The Story of Pluto The Forgotten Planetoid

Table Of Contents:

  1. A Complicated Past
  2. Pluto is Demoted to Planetoid
  3. Forgotten By Science
  4. It's Been a Long Journey
  5. What Will We Find on Pluto?
  6. A Brighter Future

Discovered back in 1930, Pluto has been one of the more mysterious objects in the Solar System for the past 85 years—so much so that it's yet to have been visited by any man-made object. In 2006, NASA's New Horizons spacecraft began its mission to Pluto by becoming the fastest spacecraft ever launched into deep space. Now, five billion kilometers and nearly a decade later, New Horizons is set to conduct the first-ever flyby of Pluto by a man-made object on July 14th, 2015.

When this historic flyby occurs, New Horizons will attempt to take as many high-quality snapshots of Pluto's surface as possible. The images it sends back should answer some of our questions about the forgotten planetoid, and in the process will provide us with a whole stack of new and intriguing questions. 

Unfortunately, the window of opportunity for this mission will be short. New Horizons' high velocity means that it won't be able to enter into a stable orbit around Pluto and will instead pass by the planetoid on a path that eventually takes it out of the Solar System. Before that happens, the NASA team hopes to make a trajectory adjustment and conduct a flyby of a another object out beyond Pluto. 

The story of Pluto features its complicated past, the loss of its planetary status, the lack of any prior exploration missions, and finally, how New Horizons is just days away from completing its incredibly long journey and capturing the first-ever up-close glimpse an unexplored world residing deep in the outer Solar System. Although Pluto has endured through billions of years of existence in the outer reaches of the Solar System, this story begins in 1930. 

In the early 20th century, Astronomers had such a hard time seeing Pluto in their telescopes that are at least 14 known pre-discovery images in which the tiny planetoid can actually be identified, dating as far back as 1909. Nobody noticed that this faint recurring object existed, or that it was a planet residing in the solar system. Finally, in 1930, Clyde Tombaugh processed these images at the Lowell Observatory and found what astronomers had been after for decades:

Discovery of Pluto, Lowell Observatory
That's no star... It's Pluto!
Image Modified from: Lowell Observatory Archives.


The search for Pluto originally began with the search for a 'Planet X' around the turn of the 20th century. After the discovery and subsequent observations of Neptune in the mid-late 19th century, astronomers hypothesized that there must exist a ninth planet, dubbed Planet X, out past Neptune. After all, Neptune's existence had been predicted prior to its discovery through observing the motions of the other planets, and with Planet X it was expected to be no different.

Victorian-era astronomers calculated that the mass of the outer planets they were observing didn't match their orbital characteristics. The Planet X hypothesis claimed that observations of Uranus' orbit could only be explained by the existence of an additional planet further out in the Solar System, an Earth-sized object whose gravity was occasionally tugged on Uranus—just a tiny bit. Eventually, they found something.


When Pluto was discovered decades later, it was praised as the missing Planet X. Its mass was greatly over-estimated as being roughly equal to that of Earth's, a figure adopted in order to account for the perceived discrepancies in Uranus' orbit. Then, over decades of observation, Pluto's estimated mass gradually fell by several orders of magnitude. Again, something was missing.

In 1978, with the discovery of Pluto's moon, Charon, astronomers were able to correctly calculate Pluto's mass at around 0.22% that of Earth's mass. By comparison, the Earth's Moon is 1.2% the mass of Earth—six times heavier and triple the volume of Pluto. And that makes Pluto only the 17th most massive known object in the Solar System. 

Discovery of Charon, 1978

Discovery of Pluto's Moon, Charon
Astronomers deduced that a planet shouldn't rapidly change shape.
Image Modified from U.S. Naval Observatory.

With this Plutonian revelation, the search for Planet X continued until, after the 1989 flyby of Neptune by Voyager 2, astronomers realized that Neptune's mass had been over-estimated by approximately the mass of Mars. After this adjustment, there was no longer a need for the existence of a larger planet beyond Neptune to account for Uranus' orbital characteristics, and Pluto remained the largest known object beyond Neptune for the time being. 

Astronomy is a science that requires precise measurement and calculation. This final model of the Solar System was now one of perfectly balanced harmony... except for one problem.

