On August 24, 2006, Pluto ceased to be a planet. No change occurred in space, but rather in Prague (Czech Republic) where the International Astronomical Union (IAU) approved a definition of a planet that left out the one that for 76 years had featured as the ninth in the Solar System. But to this discussion has been added the scientific debate about the possible existence of a giant world beyond Neptune, which, to the indignation of some, is attempting to seize the title of Planet Nine.
Since the discovery of Pluto by Clyde Tombaugh in 1930, no new name has been added to the list of our planetary neighbours. But when, in 2005, astronomer Mike Brown from Caltech (USA) discovered Eris, a new trans-Neptunian object (TNO) that exceeded the mass of Pluto, the IAU recognised the need to approve a definition of “planet” to bring order to the taxonomy of the Solar System. The result was the expulsion of Pluto from the club of planets.
That decision sparked an outcry that has continued to reverberate until today and that has among its most ardent supporters Alan Stern, principal investigator of NASA’s New Horizons mission, which in 2015 visited the vicinity of Pluto, today officially a dwarf planet. The controversy was rekindled when Brown and other scientists attached the nickname of Planet Nine to a possible giant world that could exist beyond Neptune, as suggested by certain mathematical models.
“My view is that calling this object Planet Nine is insensitive to Dr Tombaugh’s incredible legacy and diminishes it, and therefore should not be used,” Stern tells OpenMind. “There are many generic terms that would work, beginning with Planet X.” On July 29, Stern and 34 other scientists signed a letter in the Planetary Exploration Newsletter in which they defended this position.
Another of the signatories is astrobiologist David Grinspoon of the Planetary Science Institute, co-author with Stern of the book Chasing New Horizons: Inside the Epic First Mission to Pluto (Picador, 2018). For Grinspoon, not only is the use of the term Planet Nine “obnoxious,” but in his opinion it is also the fact that Brown has adopted the alias of Pluto Killer. “The folks publicizing the term Planet Nine are the same guys who have bragged about killing Pluto,” he tells OpenMind. “I believe there’s some ego wrapped up in it.”
Another large planet in our solar system
Even so, Grinspoon remarks that: “this is obviously not the most important issue in planetary science today.” Beyond the terminological clashes, the scientific debate centres on the existence of Planet Nine or X. “The possibility of another large planet being found in our solar system is an absolutely thrilling one,” admits Grinspoon.
Since 2014, several scientists, including Brown, have provided evidence that a planet of about ten times the Earth’s mass, with a size similar to Neptune and with a distance to the Sun roughly 20 times greater than that of Neptune, could explain the detected orbital anomalies in several TNOs, among them Sedna, discovered by Brown himself in 2004. These objects have their orbits grouped in space and inclined to the plane of the planets, something that could correspond to the gravitational influence of a great distant planet. Last May, a new study conducted by the University of Michigan (USA) discovered that the orbital inclination of another TNO, provisionally called 2015 BP519, also fits with the hypothesis of Planet Nine/X. “This adds to the circumstantial evidence for the existence of this proposed new member of the Solar System,” the authors wrote.
However, the hypothesis of the large trans-Neptunian planet also has its detractors. Last June, astrophysicist Ann-Marie Madigan, of the University of Colorado in Boulder (USA), presented a hypothesis that she proposed for the first time in 2015 and that offers an alternative explanation to the orbital anomalies of many TNOs. According to Madigan and her collaborators, the collective gravity of a multitude of small objects could be causing the effect on the other larger ones.
For the astronomer from the Complutense University of Madrid (Spain) Carlos de la Fuente Marcos—who, in 2014, together with his brother Raul, was among the first to show that the orbital anomalies of some TNOs could be due to large trans-Neptunian planets—the work of Madigan (not yet formally published) has weak points, such as the likelihood that the collective mass of objects reaches the necessary threshold to cause the described effects and maintain them over time.
Looking for a needle in a haystack
De la Fuente Marcos explains to OpenMind that the hypothesis of the large trans-Neptunian planets is currently in good health, thanks to the multiple lines of evidence that “seem to continue indicating that there are massive disturbing agents located beyond Pluto.” The astronomer adds that Brown’s forthcoming new work will show that “the probability that available observations are incompatible with the presence of his Planet Nine is well below 1%.” “We can anticipate that our collaborators from the Institute of Astrophysics of the Canary Islands have managed to observe several of the most distant objects and there are very interesting trends in the data obtained,” he adds.
In any case, Planet Nine/X will not go from paper to reality until astronomers are able to see it. Paradoxically, it is easier to discover a planet light years away than within the confines of our Solar System; exoplanets are detected by their transit in front of their star, something that in this case is not possible. On the other hand, the light that a distant trans-Neptunian planet would receive from its star, the Sun, may be too dim to observe it directly, although the signature of its internal infrared heat may be found.
Another difficulty is that most telescopes have too small an observation field; it has been compared to looking for a needle in a haystack by gazing through a straw. Brown and his collaborators track the sky with the Subaru telescope in Hawaii, whose observation field is much wider. De la Fuente Marcos is confident that “a discovery will be made relatively soon,” and that with the Large Synoptic Survey Telescope currently under construction in Chile, which will be operational in 2022, “its long-term discovery is practically guaranteed, assuming that it exists.” But while Brown gave the search a duration of five years back in 2016, others have begun to get impatient: “We do know the search has been unsuccessful for a long time,” says Stern.
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