Is there an unseen planet — a "planet X" or "Planet Nine" — in the outer solar system? That possibility is supported by calculations in a new study that shows the hypothetical planet could ...

A world seen orbiting a 3-million-year-old star about 520 light years from Earth is one of the youngest known planets, offering a window into early planet formation. The star is an early-stage ...

Now, researchers suggest these mysteries could point to the existence of an undiscovered Earth-like planet. Decoding the ...

Sure, we discovered Pluto and other dwarf planets beyond it, but nothing Earth-sized or larger. If there is some planet nine, or "Planet X" lurking out there, we have yet to find it. But there is ...