Are there Nine Planets in our Solar System (still not Pluto - sorry)?
The Situation Before 2006
In 1906, there were eight known planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Then, in 1930, Pluto was discovered and officially named the ninth planet - coming after Neptune in terms of distance from the Sun. In the late 1990s, however, astronomers started arguing about whether Pluto was indeed a planet.
Pluto’s Demotion to Dwarf Planet in 2006
In August 2006, the International Astronomical Union (IAU) laid out an official definition for what constitutes a fully fledged planet. These criteria are:
The celestial body must orbit a star (in our solar system this would be the Sun).
The celestial body must have sufficient mass to assume a round shape.
The body must have enough mass that its own gravity has cleared away any other objects of a similar size near its orbit.
Pluto did not meet this third criterion because it has not cleared its neighboring region of other objects, meaning that its mass is substantially less than the total mass of other objects in its orbit. Therefore, since August 2006, Pluto has been classified as a “dwarf planet” - like Eris (discovered in 2005), Makemake and Haumea in the outlying Kuiper Belt, as well as Ceres in the Asteroid Belt (between Mars and Jupiter). Today, there are officially five dwarf planets recognized in our solar system, but there may be many more yet to be discovered.
The heart-shaped glacier that dominates Pluto, can be seen toward the bottom right. NASA
Not all scientists agreed with these classifications, arguing that other factors should also be considered including presence of atmosphere, a magnetosphere and geological activity. The discussions reached new heights after NASA’s New Horizons reached Pluto and its moons in 2015 and scientists were astounded by Pluto’s beauty and complexity, including its giant heart-shaped glacier.
At the time, NASA’s chief administrator Charles Boden said that he hoped the scientists would reconsider the classification and Pluto’s downgrading because he considered Pluto a “planet”.
Is there a Planet Nine in our Solar System?
Irrespective of ongoing discussions about Pluto’s status, our solar system is currently officially said to have eight planets.However, we may yet still have nine planets.
Artist's illustration of Planet Nine, a world about 10 times more massive than Earth that may lie undiscovered in the far outer solar system. (Image credit: Caltech/R. Hurt (IPAC)
Artist's illustration of Planet Nine, a world about 10 times more massive than Earth that may lie
undiscovered in the far outer solar system. (Image credit: Caltech/R. Hurt (IPAC)
Scientists have not seen the ninth planet. However, they have inferred its existence by its gravitational effects on other objects in the Kuiper Belt. This vast region at the fringe of the solar system beyond Neptune is believed to contain hundreds of thousands of icy rocks left over from the birth of the solar system. Pluto is the best-known and the largest of the known Kuiper Belt objects which all have highly elliptical or oval orbits that align in the same direction.
In 2016 scientists from the California Institute of Technology published their hypothesis that an unseen massive object is causing the pulling effect on the Kuiper Belt objects. Their research was based on mathematical models and computer simulations using observations of smaller Kuiper Belt objects with orbits that aligned in a similar way. According to their estimates, “Planet Nine” would have to be about five to 10 times the mass of Earth (and 5,000 times the mass of Pluto) and orbit about 400-800 au [astronomical units] from the Sun which would be between 300 and 1,000 times farther than the orbit of Earth.
Gravitational pull [The six most distant known objects in the Solar System with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Also, when viewed in 3D, they tilt nearly identically away from the plane of the Solar System. Caltech’s Konstantin Batygin and Mike Brown have shown that a planet with 10 times the mass of the Earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. (Courtesy: Caltech/R. Hurt (IPAC))
Based on their idea of its mass and also its position in space, scientists have also come up with a theory of what Planet Nine looks like. Apparently, it is either a rocky super-Earth, or a gaseous mini-Neptune - also sharing characteristics with Uranus or Neptune, making it an icy planet with a solid core.
Why has Planet Nine not yet been found?
A planet this far away in the outer solar system far beyond Pluto would be extremely difficult to spot in normal optical sky searches because of its faintness. Best estimates suggest that it could be 600 times fainter than Pluto, meaning that it is unlikely to be reflecting much light from the Sun. As such it will remain in shadow, hidden within the sparking starfield of the Milky Way.
To aid their search for the elusive Planet Nine, scientists have turned to telescopes that can search the cosmos in millimeter wavelengths, such as the Atacama Cosmology Telescope (ACT) in Chile. However, the best hope lies with the even more sensitive Vera Rubin Observatory, which is currently still under construction in Chile.
What other explanations are there?
As astronomers continue to come up empty in their search for Planet Nine, attitudes towards its existence vary widely. Enthusiasts remain committed and believe it is simply a question of time before Planet Nine is finally discovered.
Another hypothesis proposed in 2019 suggests that Planet Nine might not be a planet at all, but a primordial black hole that formed from gravitational perturbations very shortly after the Big Bang and that our solar system later captured.
The search is still on, but the question remains whether Planet Nine may forever remain a mystery.
