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Current Science at NOAO

Kuiper Belt: the Final (Solar) Frontier

Marc W. Buie, Lowell Observatory
Cindy Weehler, KPNO/NOAO RET

June, 2000  |  "It's like looking for a black cat in a dark room, that isn't really there." That's how planetary astronomers might describe the search for Kuiper Belt Objects (KBOs) which inhabit the outer regions of our solar system. Until recently the Kuiper Belt was only a theoretical region. About fifty years ago Gerard Kuiper proposed that there was as area beyond Neptune and Pluto that held short period comets. His theory was ignored until the 1970's because these objects are so hard to see. (They, like the planets, can reflect only the light they receive from the Sun but their distance is so much greater that they are millions of times fainter than what the human eye can see.) However, in 1992 the first KBO was discovered and approximately 250 have now been identified. This is an exciting, recent development in planetary astronomy because KBOs are believed to contain primitive material from the original formation of the solar system. By studying them we may be able to observe what matter in the early solar system was like before it formed into planets.

Caption: This diagram shows the locations of the known KBOs in relation to the outer planets' orbits. Astronomers are surveying and mapping these objects in the first step towards understanding them.

Dancing Partners

Kuiper Belt Objects are also fascinating because they orbit in the outer solar system, extending from Neptune to... well, it's not known how far. Understanding their movements and placement in the solar system will help us understand why the solar system formed as it did. They extend beyond Pluto, the ninth and last planet, which has an oddball orbit that is eccentric (elongated) and tilts at a 17-degree inclination to the otherwise almost flat plane of the solar system in which the other planets travel. In fact, Pluto's orbit is so eccentric that it is sometimes closer than Neptune, making it the eighth planet from the Sun. Pluto's orbit is due to a sort of dance step that two orbiting bodies choreograph when they are close together and one is much more massive than the other (hence, has more gravitational force). The larger body, in this case Neptune, has a strong gravitational pull on the smaller, Pluto, and would pull it out of a stable orbit if Pluto weren't orbiting twice for every three of Neptune's revolutions around the Sun. This synchronized periodic motion is called resonance and also applies to some- but, surprisingly, not all- of the Kuiper Belt objects that orbit near Neptune.

KBO or Planet?

Lowell Observatory on Mars Hill in Flagstaff, Arizona, is one of several institutions seeking to survey and map the Kuiper Belt's inhabitants. Most KBOs are probably a few hundred kilometers in diameter but some may be as big as Pluto. Can they be called planets too? As interesting as it is to speculate about, this illustrates why scientists feel they need more information before changing the definition of what a planet is, as some have proposed. A classification scheme is only as good as the data used to make it, and more is needed. Coincidentally, Lowell Observatory is also where Pluto, the most famous occupant of the Kuiper Belt, was discovered by Clyde Tombaugh in 1930.

How is it Done?

How would you find a small, dark, distant object at the edge of the solar system? At Lowell, astronomers Millis, Buie, and Wasserman are using a technique in which two frames are taken in one part of the sky at different times. The first picture is color-coded red; the second cyan (the colors green and blue combined). These are the three primary colors composing white light (as opposed to the red, blue and yellow of pigments) and when they are combined they form white light like that which we see in the Sun. When these two images are combined all the objects in the same place in both pictures appear white, while any object that moved appears red in one picture and cyan in the other. The beauty of this method is that it requires the exposure of only two frames on the telescope, allowing much more of the sky to be surveyed during a given night. Since time on telescopes is expensive and hard to come by, the more images that can be taken the faster large numbers of objects can be surveyed. This technique uses the natural color- distinguishing ability of the human brain to do the work of searching the data. Astronomers can then quickly analyze large amounts of data after the images are taken, freeing up telescope time for taking frames of more areas of the sky.

The Kitt Peak Connection

Teams of astronomers from Lowell Observatory, the Massachusetts Institute of Technology, and Ohio State University, are using the Kitt Peak National Observatory's Mosaic camera with the Mayall 4-m telescope on Kitt Peak to identify the KBOs. The Mosaic camera attaches to the telescope to record the image. It uses a charge-coupled device (CCD), which is a silicon chip with millions of light-sensitive detectors able to pick up whatever light is in the area of the sky the telescope is pointing to. Even with millions of light-sensors, a CCD is too small to capture much of the sky in each frame. The Mosaic, developed by astronomers at NOAO, consists of eight chips pieced together, vastly increasing the amount of sky that can be imaged in each frame. This allows 15 to 20 new KBOs to be discovered on any clear night. With this potentially large statistical sampling astronomers hope to determine the spatial distribution of KBOs, their relative proportions in various orbits, their physical properties, and what they can tell us about circumstellar dust disks around other stars.

How did it all start?

It is hoped that the Kuiper Belt is analogous to the planet forming circumstellar dust disks seen around other stars, which are also probably the remnants of solar system formation. They are in the same relative location as the Kuiper Belt and may be continuously supplied with material from colliding KBO-like objects in a model similar to our early solar system. Ironically, we can see the dust from collisions around distant stars more easily than we can in our own solar system. The asteroid belt, which is located between Earth and the Kuiper Belt, creates dust from its own collisions and blocks our view. By studying the dust disks around distant stars we may learn more about our own Kuiper Belt as well.

What does a KBO look like? Should you imagine a dark, sooty, comet-like object? A smaller version of spherical Pluto? A rocky, potato-shaped asteroid? Astronomers can only guess with the rest of us at this point.

But Why Do It?

The important goal now is to map as many KBOs as possible. Astronomers look back twenty years to what we knew of the asteroid belt between Mars and Jupiter when only 200 were known. This small sampling could not have described the dynamic distribution of these objects and the complexity of their orbits now revealed after mapping 50,000 of them. By discovering and mapping the orbits of 50,000 KBOs scientists hope to gain as much information about their dynamics.

Caption: The three graphs above plot the locations of asteroid orbits when a) only 200 were known, b) 2000 were known, and c) 50,000 were known. Eccentricity (y axis) is the degree of tilt that the object's orbit has to the plane of the solar system. Semi-major axis (x axis) can be thought of as relative distance from the Sun. The greater amount of data in c shows that asteroids cluster in families within the main asteroid belt (the dark clumps) and don't cluster in other areas (the light, vertical lines). This can't be detected in the first two graphs, which have fewer objects. The areas without asteroids show places where asteroids don't have a stable resonance with Jupiter, the large nearby planet whose gravity stirs up the asteroid belt.

Images taken with the WIYN telescope on Kitt Peak are being used to verify the Lowell group's KBO candidates, along with other investigators using their own telescopes. Orbit refinement will be the more time-consuming step in the necessary follow-up studies of this survey.

The investigators are: Marc Buie, Robert Millis and Larry Wasserman from Lowell Observatory; Jim Elliot from the Massachusetts Institute of Technology; and Mark Wagner of Ohio State University.

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