Astronomers are still searching for Planet Nine at the edge of the Solar System

The hypothesis of a large planet Nine (or 9), as it was provisionally named, first appeared in 2016, and astronomers Konstantin Batygin and Michael E. Brown, who work at the California Institute of Technology, came up with it at the time.

Both are trying to prove the existence of this planet to this day – and that’s why they are still collecting more and more evidence, but unfortunately only indirect so far. Direct observation, and thus the discovery of such a body, requires very powerful instruments and advanced observation and analytical methods.

This is because a distant planet would emit very little light reflected from the Sun, mostly in the (invisible to us) infrared or microwave region, and it would move very slowly relative to the stellar background.

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Due to the lack of direct evidence, most astronomers are so far skeptical of the “Planet 9” hypothesis.

Let us recall that the planet Neptune and the dwarf planet Pluto were also discovered in a similar way at one time. Astronomers first predicted their existence mathematically thanks to their gravitational effects on the movements of already known planets, and only then was their existence confirmed by direct observation (in the case of Pluto, it was decades of searching that passed between the detection of deviations or disturbances in the planets’ orbits and the direct observation of Pluto).

As far as Planet Nine is concerned, we are still in the first phase, when we suspect that something is “off” at the edge of the Solar System, but we don’t know what. The whole matter may have an alternative explanation.

Peculiarities of the movement of some objects in the Kuiper belt

According to Batygin and Brown, the unknown Planet Nine dominates the Kuiper Belt region so much that it was able to “drive away” or resonantly “flatten” at least six objects that move beyond the orbit of the planet Neptune (so-called trans-Neptunian objects, TNO). Their distinctly elliptical orbits all point in a similar direction and form a similar angle to the basic plane of the Solar System (the ecliptic) in which most planets move.

Usually, the aphelion (farthest points from the Sun) of elliptical orbits spontaneously turn slowly around the Sun at different speeds. Therefore, it is strange that the orbits in question kept a “tight formation” so perfectly, when they should be scattered more or less randomly within all angles.

Already in 2016, astronomers managed to explain this phenomenon precisely on the basis of a model that includes as a necessary component the gravitational force of the unknown Planet Nine, whose perihelion (the closest point of the elliptical orbit around the Sun) is located on the opposite side of the perihelion of the Kuiper belt objects influenced by it.

This does not mean that the observed data cannot be explained in some other way, but it is quite unlikely. However, some astronomers dispute this conclusion, arguing instead that the clustering of TNO orbits is due to observational bias resulting from the difficulty of observing and systematically tracking these objects.

Another possibility is the hypothesis that instead of Newton’s gravity in the practice of the Solar System, a slightly different, so-called modified theory of gravity works, which will explain the given phenomena even without another planet.

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However, the action of Planet Nine also explains the atypical orbits of some bodies like the dwarf planet Sedna. In addition, this model predicted the existence of another class of bodies on very exotic orbits, both perpendicular to the direction of Planet Nine’s elliptical orbit and completely perpendicular to the plane of the ecliptic. And indeed, astronomers have discovered five bodies whose orbits have exactly this character.

More indirect evidence was provided by Batygin, Brown and others in a paper this April, in which they focused on objects whose motion is unstable because they interact with the orbit of the planet Neptune. Their instability meant that the relevant analysis was much more complex. According to the authors, the best explanation for their existence and the nature of their orbits is that they come from another planet, corresponding to Planet Nine. The Independent drew attention to this recently.

Their team performed a series of simulations that included the influence of various factors on the development of the orbits of these objects. In addition to the strong influence of giant planets in their vicinity, such as Neptune, they also took into account subtle gravitational influences coming to us from the Milky Way or the influence of stars that have approached the Solar System in the past.

“The model that included planet 9 gave the best explanation statistically,” Batygin said.

Although there are other explanations for the behavior of these objects – including the assumption that other planets that are now missing in the given places also once influenced their orbits – the model including Planet Nine remains the best explanation, according to the authors.

Work in recent years has yielded an improved set of predictions for the orbit and mass estimate of Planet Nine as well. Additionally, in a 2021 book, Batygin and Brown showed how Planet Nine’s gravity can “unfreeze” the orbits of objects in the so-called inner Oort cloud (hypothetical cloud of comets – editor’s note) and direct them closer to the Sun, e.g. to the region of the outer Kuiper belt.

We will have more data in our hands when the Vera C. Rubin Observatory begins its operations, which will have a large all-sky telescope with a diameter of 8.4 meters and thus “pick up” many objects at once.

It is currently under construction in Chile, and when it starts working, it will be able to scan very efficiently and precisely precisely the distant and dimly lit trans-Neptunian objects.

