In this artist’s illustration, Planet Nine, a theoretical planet 10 times the mass of Earth, is shown hovering in the depths of our outer solar system. With the ring around the Sun showing the orbit of Neptune, Planet Nine lies nearly 20 times as far away. Credit: ESO/TomRuen/nagualdesign
All right, I know. I was extremely skeptical too. And I’m still not completely sold. But I have to admit that the evidence is mounting. There just may be a so-called Planet Nine lingering in the dark, icy graveyard that is our outer solar system. And if it’s there, its subtle influence could help explain some of our solar system’s most mysterious characteristics — like the highly elliptical orbits of comets and asteroids, or the slight tilt of the plane of our solar system with respect to the Sun.
According to NASA, Planet Nine (or Planet X) “is an informal nickname for a predicted but undiscovered world that may exist in the outer solar system.” The hypothesized world, estimated to be around 10 times the mass of Earth, is expected to reside in the depths of our outer solar system, nearly 20 times farther away than our eighth planet, Neptune. Based on that distance, it would take Planet Nine 10,000 to 20,000 years to orbit the Sun just once.
Unfortunately, all of the evidence right now for Planet Nine is circumstantial. That is, the existence of the planet would explain some unanswered questions we have about our solar system, but it has yet to be directly detected. However, when you take all of the evidence together, it’s almost harder to imagine a solar system without Planet Nine rather than with one.
What is the evidence?
The first pieces of evidence for Planet Nine came from a January 2016 paper published in the Astronomical Journal by planetary astrophysicist Konstantin Batygin and astronomer Mike Brown, both from Caltech.
- There are at least six known objects in the Kuiper Belt— a circumstellar region of icy bodies that stretches from Neptune to interstellar space — that all have elliptical orbits aiming in the same direction.
- The objects are not only pointing the same way, but they are also all tilted 30 degrees below the solar system’s orbital plane. This may not seem like much, but the researchers calculated that the odds of six Kuiper Belt Objects (KBOs) randomly traveling in a cluster this tight to be one in a thousand.
- Computer simulations of the solar system that include Planet Nine show that some KBOs should have orbits perpendicular to the solar plane— the flat disk around the Sun in which all the planets’ orbits lie. Brown found that there are five examples of these eccentric KBOs already known to astronomers.
Six months after they published their first paper, Batygin’s graduate student, Elizabeth Bailey, led a study investigating a longstanding mystery of our solar system: Why is the solar system’s orbital plane misaligned with the Sun’s equator by about 6 degrees?
- By including Planet Nine in analytical models, the team showed that Planet Nine would naturally influence the orbits of the other planets. Over the 4.5- billion-year history of the solar system, this effect would be enough to cause the entire solar plane to wobble like a top, explaining the misalignment.
- Finally, Planet Nine’s orbital influence would also explain the existence of distant KBOs that orbit in a direction opposite that of everything else in the solar system. “No other model can explain the weirdness of these high-inclination orbits,” Batygin said. “These things have been twisted out of the solar system plane with help from Planet Nine and then scattered inward by Neptune.”
Researchers believe that Planet Nine can explain the anomalous orbits of distant Kuiper Belt objects. This graphic summarizes what we think we know about Planet Nine. Space.com/Karl Tate
So, would Planet Nine pose a threat to Earth?
No.
Let’s be crystal clear on that. There is absolutely zero chance that Planet Nine (if it exists) has the potential to bust out of its orbit and come barreling through the inner solar system, wreaking havoc on Earth. The only thing Planet Nine can do is nudge a few comets or asteroids into more elliptical orbits, sending them through the inner solar system. Although these objects do pose a potential threat to Earth, they are not a new danger. Just ask the dinosaurs.
What do other astronomers think?
Although not everyone is sold on Planet Nine, there is no question that the theory has gained traction over the last few years. Yet, there are still many doubters out there who hope to find less fanciful explanations for the evidence. Cory Shankman, a Ph.D. student from the University of Victory, is one such doubter.
Shankman and his team recently analyzed the data from a sky-mapping project called the Outer Solar System Origins Survey, which discovered more than 830 trans-Neptunian objects — or minor planets beyond Neptune that range from about 30 miles (50 km) to over 600 miles (1000 km) across. Shankman suggests that a random distribution of these trans-Neptunian objects may account for the supposed evidence of Planet Nine.
What next?
Currently, Batygin’s team is searching the skies for Planet Nine with the Subaru Telescope at the Mauna Kea Observatory in Hawaii, one of the best tools for wide-field searches for dim and distant objects. If discovered, Planet Nine would not only replace the holes in our hearts left by Pluto’s demotion, but it would also show that our solar system contains a super-Earth, which exoplanetary surveys have proven are extremely common in our universe.
But most importantly, the discovery of Planet Nine would put to rest some major questions astronomers have about our solar system. “There are now five different lines of observational evidence pointing to the existence of Planet Nine,” Batygin said. “If you were to remove this explanation and imagine Planet Nine does not exist, then you generate more problems than you solve. All of a sudden, you have five different puzzles, and you must come up with five different theories to explain them.”
If you would like to help find Planet Nine, I strongly suggest you check out Backyard Worlds: Planet 9, a citizen science project started by Jacqueline Faherty and Marc Kuchner.