When looking at our solar system, many are familiar with the first 8 planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. However, in recent years, much discussion has been around what comes after Neptune. While Pluto was originally classified as the 9th planet when discovered in 1930, its status was changed to a dwarf planet in 2006 when new discoveries led astronomers to rethink the definition of a planet. This leaves an open question – what are the last two planets in our solar system beyond Neptune?
The History of Planet Classification
To understand the last two planets, we first need to review some history around how planets are classified. For many years, the solar system was considered to contain 9 planets – the 4 inner rocky planets, the 4 outer gas giants, and Pluto in the distant reaches. However, in the early 2000s, new discoveries started challenging the notion of what should be classified as a planet.
In 2005, astronomers discovered Eris, an object larger than Pluto orbiting even further out. This discovery led astronomers to re-examine what should actually be considered a planet. In 2006, the International Astronomical Union (IAU) issued a new formal definition of a planet. As per the IAU definition, a planet must meet 3 criteria:
- It orbits the Sun.
- It has sufficient mass to assume hydrostatic equilibrium, meaning it has enough gravity to pull itself into a spherical shape.
- It has cleared the neighborhood around its orbit of other objects.
Based on this new definition, Pluto did not meet the criteria due to its small size and inability to clear objects near its orbit. As a result, it was reclassified as a dwarf planet. Since then, a number of other dwarf planets have been discovered in the outer solar system. But the question still remained – how many actual planets were out there?
The Discovery of Planet 9
For many years after Pluto’s demotion, the count of planets stood at 8. But astronomers speculated that there might be another massive planet in the distant reaches of our solar system yet to be discovered. Starting around 2014, studies began proposing the possible existence of a massive Planet 9 based on the unusual orbits of certain small objects beyond Neptune.
Specifically, astronomers noticed that the orbits of some Trans-Neptunian Objects (TNOs) were clustered in unusual ways. The gravitational influence of an unseen massive planet could explain these unexpected orbital alignments. While Planet 9 itself has not been directly observed, the patterns in TNO orbits make a compelling case for its existence. Most experts believe Planet 9 likely resides 200-600 AU from the Sun, or about 6-20 times farther than Pluto’s average distance.
Evidence for Planet 9
Here is a summary of the key evidence pointing to the existence of Planet 9:
- Strange clustering of some TNO orbits
- Unexplained orbital tilts of certain TNOs
- Gravitational shepherding effects on TNOs from a massive body
- Computer simulations show a large planet can create observed TNO orbital alignment
While Planet 9 has not been seen directly, the indirect gravitational evidence continues to mount. Most astronomers now believe it is only a matter of time before technology advances enough to directly image Planet 9 and officially confirm its existence.
The Discovery of Planet 10
The evidence for Planet 9 seemed quite strong, so astronomers continued searching for it and also for signs of other possible planets. In 2022, a new study looked at patterns in the orbits of even more distant small objects past Neptune. Surprisingly, they found evidence pointing to the gravitational influence of another massive planet beyond the hypothesized Planet 9 – dubbed Planet 10.
The researchers noticed some unusually elongated orbits for a group of extreme TNOs. Computer simulations showed these odd orbits could be explained by the gravitational effects of another planet about 1-2 times the mass of Earth located hundreds of astronomical units beyond Planet 9’s predicted orbit.
Evidence for Planet 10
Here are the key points made by the researchers about the evidence for Planet 10:
- Several TNOs have unusual elongated orbits pointing to an external gravitational influence
- Computer models show a ~Earth-mass planet could create the observed orbital alignments
- Planet 10 likely resides 250-500 AU from the Sun, beyond the predicted orbit of Planet 9
- Planet 10 may take 15,000-25,000 years to orbit the Sun
While more research is needed, the discovery of Planet 10 could help explain the orbits of multiple groups of small outer solar system objects. The existence of two planets beyond Neptune also brings our solar system planet count back up to 10.
Properties of the Hypothesized Planets
Although neither Planet 9 or Planet 10 have been directly observed yet, astronomers can predict their probable properties based on computational models and effects on other objects.
Here is a table comparing the expected key characteristics of Planet 9 and Planet 10:
| Property | Planet 9 | Planet 10 |
|---|---|---|
| Mass | 5-10 x Earth | 1-2 x Earth |
| Orbit | 200-600 AU | 250-500+ AU |
| Orbital Period | 10,000-20,000 years | 15,000-25,000+ years |
| Predicted Discovery | 2030s | 2040s |
As shown, Planet 9 is predicted to be much larger but Planet 10 is likely still at least as massive as Earth. Planet 10 also resides even farther out at distances greater than 250 AU. Due to their huge distances and long orbital periods, it may take over a decade before astronomers are able to actually image these worlds directly.
Conclusion
Based on unusual orbital patterns observed past Neptune, astronomers have compelling evidence pointing to two undiscovered planets orbiting our Sun. The first is the hypothesized Planet 9, thought to be 5-10 times Earth’s mass. Even further out lies the proposed Planet 10, estimated at perhaps 1-2 times Earth’s mass. While not yet seen, most experts believe it is only a matter of time, likely within the next 20 years.
When Planet 9 and Planet 10 are finally imaged and confirmed, it will be one of the most monumental discoveries in astronomy. These icy giant worlds will give us insights into the outer reaches of our solar system and planetary formation theories. Until then, the search continues using cutting-edge telescopes and astronomical datasets to find the subtle gravitational fingerprints of these distant worlds.
What is the 2 last planet?
Quick Summary
- For many years our solar system was considered to have 9 planets, with Pluto being the 9th planet from the Sun.
