Planetary Sciences News

Six Things Dwarf Planets Have Taught Us About the Solar System

It's been 10 years since Pluto was reclassified as a dwarf planet. But no matter the label, it and its dwarf planet cousins continue to stun researchers with their complexity.


Classrooms across the world received some bad news on 24 August 2006. Pluto—the celestial body discovered in 1930 and named by an 11-year-old girl, the “pizzas” in the planet mnemonic “My very educated mother just served us nine pizzas”—had been officially stricken from the solar system’s family of planets and reclassified as a “dwarf planet.”

The discovery of the slightly more massive object Eris inspired the International Astronomical Union’s (IAU) decision. Proponents of the change insisted that if Pluto got to keep the label “planet,” so too should similarly sized objects—like Ceres, for instance, which was then considered a large asteroid.

A dwarf planet, by IAU’s new definition, must directly orbit the Sun. It must be massive enough for gravity to pull it into a roughly spherical shape. But unlike regular planets, dwarf planets haven’t cleared other smaller celestial debris out of its orbital path.

A “Pearls Before Swine” comic strip by Stephan Pastis.
Pluto’s “demotion” has itself become a humorous meme in popular culture, as represented by this Pearls Before Swine comic strip. Credit: Stephan Pastis/UniversalUClick

As more objects got discovered, Pluto’s new label “dwarf planet” stuck. Then the jokes began.

Facebook group pages popped up, with snarky titles of “When I was your age, Pluto was a planet!” Angry Pluto enthusiasts wrote hate mail to astronomer Mike “Pluto Killer” Brown, one of the scientists who found Eris. Astrophysicist Neil deGrasse Tyson, a strong proponent of the dwarf planet label, received his own stream of hate mail from crushed 6-year-olds; Twitter users still hurl insults at him occasionally.

@neiltyson Quick question from a HS Student: What gives you the right to demote #pluto?  What discoveries have you made? #realtalk

— Matthew Brewer (@mbrewer_SL) February 4, 2016

Although some consider the reclassification a “demotion,” Pluto and its cousins Ceres, Makemake, Eris, Haumea, and others continue to dazzle scientists with their strange features and surprising geology. But perhaps more critically, these dwarf planets also trace a trail of scientific breadcrumbs that scientists can follow back in time to understand the origins of the solar system.

“I like to think of Pluto being the dwarf planet that showed us how the solar system’s architecture came to be,” said Renu Malhotra, a planetary scientist at University of Arizona’s Lunar and Planetary Laboratory.

Here are six such revelations about the solar system that we gained from studying dwarf planets.

1. Dwarf Planets Are as Complex as Regular Planets

When the New Horizons probe passed by Pluto more than a year ago, scientists found a complex system with areas of geologically young surface and evidence of active geology. Pluto, the images revealed, wasn’t just a chunk of rock orbiting in space. “Even I underestimated what we would find,” said Alan Stern, principal investigator of the New Horizons mission.

An image of Pluto’s diverse surface features.
A mosaic of Pluto’s complex surface taken by the New Horizons probe from about 15,000 kilometers away as it approached Pluto on 14 July 2015. Scientists are working to understand the origins of these unexpectedly diverse features. Credit: NASA/JHUAPL/SwRI

Pluto continues to stun scientists with its unexpected surface features, but its newly revealed complexity is just the beginning. Makemake has no atmosphere. Haumea spins faster than any other known large object in the solar system. Eris might have a thin, icy surface. Ceres hosts mysterious bright spots.

“The fact that these objects can be every bit as complicated as terrestrial planets is a headline,” Stern said. “It should be written in as big a point size as we can write it, because it was completely unexpected.”

2. Dwarf Planets Reveal Neptune’s Orbital Origins

Graphic representation of Pluto’s and Neptune’s orbits.
Scientists calculated that in the early solar system, Neptune migrated out to its current position and nudged Pluto into a resonance orbit. Although Pluto sometimes crosses Neptune’s orbital path, the resonance protects the two planets from colliding. Credit: NASA/JPL

By studying the particular orbital relationship between Pluto and Neptune, scientists figured out how Neptune got to its current position in the solar system. The two bodies are inextricably locked in an orbital resonance: Every time Neptune orbits the Sun three times, Pluto orbits twice, which means that even though Pluto may occasionally cross Neptune’s orbital path, they will never meet.

Scientists have always known about this resonance, but it was Malhotra who realized its significance. In a 1995 paper, Malhotra calculated that the only way Neptune and Pluto could have ended up in this resonance was if they both had formed closer to the Sun, then migrated out.