A rift in the scientific community, with some considering Pluto to be a planet and others considering Pluto to be a different class of object altogether. Eventually, Pluto lost its planetary classification, being re-branded as a 'dwarf planet' or 'planetoid'. In many ways, the lifetime of Pluto has mirrored a typical human lifetime—admittedly, a somewhat depressing one. Here's the timeline for the story of Pluto:


Discovered in 1930 and named after the god of the underworld, Pluto's arrival was met with a significant level of optimism from within the scientific community. Pluto became the first supposed Earth-like planet discovered in the outer-reaches of the Solar System, meaning that smaller worlds now outnumbered the gas giants (in quantity, but not mass). If Earth could be thought of as humanity's luxury penthouse suite in the city-center, then Pluto would now serve as our quiet countryside villa—only a bit harder to get to. 

1950: PLUTO Enters Adulthood

As Pluto's discovery celebrated its 20th birthday, Gerard Kuiper, now considered to be the father of modern planetary science, revised the estimated mass of Pluto to about 10% the mass of Earth. This somewhat diminished the earlier excitement surrounding the discovery of the ninth planet (now also the smallest known planet), and confounded astronomers who still insisted on there existing an Earth-sized Planet X out past Neptune. Pluto no longer appeared to live up to its 'god of the underworld' namesake if a larger Planet X existed out beyond its orbit (Voyager 2 later disproved this theory.).

1978: Pluto Has a Mid-Life Crisis

As Pluto turned 48 years old, Charon was discovered by James W. Christy at the U.S. Naval Observatory. Through observing the motions of Pluto and Charon, astronomers were able to definitively calculate that Pluto was in fact only 0.22% the mass of Earth, significantly smaller than originally thought.

This became a bit of an unfortunate birthday for Pluto, as its revised mass sort of put it into a whole new class of objects—too small to be classified as a planet, yet too large to be classified as a comet or an asteroid. Pluto had just celebrated its 48th birthday, and nobody really knew what to call it anymore. The poor thing was left feeling tiny and insignificant, but at least it now had a friend named Charon to keep it company.


The final decision came after Pluto turned 76 years old. By this point, various spacecraft had visited every other planet in the Solar System—all except for the long-forgotten Pluto. Finally, a space-launch, and New Horizons blasted off towards the neglected little planet at the edge of the Solar System. 

At the same time, astronomers had observed that Charon was sufficiently massive that Pluto was actually orbiting a point in space beyond its own surface, known as a barycenter. Planets are supposed to have their barycenters contained within their volume, as the eight other planets in our Solar System do, otherwise they're technically considered a binary system.

The Pluto-Charon Barycenter:

Pluto-Charon barycenter
Pluto and Charon love to dance.

This meant that Pluto and Charon were both orbiting a point in space between themselves, and that point in space was orbiting the sun—not Pluto. The space bureaucracy caught on to this and decided that planets are supposed to orbit stars, not empty space. 

In 2006, a few months after New Horizons left Earth on its ground-breaking mission to Pluto, the International Astronomical Union (IAU) declared that Pluto would no longer be classified as a planet, and that it instead fell into a category labeled interchangeably as either 'Planetoid' or 'Dwarf Planet'. Either way, Pluto was demoted and Neptune officially became the most distant known planet from the sun. 

Imagine the difficulty of explaining to New Horizons that the planet it was just sent out to explore wasn't a planet anymore. In fact, try explaining it to Pluto. 

2015: Pluto Will Receive a Visitor

Now that Pluto is 85 years old, our envoy has nearly arrived. Fortunately, recent decades have been kind to Pluto; whereas earlier observations of Pluto's home region hadn't revealed anything of significance, we now know of multiple other Pluto-like planetoids residing deep in the outer Solar System.

New Horizons under construction
NASA's New Horizons spacecraft, first envoy to Pluto.
Image Credit: NASA.

There could be as many as hundreds of other planetoids that are nearly identical to Pluto, with a handful of these already detected and identified by Hubble. And New Horizons isn't planning on visiting just one planetoid on its long voyage—the team at NASA would like to find as many other objects as possible once they're through with Pluto. 

Throughout Pluto's short life, it'd been engaged in a heated back-and-forth struggle for status within the Solar System. For the first fifty years after its discovery, Pluto was happily settled in as the ninth and furthest planet from the sun—the rightful 'god of the underworld'. Then, in 1979, Pluto's orbit crossed inside of Neptune's orbit and the two swapped places, with Pluto moving up to the place of eighth planet from the sun.


For twenty years, from 1979 to 1999, Pluto remained the eighth planet from the sun and Neptune settled into ninth place. It was shortly afterward that the IAU made certain that Neptune would forever remain the eighth planet from the sun, regardless of whether or not Pluto occasionally moved inside of its orbit. Pluto was disqualified from planetary competition, and subsequently from the competition for Solar System domination.