The orbit and dimensions of the predicted Planet Nine

As for the unknown “Nine”, according to Batygin and Brown, it should be a “Super-Earth”, possibly a rocky or solid core of a former gas giant, about 5-10 times heavier than Earth. It will probably be a frozen and very dark world. According to researchers, it should orbit the Sun in an elliptical orbit at distances of 340 to 560 AU (one astronomical unit AU is approximately 149.6 million km in size and corresponds to the average distance of the Earth from the Sun).

At the same time, they estimate that its orbit should have an inclination of about 16 degrees with respect to the plane of the Solar System. Its orbital period should be something between 10-20 thousand years.

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Such a distance is astonishing for us today and, unfortunately, still rather unattainable with current instruments and methods of observation. So far, the most distant object in the Solar System that we have observed is a body nicknamed FarFarOut with a size of 400 km. We observe it at a distance of 132 AU from the Sun. To give you an idea, Neptune orbits the Sun at a distance of about 30 AU.

However, planetary scientists Patryk Sofia Lykawka from Japan’s Kindai University and Takashi Ito from the National Astronomical Observatory of Japan research center came up with a slightly different variant, according to which the observed behavior of trans-Neptunian objects could also be explained by the presence of a smaller terrestrial planet, more similar to Earth, at least in terms of size. It would also orbit a bit closer – at a medium distance between 250 and 500 AU.

According to Batygin and Brown, Planet Nine could be the core of a giant planet that was ejected from its original closer orbit by Jupiter during the formation of the Solar System.

Others believe that the planet could have come from another star, i.e. that it was once a wandering planet. Or that it formed on an even more distant orbit and was pulled into its current eccentric orbit around the passing star. Other authors believe that it is not a planet, but a primordial black hole (of the same mass) from the beginning of the universe, which is why we do not see it.

Hypothetical Planet Nine may be an ancient black hole

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Although sky surveys such as WISE (Wide-field Infrared Survey Explorer) and Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) have not detected Planet Nine, they have not yet ruled out even the existence of a Neptune-type object in the outer part of the Solar System. However, until Planet 9 is observed, its existence remains purely hypothetical.

The idea of another unknown planet on the outskirts of the Solar System is not new. Similar serious speculations, based on models of celestial mechanics, appeared among astronomers in 2014. In December 2015, a group of astronomers even interpreted some observations of the Atacama Desert Radio Telescope Array (ALMA) as microwave radiation from an unknown planet moving on the outskirts of the Solar System. And other physical models also predicted the existence of a fifth large planet, which could be “thrown” to the edge of the system by the gravitational effects of other large planets. For now, however, astronomers can only calculate the approximate orbit of the hypothetical Planet Nine and the areas where it is most likely to occur, they do not know its specific position at a given time.

The Kuiper Belt, the Scattered Disk and the Oort Cloud

The space directly beyond the orbit of Neptune (it orbits at a distance of about 30 AU, astronomical units from the Sun) is called, after Gerard Kuiper’s classic study of this area, the Kuiper belt. It is estimated that there are around 100,000 smaller objects (only about 1,000 have been cataloged so far) with a diameter greater than 100 km, but the total mass of the belt is estimated to be at most one tenth of the mass of Earth. These bodies will orbit the Sun in roughly a few hundred to thousands of years. Although most of the dynamical structure of the Kuiper belt can be understood within the framework of the gravitational influence of the Solar System with eight known planets, bodies with orbital periods longer than ~4000 years show (according to Batygin and Brown) a peculiar orbital arrangement that defies this conventional explanation.

Also orbiting here is Pluto, which, with a diameter of about 2,300 km and a mass two thousand times smaller than Earth, is the largest body in the Kuiper belt.

When we go even “deeper” into space, we pass into the region of the so-called scattered disk, whose objects can be more than 100 AU from the Sun, i.e. about 14,960 million km. This also includes the largest known object from the region beyond Neptune, the dwarf planet Eris, which actually deprived Pluto of its planet status because it is larger.

Eris was discovered in 2003 and orbits the Sun in 557 years. The Scattered Disk region is also thought to be the source of short-period comets. But the most distant part of our planetary system is the Oort cloud.

A hypothetical Oort cloud

So far, this is rather just a calculated spherical cloud, made up of about a trillion small frozen cometary nuclei. Some from here sometimes head into the planetary region and become comets, others migrate between the Oort Cloud and the outer boundary of the Kuiper Belt.

An example of such a “migrant” is the planet Sedna, which takes an incredible 10,800 years to orbit the Sun.

Although the level of our observational equipment continues to grow, the question of whether we have discovered all the large bodies in our system is still valid. We do not have 100% certainty.