December 8th: What is the Kuiper Belt?

Date: December 8, 2009

Title: What is the Kuiper Belt?

Podcaster: Susan, Amanda and Kevin Murph


Description: Susan Murph from the How to Grow Your Geek Podcast and her children Amanda and Kevin discuss Pluto and the Kuiper Belt.

Bio: Susan is also a life-long sci-fi, fantasy and science geek, and loves to incorporate her favorite hobbies into her current career of raising her two kids as a stay-at-home mom. She believes that including her kids in her hobbies not only strengthens her relationship with them, but also benefits their development of useful skills such as critical thinking, logic, creativity and reading comprehension, just to name a few. Susan currently hosts and produces the “How to Grow Your Geek” podcast.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Rossiter & Associates (, a process improvement consulting firm specializing in energy efficiency for oil refining, petrochemicals and chemical operations. Rossiter & Associates – Energy Efficiency by Design.


Kuiper Belt Objects – What are they?

Susan: Hi, my name is Susan Murph, and I am the host of How to Grow Your Geek: Parenting and More! My daughter Amanda and I enjoy studying space and astronomy, and my son Kevin is now taking a really cool astronomy class at school that has gotten him very interested as well, so he wanted to help too. Today, in honor of the birthday of Gerard Kuiper, we wanted to talk about the belt of objects that bear his name.
Kevin, what is the last object in the solar system?

Kevin: Is it Pluto?

Susan: That was what we thought . But ever since the discovery of Pluto in 1930, there were a few people who had the idea that perhaps Pluto was not alone out there.

Amanda: What did they think was out there with Pluto?

Susan: That is a good question, and the answer begins in 1943, with an article in the Journal of the British Astronomical Association. Kenneth Edgeworth hypothesized that, when our solar system first formed, in the region beyond Neptune, the material was too spread out to condense into planets, and instead condensed into a lot of smaller bodies. He also believed that from time to time, some of those bodies enters the inner solar system, becoming what we call comets.

Kevin: So there are a lot of small bits out there? Like a ring?

Susan: That is a great question, and some people thought that might be true. In 1951, in an article for the journal Astrophysics, Gerard Kuiper wondered if a disc of material had formed early in the Solar System’s evolution. However, he did not believe that such a belt still existed. Kuiper was operating on the assumption common in his time, that Pluto was the size of the Earth, and had therefore scattered these bodies out of the Solar System. We now know that Pluto is much smaller, so that changed the possibility of Pluto cleaning out its orbit. Plus, we have started to find these bodies, some even closer than Neptune.

Amanda: We have found chunks of material inside the solar system? Where?

Susan: In 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. Today we know that there is an entire population of comet-like bodies, called centaurs, that exist between Jupiter and Neptune. The centaurs’ orbits are unstable and have dynamical lifetimes of a few million years. From the time of Chiron’s discovery, astronomers speculated that they therefore must be frequently replenished from somewhere. Further evidence for the belt’s existence later emerged from the study of comets.

Kevin: What have we learned from studying comets?

Susan: We know that comets only last a short period of time compared to stars and planets. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space, eating them gradually away. In order for us to still see comets in the Solar System, there must constantly be a new supply of comets coming from somewhere. One such area of replenishment is the Oort cloud; which is believed to be the point of origin for long period comets, those, like Hale-Bopp, with orbits lasting thousands of years between visits close to Earth.

Amanda: But wait, aren’t there comets that come back a lot more often than that? Like Halley’s Comet?

Susan: There is another group of comets called short period or periodic comets; those, like Halley, with orbits lasting less than 200 years. But these comets were confusing. Astronomer Julio Fernandez speculated that a much closer comet belt would be required to account for the observed number of short period comets. Then in 1988, the Canadian team of Martin Duncan, Tom Quinn and Scott Tremaine ran a number of computer simulations to determine if all observed comets could have arrived from the Oort cloud. They found that the Oort cloud could not account for all of the short-period comets. With a belt as Fernandez described it added to the formulations, the simulations matched observations. Reportedly because the words “Kuiper” and “comet belt” appeared in the opening sentence of Fernandez’s paper, Tremaine named this theoretical region the “Kuiper belt.” But so far, the math was the only evidence they had.

Kevin: When did that change?

Susan: In 1987, astronomer David Jewitt, then at MIT, became increasingly puzzled by “the apparent emptiness of the outer Solar System.” He encouraged then-graduate student Jane Luu to aid him in his endeavor to locate another object beyond Pluto’s orbit, because, as he told her, “If we don’t, nobody will.” Finally, after five years of searching, on August 30, 1992, Jewitt and Luu announced the “Discovery of the candidate Kuiper belt object” 1992 QB1; Six months later, they discovered a second object in the region, 1993 FW. So the discovery of the inhabitants of the Kuiper belt had begun!

