How Do You Discover A Planet?

Out in the furthest reaches of our Solar System, twenty times further from the Sun than Neptune, a massive unknown planet may be lurking. At ten times the mass of the Earth, the gravitational pull of the massive planet has herded the orbits of an obscure group of icy objects into a strange alignment. Last January, Caltech scientists Mike Brown and Konstantin Batygin announced the likely existence of this new planet, dubbed Planet Nine, in a paper published in the Astronomical Journal. Though the mysterious planet has yet to be seen through a telescope, evidence of its existence has been growing for years. The next major Solar System discovery is upon us. When astronomers finally capture an image of Planet Nine, it will mark the only discovery of a new planet in our Solar System in living memory. But why do we think Planet Nine is really there?

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Other than Earth, the first six planets known to humankind were discovered simply by looking up. The closest five, Mercury, Venus, Mars, Jupiter, and Saturn were visible to ancient humans with the unaided eye for thousands of years. Our ancestors, reading the skies night after night, noticed them as bright lights that moved from night to night against the backdrop of fixed stars and named them planets, from the Greek word for wanderer. At this time, the Sun and Moon were also considered planets, for they too wandered among the stars, but as the centuries passed, we came to better understand our place in the universe. In 1543, Copernicus published his heliocentric theory, correctly positioning the planets as our celestial siblings orbiting the Sun just as the Earth does. 

The next big discovery in the solar system came in 1781, when Englishman William Hershel observed Uranus, a planet invisible to the unaided eye, with a massive homemade telescope. He meticulously studied the new planet, night after night, until he realized that it too wandered through the starry skies. Though Uranus had been spotted by previous generations of astronomers, because of its great distance from the sun, the planet’s wandering motion had never been observed, so Uranus was simply assumed to be a star. Even Hershel was skeptical: he initially believed the seventh planet was a comet—after all, no new planets had ever been documented in recorded history—but careful study of its orbit allowed astronomers to conclude Uranus was a planet in its own right.

Neptune and Planet Nine are different. Just as you can infer the presence of a breeze on a windy day by observing tossing trees and dancing leaves outside your window, scientists first detected Neptune and Planet Nine not through direct observation, but through their effects on objects that can be seen. In the 1840s, English and French astronomers predicted Neptune’s location after observing anomalous deviations in the orbit of Uranus. Again, the astronomy community was skeptical at first. But, using mathematics and the laws of physics, the planet’s location in the sky was pinpointed, and telescopes appropriately aligned to the predicted spot quickly sighted the eighth planet, just as promised.

Humanity’s exploration of the outer reaches of the Solar System didn’t end with Neptune. In the last century, astronomers began probing a region of space called the Kuiper Belt, a swath of tiny icy objects just beyond the orbit of Neptune. The giant blue planet dominates the Kuiper Belt gravitationally, shaping the orbits of the nearby Kuiper Belt Objects. For instance, the Kuiper Belt’s most famous resident, Pluto, is locked into its orbit by a gravitational relationship with Neptune known as resonance. For every two orbits Pluto makes, Neptune makes three. This synchronization regulates Pluto’s motion along its orbit, just as a parent regulates the motion of a child on a playground swing by pushing in harmony with the swing.

While looking for Kuiper Belt Objects, astronomers made a peculiar discovery—Sedna, a strange, icy body about half the size of Pluto. Sedna orbits the sun in a highly elliptical path that takes it from twice to thirty times as far from the Sun as Neptune. At these distances, Sedna is much too far away to be a member of the Kuiper Belt, floating peacefully billions of miles away from Neptune’s region of influence. Usually, the smaller members of the Solar System start out with circular orbits and, over time, find themselves on extremely elliptical orbits after close encounters with massive planets. Like a slingshot, big planets inject energy into the orbits of small bodies during close encounters and send them rocketing into strange new orbits or even out of the Solar System entirely. But no known object could have explained how Sedna came to have such a strange orbit, since it never came close to any known planets. So, for years, astronomers assumed its existence spoke to a freak event like a close gravitational encounter with a passing star—a one in a billion anomaly.

Yet similar objects kept being discovered. In astronomer’s parlance, these new Sedna-like objects all had high perihelia and high major axes. That is, their closest approaches (perihelia) to the Sun were well beyond the orbit of Neptune, and the long (major) axes of their oval orbits were many times larger than their shorter (minor) axes. In other words, the objects had highly elliptical orbits, and they never got very close to Neptune or the Sun. Most telling of all, the long axes of the orbits of all of these objects pointed roughly in the same direction, an eerie coincidence Mike Brown described as “like having six hands on a clock all moving at different rates, and when you happen to look up, they're all in exactly the same place.” Since the chances of such an alignment occurring by accident are low, at about 0.007 percent, the Caltech scientists suspected something was missing from current models of the solar system.

Orbital Parameters: The orbits of Sedna and two of its sibling objects are shown above. Sedna’s important orbital parameters are labelled. The long axes of the orbits of the three objects point in roughly the same direction, a major clue to Batygin and Brown that our current models of the Solar System might be missing something.

