Unlocking the Secrets of the Cosmos: Can the Vera Rubin Observatory Unveil Planet Nine?

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Rachel Feltman: For Scientific American’s Science Quickly, I’m Rachel Feltman. Unless your age skews towards the younger end of our audience, you probably grew up learning about the nine planets in our solar system. Naturally, unless you’ve been living in isolation since 2006, you’re also aware that we presently recognize only eight planets. Apologies to all Pluto enthusiasts.

However, you may have caught whispers surrounding the enigmatic Planet Nine. This speculative additional planet has surfaced intermittently in the media for over ten years. With the advent of a new observatory launching in 2025, the reality about Planet Nine could finally be within our grasp.

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Joining us to elaborate is Clara Moskowitz, the senior editor focusing on space and physics at Scientific American.

Thank you so much for being here today to discuss.

Clara Moskowitz: Thank you for having me.

Feltman: Let’s start with the fundamentals. Many people have probably heard of Planet Nine in passing. It’s a captivating notion, but when we refer to Planet Nine, what exactly do we mean?

Moskowitz: We’re referring to this possible planet—its existence is still uncertain—that might reside within our solar system.

When you think about it, it’s quite a radical notion that there could be another entire planet in our solar system that we have yet to discover. Of course, we are well-acquainted with the eight recognized planets. Then there’s the contentious status of Pluto. So, how could we have overlooked an entire world in our cosmic vicinity? Additionally, this hypothetical Planet Nine isn’t something minuscule. It could be between five to ten times the mass of Earth. Therefore, if it does exist, it would be a significant part of our solar system.

Feltman: Indeed, I have numerous inquiries regarding that. But I think now is an excellent time to pause—perhaps for those whose instinctive response to Planet Nine is something along the lines of, “We already have a ninth planet. It’s Pluto. How could they?”—could you remind us what led to Pluto’s contentious reclassification and why this theoretical Planet Nine would still qualify as a planet if it truly exists?

Moskowitz: Absolutely. These two narratives are interconnected, as the entire saga of Pluto is what ultimately prompted the concept of Planet Nine. Allow me to elaborate on how that came to fruition.

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Moskowitz: The troubles with Pluto started in 2005 when astronomers uncovered a sizable object in the solar system known as Eris. Interestingly, Eris is actually larger than Pluto, albeit much farther away, which is why it hadn’t been observed until then. It lies approximately 68 times the distance from the sun compared to Earth. With the discovery of Eris, astronomers began contemplating, “If this entity is larger than Pluto, shouldn’t it also qualify as a planet?” So they pondered, “What is our formal definition of a planet?” They realized there wasn’t a clear, established one.

Feltman: [Laughs] It was more of a feeling, a conceptual state.

Moskowitz: Precisely, exactly. Subsequently, astronomers began discussions and a group called the International Astronomical Union established guidelines. They determined that new criteria were essential for defining what constitutes a planet. Thus, in 2006—right after Eris’s discovery—they had to formulate rules to delineate which celestial bodies qualified as planets.

There are three conditions. The object must orbit a star, which is clearly logical. It has to possess sufficient mass for its gravity to shape it into a sphere. If it is too small and appears irregular, resembling asteroids, it cannot be classified as a planet. Lastly, it must clear its orbital path, indicating it has enough gravitational pull to displace other rocks or asteroids from its trajectory.

It was in fact this last condition that caused Pluto to be reclassified as it shares its vicinity with numerous other rocks orbiting in tandem with it. Thus, it simply lacks the required size. Pluto was subsequently categorized as a dwarf planet, along with Eris and several other similar objects discovered during this period.

Feltman: Indeed. Many people experienced a strong emotional response to that, but it seems we would have had a perpetually expanding list of planets had we not solidified a strict definition. I can understand why that had to occur.

Moskowitz: Exactly. This sets the stage for the emergence of the Planet Nine hypothesis since they later discovered an object named Sedna.

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Moskowitz: Sedna is yet another object of similar size, positioned extremely far out. The closest it comes to the sun is about 76 times the distance between the Earth and the sun. Following this, astronomers identified several other objects akin to Sedna.

The peculiar aspect regarding these bodies is their bizarre orbital patterns. Their orbits are highly elongated and distant, and it was later identified that they also seem to be inclined at an unusual angle relative to the rest of the solar system’s planets. They are simply peculiar, yet numerous objects exhibit this behavior, leading scientists to struggle to comprehend why there are so many entities with such extreme, unconventional long orbits unless there is something concealed out there influencing their trajectories—modifying their paths through its gravitational force. This hidden entity would need to be significantly substantial.

Feltman: Now that scientists have dedicated over a decade to investigating the possibility of Planet Nine, what evidence exists to substantiate that this kind of celestial body actually exists?

