Exploring the Outer Solar System Jane Luu

Transcription

1 Oslo lect 1 1 Exploring the Outer Solar System Jane Luu When I was asked to give this talk, I did not know what I should talk about. Then I started thinking about Norway, which led naturally to some reading on the Vikings, and then I knew what I was going to say. What I will talk about is the story of the Kuiper Belt. The story should appeal to many people because it is a story about exploration with a happy ending -- but it might appeal particularly to Norwegians because Norwegians know something about exploration. With its long coastline and rugged interior, it is understandable that Norway produced many explorers. Probably the best known of these explorers, the one everyone has heard of, is Roald Amundsen. He is most famous as the first person to reach the North Pole, but I discovered recently that he also was the first person to sail through the Northwest passage, the waterway linking Europe with Asia. That is an impressive feat, but as I read more, it became clear that Amundsen was simply continuing the tradition from a long line of Norwegian explorers: the Vikings. The word Viking usually conjures up visions of big men in horned helmets, raiding and pillaging the coast of Europe. To my surprise, my reading revealed a much complex picture of the Vikings. Reports of Viking attacks mostly came from churches and monasteries, who were of course the main victims of these attacks because they had the largest concentrations of wealth at the time. Thus although the reports were surely true, they only told part of the story. Less biased reports would indicate that the Vikings were also settlers and traders, and that their ventures away from their homeland started as early as the year 800, while the rest of Europe was still in the Medieval Ages. They established commerce between Scandinavia and the rest of Europe, they settled Iceland and Greenland, crossing the ocean in modest open boats called knarr. We have all heard of Leif Eriksson, probably the first European to set foot on America, almost 500 yrs before Columbus. He was following in the footstep of his father, Erik the Red, who settled Greenland. There is no doubt that the Vikings terrorized Europe, but they were also fearless explorers. They headed into the stormy North Atlantic ocean in small open boats, risking their lives in search of new horizons, at a time when most of the world was unknown and unmapped. I am honored that their descendants are giving us a prize for the exploration of the outer solar system. I was lucky that when I started working in astronomy, I had the opportunity to work with someone almost as brave as the Vikings and fortunately less violent. I met Dave Jewitt when he was pondering about the outer solar system, and we set out on our exploration of the outer solar system. Here is how the whole thing happened. Unlike many astronomers, I did not study astronomy as an undergraduate; in fact, I was not aware of astronomy at all. There was no astronomy research at Stanford at the time, and the projects I worked on were always in the basement of the physics building, with no windows, so I was utterly unaware of heavenly bodies. After graduating from Stanford, I worked briefly at the Jet Propulsion Laboratory (JPL). Now the halls of JPL were filled with beautiful images of the planets and satellites that were sent back by various spacecraft, and for the first time, I really became aware of the contents in the solar system. The objects in the pictures looked so exotic, with

