What Has the Kuiper Belt Taught Us About The Solar System?

Over 4 billion miles (6.7 billion km) from the Sun, the Kuiper Belt is a vast zone of frozen worlds we still know very little about. Image: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)

Today marks the 20th anniversary of the discovery of the first Kuiper Belt Object, 1992QB1. KBOs are distant and mostly tiny worlds made up of ice and rock that orbit the Sun at incredible distances, yet are still very much members of our Solar System. Since 1992 over 1,300 KBOs have been found, and with NASA’s New Horizons spacecraft speeding along to its July 2015 rendezvous with Pluto and Charon (which one could argue are technically the first KBOs ever found) and then onwards into the Belt, we will soon know much more about these far-flung denizens of deep space.

But how has the discovery of the Kuiper Belt — first proposed by Gerard Kuiper in 1951 (and in a fashion even earlier by Kenneth Edgeworth) — impacted our current understanding of the Solar System? New Horizons Principal Investigator Alan Stern from the Southwest Research Institute recently discussed this on his mission blog, “The PI’s Perspective.”

First, Stern lists some of the surprisingly diverse physical aspects of KBOs that have been discovered so far:

  • Some are red and some are gray;
  • The surfaces of some are covered in water ice, but others (like Pluto) have exotic volatile ices like methane and nitrogen;
  • Many have moons, though none with more known moons than Pluto;
  • Some are highly reflective (like Pluto), others have much darker surfaces;
  • Some have much lower densities than Pluto, meaning they are primarily made of ice. Pluto’s density is so high that we know its interior is about 70% rock in its interior; a few known KBOs are more dense than Pluto, and even rockier!

But although these features are fascinating in themselves, just begging for further exploration, Stern notes that there are three very important lessons that the Kuiper Belt has taught us about the Solar System:

1. Our planetary system is much larger than we had ever thought.

“In fact, we were largely unaware of the Kuiper Belt — the largest structure in our solar system — until it was discovered 20 years ago,”  Stern writes. “It’s akin to not having maps of the Earth that included the Pacific Ocean as recently as 1992!”

2. Planetary locations and orbits can change over time.

“This even creates whole flocks of migration of planets in some cases. We have firm evidence that many KBOs (including some large ones like Pluto), were born much closer to the Sun, in the region where the giant planets now orbit.”

3. Our solar system, and likely others as well, was very good at making small planets.

“Today we know of more than a dozen dwarf planets in the solar system, and those dwarfs already outnumber the number of gas giants and terrestrial planets combined. But it is estimated that the ultimate number of dwarf planets we will discover in the Kuiper Belt and beyond may well exceed 10,000. Who knew?”

And with a little jab at the whole Pluto-isn’t-a-planet topic, Stern asks: “And which class of planet is the misfit now?”

Read: Was Pluto Ever REALLY a Planet?

The discovery of the Kuiper Belt has shown us that our solar system — and very likely planetary systems across the galaxy, even the Universe — aren’t neat and tidy things that can be easily summed up with grade-school models or chalkboard diagrams. Instead they are incredibly diverse and dynamic, continually evolving and consisting of countless, varied worlds spanning enormous distances… yet still connected through the ever-present effects of gravity (not to mention the occasional-yet-unavoidable collision.)

“What an amazing set of paradigm shifts in our knowledge the Kuiper Belt has brought so far. Our quaint 1990s and earlier view of the solar system missed its largest structure!”

– Alan Stern, New Horizons Principal Investigator

Read more about the New Horizons mission here.

 The first KBO identified, 1992 QB1 (European Southern Observatory)

18 Replies to “What Has the Kuiper Belt Taught Us About The Solar System?”

  1. Not to pick nits, but Gerard Kuiper was only 25 years old in 1930. It
    was in 1950 that he first suggested that the solar system did not
    abruptly end at Pluto, and 1951 when he formally proposed the existence
    of an extensive reservoir of cometary bodies beyond Neptune.. Kenneth
    Edgeworth made a similar, but less well developed proposal in 1943, and
    other astronomers expressed similar thinking even earlier. In fact,
    Isaac Newton in Principia noted his belief that most comets came from
    the outer reaches of our solar system.

    1. That’s right, Kuiper’s proposal of an icy belt beyond Neptune was published in 1951. Pluto was discovered in ’30 — although not then thought to be a KBO — and I got the dates mixed. Repaired.