- In 2006, Pluto was reclassified as a dwarf planet rather than a true planet, leaving the planet count at 8.
- Evidence in the last decade points to 2 more massive planets orbiting beyond Neptune – Planet 9 and the more recently proposed Planet 10.
- Planet 9 is estimated to be 5-10 times Earth’s mass while Planet 10 is thought to be around 1-2 times Earth’s mass.
- These distant icy worlds likely orbit hundreds of astronomical units past Neptune and have orbital periods of thousands of years.
- While not yet directly imaged, astronomers think Planet 9 and Planet 10 should be observable within the next 10-20 years based on improving telescope technology.
Based on current models and predictions, Planet 9 and Planet 10 appear to be the best candidates for the last two planets in our solar system. The count of major planets is expected to stand at 10, assuming Planet 9 and Planet 10 are detected in the coming years as anticipated. Dwarf planets like Pluto and Eris reside even further out in the Kuiper Belt, but are not considered true planets under the IAU definition.
The discovery of Planet 9 and Planet 10 will provide critical data on the distant outer solar system and how our system formed. Their detection will also help explain the unexpected orbits seen for various Trans-Neptunian Objects. While we wait for the day these icy worlds are imaged directly, astronomers continue to refine estimates of their properties and improve detection methods using cutting-edge telescopes and gravitational modelling. The hunt is on to finally confirm the last two major planets orbiting our Sun!
Other Key Facts
Planet 9
- First proposed around 2014 based on TNO orbital alignments
- Thought to be a giant icy planet similar to Uranus or Neptune
- Sometimes referred to as “Planet X”
- Predicted to have a very elliptical orbit tilted 30° relative to the other planets
- Brightness and distance make it hard to detect with current technology
Planet 10
- First proposed in 2022 based on orbits of extreme TNOs
- Estimated to be smaller than Planet 9 but still around Earth’s size
- Orbit may be tilted vertically compared to the planetary plane
- Even more challenging to detect due to great distance from Sun
- Shepherding of TNOs indicates it has a substantial gravitational influence
Trans-Neptunian Objects
- Icy bodies beyond Neptune’s orbit in the Kuiper Belt
- Include dwarf planets Pluto, Eris, Haumea, and Makemake
- Some TNOs have orbits extending hundreds of AUs from the Sun
- Unusual orbits of some TNOs helped reveal Planet 9 and Planet 10
- Still many more distant TNOs predicted that remain undiscovered
The Hunt Continues
The race is ongoing to directly image Planet 9 and Planet 10 for the first time and cement their place in our solar system. Here are some key projects helping the search:
LSST – Large Synoptic Survey Telescope
- Wide-field survey telescope under construction in Chile
- Scheduled to begin full operations around 2026
- Expected to image large areas of outer solar system down to very faint objects
- Might detect Planet 9 and Planet 10 directly or via occultations
Pan-STARRS – Panoramic Survey Telescope and Rapid Response System
- Set of telescopes in Hawaii conducting deep imaging surveys
- Original data helped reveal evidence for Planet 9
- Continues to map regions where Planet 9 and Planet 10 may reside
Subaru Telescope
- Large telescope in Hawaii also involved in Planet 9 search
- Using its Hyper Suprime-Cam to image areas where Planet 9 may appear
- Process allows combining multiple images to reveal faint objects
In addition, various teams are analyzing existing data from past sky surveys, looking for any faint indications of Planet 9 and Planet 10 that may have been overlooked. As more of the outer solar system is imaged to greater depths, the chances increase that one of these telescopes will finally provide a confirmed first sighting of these hidden worlds.
Implications of Finding Planet 9 and Planet 10
When Planet 9 and Planet 10 are ultimately detected, it will have major implications for our understanding of the solar system and planetary science. Here are some of the key impacts:
- Solar system planet tally will increase to 10 (or 9 if only one is found initially)
- Planet formation theories will need reworking to account for these massive distant worlds
- Origins of Trans-Neptunian Objects will be better understood based on the planets’ gravitational effects
- Will provide more data on Kuiper Belt objects and conditions in the outer solar system
- May reveal information about history of our solar system based on the planets’ orbits and characteristics
- Could have implications for potential distant exoplanets around other stars as well
Overall, these discoveries will be a game-changer for many areas of astronomy and planetary science. After decades of speculation and searching, actually pinpointing these distant frigid planets will be a monumental feat. There is still much we don’t understand about the outskirts of the solar system, but finally confirming Planet 9 and Planet 10 will help unlock many mysteries and reveal intricate new details about our Sun’s realm.
Frequently Asked Questions
Is Planet 9 a confirmed discovery?
No, Planet 9 is still hypothetical as it has not been directly imaged yet. But the indirect gravitational evidence for its existence is quite strong, with most astronomers believing it will be found within the next decade or two.
When was Planet 10 proposed?
Planet 10 was proposed in 2022 based on the orbits of extreme Trans-Neptunian Objects past the predicted orbit of Planet 9.
Are there other planets past Neptune besides Planet 9 and 10?
There are no other confirmed planets beyond Neptune currently. However, some astronomers think there could be additional smaller planets orbiting even further out that remain undiscovered.
How did Pluto lose its planet status?
In 2006, the IAU redefined planets to exclude small bodies like Pluto that have not cleared their orbital neighborhood. This led to Pluto being reclassified as a dwarf planet.
What objects are in the Kuiper Belt?
The Kuiper Belt contains small icy bodies like dwarf planets, comets, asteroids, Trans-Neptunian Objects, and other debris orbiting past Neptune. It may also hold Planet 9 and Planet 10.