10 Theoretical Planets Suggested by Scientists

Neptune was once thought of as a theoretical planet—it was believed to exist despite never being observed. In fact, numerous other hypothetical planets have been suggested. Some were dismissed, but others may have existed in the past, and could still be out there today.

10. Planet X

In the early 19th century, astronomers were aware of all the major planets in our solar system, except for Neptune. They also understood Newton’s laws of motion and gravity, which allowed them to predict planetary movements. However, when these predictions were compared with actual observations, it became clear that Uranus wasn’t following its predicted path. French astronomer Alexis Bouvard speculated that the gravitational influence of an unseen planet might be causing Uranus to deviate.

After Neptune's discovery in 1846, many astronomers investigated whether its gravity could account for Uranus's movement. It couldn’t. Could there be another hidden planet? Many suggested the existence of a ninth planet. American astronomer Percival Lowell was the most passionate advocate for this mysterious ninth planet, which he named Planet X.

Lowell constructed an observatory with the aim of discovering Planet X, but he never succeeded. Fourteen years after his death, an astronomer at Lowell’s observatory found Pluto. However, Pluto wasn’t massive enough to explain Uranus’s motion, so the search for Planet X continued. The quest didn’t end until the Voyager 2 probe flew by Neptune in 1989, revealing that astronomers had previously miscalculated Neptune’s mass. This new measurement of Neptune’s mass finally clarified Uranus’s trajectory.

9. A Planet Between Mars and Jupiter

In the 16th century, Johannes Kepler observed a significant gap between the orbits of Mars and Jupiter. He speculated that a planet might occupy this space but did not actively search for it. After Kepler, astronomers began recognizing a pattern in the planetary orbits. The orbital distances, from Mercury to Saturn, approximately follow the sequence 4, 7, 10, 16, 52, and 100. If you subtract 4 from each number, you get 0, 3, 6, 12, 48, and 96. Notice that 6 is double 3, 12 is double 6, and 96 is double 48. There is also a peculiar factor of four between 12 and 48.

Astronomers began wondering if there might be a missing planet between 12 and 48, specifically at 24—between Mars and Jupiter. As German astronomer Johann Elert Bode noted, “After Mars, there follows a space of 4+24=28 parts, in which no planet has yet been seen. Can one believe that the founder of the universe had left this space empty? Certainly not.” When Uranus was discovered in 1781, its orbit perfectly fit the pattern. It seemed to confirm the idea of a natural law, now known as Bode’s Law or the Titius-Bode Law, though the gap between Mars and Jupiter remained.

A Hungarian astronomer, Baron Franz von Zach, became firmly convinced that Bode’s Law was valid and that it indicated an undiscovered planet between Mars and Jupiter. He spent years searching for it, but without success. In 1800, he organized a more systematic search involving several astronomers, including Italian Catholic priest Giuseppe Piazzi, who in 1801 discovered an object with the exact orbital distance.

The object, named Ceres, was too small to be classified as a planet. For many years, Ceres was regarded as the largest asteroid in the asteroid belt. Today, it is classified as a dwarf planet, similar to Pluto. Incidentally, Bode’s Law was eventually discarded when Neptune’s orbit was found to contradict the pattern.

8. Theia

Theia is the name given to a hypothetical planet roughly the size of Mars, which might have collided with Earth about 4.4 billion years ago. This impact could have caused Theia to disintegrate, with the debris forming the Moon. The name was proposed by English geochemist Alex N. Halliday, named after the Greek titan Theia, who was the mother of the Moon goddess Selene.

It’s important to note that the origin and formation of the Moon remains an area of ongoing scientific investigation. While the Giant Impact Hypothesis, involving Theia, is the most widely accepted explanation, it is not the only one. The Moon could have been captured by Earth’s gravity, or perhaps the Earth and Moon formed together as a pair. There could be other possibilities too. It’s also significant that the early Earth was likely struck by many large bodies, and Theia might just be the one responsible for creating the Moon, assuming that’s the case.

