The so-called extreme trans-Neptunian objects (ETNOs) are orbiting the Sun at heliocentric distances larger than 150 AUs, and their discovery a decade ago was soon recognized as a turning point in our knowledge of the outer Solar System. The currently tally stands at 21 ETNOs, and only one, Sedna, has been spectroscopically observed. In the last years several studies have suggested that the dynamical properties of the ETNOs could be better explained if one or several planets of several Earth masses are orbiting the Sun at hundreds of AUs. In 2016, Brown and Batygin used the orbits of seven
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