If you mean the latest Planet X idea, they aren’t. Nemesis (hypothetical star) is postulated to be 1.5 light years away with an orbital period of 26 million years. It almost certainly doesn’t exist as it was postulated to be a red dwarf or a brown dwarf (half way between a planet and a star). The red dwarf idea was ruled out by the WISE space telescope infrared search. Also most brown dwarfs were ruled out too as it could spot the more usual 150 K brown dwarf out to ten light years away.

The one remaining possibility for Nemesis is an unusually ultracold brown dwarf. That could exist but it’s not likely because such stars are rare. A SEARCH FOR A DISTANT COMPANION TO THE SUN WITH THE WIDE-FIELD INFRARED SURVEY EXPLORER

While this latest idea has orbital period 10,000 to 20,000 years if it exists, and they suppose it to be much smaller, about the size of Neptune perhaps,

• Would New Planet X Clear Its Orbit? - And Any Better Name Than "Planet Nine"?

And please sign my petition here: Ban dramatized reporting of "doomsday" stories - which make vulnerable people suicidal!