Many astronomers dream of discovering a new planet, but Mike Brown is perhaps the only one who takes pride in having “killed” one. His research led to the demotion of Pluto from its status as the ninth planet of our solar system, a decision that sparked public outrage. People questioned, how could you rewrite our childhood memories and alter our planetariums?
Around a decade ago, Brown’s then 10-year-old daughter suggested a way he might redeem himself: by discovering a new planet. Brown recalls, “I laughed when she proposed it, thinking to myself, ‘That’s never going to happen.’”
Nevertheless, Brown might be close to making his daughter’s suggestion a reality. Research conducted by him and his peers over the last ten years indicates unusual occurrences in the outer solar system: certain distant celestial bodies are found in peculiarly shaped orbits, seemingly organized by an unseen gravitational force. Brown believes this force could be due to a ninth planet, larger than Earth yet smaller than Neptune.
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As of now, this hypothetical “Planet Nine” remains undiscovered. If it exists, it’s too remote and dim for nearly all current telescopes to detect. However, this is about to change with the upcoming deployment of the Vera C. Rubin Observatory in Chile. This facility is expected to uncover millions of celestial phenomena, from distant supernovae to near-Earth asteroids, and importantly, thousands of new objects in and beyond Pluto’s orbit.
Should Brown’s proposed hidden world exist, the Rubin Observatory is likely to find it, or at least provide strong indirect proof of its existence. “Within the first couple of years, we’re going to resolve this question,” asserts Megan Schwamb, a planetary astronomer at Queen’s University Belfast in Northern Ireland, hinting that the solar system may be deemed to have a ninth planet once again.
Pluto was discovered in 1930 and always appeared as a solitary figure on the edge of our solar system. However, in the early 2000s, astronomers began to realize that Pluto had company: other similar frost-covered worlds were being identified in that distant region. In 2005, using the Palomar Observatory in California, Brown and two colleagues discovered a distant celestial body named Eris, which was slightly larger than Pluto and located 68 times further from the sun than Earth.
“The moment I calculated the size of Eris, I knew the gig was up,” Brown recounts. It was a pivotal moment: either Eris would be classified as a new planet, or Pluto would lose its planetary status.
Discovering a ninth planet would be a monumental event, potentially reshaping our understanding of the solar system’s history.
In 2006, the International Astronomical Union set the criteria for a celestial body to be considered a planet: it must orbit a star, have enough mass for its gravity to form a nearly round shape, and have cleared its orbit of other debris. Pluto, which shares its orbital path with numerous other objects, failed to meet the last criterion and was reclassified as a “dwarf planet.” However, this did not diminish the interest in Pluto or its distant neighbors among astronomers.
Pluto and Eris are part of the Kuiper belt, a doughnut-shaped area filled with icy debris remnants from the early solar system. There are numerous other bodies like them, known as trans-Neptunian objects, but they are difficult to observe.
In the early 2000s, Brown, along with Chadwick Trujillo of Northern Arizona University and David Rabinowitz of Yale University, discovered several of these, including Sedna in 2004. Sedna orbits the sun at a distance of 76 astronomical units (AU), making it one of the most remote objects ever detected in our solar system at the time. It resides beyond the Kuiper belt and was only visible as a tiny, shifting dot among the stars. Sedna, categorized as an extreme trans-Neptunian object (ETNO), played a crucial role in the hypothesis of Planet Nine, although its significance wasn’t recognized initially.
In 2014, Trujillo and Scott S. Sheppard of Carnegie Science in Washington, D.C., published a paper on Sedna and another distant object called 2012 VP113, whose closest approach to the sun is an astonishing 80 AU. Both objects seemed to traverse back and forth across the heliopause—the boundary where the solar system meets interstellar space. “These two objects are in a class of their own,” Sheppard remarks. They seemed unexplainable.
Sedna and 2012 VP113, along with a handful of other peculiar objects, follow orbits so elongated and distant that they must be influenced by some significant gravitational force, but at these distances, the sun’s gravity is the only expected influence. “These objects are in a dead zone,” Sheppard says. He and others speculated that an unseen gravitational actor was needed to explain these mysterious travelers. In 2014, Sheppard and Trujillo suggested that these objects’ peculiar orbits could be due to a hidden planet, possibly two to five times the mass of Earth, that was gradually altering the shapes and positions of their original orbits over time.
The best approach to verify this theory was to use these ETNOs and their orbits as gravitational probes of the outer solar system, explains Sheppard. Brown found this idea intriguing and brought Sheppard and Trujillo’s study to Caltech astronomer Konstantin Batygin, a theorist who delves into the underlying reasons behind the solar system’s structure. “I take deep joy in solving observational puzzles,” Batygin says. “For me, the excitement is in making the calculations and testing them against the data.”
Brown and Batygin pondered over six ETNOs and noticed something strange. Unlike the eight known planets, whose orbits are roughly circular and lie along the same flat plane known as the ecliptic, these six objects—including Sedna—had elliptical orbits and were tilted about 20 degrees relative to the ecliptic. Moreover, they all made their closest approaches to the sun in the same region of space. They were too far out to be within Neptune’s gravitational influence, but something seemed to have shaped their orbits.
Computer models by Brown and Batygin suggested that the most plausible explanation was a hidden planet with a mass five to 10 times that of Earth, orbiting as far as 700 AU away. This world, perhaps exiled from the inner solar system during its chaotic early years, managed to maintain a gravitational connection to the sun. As it moved through the distant darkness, it exerted its own gravitational pull on those six passing objects, herding them into similar, unusual orbits.
