Mike Brown is a Caltech astronomer who is fixated with trans Neptunian objects, that is chunks of matter in orbit around our Sun at a distance larger than Neptune’s major axis of 30 astronomical units (that is, 30 times the distance Earth-Sun). Among many such chunks, he and his team discovered the dwarf planet Eris, which at some moment was thought to be bigger than Pluto. This story triggered a long debate about what is the proper definition of a planet, and eventually led to dismissing Pluto as a planet at all, on the basis that its size is about 1/6 of our Moon, and its orbit is very eccentric, crossing that of Neptune at two points (it is calculated that a precious kinematic resonance has avoided up to now a collision between the two). Since then, Mike is referred (and calls himself) “the man who killed Pluto”. However, he thinks he did instead a great service to this beautiful brown-grey ball, as it was shown in the greatest details by the fantastic travel of the New Horizon mission. Instead of being dismissed as the weirdest and smallest planet of the Solar System, Pluto is now promoted to the largest (known) object of the Kuiper belt. In fact, the existence of a trans-Neptunian population was firstly hinted by F. Leonard, after Pluto’s discovery by Clyde Tombaugh in 1930. That same year, the astronomer Armin Leuschner suggested that Pluto “may be one of many long-period planetary objects yet to be discovered.” Other astronomers followed, proposing that in the vast region beyond Neptune the material within the primordial solar nebula would have been too widely spaced to condense into planets, and so rather it should have condensed into a disc of a myriad smaller bodies. Until, in 1951, Gerard Kuiper formulated a consistent theory on a similar disc having formed early in the Solar System’s evolution, at a distance between about 38 to 50 a.u.

But Mike Brown has not yet finished surprising the world of astronomy. In 2016, he and Konstantin Batygin announced their observation of another Kuiper belt’s dwarf planet, 2012-VP113, whose orbit lines up almost perfectly with the orbits of five other extreme trans-Neptunian objects. Although it could just be a coincidence, the sheer probability of such a clustering of celestial objects occurring, without something or somebody tugging them into place, is vanishingly small. The cluster of dwarf planets appears to be circling the Sun on super-elongated orbits at average distance of 149 billion km form the Sun, 75 times more distant than the already faraway Pluto, and takes between 10,000 and 20,000 Earth’s years to complete the round. Two years later, another trans-Neptunian object, 2015-BP519, was also calculated to be on an unusual trajectory. All these objects have in common their strangely inclined orbits, in a plane different from the ecliptic (the plane in which the orbits of the now-8 Solar planets lie). Yet another “strange” object passing in that same region of space is the dwarf planet Sedna, with size about 2/3 that of Pluto. This one has an orbit that only at its closest point barely touches the outer Solar System, and at the maximum elongation reaches almost 900 a.u. What makes it “strange” is that also Sedna’s orbit lies more or less in the same plane as the dwarf cluster. And all these objects appear to have their perihelion around a common, relatively “small” region of the outer space. As Brown told in an interview to the Guardian, “We saw a strange signal in the data that meant something odd was going on in the outer Solar System: all of these distant objects were lined up in a weird way and that shouldn’t possibly happen. We worked through the mundane explanations, but none of them worked out. It’s like having six hands on a clock, all moving at different rates, and whenever you happen to look up, they’re always in exactly the same place. The odds of that happening randomly are about 1/14,000. So, we thought something else must be shaping these orbits.”

In their highly cited paper (more than 1 million downloads), Batygin and Brown came to the conclusion that the two most likely explanations for what exactly that “something else” could be, are  either a yet-undiscovered mass of debris floating through the Kuiper belt, or a hidden planet. They ruled out the Kuiper belt debris hypothesis, because in order for this debris to have enough force to push the six dwarf planets into the same inclined orbital plane, the Kuiper belt would have to be possibly 100 times more massive than it actually is. Even at their closest distance from the Sun, these “extreme trans-Neptunian objects”, or eTNO, stay so far from Neptune that it cannot be responsible for shaping their odd orbits. More likely, they say, should be the gravitational force from an unseen planet so distant it’s been eluding us for centuries, that could have forced the six rocky objects into a line. Planet Nine – also referred to as Planet X – should be a massive object, 5 to 15 times the Earth, in an elliptical orbit far beyond Pluto, at an average distance from the Sun estimated to be anywhere between 300 and 1,000 a.u.

