters again raises the thorny question of what can be called a planet and what cannot. So far there is no good definition of exactly what is a planet, said astronomer Brian Marsden of Harvard University's Minor Planet Center. The matter only gets more confusing when you add KBOs and free-floating planets discovered outside our solar system.

Historically, Pluto was designated a planet when it was discovered in the 1930s because it was thought to be much larger than it is, Marsden said. The 900-kilometer-wide asteroid Ceres was also considered a planet when it was discovered, until its siblings were turned up and revealed the existence of the asteroid belt between Mars and Jupiter.

If 2001 KX76 is any indication of larger KBOs out there, it might also lead to the demotion of Pluto from puniest planet to king of KBOs, said Marsden.

Millis prefers a third alternative: "There may exist a new class of planets."