Unlike the astronomers who deduced the existence of Neptune using meticulous calculations performed by hand, Brown and Batygin needed to use computing power to make their predictions. Neptune’s discovery involved predicting the location of an unknown planet based off of deviations in the orbit of just one object, namely Uranus, but Planet Nine’s presence needed to be inferred from a collection of objects with a complicated and chaotic gravitational dynamic. In complex systems like this one, it is much faster and easier to attempt to study the system using the brute force of computation. Brown and Batygin developed a simulation that used trial and error in order to deduce the placement of the hypothetical planet in the solar system. By figuring out which arrangements of planets didn’t produce the observed configuration of the solar system, the researchers could narrow down in which regions of the solar system the new planet might be located.

One run of the simulation might go like this: a possible orbit for Planet Nine is specified in detail, and placed into the known model of the solar system. The simulation begins, and the computer predicts the positions of all the planets, moons, and minor solar system bodies over the course of millions, or even billions, of years. When the simulation ends, the outcome is compared to the observable parameters of the solar system today, including the positions of the planets, the alignment of Sedna and its siblings. Sometimes, the end result is dramatically different from the solar system today. In the wrong alignment, an interaction with Planet Nine could fling one of the other eight planets from the solar system via the slingshot effect. Usually, the differences are more subtle. There might not be any Sedna-like objects, or they are not aligned anymore, or a subtle detail of their orbits doesn’t match the real state of Solar System. If there are any differences between the simulation’s prediction and the observed state of the solar system, the initial guess for Planet Nine’s orbit is ruled out.

Batygin and Brown first hypothesized that Planet Nine would be located on the same side of the Solar System as the anomalous Sedna-like objects. They reasoned those long major axes that puzzled astronomers ought to be pointed towards the location of the unknown planet, as if Planet Nine were shepherding Sedna and company into alignment. However, this initial guess didn’t fit the model perfectly, producing a different alignment than the one observed. Then, in a lucky guess, the researchers started the simulation with Planet Nine on the opposite side of the sun. In this new configuration, with the long axes of the objects’ orbits pointing away from Planet Nine, the output of the simulation perfectly matched what we see in the sky today.

A Possible Orbit for Planet Nine: Batygin and Brown’s model places Planet Nine on the opposite side of the sun from Sedna and its siblings. The long axes of the Sedna-like objects’ orbits point in the opposite direction as the long axis of Planet Nine’s orbit.

Placing Planet Nine on the opposite side of the Solar System from Sedna allows for a stable and accurate configuration of orbits. The model also had an unexpected consequence: it predicted a new class of objects astronomers should expect to see. This new group should contain small bodies with orbits perpendicular to the orbits of Sedna and its siblings. Finding these predicted objects would be a key test for the Planet Nine hypothesis. A scientific model can be useful for explaining observed phenomena, but it derives most of its power from its ability to correctly predict new aspects of a situation. Brown and Batygin scoured the catalogue of known minor members of the outer solar system, and, just as the model suggested, four objects with the predicted orbital properties were found.

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Simulations can only take astronomers so far. Though Batygin and Brown have deduced by simulation thus far that Planet Nine is ten times heavier than Earth and completes its orbit in tens of thousands of years, the planet technically hasn’t been discovered, as no astronomer or telescope has spotted it. Direct detection is crucial in hunting for unknown planets in our Solar System. After the discovery of Neptune, astronomers sought to find more planets through similar indirect means. In the early twentieth century, supposed deviations were again found in the orbit of Uranus, leading to the prediction of Planet X. The search for that planet uncovered Pluto. But Pluto turned out to be too small to impact Uranus’s orbit in any way, and the deviations were later discovered to be observational errors. Direct imaging of Planet Nine should be feasible for a planet this close; compared to extrasolar planets orbiting other stars, Planet Nine is in our cosmic backyard. We should be able to spot it. So why haven’t we found it yet?

One possibility is that Planet Nine has already been spotted. Just as Uranus had been mistaken for a star before Herschel discovered its wandering motion in the sky, Planet Nine might be misclassified in some comprehensive sky survey as an obscure star. Planet Nine would be far enough away for this to be a possibility. Even with the most powerful telescopes, it takes 24 hours for Planet Nine to move noticeably far enough to rule out the possibility that the distant point of light is a fixed star. And there are further complications. A large swath of Planet Nine’s orbital path takes it through the plane of the Milky Way. If the planet is located in these unfortunate regions, astronomers will be searching for a faint, wandering dot against the backdrop of some of the most densely star-packed regions of the night sky. The busy background will make it difficult to spot the planet, like looking for a tiny ant crawling across a static-filled television screen.

Despite these challenges, we must continue the search for Planet Nine. While Brown and Batygin hope their prediction is correct, the only way to know for sure is to take to the skies, scanning the predicted orbital path and hoping for a glimpse of the elusive planet. If Planet Nine is really there—and Brown predicts we will know the answer within the year—we will recognize the planet the same way humanity has been recognizing planets for thousands of years. Somewhere up there, Planet Nine may be a faint, faraway dot, but it, just like the other planets in our Solar System, will be a wanderer amongst the silent, distant stars.  

