- Astronomers are hunting for a missing planet in the solar system beyond Pluto.
- They've found indirect evidence for the existence of the hypothetical world, often called Planet Nine.
- But Alan Stern, who leads NASA's New Horizons mission that visited Pluto, says the name Planet 9 is "insulting" and suggests we should instead use Planet X.
- The two scientists who nicknamed Planet Nine disagree, saying that Planet X's history is already "more or less done."
The search is heating up for a planet beyond Pluto - not just over the evidence, but also over the hypothetical world's name.
No one knows for sure if there really is a missing planet out there, since astronomers have yet to identify it in telescope images. However, they keep turning up indirect evidence that it exists.
A study released in May, for example, suggests a rocky object dubbed 2015 BP519 orbits the sun on a bizarrely high and eccentric path. A planet about 10 times the mass of Earth and 20 times farther out than Neptune, the researchers conclude, is perhaps the best explanation for what kicked the rock onto its wild course.
That ostensible world is most often called Planet Nine, and it was formally proposed in 2016 - a decade after the International Astronomical Union decided to classify Pluto as a dwarf planet rather than counting it as the ninth planet in our solar system.
But as we learned by asking scientists about the evidence surrounding Planet Nine, the name has its critics. And it all comes down to where you fall in the Pluto debate.
Alan Stern, a planetary scientist who leads NASA's New Horizons mission, told Business Insider in an email that he is "strongly against" the term Planet Nine.
"It is an effort to erase Clyde Tombaugh's legacy and it's frankly insulting," he said, referring to the astronomer credited with the discovery of Pluto, which was considered the solar system's ninth planet for more than 75 years. "I hope you will change to calling this Planet X, as most responsible writers do."
Stern is an ardent defender of Pluto's planethood. In 2015, shortly after New Horizons flew past Pluto, he told Business Insider that the world's demotion from its former status as a planet was "bulls---." In 2017, he and five other researchers asked the IAU to change its definition of a planet to mean any mostly round object that wasn't a star. He also recently authored a perspective on the issue in the Washington Post.
But the two scientists who proposed the existence of Planet Nine and named it in 2016 - Mike Brown and Konstantin Batygin - disagree with Stern's critique and his historical argument.
Planet X vs. Planet 9
The term Planet X has deep roots in the history of astronomy, dating back to the discovery of Neptune in 1864.
Some oddities were found in the orbits of Neptune and Uranus that astronomers couldn't explain. In 1903, Percival Lowell - the astronomer who'd go on to lay the groundwork for Pluto's discovery - spoke about a possible ninth planet "X" that could be causing those abnormalities. In 1905, he started using the Lowell Observatory to hunt for it.
Lowell spent the next decade taking hundreds of photos of space, but never identified that mystery planet. He died in 1916.
In 1930, astronomer Clyde Tombaugh, who eventually worked at Lowell Observatory, found the world his predecessor had proposed and searched for. Tombaugh named the object Pluto in part because he later found it in Lowell's old photographs; the name starts with P and L - Lowell's initials.
Even after Pluto's discovery, though, scientists thought there could still be another undiscovered Planet X influencing Uranus' path. That is, until Voyager 2 flew past Neptune in 1989. Scientists used the event to figure out that Neptune was less hefty than previously thought (a difference roughly equivalent to Mars' mass), and that revision solved the perceived discrepancies in Uranus' orbit.
Batygin and Brown said this is why they and many other astronomers avoid the use of the term Planet X.
Magnolia Pictures
A still from "Melancholia," a movie about a rogue planet that destroys Earth.
"'Planet X' is not a generic reference to some unknown planet, but a specific prediction of Lowell's which led to the (accidental) discovery of Pluto. Our prediction is not related to this prediction," Brown told Business Insider in an email.
Batygin put it a different way: "The reason we call it Planet Nine is becuase the story of Planet X is more or less done," he said. "Planet Nine is the most descriptive set of words to describe what we are after."
He added that the term does not suggest any "disrespect or sleight to Clyde Tombaugh's legacy" as the person who found Pluto and discovered the Kuiper Belt, a region of the outer solar system inhabited by small, icy worlds.
Batygin opposes the term Planet X for another reason, too: he wants to distinguish the work astronomers are doing from wild claims of a hypothetical world that could slam into Earth.
"Crazy people speculate about Planet X coming to destroy Earth. I get these crazy emails all of the time," he said. [Planet Nine] is another way to discern between the nut jobs and what we're doing."
Stern contends, however, that Planet X "is widely recognized as a generic term for undiscovered planets" and thus appropriate.
Finding and exploring new worlds beyond Pluto
An illustration of Ultima Thule, also known as Kuiper Belt object 2014 MU69.
Stern said he and his colleagues don't think the case for a planet beyond Pluto (that fits the current IAU definition) is very strong.
"The fact they've now been looking for two years and found nothing doesn't help," he said.
Brown and Batygin shrug off this criticism, as it arguably took more than 25 years' worth of effort to find Pluto.
"The Planet Nine theory is in quite good shape," Batygin said. He and Brown are currently using the Subaru telescope atop Mauna Kea in Hawaii to conduct their search, but that comes with its own challenges.
"The observational search for Planet Nine is proving very difficult, basically because of weather," Batygin said. "Last year we lost more than half of our observing season to being clouded out or hailed on, but we're back on the telescope this fall. We'll keep on gong as long as the theory is strong."
The duo also remains unconvinced that a gaggle of icy objects tugging on one another could explain the bizarre orbit of 2015 BP519, the rocky object beyond Neptune. That suggestion came from astronomers at the University of Colorado earlier this week, but Batygin said if that were the case, those icy objects would need a combined weight many times the suspected weight of the entire Kuiper Belt.
"What they're proposing is a contradiction with what we know about the Kuiper Belt," Batygin said.
NASA
The New Horizons probe before launch.
Stern, who co-authored a book titled "Chasing New Horizons: Inside the Epic First Mission to Pluto", is currently beginning a new phase of Kuiper Belt exploration with New Horizons. The nuclear-powered spacecraft recently woke up to prepare to explore Ultima Thule: a strange icy object beyond Pluto.
"IT'S HAPPENING! IT'S HAPPENING! New Horizons is awake after a nearly 6 month hibernation. Flyby preparations for Ultima Thule begin shortly!" Stern tweeted on Tuesday.
New Horizons is expected to visit Ultima Thule on January 1, 2019, which would make it the most distant object ever visited by a spacecraft.