Most people are familiar with the eight known planets of the solar system. However, it is almost certain that billions of years ago from now, the solar system formed more planets than these eight. While most of them have disappeared or left the solar system by now, could it be possible that a few have remained and survive to this day?

The answer to this question may come from what are known as trans-Neptunian objects (TNOs). As the name implies, TNOs are small celestial bodies that orbit the Sun at a greater average distance than the orbit of Neptune. In particular, the distant Kuiper Belt, the region located beyond 7.5 billion kilometers (or 50 astronomical units) from the Sun, contains many TNOs. While these objects represent the remnants of planetary formation in the outer solar system, their orbits and distribution might very well reveal the presence of undiscovered planets.

In a recent study published in The Astronomical Journal on 25 August 2023, Associate Professor Patryk Sofia Lykawka from Kindai University in Japan and Associate Professor (Senior Lecturer) Takashi Ito from the Center for Computational Astrophysics at the National Astronomical Observatory of Japan (CfCA/NAOJ) tackled this puzzle. Based on theoretical analysis of observations coupled with cutting-edge computer simulations, they came to the remarkable conclusion that an Earth-like planet (a planet 1.5-3 times as massive as Earth) may be lurking in the distant Kuiper Belt!

The researchers began by analyzing in detail the orbital structure of the distant Kuiper Belt, which exhibits several unexplained anomalies. For instance, there is a large population of detached TNOs, whose orbits are beyond Neptune's gravitational influence. Additionally, there is a significant number of TNOs with highly inclined orbits along with a population of "extreme TNOs" whose orbits are extremely difficult to explain with the current models for the formation of the solar system and the Kuiper Belt.

Based on these analyses, the researchers theorized that another planet besides the four giants (Jupiter, Saturn, Uranus, and Neptune) must have influenced the formation of the Kuiper Belt. To test their hypothesis, they conducted a multitude of computer simulations using the research computers installed in Lykawka's laboratory and the general-purpose PC cluster at NAOJ, using models of the early solar system that existed about 4.5 billion years ago.

Herein, the researchers considered interactions between the four giant planets, a hypothetical Kuiper Belt planet, and a disk of small objects representing the primordial distant Kuiper Belt. After each simulation was completed, the resulting populations of TNOs following the lapse of 4.5 billion years were compared to those gathered from modern observations to see if any of the models explained the anomalies in the distant Kuiper Belt.

Remarkably, the simulation's best results suggested that there should be an undiscovered planet with a mass 1.5-3 times that of the Earth orbiting the Sun at distances between about 200 and 500 (or even ~200-800) astronomical units. Thanks to the palpable mass and an inclined orbit of about 30 , such a planet could have generated the large number of detached TNOs, the highly inclined TNOs, as well as the extreme TNOs with peculiar orbits, in line with our current observations.

The discovery of a new Earth-like planet in the solar system would no doubt have profound implications, as Dr. Lykawka explains: "First, the solar system would officially have nine planets again. Moreover, much like what happened in 2006 when Pluto was demoted from the planet category, we would need to refine the definition of a 'planet,' since an Earth-like planet located far beyond Neptune would likely belong to a new class of planets. Finally, our theories of solar system and planet formation would also need revision."

Now that the prediction has been made, it is time to search for this Earth-like planet in the distant Kuiper Belt. According to Dr. Lykawka, future Japanese or international astronomical surveys might be able to detect this new planet in less than a decade. Many new extreme TNOs could be discovered in the process, providing valuable insights into the trans-Neptunian region. "A more detailed knowledge of the orbital structure in the distant Kuiper Belt will provide us with a better understanding of the formation of the outer solar system, which would also reveal the conditions under which the planets formed. Even the discovery of a single or a few such new TNOs could revolutionize our theories about how the solar system formed."

Research Report:Is There an Earth-like Planet in the Distant Kuiper Belt?

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