Scientists theorize that in the early days of the solar system, the gas giants, Jupiter, Saturn, Neptune, and Uranus, migrated inward toward the Sun and knocked out leftover debris. This gravitational push on planetary debris ended up changing the planets’ orbits as well, sending Neptune farther out. Neptune’s gravitational force encountered Pluto’s, and the two bodies pushed and pulled at each other until they fell into a resonance orbit. Astronomers detected the same effect in other bodies, including a new dwarf planet announced to the world this year.

3. Dwarf Planets Give Us a Peek into the Early Solar System

Dwarf planets are handy guides to the ancient solar system. For instance, all the Kuiper belt dwarf planets—Pluto, Haumea, Makemake, and Eris—have moons that scientists suspect formed from high-impact collisions, said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D. C. Haumea in particular is the only known Kuiper belt object to have a “family” that orbits along with Haumea and its moons, meaning that the debris kicked off by an impact long ago didn’t have enough energy to escape Haumea’s gravitational pull.

The presence of such moons is further evidence of an early period of “late heavy bombardment” of objects in the solar system. Scientists think that during this time, about 3.8–4 billion years ago, gravitational interactions between Jupiter, Saturn, and Neptune sent comets and asteroids sprawling across the solar system to collide with planets.

In the last 2 years, Ceres has also provided various windows into the past. In 2015, NASA’s Dawn probe headed to the dwarf planet after visiting the asteroid Vesta. There, scientists detected ammonia-rich clays in Ceres’s surface.

Ammonia itself isn’t stable at the temperatures found on Ceres (130–200 kelvins), but it is plentiful in the outer solar system. So how did the molecule get there? Scientists have formulated different hypotheses, said Carol Raymond, deputy principal investigator for Dawn. Either Ceres formed in the outer solar system, during its early days, and got kicked inward by a chaotic migration of the gas giants, or Ceres formed in the asteroid belt, and somehow ammonia-rich material from the outer solar system made its way inward.

Further study of Ceres will help clarify details of solar system formation, Raymond said.

4. Dwarf Planet Candidates Helped Scientists “Find” Planet 9

Thanks to a handful of debris orbiting farther away than Pluto, scientists this year found evidence that a rocky, Neptune-sized planet may lurk beyond the gaze of even our most powerful telescopes.

The story began in 2003, when Brown and his team at the California Institute of Technology (Caltech) discovered Sedna, a dwarf planet candidate that orbits far beyond the Kuiper belt, Pluto’s neighborhood of large, icy bodies 30 astronomical units (AU) away. Sedna maintains a steady orbit and comes within only 76 AU of the Sun at its closest approach.

Graphic representation of the orbits of several large objects and the proposed Planet 9.
The orbits of Planet 9 and the dwarf planets it supposedly influences. Scientists calculated that only a Neptune-sized planet could keep these objects in their peculiar, angled orbit. The diagram was created using WorldWide Telescope. Credit: Caltech/R. Hurt (IPAC)

Since then, scientists have spotted several more objects near Sedna, including 2012 VP113, found by Sheppard and colleague Chad Trujillo of Hawaii’s Gemini Observatory. The pair noticed that their new object and the rest of these far-away objects had similar, steady orbits.

Back at Caltech, after reading Sheppard’s and Trujillo’s work, Brown and his colleagues set out to find the cause of such clustering, and after many hours of poring over models and simulations, they officially proposed that only a planet-sized body could exert enough gravitational pull to keep the far-away cluster of dwarf-planet-sized objects in steady orbits. This hypothesized planet was deemed Planet 9 (sometimes called Planet X).

“Right now we’re doing surveys trying to find more dwarf planets,” Sheppard said. “If we find more and more of these, they can lead us to the much bigger, major Planet X.”

5. Ceres (We Hope) Will Help Us Understand Icy Ocean Moons

Kuiper belt dwarf planets aren’t the only thing keeping scientists busy. Dawn mission scientists recently discovered that regions of Ceres contain higher concentrations of carbonate minerals than anywhere outside of the Earth’s ocean floor. These minerals reveal that Ceres is like a “fossilized” ocean world, Raymond explained. They could be the remnants of a vast ocean that once existed on the dwarf planet.

In Ceres’s geologically young Occator crater, scientists figured out that mysterious bright patches come from sodium carbonate, a highly reflective mineral found in hydrothermal environments under Earth’s oceans. This means that at some point in Ceres’s history, hydrothermal processes must have pushed this material to the surface, Raymond said.