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Originally written for Caltech's science writing class, En/Wr 84.

It's Time to Demote Pluto... Again

Originally published in the California Tech, page 3.

In 2006, the International Astronomical Union (IAU) voted to reclassify Pluto, stripping it of its status as a planet while establishing a new class of celestial bodies: dwarf planets. While it makes sound scientific sense to demote Pluto from its planetary status, I argue that we should go even further by destroying the designation dwarf planet — a category which is both scientifically useless and pedagogically confusing. Instead, we should call Pluto what it really is: a Kuiper Belt Object.

So why, according to the IAU, isn’t Pluto a planet? The IAU has established three criteria for evaluating whether or not an object is a planet or dwarf planet. According to resolution B5¹, a planet is “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to … [assume] a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood [sic] around its orbit.” The first criterion is fairly straightforward, separating the notion of planet from the notion of a natural satellite or moon. The second is a statement about the size of an object — a planet must be big enough for gravity to be the dominant force sculpting its shape. And the third is, well, confusing. Unfortunately, the third criterion is critical. It is the qualification Pluto fails and it establishes the difference between a planet and a dwarf planet. Planets have “cleared their neighborhood” and dwarf planets haven’t.

What the “clearing its neighborhood” criterion comes down to is gravitational influence. Planets, especially large ones like Jupiter, have gravitationally dominated their orbits. Anything that passes too close to Jupiter will either crash into the planet or be ejected from the solar system. In this way, Jupiter clears its neighborhood. The same process works for the other seven planets as well. However, Pluto exists in a belt of similar objects — the Kuiper Belt — where its diminutive size is enough to make it round, but not enough to clear its orbital path.

Because this process is inherently gravitational, it means that the IAU criteria establish two separate size thresholds, both of which must be passed in order to be a planet. Dwarf planets, only passing the “roundness” criterion, exist in a sort of in-between size category, a poor consolation prize to satiate angry Plutophiles. The dwarf planet distinction fails on two counts: it groups together objects with very little in common (other than roundness) and it fails to group together objects that share important physical properties and histories.

There are five objects in the solar system that qualify as dwarf planets: problematic Pluto, our own Mike Brown’s Eris, Ceres (the largest object in the asteroid belt) and the two obscure additions of Haumea and Makemake. Pluto, Eris, Haumea and Makemake are all residents of the Kuiper Belt, which lies beyond Neptune’s orbit, while Ceres is located much closer to the sun as the largest resident of the asteroid belt, which is between the orbits of Mars and Jupiter. While the Kuiper Belt Objects (KBOs) in this group have much in common with each other, they are vastly different from Ceres, both in composition and history. Ceres is primarily rocky; the KBOs are icy. Ceres, as a member of the asteroid belt, has primarily been influenced by Jupiter, while the KBOs’ histories are heavily shaped by the influence of Neptune.

Lumping Ceres together with these other objects has real consequences: it leads to the impression that Ceres is located in a completely different region of the solar system. As an astronomy outreach educator, I have encountered several aspiring amateur astronomers who mistakenly believed Ceres orbited beyond Neptune. The category dwarf planet, then, is misleading, a term that obscures truth.

A categorization that makes more sense is to group Ceres with objects that share its composition and history — the asteroids. Ceres may be an exceptionally large member of the asteroid belt, but this does not warrant the distinction of “dwarf planet.” Similarly, Pluto, Eris and their lesser known cousins should be classified alongside the rest of the Kuiper Belt Objects, with which they have more in common than with outlier Ceres.

Superstar astrophysicist Neil deGrasse Tyson advocates for a similar zone-like division of the solar system², separating asteroids and Kuiper Belt Objects. He even goes as far as splitting the planets into two categories — inner planets and outer planets — a division that again reflects the shared composition and history of the rocky planets and gas giants. He implemented this categorization in his design for the solar system exhibit in the Hayden Planetarium before the discovery of Eris, before Pluto’s demotion was even up for discussion.

But this division makes pedagogical sense. Simply memorizing a list of planets is not instructive and leaves learners with a rigid and inflexible understanding of science, as the backlash against Pluto’s reclassification demonstrates. Teaching the solar system as a collection of different classes of objects opens up a more flexible understanding of science and leads naturally to scientifically relevant questions. Why, for instance, is it that the inner planets and outer planets are separated by belt of asteroids? Why are inner planets rocky and outer planets gassy? Such inquiries are more reflective of the true nature of science. Science is not simply a list of facts, but a systematic way of asking questions and organizing knowledge. Shouldn’t we, as scientists, strive for terminology that accurately reflects the exciting, ever-changing processes by which we discover it in the first place?

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I originally wrote this piece for En/Wr 84, Caltech's science writing course. I plan on posting more material from that class soon.

¹ You can read about the IAU definition here.
² See Tyson's The Pluto Files