Almost a decade after astronomers proposed the existence of Planet 9, an extra invisible planet in the outer reaches of the solar system, they still haven’t agreed whether it is real or not. Now, new research from Caltech astronomers has just uncovered an extra line of evidence in favor of the hidden planet. Their computer simulations require Planet 9’s gravitational pull to explain how small chunks of rock and ice around Neptune’s orbit end up close to the Sun.
“There’s an open question about why certain objects in the solar system act the way they do, and we can’t explain it, but if we add Planet Nine to the model, it all makes sense,” says Juliette Becker, an astronomer. at the University of Wisconsin, Madison, not affiliated with this new work.
These objects are Trans-Neptunian Objects (TNOs) – pieces of debris in the outer solar system, beyond Neptune and even Pluto. Until the 2000s, astronomers had not spotted many TNOs – especially more distant ones. They are incredibly faint, a result of their small size and enormous distances from Earth, and are difficult to see. However, once astronomers built a more substantial catalog of observed TNOs, they began to notice some strange trends.
A group of TNOs were clustered together, sharing similar orbits, as if they were being fought over by something, like a group of sheep by a shepherd. These eccentrics orbited at very high angles compared to other TNOs and were aligned in the same direction. Some astronomers, including the same Caltech team behind the new evidence, stated that the most likely explanation for these observations was the existence of Planet 9 acting as a massive object that acts as a gravitational shepherd for TNO’s sheep.
However, other astronomers thought that Planet 9 was a strange solution to the puzzle at hand, finding other ways to explain the unexpected observations. Some suggested that clusters of TNOs could be a natural result of the formation of the solar system, with no need for Planet 9. Others thought that the shepherd was actually a small black hole rather than a giant planet. More recently, two astronomers in Japan proposed that a different planet, rather than Planet 9, could be lurking in the Kuiper Belt.
Theories abound to explain the observed orbits of TNOs – and astronomers have spent the last eight years discussing and debating which ones make the most sense. This is not an anomaly, but rather an illustration of the scientific process. Scientists iteratively and collaboratively improve our understanding of a natural phenomenon, exploring all the evidence to find the best explanation for an observation.
Now, the Caltech team has just shown how Planet 9 could be needed to explain a different group of TNOs, which were somehow hurtled toward the Sun. An object on a path that crosses Neptune’s orbit, dives toward to the Sun and back shouldn’t be able to stay that way for long. If we see objects in this type of orbit, something must be pushing them there – perhaps all the way to Planet 9.
“If Planet Nine exists, it would occasionally pull the orbits of distant trans-Neptunian objects closer to the Sun, to the point where they would intersect Neptune’s orbit. Without Planet Nine, these objects cannot be pushed into Neptune very often,” explains Konstantin Batygin, Caltech astronomer and lead author of the new paper.
“Planet Nine would replenish the population of these objects as they become depleted, explaining why we can see them today, when the Solar System is relatively old,” adds Becker.
Over the years of theories, some astronomers have become fascinated by the idea of actually locating Planet 9 in the night sky. Despite the evidence of its gravitational influence, seeing is still believing and many of us won’t be satisfied until we have concrete proof that Planet 9 is there in our telescopes.
Batygin and co-author Mike Brown, also an astronomer at Caltech, have been hunting for Planet 9 using huge data archives obtained by night sky surveys from the Pan-STARRS1 facility atop Haleakala in Hawaii, the Dark Energy Survey found. in Chile, and the Zwicky Transient Facility, near San Diego. Yale astronomers even used the TESS exoplanet-hunting satellite to scan the sky for Planet 9. Unfortunately, no one has seen the elusive extra planet yet.
“Simply put, Planet 9 is very distant and extraordinarily dark,” says Batygin. “It is difficult to appreciate the challenge of detecting it directly without seeing firsthand how complex the observation process is, especially when looking for the proverbial needle in the haystack.”
The Vera Rubin Observatory in Chile – currently scheduled to begin operations in early 2025 and equipped with the largest digital camera ever made for astronomy – will provide an excellent opportunity to continue the search for Planet 9. Astronomers have eagerly awaited this facility for years. , even citing him in a PopSci 2020 article as the key to solving this mystery once and for all.
What if there are no planets to be found, even with a bigger and better observatory in the case? “If it doesn’t exist, then we’ll need to find individual explanations for all these different observations,” says Becker. “I am continually amazed at how many solar system puzzles the existence of Planet Nine would solve.”