An image of the bright spot in Ceres’s Occator crater, taken by NASA’s Dawn probe.
Scientists found evidence of carbonate minerals in the bright spots of dwarf planet Ceres’s Occator crater. Stripes on the inset represent where the spectrometer frames lie, whereas red signifies a high abundance of carbonates and gray indicates a low abundance. These carbonate minerals mean that Ceres may have been covered once by an ocean. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/ASI/INAF

If hydrothermal processes are confirmed, Ceres’s surface may be analogous to the current seafloors underneath the solar system’s ice-covered moons. Astrobiologists yearn to peek below the icy shells of Jupiter’s and Saturn’s moons Europa and Enceladus because there are vast oceans underneath, and life needs water to proliferate.

Ceres is similar to these moons because 25% of the dwarf planet is water ice. Plus, its seafloor-type conditions are “where all of the elements necessary for habitability occur together,” Raymond said.

6. Dwarf Planets Are Prolific

Pluto holds a special place in the Internet’s collective heart but may not be so special in the solar system. Currently, there are six dwarf planets officially designated by the IAU: Pluto, Ceres, Eris, Makemake, Haumea, and 2015 RR245, discovered in July. Since scientists started looking deeper into the Kuiper belt, they have found at least 20 more similarly sized objects, Sheppard said.

A graphic representation of the officially designated dwarf planets.
There may be only six officially designated dwarf planets (missing from the image is 2015 RR245, announced this year), but many more dwarf-planet-sized objects exist. They may even be the dominant class of objects in the solar system. Credit: Konkoly Observatory/András Pál, Hungarian Astronomical Association/Iván Éder, NASA/JHUAPL/SwRI

And there may be dozens more out there. “We discovered that dwarf planets are the most populous class in the solar system,” Stern said. Other solar systems may be like ours, too, he added.

This population revelation, along with the surprising geological and atmospheric complexity found on dwarf planets, means that the field could be “at the very beginning of a paradigm shift and a revolution,” Stern said. Perhaps, he continued, it’s the classic large planets that are the “oddballs” of planetary formation.

He wonders, “Who’s the misfit now?”

—JoAnna Wendel, Staff Writer

Correction, 18 August 2016: This post was updated to reflect that dwarf planets may be common in other solar systems.

Citation: Wendel, J. (2016), Six things dwarf planets have taught us about the solar system, Eos, 97, Published on 17 August 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • Catie Gray

    I want to know who decided to name a dwarf planet Makemake? Because regardless of how its actually pronounced I’m always going to read it as Make-make.

  • davidlaing

    Much of this tends to substantiate a hypothesis I presented in my Earth science text “The Earth System” (1991, Wm C Brown, pp. 465-9). In this, I proposed that the nine Solar System planets (eight existing plus the asteroid belt representing a shattered planet) were formed inside the collapsing Solar Nebula BEFORE Sun commenced thermonuclear “burning.” Specifically, the protoplanets were “spun off” sequentially from Proto-Sun’s distended equator undergoing Poincare fission when it reached a critical speed of rotation determined by its density, which I assumed was about 1.6, the same as present Neptune, the first planet to be spun out. Fission would have slowed Proto-Sun’s rotation and transferred some of its angular momentum to proto-Neptune, whose spin rate would have accelerated along with Proto-Sun’s due to ongoing deposition in the collapsing Solar Nebula. Proto-Neptune’s outward-spiraling path, due to angular momentum transfer from the increasing spin rate of Proto-Sun, would thus have tended to clear some of the debris from the dust cloud. Proto-Neptune’s accelerating spin rate would, in turn, be responsible for the eventual fissioning of that body, which would have produced Neptune’s natural satellites, which lacked sufficient gravity to accumulate much dust and thus to develop accelerating spin rates, and so they remained spin-orbit coupled to Proto-Neptune.

    This same sequence would have been repeated eight times before Proto-Sun became large enough to commence thermonuclear burning, which put an end to fission events and blew away the first gaseous envelopes of the rocky planets. In the text, I explained how this would explain the fact that Sun retains a minuscule amount of the total Solar System’s angular momentum, the rest residing in the outward-migrating protoplanets, why the orbits of the true planets are nearly circular, coplanar, and geometrically spaced, why planetary atmospheres increase in molecular weight from Jupiter outward, and the Great Bombardment as the natural result of the fissioning of satellites from their primaries, producing debris that fell back on both bodies, disproportionally affecting the planet-facing sides of the satellites, thus greatly reducing the need for a high population of Solar System-wide “loose cannon” asteroids at that time.

