Planet X... Again

Studying perturbations in the orbit of Uranus led to the discovery of Neptune in 1846-- a spectacular confirmation of Newtons's Laws of Gravity. A similar search for another "Planet X" even farther out led to the discovery of Pluto in 1930. Pluto, it turned out, wasn't big enough to gravitationally affect much of anything, and the oddities in Neptune's orbit were eventually explained in other ways. The universe can be subtle.

Now, a team of Japanese researchers are calling for another search for Planet X. They say the peculiar orbits of some Kuiper Belt objects-- bodies that orbit the Sun beyond Pluto-- can best be explained by a large body in an orbit 100-200 times the Earth-Sun distance. They calculate the body would be between 30-70 percent of Earth's mass, which would make it Mars' size or larger, but smaller than Venus.

Such a world, if it exists, would likely be made mostly of rock-hard water ice. That kimd of huge water resource, conveniently in one place, could well eventually be humanity's last base in the Solar System before we head to the stars.