Moskowitz: The evidence is embedded within these other objects,these extreme trans-Neptunian entities that we observe, and the reality that their trajectories are so peculiar. Like I mentioned, they possess exceptionally large, exceedingly elongated paths of a completely different scale than the planets found in our solar system.

So that’s one aspect, and then the other aspect pertains to the inclination of their orbits. In our solar system, we have what is referred to as an ecliptic. It serves as a plane, where all eight planets traverse in this roughly flat manner. You can visualize them all orbiting along the same flat plane. Meanwhile, these other bizarre celestial bodies located far out there are moving at this complete angle. And it’s like, that’s quite challenging to clarify through physics because we assume the planets would have all emerged from a circumstellar disk that was revolving around the sun post formation of the sun.

Therefore, after a star’s inception, there’s still an abundance of debris that failed to coalesce into the star itself. All that material commences orbiting the star—the young star—in this ring-shaped disk, ultimately leading to the formation of the planets within the star’s planetary system. So it stands to reason that they would all remain within a plane, and it seems illogical to have a considerable population of other entities orbiting far outside and at a complete angle to that plane. For such a scenario to occur, you would need some other substantial object exerting influence on everything, which brings us to the concept of Planet Nine.

Feltman: So scientists have discovered these entities that might imply the existence of a Planet Nine, but what alternative explanations might exist?

Moskowitz: Exactly, so, you know, we haven’t directly observed the planet, so we can’t definitively say it exists. One straightforward possibility is that our understanding of these extreme, distant objects or this segment of the solar system might still be limited because they are exceedingly challenging to detect. It’s plausible that this small collection of unusual objects we’ve identified just happens to be peculiar, and in reality, there’s a plethora of additional objects present with far more conventional orbits—and that we’re merely examining, in a sense, outliers and noting this trend, while reaching for an explanation, but perhaps, you know, it’s much more ordinary than we initially believed.

There are other theories as well, such as the possibility of a rogue planet or a wandering star that passed by our solar system at some point, dragging Sedna and these other objects into their strange orbits long ago. Thus, they could have been influenced by some passing entity that no longer belongs to our solar system. Astronomers have even suggested that there might be a concealed black hole lurking in the distant corners of our solar system, exerting a pull on these items into unusual locations. It’s somewhat of a complete concealed domain where we lack real knowledge of what exists.

Feltman: So what types of research are being conducted to address these inquiries and, you know, explore what lies beyond?

Moskowitz: The extremely thrilling aspect of this narrative is that it presents a significant enigma that we are almost assuredly on the brink of resolving in one way or another very soon, due to the emergence of a colossal new telescope designated to commence operations this year, known as the Vera Rubin Observatory.

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Moskowitz: It boasts the largest camera globally, situated in Chile atop a mountain, and is set to activate this year. It is anticipated to achieve its first illumination in July.

This technology is going to revolutionize everything. The modus operandi of the Rubin Observatory will involve scanning the sky every few days, meticulously mapping the entire southern sky repeatedly. This method is ideally suited for discovering more objects out there—potentially uncovering Planet Nine itself, if it exists—but in either scenario, it will help identify many more Sedna- and Eris-like entities. Currently, spotting these items is quite challenging. They are incredibly distant and dim. However, Rubin is substantially larger than any telescope previously available, enabling the production of maps that will help track movements from night to night, allowing identification of these objects’ orbits.

This undertaking should significantly enrich our catalog of outer solar system entities. We are poised to identify many more than we currently possess. Soon, we will be able to discern whether the few we are aware of are genuinely anomalous or if they are commonplace. If a large quantity of objects follows these unusual orbits but remains aligned with the trend, it will bolster the case for the existence of a hidden planet. Conversely, if we uncover numerous objects exhibiting entirely random paths, devoid of trends implying a single entity is influencing their trajectories, it would begin to challenge the notion of Planet Nine.

As I mentioned, the most exhilarating outcome would be spotting the planet itself, although that isn’t guaranteed, even if it does exist, as it may be in a location that’s tough to detect. We are still discussing extreme distances, after all. The speculation around Planet Nine posits that it might be as far as 700 times the distance between Earth and the sun. This is just astonishing—Neptune, the furthest recognized planet in our solar system, is only 30 times the distance from Earth. Therefore, we’re possibly looking at an object that’s 700 times that distance, which might only be five or ten times the size of Earth. Consequently, spotting it will be exceedingly difficult, even with Rubin—but we can maintain hope. We might strike it lucky.

Feltman: Yes, that would be incredibly thrilling. But barring that, which would obviously be phenomenal, what do you think might be necessary for this concept to gain widespread acceptance as a probable fact within the astronomical community? What sorts of evidence are we seeking to decisively support this?

