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The artist's imagination of the ninth planet (photo source: Wikipedia) is there an undiscovered ninth planet in the solar system? This problem has plagued scientists for years because they have observed the highly unusual orbits of some small celestial bodies outside the solar system. However, some scientists believe that perhaps these phenomena are not caused by a hidden planet, but because of a deviation in our understanding of gravity.

The outer edge of the solar system has an asteroid belt-like region called the Kuiper belt belt. Its range starts from the orbit of Neptune (about 30 astronomical units from the sun, 1 astronomical unit is equal to the average distance between the earth and the sun), and extends to about 50 astronomical units from the sun. Like the main asteroid belt, the Kuiper belt mainly contains many small celestial bodies. The Kuiper Belt also contains at least four dwarf planets, namely Pluto, Priscilla, Ornithos and Eris.

In 2016, scientists discovered that some small objects in the Kuiper Belt have highly unusual orbits, which they speculate may be due to the gravitational pull of an undiscovered planet. The hypothetical planet, known as the Ninth Planet, is estimated to be about 5 to 10 times the mass of Earth, 2 to 4 times the diameter of Earth, and the long half axis of its orbit is about 400 to 800 astronomical units.

But recently, a team of scientists published a paper in the Journal of Astronomy (The Astronomical Journal), pointing out that the abnormal orbits of Kuiper Belt objects may not be caused by a hidden planet, but because of a deviation in our understanding of gravity.

They studied the effect of the Milky way on Kuiper Belt objects in the solar system if gravity followed a theory called modified Newtonian Dynamics (Modified Newtonian Dynamics).

What is modified Newtonian dynamics? Modified Newtonian dynamics was proposed by Israeli physicist Maud Hay Milgrom (Mordehai Milgrom) in 1983 to explain why stars at the edges of galaxies rotate faster than expected.

According to Newton's law of gravity, stars in the edge of the galaxy should rotate at a slower speed. However, observations show that these stars actually rotate at a fairly high speed. This means that there must be an invisible material in the galaxy to provide extra gravity.

This invisible matter is called dark matter, and it is assumed by the current mainstream gravitation theory. However, dark matter has not been directly detected so far, and its nature and distribution remain a mystery. Modified Newtonian dynamics proposes another possible explanation without introducing the concept of dark matter: Newton's law of gravity only applies to high acceleration, while at low acceleration, gravity changes, resulting in a different effect than expected.

Specifically, at very low acceleration, the gravitational force between objects is inversely proportional to the distance between them, not to the square of the distance. At this point, the gravitational pull between objects is stronger than predicted by Newton's law of gravity, which explains why stars at the edges of galaxies can rotate so fast.

How does modified Newtonian dynamics affect the solar system? Although the modified Newtonian dynamics can well explain the observations on the galactic scale, can it also be applied to the solar system? This is what Harsh Mathur, a physicist at case Western Reserve University, and Katherine Brown, a physicist at Hamilton College, want to explore.

To explore this question, they studied the gravitational effects of the Milky way on objects in the Kuiper Belt of the solar system. This effect is negligible within the framework of Newton's law of gravity, but it may have a significant effect in the framework of modified Newtonian dynamics.

They found that on a long enough time scale, the long axes of some celestial orbits in the Kuiper Belt gradually align toward the center of the Milky way, and the aphelion points toward the center of the Milky way. This alignment can cause the orbits of these celestial bodies to appear abnormal, creating the illusion of the existence of the ninth planet.

Mathur and Brown said the alignment was "eye-catching". Their research not only shows that modified Newtonian dynamics can be applied to the solar system, but also reveals that anomalies at the edges of the solar system may be due to a deviation in our understanding of gravity, rather than the existence of an undiscovered planet.

However, they also admit that the study has many uncertainties and limitations, such as insufficient data and controversy over the revision of Newtonian dynamics itself. For example, although modified Newtonian dynamics can explain the abnormally high rotation speed at the edge of galaxies, it does not apply to all galaxies, and scientists still need dark matter to explain. Nor can it explain some phenomena that can be explained by dark matter theory, such as the temperature fluctuation of cosmic microwave background radiation and the formation of large-scale structure of the universe. Moreover, it also conflicts with general relativity.

Therefore, their research needs more observations and experiments to verify. "regardless of the results, this work shows that the outer edge of the solar system is an important experimental site for exploring the basic problems of gravity and physics." Brown said.

References:

Https://iopscience.iop.org/article/10.3847/1538-3881/acef1e

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