Amanda: So, what is the Kuiper Belt made of? I would think that way out there it would be very cold!

Susan: You would be right! Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices; a mixture of light hydrocarbons (such as methane), ammonia, and water ice, a composition they share with comets. The low densities observed in those KBOs whose diameter is known, is consistent with an icy makeup. The temperature of the belt is only about 50K, so many compounds that would be gaseous closer to the Sun remain solidly frozen.

Kevin: So what about Pluto? I thought it was part of this Kuiper belt?

Susan: And it is! The discovery of these large KBOs in similar orbits to Pluto led many to conclude that Pluto was not particularly different from other members of the Kuiper belt. Not only were these objects close to Pluto in size, but many also possessed satellites, and were made of similar materials such as methane and carbon monoxide. Thus, just as Ceres was considered a planet before the discovery of its fellow asteroids in the belt between Mars and Jupiter, some began to suggest that Pluto might also be reclassified into a category with its fellow Kuiper belt objects.

Amanda: So that is what made everyone argue so much about whether Pluto is a planet or not?

Susan: Actually, the issue was brought to a head by the discovery of Eris in the scattered disc far beyond the Kuiper belt, which turned out to be 27 percent more massive than Pluto. The question came up whether or not Eris would be considered a planet. In response, the International Astronomical Union (IAU), was forced to define a planet for the first time. After many discussions, they included in their definition that a planet must have “cleared the neighborhood around its orbit.” As Pluto shared its orbit with so many KBOs, it was deemed not to have cleared its orbit, and was thus reclassified from a planet to a dwarf planet, at least for now.

Kevin: So what do we call all of these things?

Susan: Good question Pluto, Eris, and all of the other objects outside Neptune’s orbit are called collectively Trans-neptunian objects. That includes the Kuiper belt objects, the scattered disc, and all the dwarf planets except for Ceres, which lives in the asteroid belt between Jupiter and Mars.

Amanda: What are the dwarf planets we know about so far?

Susan: As of 2009, only five objects in the Solar System, Ceres, Pluto, Eris, Makemake and Haumea, are considered dwarf planets. However, a number of other Kuiper belt objects are also large enough to be spherical and could be classified as dwarf planets in the future.

Kevin: Hey wait, doesn’t Pluto have a moon?

Susan: Yes, Pluto has its satellite Charon. Eris, Pluto, and Haumea possess satellites, and two have more than one. A higher percentage of the largest KBOs possess satellites than the smaller objects in the Kuiper belt, suggesting that a different formation mechanism was responsible. There are also a high number of binaries, which are two objects close enough in mass to be orbiting “each other,” in the Kuiper belt. The most notable example is the Pluto-Charon binary.

Kevin: Are we studying the Kuiper Belt right now with a spaceship?

Susan: Great question – On January 19, 2006 the first spacecraft mission to explore the Kuiper belt, New Horizons, was launched. The mission, headed by Alan Stern of the Southwest Research Institute, will arrive at Pluto on July 14, 2015 and then continue to study other Kuiper belt objects. Any KBO chosen will be between 25 and 55 miles in diameter and, ideally, white or gray, to contrast with Pluto’s reddish color. John Spencer, an astronomer on the New Horizons mission team, says they haven’t yet chosen another target, as they are awaiting data from the Pan-STARRS survey project to ensure as wide a field of options as possible. The Pan-STARRS project, due to come fully online by 2009, will survey the entire sky with four 1.4 gigapixel digital cameras to detect any moving objects, from objects close to Earth all the way out to KBOs.

Amanda: Do other stars have Kuiper-like belts?

As of 2009, astronomers have seen dust disks believed to be Kuiper belt-like structures around nine stars other than the Sun. They appear to fall into two categories: wide belts, with radii of over 50 AU, and narrow belts (like our own Kuiper belt) with diameters of between 20 and 30 AU and relatively sharp boundaries. Most known debris discs around other stars are fairly young, but a few we have seen with the Hubble Space Telescope are old enough (roughly 300 million years) to have settled into stable configurations. Beyond this, 15-20% of solar-type stars have an observed infrared excess which is believed to indicate massive Kuiper Belt-like structures.

So yes, we think that these may exist in other solar systems, and we look forward to studying them as well as our own!

Amanda: We hope you have enjoyed our presentation, and if you would like to hear more from us, please visit

Kevin: Thanks so much, and please have a great Year of Astronomy!

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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