  2. Conventional understanding of planetesimal formation has been superseded by a paradigm shift, demonstrating that 100+ km planetesimals form rapidly by gravitational collapse, not slowly by accretion:
    David Nesvorny, A. N. Youdin , D. C. Richardson, 2010, Formation of Kuiper Belt Binaries by Gravitational Collapse, arXiv:1007.1465v1

    1. I don’t know the consensus of course, but this is new work. They themselves note they can’t predict every binary, for example Pluto-Charon. “Much work
      remains to determine the relative roles of GI [Gravitational Instability; new model] and HC [Hierarchical Coagulation; old model] in the Solar System and beyond.”

  3. These exciting revelations are always bittersweet: They make the universe more comprehensible to me, but they also further outdate all my astronomy books!
    (My first, The Golden Library of Knowledge, “The Moon” (1959), is good for a chuckle!)

  4. “And which class of planet is the misfit now?”

    Wrong question. Dwarf planets are misfits and they are not planets. 🙂 Keep trying, godlike Alan.

    1. 10,000+ > 8. I’m all on board the Pluto is not a planet/new classification/dwarf planet bus, but the numbers speak for themselves per literal definition of, “Misfit”. Correct is correct.

      1. Grains of sand on earth > 10 000 dwarfs.

        Correct is correct, but you haven’t noticed that godlike Alan pulled a little trick there. He’s trying to sell them as planets.

  5. Surely Stern would agree that the Oort cloud is the larger structure of the solar system? The again, it is outside the system boundaries that Voyager 1 is currently passing.

    I especially enjoyed the link to Cooper’s KBO model. It discusses how and why different KBO populations look differently, and add a nice astrobiological perspective on the habitability potential of these bodies. The Kuiper Belt with its populations and objects is where the models of the evolution of the outer protoplanetary disk (the Nice model) meet the models of the disk environment (the birth of the solar system).

    That can be especially seen in a new model of the solar system, where they uncouple the supernovae that seeds the molecular cloud with 60Fe from the star that created the shell that originated the Sun and its companions.

    It is very relaxed compared to earlier solar system formation models and very predictive to boot, especially on the Kuiper Belt properties. Besides predicting our CAI radioactive 60Fe and 26Al record, together with earlier work it predicts formation of “the Sun in a relatively small cluster … in agreement with dynamical requirements, i.e. stability of planetary orbits, existence of the Kuiper belt object Sedna, and formation of the Oort cloud (Adams 2010; Brasser et al. 2012).”

    When Stern navigates New Horizon to the Kuiper Belt, he can take comfort in that he is also navigating into observations of the very early dawn of our system.

    1. Thanks for bringing up the Oort cloud. After reading that there are possibly trillions of objects there, I wondered why it wouldn’t be considered the largest structure. The Kuiper Belt and Oort cloud may someday be the home of mankind, on the way to the stars.

  6. Great article, not to nit-pick, but LITERALLY, wasn’t Pluto the 1st KBO discovered? ….just sayin’….

    1. Stern doesn’t say, but what immediately comes to mind is the Nice model of our planetary disk evolution, and checking an encyclopedia entry on Pluto confirms this.

      The Nice model, with its modern elaborations of Grand Tack and Ejected Giant(s), is a very predictive model for the outer system. It predicts a lot of other observations but also the present inner edge of the Kuiper belt, its cold and hot populations, its occupied resonances and, I had forgotten this, specifically Pluto’s capture by Neptune into a resonant orbit.

      These family of models predicts that the planets and the Kuiper belt initially formed within ~ 30 au. The Grand Tack between Jupiter and Saturn stopped their inwards migration, predicting the size of Mars and the inner system, and the inner and outer asteroid belt populations. The two gas giants’ subsequent resonant scattering outwards with the ejection of 1 or 2 ice giants predicts an initially sufficiently thick disk, but also that the Kuiper belt objects were forced out to ~ 50 au.

      And that was when Pluto was caught in the resonance. “It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune’s migration perturbed it into a resonant capture.[122]”

      It is by the way awesome, I think, how the Nice model on the one end constrain the Kuiper belt inner edge out to where we find it and predicts Pluto’s orbit to boot. While on the other end supernova origin models of the initial cluster our Sun was born in constrain the Kuiper belt outer edge in to where the Nice model starts out and predicts Sedna’s orbit to boot. (See my previous comment on some of the latter. It may be a hot subject, that was the 2nd such paper I’ve seen in press release lately.)

      The system looks like a mess, and people used to look at the various orbital distances and put them in various relationship to see if it meant something. Then came various observatories and missions and transformed that into a profound (related to other system’s formation) and surprisingly complete picture.

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