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Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun. The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modeling...

There might be a ninth planet in the solar system after all - and it is not Pluto. Two astronomers reported on Wednesday that they had compelling signs of something bigger and farther away — something that would definitely satisfy the current definition of a planet, where Pluto falls short. "We are pretty sure there's one out there," said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology. What Dr. Brown and a fellow Caltech professor, Konstantin Batygin, have not done is actually find that planet, so it would be premature to revise mnemonics of...

Scientists have been wondering whether a "Planet X" exists in the dim regions far beyond the known planets, but it has remained largely speculative.... That started to change in March 2014, when a pair of astronomers announced that they’d discovered a brand-new dwarf planet, 2012 VP113, beyond the well-populated edge of the Kuiper belt, whose main mass stretches from Neptune’s orbit around 30 astronomical units (or 30 times the Earth-Sun distance) out to 50 astronomical units. It wasn’t the only such object: Sedna, a 600-mile-wide rock discovered in 2003, also boasted this far-out orbit, and it seemed to be making...