Moskowitz: I believe that if we gather sufficient observations of other entities that all display comparable orbits that are significantly different from those in the inner solar system, we could utilize these objects as gravitational indicators to effectively validate the existence of a large planet, even if we cannot visualize it. Should we have ample data points, sufficient objects out there that we can input all their orbits into a computer model and demonstrate that the only way they could be behaving this way is if another planet exists in that particular position with that certain mass, many individuals would be quite convinced, regardless of whether we catch a glimpse of Planet Nine itself.

Feltman: Yes, although if they gather enough evidence indicating that there is, without a doubt, a significant gravitational influence in play and we still lack direct observation of the planet, I do dread to consider how people online might react to whatever they believe is out there [laughs], but I suppose that’s always a possibility [laughs].

Moskowitz: [Laughs] Indeed, that is always a possibility. Personally, I find the notion of a little black hole operating in the shadows, causing all this chaos, quite appealing, you know? So, it won’t be a conclusive end. Yes, there will likely be much debate, with some asserting, “I’m convinced,” while many others proclaim, “I remain unconvinced.” You know, that’s simply the nature of these matters.go. But it’s just incredibly enjoyable to be at this moment in time where something significant is on the horizon, you know, and we should have a lot more insights coming soon.

Feltman: Indeed, and it appears that the Rubin will be monumental for astronomy as a whole. Also, fantastic name. I penned numerous articles advocating that Vera Rubin deserved a Nobel Prize. What other, you know, inquiries are we going to delve into with this telescope that you’re enthusiastic about?

Moskowitz: Oh, absolutely, I mean, it’s going to be truly remarkable to obtain these maps, I mean, because we’re going to encounter a plethora of new objects. I mean, it’s going to be exceptionally beneficial for locating potentially hazardous asteroids, right, nearby or on a collision course with us. So that’s a significant, crucial matter. You know, to be able to examine what’s moving out there, what’s soaring around us, is still quite challenging. It’s still—you know, you’re searching for these needles in haystacks, and Rubin should create a massive impact.

And then, you know, some of the most pressing questions about the universe, like dark matter and dark energy—Vera Rubin, whom you mentioned, is this extraordinary, remarkable scientist who provided the strongest evidence we have that there’s likely this invisible matter that we cannot detect out there in the cosmos referred to as dark matter. And we’ve been in pursuit of it for years, but we still lack knowledge of its composition. More astronomers are becoming convinced of its existence, yet we don’t know what it is. Thus, Vera Rubin telescope should aid us in obtaining much more accurate measurements by examining, once again, the movement of all these entities in our solar system, but our—mostly within our galaxy and in the universe itself—is going to provide us methods to trace gravity and track the presence and arrangement and distribution of dark matter, and that should assist us in illuminating what it might potentially be.

There’s another component we didn’t mention, which is a kind of amusing and humorous facet of this tale, which is that the individual who is arguably the greatest advocate of Planet Nine, you know, the person most convinced that a new planet exists in the solar system, is also the one accountable for demoting Pluto as a planet in the solar system.

Feltman: Yeah, the notorious Mike Brown [laughs].

Moskowitz: The notorious Mike Brown from Caltech [the California Institute of Technology] is a significant part of this narrative. He is the individual who discovered Eris and consequently compelled the International Astronomical Union to alter its definition of a planet. And then he’s the one who began suggesting, “Look, all these objects are indicating that there’s another planet out there.” And, you know, around this time, I believe he had a daughter who was around eight or ten, and she remarked, you know, “Maybe you can actually atone for the immense harm you caused by demoting Pluto if you locate another planet.” And he’s somewhat on a mission now to accomplish that, so part of you has to cheer for him to succeed, purely for that reason.

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Feltman: [Laughs] Yeah, totally. Thank you so much for joining us to discuss space. It’s always such a delight to have you.

Moskowitz: Yes, it’s absolutely my pleasure. Thank you very much.

Feltman: That concludes today’s episode. We’ll return on Monday with our trusty science news roundup. In the meantime, please do us a favor and leave us a quick rating and review wherever you tune into this podcast. You can also direct any questions or comments to sciencequickly@sciam.com.

Science Quickly is produced by me, Rachel Feltman, in collaboration with Fonda Mwangi, Kelso Harper, Madison Goldberg, and Jeff DelViscio. This episode was reported and co-hosted by Clara Moskowitz. Emily Makowski, Shayna Posses, and Aaron Shattuck verify our show’s content. Our theme music was created by Dominic Smith. Subscribe to Scientific American for more current and comprehensive science news.

For Science Quickly, this is Rachel Feltman. Have a wonderful weekend!