    The present article adds several compatible observations to this scenario, and some not so much. First, the non-planetary bodies are in resonantly locked, but highly eccentric orbits that are not coplanar with the planetary orbits, signaling a formative mechanism that was distinct from the planets. Second, Haumea, with its rapid spin rate, is a highly distended oblate spheroid, near the critical fissioning ratio (about 0.6) for its density, and it is accompanied by a large number of natural satellites and much debris from the fissioning process. This leaves unanswered the question of how Haumea developed its rapid spin, but the existence of natural satellites on all other Oort Cloud planetoids suggests that nebular collapse could also have affected all these outer, non-planetary objects, as well. Still, Haumea’s unusual spin remains unaccountable, although impact, facile though it might be, might have something to do with it. Third, “Planet X” seems to me a bit of a stretch, especially so given its highly eccentric orbit about Sun, and fourth, “life on other worlds” is a bit tired, all known evidence pointing to life being an exclusive property of the Earth system.

    Al-in-all, a very thought-provoking article!

  • Several points need to be made here. First, the IAU did not coin the term “dwarf planet.” That was done by Alan Stern back in 1991. However, the IAU, or rather the four percent of the IAU who voted on this, did misuse Stern’s term by stating that dwarf planets are not planets at all but another type of object entirely. This makes absolutely no sense. Stern coined the term to designate a third class of planets in addition to terrestrials and jovians, not to create a class of non-planets. His intent was that the same way dwarf stars are still stars, and dwarf galaxies are still galaxies, dwarf planets are still planets.

    Dwarf planets have the same complexity, structure, and processes as their larger terrestrial counterparts. The only difference between the former and the latter is that dwarf planets are smaller.

    Second, the notion that classrooms around the world are required to follow the IAU’s dictates is false. The IAU defines terms only for its own use. The media erroneously acted as though their decree somehow changed reality and became an edict the entire world must follow. Not only did just four percent of the IAU vote on this, most of whom are not planetary scientists but other types of astronomers, but also, an equal number of professional planetary scientists immediately signed a formal petition rejecting the IAU decision–a petition led by Stern. This means the questions of what constitutes a planet and how many planets our solar system has remain matters of ongoing debate. Science is not determined by decrees from “on high.”

    The many amateur astronomers and members of the public who joined the dissenting professional planetary scientists, including via Facebook groups, did so based on rejection of the idea that an object has to clear its orbit to be considered a planet. We instead favor the geophysical planet definition, according to which a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or orbiting another planet. If an object is large enough and massive enough to be rounded by its own gravity (hydrostatic equilibrium), according to this definition, it is a planet. Therefore, Ceres, Pluto, Haumea, Makemake, and Eris are all planets of the dwarf planet subcategory.

    The media really needs to stop enabling Mike Brown in calling himself the “Pluto Killer.” Brown did not “kill” planet Pluto; he is not even a member of the IAU. What he did do was discover another dwarf planet.

    Item number four in your list is also problematic. First, no large Neptune-sized planet has been found in the outer solar system. Second, it is misleading and erroneous to refer to such a hypothetical planet as “Planet Nine,” as there is an ongoing debate about how many planets our solar system has, and according to advocates of the geophysical definition, we already have more than nine planets. Brown deliberately chose this term as a snub to those who reject the IAU definition and its subsequent claim that our solar system has only eight planets. Sheppard’s reference to this hypothetical world as “Planet X” is far more appropriate, as this has been the traditional term used to refer to an unknown but hypothesized planet.

    Finally, Dawn has shown that even Vesta, which is not quite in hydrostatic equilbrium but close to it, is far too complex to be called an asteroid. Objects like Vesta and Pallas should be assigned an intermediate category between asteroid and dwarf planet, such as proto-planet or sub-dwarf planet.

    • Ulrich 尼托 Knittel

      this is a very useful contribution, just the fact that we could end up with a dozen or more planets does not require the introduction of the term “dwarf-planet”, we also do not think about min-moon for the tiny things that orbit Jupiter etc.

  • Dagger

    “Other galaxies may be like ours, too, he added.”

    Not to be that guy, but I think he meant (or in fact said) “other solar systems.”

    • Rick Otto

      EOS uses science journalists instead of research scientists to write a number of their articles. Journalists tend to write more clearly than scientists, but might get a fact off now and then. I thought this was pretty interesting article.

      • Dagger