The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Topics of exciting research, astronomical adventures, and scientific news about the universe are analyzed in depth.

For more than five years, astronomers have been searching the sky for a ninth planet in the solar system: a large, dark planet that would orbit beyond Neptune. But a new study now argues that there are no solid reasons to believe in the existence of such a planet.


Planet 9 made headlines in 2014 when astronomers Chad Trujillo and Scott Sheppard Carnegie Institution of Washington a study on the orbits of several small bodies in the solar system, of which orbit beyond Neptune called Trans-Neptunian Objects, or TNOs for short. These TNOs are small rocks that, due to their great distance from Earth and its darkness, are very difficult to observe. The largest known bodies of this style, the dwarf planets Pluto and Eris, are much closer to Earth than most of the trans-Neptunian family.

Trujillo and Sheppard showed that the orbits of a few TNOs they had observed were not randomly oriented, as might be expected, but that their perihelion (the points of closest approach to the Sun) were remarkably aligned. This led astronomers to hypothesize that a ‘Planet 9’, unknown to date, could be responsible for this alignment of the orbits.


In 2016, Mike Brown and Konstantin Batygin (Caltech) carried out a new study in which they found that Neptune was able to explain some of these alignments, but not all. Also, they calculated the orbits of other more distant TNO bodies (which are not under the influence of Neptune) and not only observed an alignment of the orbits but found that such orbits were coplanar. According to astronomers, the probability that these alignments happened by chance was extremely small, and they thus became fervent followers of the Planet 9 hypothesis, which led them to star in many headlines in the press of the time, of course, these Chronicles. del Cosmos also echoed the issue.

The new planet in the solar system, Planet 9, would be of the super- Earth-type, that is, about 5 to 10 times more massive than Earth, and would have a very distant and very elliptical orbit. Its gravitational effect would gradually disturb the trajectories of many less massive TNOs until their orbits were grouped in a relatively small region of the sky.


Kevin Napier (Univ. Of Michigan) has coordinated a large international group of astronomers (including several Spaniards) to complete a study that seriously casts doubt on the arguments used so far to justify the Planet 9 hypothesis. According to Napier and his colleagues, the clustering of the orbits of TNOs observed to date is only apparent, a kind of mirage.

This team considered a group of 14 extreme TNOs, in very distant orbits (that is, belonging to the group that influenced the Planet 9 hypothesis) but took into account that these 14 objects must be part of a much larger underlying family of objects. ‘ invisible ‘(difficult to detect with current telescopes) and tried to determine the distribution of the orbits of that entire family of trans-Neptunian objects, those that are seen and those that are not seen.

Napier and his collaborators concluded that the observed orbits are compatible with a uniform distribution for all orbits of the large underlying family of TNOs. The apparent clustering observed to date is because the observations have been concentrated in a very specific region of space.


The Planet 9 hypothesis would have its origin in a statistical bias, of the type that occurs in some surveys. Imagine for example a survey of a few hundred Spaniards that shows that 70% of us are flamenco music lovers. Flamenco here plays the role of Planet 9, attracting the interest of citizens, just as the planet attracts the orbits of TNOs due to its gravitational effect.

But let us also imagine that a finer analysis reveals that this sample of a few hundred Spaniards surveyed was dominated by 80% of Andalusians. So it turns out that the sample is not representative of all Spaniards and the 70% obtained as flamenco lovers reflect, above all, the tastes of Andalusia. Well, in a similar way, Napier’s team argues that the very small sample examined so far of TNOs is not representative of the entire family of these objects and, therefore, the Planet 9 hypothesis would have no justification.


In short, the new study does not rule out the existence of Planet 9 but argues that, for now, there is no observational evidence to justify its existence. To reach a definitive conclusion, the sample of observed TNOs would have to be considerably increased, covering a much wider area of ​​the sky than that explored so far, something that is difficult at the moment since existing telescopes cannot observe more distant objects (less bright).

The large panoramic Vera Rubin telescope, which is being built in Cerro Pachon (Chile), will have enough power to discover several hundred more trans-Neptunian objects from the year 2023, which will allow to increase the sample and reach a more convincing conclusion about the existence or non-existence of the hypothetical Planet 9. The article by Napier et al. Entitled No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects is pending publication in the scientific journal The Planetary Science Journal, the manuscript can be consulted