7. Vulcan

Uranus wasn’t the only planet whose observed motions didn’t match predictions. Mercury also displayed this issue. The discrepancy was first noted by French mathematician Urbain Le Verrier, who discovered that the perihelion, the closest point in Mercury’s elliptical orbit to the Sun, was moving faster than his calculations predicted. Although it was a small discrepancy, further observations convinced him it was real. He proposed that this anomaly might be caused by an undiscovered planet, which he named Vulcan, orbiting inside Mercury's orbit.

A series of Vulcan 'sightings' followed. Some were later identified as sunspots, but others came from respected astronomers and seemed credible. By the time Le Verrier passed away in 1877, he was convinced that Vulcan’s existence had been proven. However, in 1915, Einstein published his theory of general relativity, which was able to correctly predict Mercury's movements. Vulcan was no longer necessary, but the search for objects orbiting the Sun inside Mercury’s orbit continued. While nothing planet-sized has been found, some asteroid-sized objects, now called 'vulcanoids,' may exist.

6. Phaeton

German astronomer and physician Heinrich Olbers discovered the second known asteroid, Pallas, in 1802. He hypothesized that Pallas and the first known asteroid, Ceres, might be fragments of an ancient, medium-sized planet that was destroyed, possibly by internal forces or a cometary impact. Olbers suggested there might be other such objects, and soon after, two more asteroids—Juno in 1804 and Vesta in 1807—were discovered.

The planet that supposedly broke apart to form the main asteroid belt became known as Phaeton, named after a character from Greek mythology who drove the sun chariot for a day. However, the Phaeton hypothesis faced challenges. For instance, the combined mass of all the main belt asteroids is far smaller than that of a planet. Additionally, the asteroids exhibit considerable variety, which raises the question of how they could all have come from the same parent body. Nowadays, most planetary scientists believe that the asteroids formed through a gradual process of smaller fragments sticking together.

5. Planet V

Planet V refers to another hypothetical planet once thought to exist between Mars and Jupiter, but its potential existence is based on entirely different reasoning. The story begins with the Apollo missions to the Moon, during which astronauts brought back many moon rocks, some of which were 'impact melt rocks'—formed when an asteroid strikes the Moon with enough force to melt rock. Radiometric dating of these rocks revealed something surprising: most of them cooled during a narrow time window between 3.8 and 4 billion years ago.

Apparently, numerous asteroids or comets impacted the Moon during this period, an event known as the Late Heavy Bombardment (LHB). It was termed 'late' because it occurred after the majority of other bombardments. Big collisions were common in the early solar system, but that era had passed. This led to a puzzling question: What caused a temporary surge in asteroid impacts on the Moon?

Around a decade ago, John Chambers and Jack J. Lissauer proposed that the cause of the Late Heavy Bombardment (LHB) might have been a long-lost planet, which they named Planet V. According to their theory, Planet V originally orbited between Mars and the main asteroid belt. However, the gravitational influence of the inner planets eventually moved Planet V into the asteroid belt, where its gravity displaced many asteroids, sending them on paths that led to collisions with the Moon. Meanwhile, Planet V itself crashed into the Sun. This hypothesis has faced criticism, as not everyone agrees the LHB took place, and even if it did, there are alternative explanations to the Planet V theory.

4. A Fifth Gas Giant

One alternative explanation for the Late Heavy Bombardment (LHB) is the Nice model, named after Nice, France, where it was first developed. According to the Nice model, Saturn, Uranus, and Neptune—the outer gas giants—once orbited much closer to the Sun, surrounded by a cloud of asteroid-sized objects. As time passed, some of these objects passed close to the gas giants, triggering gravitational interactions that slowly expanded their orbits. Jupiter's orbit, however, contracted slightly. Eventually, a resonance occurred between the orbits of Jupiter and Saturn, with Jupiter completing two orbits around the Sun for every one of Saturn's orbits. This resonance set off a chain of chaotic events.

