Tyche is a hypothetical planet lying somewhere in the Oort cloud at the edge of the solar system. The planet was proposed in 1999 by three astrophysicists from the University of Louisiana. The trio suggested that Tyche is the size of Jupiter, has three times the mass of Jupiter, and orbits the Sun once in 1.8 million years.
The astrophysicists proposed Tyche to explain the existence of long-period comets. These comets take over 200 years to complete an orbit round the Sun. Astronomers used to believe that long-period comets appeared from random locations in the solar system.
However, the astrophysicists say that the comets actually come from the Oort cloud and are flung toward the Sun by the gravitational force of Tyche. NASA used its Wide-field Infrared Survey Explorer (WISE) telescope to search for Tyche between 2012 and 2014. It found nothing.