To make matters worse for Pluto, at the same time as it briefly surpassed Neptune to become the eighth planet from the sun, Voyager 1 & Voyager 2 were in the midst of their historic Grand Tour of Jupiter, Saturn, Uranus, and Neptune in greater detail than ever before. NASA controllers decided that a special flyby of Titan, a moon of Saturn, was more important than making the first-ever rendezvous with Pluto, a yet-unexplored planet. 

Voyager's path through the solar system
Pluto wasn't included on the Voyager Grand Tour.
Image Credit: NASA

Following Other Leads

NASA's scientists chose a flyby of Titan in favor of a flyby of Pluto on the basis of certainties. Although they knew very little about Pluto (its atmosphere hadn't even been discovered yet), they were certain that Titan possessed a thick atmosphere, an exceedingly rare feature for a world of its size (and distance from the Sun). As a result, data collected by Voyager 1's flyby of Titan helped the Cassini-Hyugens lander successfully splash down on Titan in 2005. 

The exploration of Titan has resulted from a sequence of promising scientific leads—we first saw what looked to be an atmosphere, then the first pictures of that atmosphere raised more questions, followed by a very expensive mission to put a lander to Titan's surface, which then raised even more questions that are yet to be answered. The follow-up to the Cassini-Hyugens lander will likely be a long-term study of Titan's surface features—a Curiosity-type mission, but built to survive on Titan's liquid-methane covered surface rather than a big red rock.

In Need of New Questions

Coming up with questions and finding adequate answers is the hallmark of successful science. Unfortunately, we haven't gotten many answers out of Pluto yet—nor have we really had the chance to ask the right questions (such as: does Pluto have a permanent atmosphere? Is life possible there?).

Nearly 24 years after Voyager 2 visited Neptune (at the time, ninth planet from the sun), New Horizons is set to make a significant impact when images of its Pluto flyby are released in mid-July. Hopefully, these images will spark a renewed interest in exploring some of the other objects that reside deep within the outer Solar System. 

According to NASA, New Horizons has already made history, traveling longer and farther than any other spacecraft to reach its primary destination: Pluto. Although other missions have traveled a further total distance, no other mission has had to travel 5 billion kilometers before reaching its primary target. It's about time someone threw a special bash for this lonely little planetoid, and we've sent the perfect little probe to do just that. 

Charon orbiting Pluto
Pluto, captured by New Horizons, 5 billion kilometers away from Earth.
Image Credit: NASA/Johns Hopkins University/Southwest Research Institute

New Horizons' Computer Malfunction

On July 4th, the story of Pluto took a dramatic twist. Just 10 days before its scheduled flyby of Pluto, New Horizons experienced an anomaly which forced the shutdown of its main computer. With communications temporarily lost, NASA scientists scrambled to return the spacecraft to normal science operations. NASA's director of Planetary Science, Jim Green, stated that:

"our mission team quickly identified the problem and assured the health of the spacecraft... Now—with Pluto in our sights—we’re on the verge of returning to normal operations and going for the gold."

Such difficulties are commonplace when it comes to space exploration, but it seems ridiculous that our first envoy to Pluto flew for 3,453 days (about 9.5 years) problem-free, only to experience an anomaly 10 days out from its destination. Had this glitch occurred one day before the Pluto encounter, New Horizons would have entirely missed its opportunity to gather data during the flyby. 

To complicate matters further, New Horizons is operating so far away from Earth that it takes 4.5 hours for any command issued by NASA controllers to reach the spacecraft—and another 4.5 hours for the results of that command to arrive back at NASA headquarters. This time-delay means that problems which could normally be cleared up within a few hours are now extended over a period of days—bad news for a spacecraft traveling at a speed of 14 kilometers per second.

Due to the small size of Pluto and the high velocity of New Horizons, the July 14th flyby will only last around 10 hours, during which time the spacecraft is pre-programmed to conduct its most important scientific observations. If something does go wrong during this small window, NASA won't even know about it until at least 4.5 hours in—about halfway through the encounter.

Record-Breaking Speed

New Horizons was the fastest man-made object ever launched from the surface of the Earth. But in order to reach Pluto within a reasonable time-frame, New Horizons performed a gravity-assist maneuver around Jupiter in order to boost its velocity even further.

New Horizons launches towards Pluto
In 2006, New Horizons became the fastest object ever launched from Earth.
Image Credit: NASA

This is accomplished by directing a spacecraft towards a planet and having it fall into the planet's gravity, where it is essentially pulled with increasing velocity towards the planet that it's approaching. Once sufficient speed has been achieved, the spacecraft maneuvers to 'slingshot' around the planet and off towards its next destination.