In a relatively short amount of time, by the standards of the solar system, dramatic changes took place. The nearly circular orbits of Jupiter and Saturn were stretched, and there were numerous close encounters involving Saturn, Uranus, and Neptune. This upheaval disturbed the surrounding cloud of smaller objects, setting off the Late Heavy Bombardment (LHB). Eventually, the planets settled into orbits that closely resemble their current paths.

The Nice model also provides explanations for other features we see in the present-day solar system, like Jupiter’s Trojan asteroids. However, the original version of the model didn’t account for everything and needed adjustments. One such modification suggested the existence of a fifth gas giant. In simulations, the same event that triggered the LHB also resulted in the ejection of this hypothetical fifth gas giant from the solar system. These simulations show a solar system similar to the one we observe today, making this theory a plausible one.

3. Tyche

A comet’s period is the time it takes to complete one orbit around the Sun. Long-period comets, those with a period of at least 200 years, may take much longer. These comets are believed to originate from the Oort cloud, a distant region far beyond the Kuiper belt that contains icy bodies.

In theory, long-period comets should appear in equal numbers from every direction in the sky. However, in practice, they seem to come from certain directions more frequently than others. Why might this be? In 1999, John Matese, Patrick Whitman, and Daniel Whitmire proposed that a large, distant object, which they named Tyche, could be the cause. They estimated that Tyche’s mass was about three times that of Jupiter and that it resided approximately 25,000 AU away from the Sun.

The Wide-field Infrared Survey Explorer (WISE) space telescope, however, surveyed the entire sky and provided disappointing news for Matese and his team. According to a NASA press release dated March 7, 2014, WISE found no object larger than Jupiter within 26,000 AU of the Sun. It seems that Tyche does not exist after all.

2. The Cause Of Sedna-Like Orbits

In 2003, Mike Brown, Chad Trujillo, and David Rabinowitz discovered Sedna, an object with an unusual orbit compared to other solar system bodies. Its closest approach to the Sun is around 76 AU, far beyond the Kuiper Cliff. It takes approximately 11,400 years to complete its elongated orbit, a highly stretched path that is unlike that of other known objects.

How did Sedna end up in its vast orbit? It never ventures close enough to the Sun to be influenced by any of the eight planets. In their original paper on Sedna, Brown et al. suggested that Sedna’s orbit might be the result of either scattering by a still-undiscovered planet, perturbations from a close stellar encounter, or the formation of the solar system within a star cluster. Remarkably, in March 2014, astronomers announced the discovery of another object with a similar orbit, currently named 2012 VP113. Its discovery reignited speculation about the presence of an unseen planet.

1. The Cause Of The Kuiper Cliff

The Kuiper belt is a doughnut-shaped ring of icy, small objects orbiting beyond Neptune. For a long time, Pluto and its moons were the only known objects in the Kuiper belt (KBOs), but in 1992, David Jewitt and Jane Luu made the groundbreaking discovery of another object in the Kuiper belt.

Since that discovery, astronomers have identified over 1,000 other KBOs, and this number continues to grow. Most of these objects are located closer than 48 astronomical units (AU), which was unexpected because astronomers had anticipated there would be many more KBOs located beyond this distance. The reason for this discrepancy is that Neptune’s gravity has likely cleared out some of the KBOs that once existed closer, while the more distant KBOs have remained largely untouched since the early solar system.

The sudden drop in the number of Kuiper Belt Objects (KBOs) beyond 48 AU is referred to as the Kuiper Cliff, and its cause remains a mystery. Various scientific teams have proposed that an unseen planet could be responsible for this feature. Patryk S. Lykawka and Tadashi Mukai examined all the existing theories about the size and orbit of such a planet, dismissed them, and came up with a new theory of their own. This hypothetical planet could explain both the Kuiper Cliff and several other observed characteristics of the Kuiper belt. Unfortunately, if this planet exists, it is predicted to be extremely far from the Sun (over 100 AU), making it difficult to detect.