But if Planet Nine exists so faraway that we have not yet been able to spot it, how such a massive world could orbit so far from the Sun? This is a heavily debated question, which lends itself to many speculations. One possibility is that the solar system was shook up in its past by a wandering star. This is the “Scholz star” hypothesis, a small binary-star system with a total mass 0.15 times the Sun, which should have passed at a distance of about 1 light-year from the Sun about 70,000 years ago, thus crossing the Oort cloud, and sending dozens of comets and asteroids rambling through the solar system. There is an interesting example that could support this hypothesis. A recently discovered exoplanet orbiting around a binary star, HD106906, situated at a distance of about 336 light-years from us, shows a large, perturbed orbit. The two stars, each of size slightly larger than our Sun, are separated by a distance of just about 0.4 a.u. and host a large, asymmetric disk of debris of size 500 a.u. (so, much larger than our Solar system) and one single planet at a distance of about 732 a.u. Computer simulations show that the planet could have been exiled into such a faraway path by another pair of rogue stars, which strayed too close to the binary system about 3 millions years before. On the other hand, according to an alternative hypothesis, the elusive Planet Nine may have established its unique and distant orbit from the Sun after being pushed out, during the solar system’s chaotic formation, when the even more massive gas giants Jupiter and Saturn were also fighting for their position. All such simulations use nothing more than Newton’s laws applied to N-body systems (with N very large) interacting via the gravitational force, the enormous complexity of the infinitely possible initial conditions hiding all the mysteries of the celestial dynamics. Actually, what is done is to start from the current positions of the planets and stars, and integrate their equations of motion back in time, looking for explanations. But because of the very big N, this can only be done numerically on large supercomputers, with all the uncertainties that we know associated with such kind of modeling.

The idea of a shadow Earth, invisible companion to our planet, is not new. A Counter-Earth or Antichthon was hypothesized by the pre-socratic philosopher Philolaus (470-385 BC), to support his cosmological view in which all objects in the universe revolve around a “Central Fire” unseen from Earth, but distinct from the Sun that also revolves around it. Along with the Central Fire, the mysterious Antichthon was the other heavenly body not visible from Earth, revolving around the Central Fire at exactly the opposite position from the Earth (in what we could call today the Lagrange point L3). Aristotle described it as “another Earth”, needed to explain the eclipses of the moon and their frequency, which could not be explained by Earth alone blocking the light of the Sun (since Earth at that time did not yet revolve around the Sun…). Aristotle, a critic of the Pythagorean numerology, also suggested that it was also introduced to raise the number of heavenly bodies around the central fire from nine to ten, which the Pythagoreans regarded as the perfect number.

However, according to an interesting paper published in July 2020 PRL, Planet Nine is not the only possible explanation for the eTNOs. Another possible explanation could be a primordial black hole, instead. Primordial black holes (PBH) are predicted to have come into existence within the first few fractions of a second after the Big Bang, but their existence has never been confirmed. If true, also the hypothetical black hole would be located at about the same distance of Planet Nine, but its diameter, for a mass about five times the Earth, would be only about the size of a baseball. A set of gravitational anomalies, recently identified by the Optical Gravitational Lensing Experiment (OGLE), may help explain why Planet Nine could actually be a tiny black hole. Based on five years of OGLE observations, at least six strange microlensing events have been found, seemingly occurring when objects roughly 0.5 to 20 times the mass of Earth acted as gravitational lenses. These objects, located about 26,000 light-years away toward the Milky Way’s galactic bulge, could correspond to an unexpected population of primordial black holes. The PRL paper convincingly shows that one PBH of this kind could have been captured into orbit around the Sun, at about the right distance to explain the orbital properties of eTNO otherwise attributed to Planet Nine. An exciting hypothesis that, I am sure, even Philolaus would have loved.

I want to conclude this last Letter before the holidays, by going back to my previous Letter of June 18 about the quirks of science publishing, to point out another funny episode that sheds a sinister light on the actual quality control of the major journals. Of course, the whole story can be dismissed as a mistake or a coincidence, but one may wonder at how many such “mistakes” have occurred in the past, and how many will occur in the future. Kary Mullis (1944-2019) was an American biochemist, Nobel-prize winner in 1993 for the invention of the polymerase chain reaction, or PCR, the technology that has revolutionized modern biology by introducing DNA amplification. In 1968, Kary was a second-year biochemistry major at Berkeley, enthusiast about all fields of science and an avid reader of astrophysics books and papers. After reading a 1966 Nature paper by Russell Stannard about time symmetries in subnuclear physics, he made up an ‘intuitive’ explanation of the origin of the universe, in which half of the matter was going reverse in time with respect to the other half. He wrote a rapid communication, with just Stannard’s paper as the single reference, and mailed it to Nature on February 21. After a brief revision, the paper was published two months later with the title Cosmological Significance of Time Reversal. According to Mullis himself, this total nonsense, spanning two pages in the most respectable scientific journal on Earth, received many reprint requests and was highlighted by the Nature-Times News Service, after which he kept receiving requests from popular press to explain the article. As a young student, Kary started being frightened of his little-deserved fame, and doubting of the science establishment as a whole. His doubts were bitterly confirmed 20 years later when, now an established scientist, his world-changing, Nobel-winning paper on PCR was rejected by both Nature and Science, and ended up published in the obscure Methods in Enzimology, a specialized journal with impact factor 1.6.

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