Spacecraft commonly employ this maneuver in order to achieve an additional 'nudge' towards an intended destination. New Horizons' gravity-assist maneuver around Jupiter accelerated the craft from its post-launch speed of 16 kilometers per second to a velocity just under 23 kilometers per second. Without that gravity assist, New Horizons wouldn't have reached Pluto until 2020

Although New Horizons reached a maximum relative velocity of nearly 23 kilometers per second, eight years of traveling away from the sun's gravitational pull has slowed the spacecraft down to approximately 14 kilometers per second—about a 40% decrease. 

Fast and Light

Becoming the fastest spacecraft of all time wasn't easy, and it did come with the sacrifices required of making the probe as lightweight as possible. One such sacrifice is that New Horizons can't achieve a stable orbit around Pluto because, even if such a maneuver were feasible, it doesn't have enough propellant to make the necessary maneuvers to do so.

And this design was intentional—any additional propellant would have slowed its launch velocity and resulted in less of a boost from Jupiter's gravity, which would have caused New Horizons to arrive at Pluto years later than planned. Extending the mission timeline would've required increasing New Horizons' cost and upgrading on-board instruments to be able to survive a much longer journey, which could have further increased weight, time, and cost needed to reach Pluto. It's a vicious cycle.

The result: New Horizons is about to complete its time-sensitive mission of arriving at Pluto in unprecedented time. And it's not a moment too late.

A Race Against Time

In 1988, astronomers observed Pluto move in front of a star and gradually dim the light of the star as it passed by, suggesting that the distant planetoid possessed an atmosphere above its icy surface. Further observations later determined that Pluto's atmosphere is primarily made up of nitrogen, carbon monoxide, and methane. They also deduced that its atmosphere won't be sticking around for much longer.

Pluto's big, beautiful heart
Pluto shows off its big pink heart.
Image Credit: NASA/Johns Hopkins University/Southwest Research Institute

When Pluto overtook Neptune to become the eighth planet from the sun from 1979 to 1999, those twenty years represented only 8% of the total time it takes Pluto to orbit the sun once. In relative terms, that would be like if Earth's orbit crossed within the orbit of Venus and we became the second planet from the sun—but only for the month of July.

Pluto possesses a highly elliptical 248 year orbit which varies from between 29 to 48 times further from the sun than Earth—which translates to a distance between 4.3 and 7.2 billion kilometers. When Pluto moves closer to the sun, its ice begins to thaw and gasses are released to form a temporary atmosphere. Then, as Pluto moves further away from the sun, its temporary atmosphere freezes and falls back to the surface.

Because Pluto's atmosphere is expected to be completely gone within a few decades, New Horizons is literally racing against the clock to catch a glimpse of the planetoid's atmosphere and any weather that it might be generating. The next opportunity to witness an atmosphere on Pluto won't come for over 200 years. 

Pluto is currently 35 times further from the sun than Earth (5.2 billion kilometers) and moving deeper into the Kuiper belt every day. In the 85 years since we discovered Pluto, it's only completed about 35% of its total orbit, and nobody knows when it'll reach the point of once again becoming too cold to support an atmosphere. New Horizons may just be the last time we see an atmosphere on Pluto for another 200 years. 

New Horizons' instruments will be primarily occupied in mapping geological features, determining precise chemical compositions, and measuring the atmospheric loss rate of Pluto. Here are some things we're hoping to catch a glimpse of:


Although many Kuiper objects are thought to be made up entirely of icy material, some, such as Pluto, are expected to be composed of rocky cores covered in a thick mantle of ice.

Curiosity loves exploring Mars
The Curiosity rover is very curious about rocks...
Image Credit: NASA/JPL-Caltech/MSSS

Because scientists suspect that Pluto's surface layer is composed of frozen gases like nitrogen, it's unlikely we'll find rocky structures on Pluto, and any that did exist should have long settled into the planetoid's core. 

However, Pluto is thought to be nearly identical in composition to one of Neptune's moons, Triton, leading scientists to believe that it could've originated in the Kuiper belt along with Pluto. That's right—Neptune's gravity is probably holding Pluto's cousin hostage, so anything that New Horizons finds out about Pluto could also tell us something about Triton.


Ever since being demoted from planet, Pluto has been categorized under many different titles, including: planetoiddwarf planet, ice dwarf, plutoid, and KBO (Kuiper Belt Object). Each of these classifications is an attempt to place Pluto into a category along with many other objects. One of these in particular, KBO's, is geared towards associating Pluto with thousands of similar icy objects occupying the same region of space—the Kuiper belt.