Since the discovery of Sedna in 2004, the idea of a massive, hidden planet had been proposed several times. However, the 2016 paper by Brown and Batygin announcing their calculations was a definitive call to action: We are confident that Planet Nine exists. Now, all we need to do is find it.
The search for a missing planet is inherently unusual. “How many planets are in the solar system?” Schwamb asks. “This should be a straightforward question, right? But it’s not!”
Discovering a ninth planet would be significant. Beyond consoling those who still lament Pluto’s demotion, such a discovery could alter our understanding of our solar system’s history. Any objects in and beyond the Kuiper belt are “relics left over after planet formation,” Schwamb explains. “They tell us about that hidden history that has essentially been erased from the solar system.” Did planets form that far from the sun, or did they migrate out there? Most models of planetary systems around other stars include some form of a mini Neptune. “It’s peculiar that we don’t have one,” she says.
If it exists, Planet Nine is large compared to Earth—Brown estimates it to be around seven Earth masses. However, it is so distant that it is beyond the detection capabilities of most telescopes. In general, observatories must choose between having a wide field of view to capture more of the night sky at once or a large mirror to collect more light from a smaller area and see distant, dim objects. Space is vast, so focusing on one tiny patch of it in hopes of spotting a single object is highly unlikely to succeed.
Jen Christiansen; Source: Caltech/R. Hurt (IPAC) (ETNO and Planet Nine orbital reference)
Many astronomers—not just Brown, Batygin, Sheppard, and Trujillo—have attempted to find more ETNOs. Several more have been discovered, including the Goblin (found around Halloween in 2015), Farout, and FarFarOut—additional gravitational probes for those hunting for Planet Nine. Yet, despite these efforts, Planet Nine itself has remained elusive.
Of course, there is a possibility that they haven’t found it not because Planet Nine is elusive but because it might not exist. Over the past decade, various alternative theories have been proposed to explain the unusual orbits of Sedna and its peers.
One theory suggests that there might be a Planet Nine, but it could be considerably smaller—perhaps the size of Mars—and located elsewhere on the outer boundary of the solar system. In 2017, Kathryn Volk, an orbital dynamics researcher at the University of Arizona, speculated that the orbits of various trans-Neptunian objects hinted at the presence of a Mars-sized world within the Kuiper belt. Subsequent observational data on other distant objects have since cast doubt on her team’s hypothesis, and although the idea of a Mars-sized Planet Nine has been discussed at astronomy conferences, Volk now expresses skepticism. “Much like the more traditional Planet Nine, they’re probably both incorrect,” she says. “I don’t think any of the existing predictions are right.”
In 2020, scientists proposed that an icy ring of primordial debris, if massive enough, could also be shaping the orbits of several ETNOs. Brown notes that we observe inclined frosty rings around other stars, but those rings are usually maintained by the gravitational influence of another large planet, making this a more complex explanation than just Planet Nine.
It has also been suggested that perhaps a passing star or a rogue planet zooming through space could have altered the orbits of Sedna and its companions long ago. In 2019, researchers even speculated whether a small black hole might be responsible. When I mention this possibility to Brown, he smiles. “I have it!” he exclaims. He disappears for a moment, then reappears holding a sphere roughly the size of a volleyball. “This is a seven-Earth-mass black hole. One of my students 3D-printed it for me.”
Brown chuckles. “What we know is that there is a seven-Earth-mass object out there. What it is, we don’t know,” he says. “It could be a planet. It could be a black hole. It could be a cat or a burrito. All of these are possibilities—some make more sense than others.” He sets down his miniature black hole. “A planet is a really mundane explanation.” After all, he notes, we frequently observe planets like that in distant orbits around other stars.
Trujillo is more cautious when considering alternative explanations. Sure, he acknowledges, they might be correct; those theories deserve investigation. “We still don’t really understand how Sedna and the other ETNOs ended up out there,” he says. But the possibility of an undiscovered large planet remains real.
While not as definitive as Brown, Batygin is optimistic. In astrophysics, “most theories are incorrect,” he observes. “The most surprising thing I’ve encountered over the past eight years regarding this particular issue is that there hasn’t been a compelling alternative.”
One of the biggest challenges to the Planet Nine hypothesis is the suggestion that Sedna and similar objects might not actually have strange orbits at all. Astronomers can’t observe every region of space clearly. If an observatory experiences bad weather during the winter, then data will be lacking for that part of the sky. ETNOs also spend most of their incredibly long orbits so far from Earth that they only reflect sunlight when they are closest to the sun. Additionally, there’s the Milky Way. Our solar system is situated on one of the arms of our spiral galaxy, and when we look toward the center, all we see is starlight. While beautiful, this starlight is a nuisance for astronomers. “Nobody finds [trans-Neptunian objects] where the Milky Way is,” says Samantha Lawler, an astronomer at the University of Regina in Canada. “You’re looking for a small, fuzzy, moving dot, and when there are so many stars in the background, it’s harder to find them.” Because astronomers only know about a limited number of Kuiper belt objects and ETNOs, some scientists skeptical of the Planet Nine hypothesis think we simply lack enough information to determine whether worlds like Sedna truly have strange orbits or just appear to for now.
“Each year we don’t find [Planet Nine], the probability of it actually existing goes down dramatically.”
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Cameron Aldridge combines a scientific mind with a knack for storytelling. Passionate about discoveries and breakthroughs, Cameron unravels complex scientific advancements in a way that’s both informative and entertaining.