Discovered by Gerard Kuiper (the same guy who figured out Pluto was not as big as Earth), the Kuiper belt is a region of the Solar System that stretches out beyond Neptune. Similar to the asteroid belt between Mars and Jupiter, the Kuiper Belt is much larger and made up of thousands objects similar in composition to Pluto—frozen nitrogen, methane, carbon monoxide, ammonia, and water. A handful of these KBO's so far discovered are around the same size as Pluto, which means that, once again, its 'god of the underworld' namesake may be under siege. Perhaps 'god of the Kuiper belt' would be more fitting. 

An Atmosphere.

New Horizons will be looking to conduct some atmospheric science, and the opportunity to study the atmosphere of an object like Pluto doesn't come around very often. We've had orbiters past and present specifically designed to conduct in-depth studies on the atmospheres of Venus, Mars, Jupiter, Saturn, and (of course) Earth, so we should check out the rest of the family for good measure as well... even if Pluto was just an orphan from the Kuiper belt.

Artist's idea of what Pluto's atmosphere may look like:

Artist's impression of what Pluto might look like
I guess they weren't that far off...
Image Credit: NASA/Johns Hopkins University/Southwest Research Institute.

One of the best ways to learn more about the Earth's atmosphere is through studying the atmospheres of other objects in the Solar System, and it's unfortunate that we don't have an orbiter capable of conducting a long-term study of Pluto's atmosphere. Being able to see it gradually dissipate would be an incredibly eye-opening observation, and could provide us with some crucial data that could potentially assist us out with terraforming at some point in the future.


New Horizons isn't going to detect life on Pluto, but it's possible that the craft could detect some form of organic molecules in the atmosphere during the flyby, or find evidence of a liquid ocean beneath its icy surface. While neither of these would be conclusive signs of life, they could significantly amplify the case for the existence of life beyond Earth.

Nothing we've discovered beyond Earth so far has provided sufficient proof for alien life, but there are a number of speculations as to whether or not life could exist on Mars, or on one of the many moons of Jupiter or Saturn. If Pluto could be thrown in with that speculative lot, we may begin seeing colossal progress towards better life-discovery missions throughout the Solar System.

In Antarctica, scientists recently drilled 740 meters down through the ice sheet and discovered fish living in an isolated sea floor environment, in total darkness. Scientists have also found evidence that Jupiter's moon Europa houses a large liquid ocean beneath a thick surface of ice, which has led to speculation that life could exist isolated beneath the icy surface of Europa. It's possible that New Horizons could find a similar situation on Pluto, which would foster further speculation that sub-surface oceans may be hot-spots for life on other worlds. 

Artist's Rendering compared to New Horizons' image:

Pluto comparison
Really, not that far off at all.
Image Credit: NASA/Johns Hopkins University/Southwest Research Institute

Sure, Pluto has been somewhat neglected by science for quite some time—but only because it's so incredibly far away and difficult to get to. And the fact that it's only been 85 years between first discovery and first flyby is actually a record-breaking turnaround time when it comes to space exploration.

Humans discovered Uranus and Neptune in 1781 and 1846, respectively. That means that Voyager 2, which conducted the first-ever flybys of those planets, came 205 years post-discovery for Uranus and 143 years post-discovery for Neptune. Besides a few asteroid and comet flybys, we've never sent a spacecraft to study an object in the solar system—in particular a planet, planetoid, or moon—that we hadn't known about for well over a hundred years before the mission even began.

This makes Pluto the most mysterious object that we've ever explored. And following this, New Horizons is going to attempt a flyby of another Kuiper belt object once it's done with Pluto, most likely 2014 Mu69, sometime in 2019—just five years post-discovery, and an object we know even less about than Pluto.

New Horizons is set to make a huge impact when it sends back the first-ever images of Pluto's surface, especially if it looks anything like the cloud-covered world in the artist's rendering above. My prediction: it's going to look like some weird cross-over of Mars and Earth, and we're really going to be wishing that we didn't have to wait another couple decades before the next mission to Pluto can arrive. And by the time it does, Pluto's atmosphere probably won't exist anymore—and may not exist again for another 200 years.

Now, New Horizons is about to show us a once-in-a-lifetime glimpse of a world with an atmosphere in the outer edges of the Solar System. This world has all the potential to shock us, to change our perspective on the Solar System, and to teach us something new about our place in the cosmos—here among all of the stars, the planets, and the underrated planetoids alike.

Check out the post-flyby followup here.

Also, 5 Life Lessons We Can All Learn From Pluto.

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