2 2 all kinds of mysterious surface features, and I remember thinking that it must be fun to study these beautiful things. So I asked a friend of mine who were the people whose job it was it was to study these images, and that was when I first heard of planetary science. This revelation stuck in my mind, and a year later, when I was applying to graduate schools in physics, I applied to the Earth, Atmospheric and Planetary Science department at MIT on a whim. Surprisingly, I was accepted, so I abandoned physics in the basement to study things in the sky. For the qualifying exam at MIT, I had to do two different research projects, so I went to talk to Dave Jewitt. I knew that he was a comet expert and thought I might do something with comets. That was when Dave told me about his idea of looking for things in the outer solar system. Dave s question was: why were the outer regions of the solar system so empty? The inner solar system (roughly speaking, the area from the Sun out to Jupiter) was quite full: it contained the terrestrial planets, vast numbers of asteroids and comets. But beyond Jupiter lay only the giant planets and the oddball planet Pluto, and 1 or 2 strange interplanetary bodies whose origin was unknown. As he saw it, there could be two reasonable explanations for the emptiness of the outer solar system: 1) The outer regions could be empty because of the gravitational influence of the giant planets. These large bodies were so good at ejecting anything nearby that everything that was near them was either deflected into the sun or other planets, or ejected completely from the solar system. Could these planets have cleared out everything between and beyond them? We did not know, and computers of the 1980 s were too slow to confirm the long-term stability of orbits among the giant planets. But it was a reasonable hypothesis that could not then be discarded. 2) Maybe outer solar system objects existed but they were too faint for surveys to date. We can see solar system bodies only because they reflect sunlight, so a solar system object becomes faint very quickly if you move it further away from the Sun. If you take an object from 1 Astronomical Unit which is equal to the Sun-Earth distance (also called AU for short) and is the yardstick we use to measure distances in the solar system -- and move it to 10 AU, it would not appear 10 times dimmer, but 10,000 times fainter than it did at 1 AU. This is because the brightness of solar system objects scales as 1/R 4, where R is the distance from the Sun. Thus bodies could exist between or beyond giant the planets, but they were so faint that no one had detected them. Dave was good at finding faint objects with a telescope. In 1982, while still a graduate student at Caltech, Dave had used one of the first charge-coupled devices (CCDs) on the Palomar 200-inch telescope to recover Comet Halley. Halley was still some four years before perihelion, and located at 11 AU, beyond Saturn. This was much too far from the Sun for its surface ices to evaporate, so no coma has yet developed, and the bare comet nucleus had the brightness 24th magnitude (for those of you who are familiar with the astronomical brightness scale). At that time it was by far the faintest object ever observed in the solar system. Dave thought that if he could see something as small as the nucleus of Halley beyond Saturn, maybe he could see objects at comparable or even further distances. Dave told me his idea of looking for objects beyond the giant planets and I had no idea how difficult it was going to be, but it sounded like fun so I said OK. But how were we going to find distant objects? At first we thought that the best way might be through their occultation of back-

3 Oslo lect 1 3 ground stars. In an occultation, a solar system object would move in front of a background star and temporarily block out its light (the star is occulted ). If you monitored the star s brightness, you would find that the star would dim very briefly, then return to its original brightness after the object had passed by. The duration of this dip in the star s brightness is an indication of how big the occulting body is; this technique has long been used for astronomers to measure the size of nearby asteroids. In our case, we just wanted to use occultations to indicate the presence of distant objects; size was a secondary issue. The whippoorwills of Arizona quickly put an end to our occultation strategy. For those who are not familiar with them, whippoorwills are nocturnal birds who live in eastern and southwestern US. Their coloring makes them hard to see during day, so they are almost never seen during daytime, but they are very active and vocal -- at night. We were observing at Kitt Peak, Arizona, at the time, and a disappointing phone call to the Arizona Audubon Society revealed that we might detect whippoorwills flying over Kitt Peak more often than we might outer solar system objects. Never mind the technical challenges of the occultation technique (short duration, rare events), occultations by distant objects were going to be too similar to occultations caused by birds. We had to come up with a new plan. We had no predictions for the numbers or brightness (or even the existence) of what we were looking for, so we decided to use two methods for our search. First, we would photograph large patches of the sky using conventional glass plates with Schmidt telescopes, with a brightness limit of 20th magnitude. This would allow us to find relatively bright objects over a large area of the sky (brightness limit: magnitude 20). At the same time, we would also use a CCD camera on the MIT 1.3m telescope on Kitt Peak, Arizona, to cover tiny patches of the sky to find much fainter things (brightness limit: magnitude 24). Both CCDs and photographic plates have their pluses and minuses. The main advantage of photographic plates is that they are big, each covering a large area of the sky. The photographic plate we used covered 25 square degrees, or about the size of 100 full moons. In contrast, CCDs were much more sensitive 100x more sensitive than photographic plates but each CCD image would only cover a tiny patch of the sky (about 1000x smaller than the area covered by a photographic plate). By using both plates and CCDs, we could detect rare, bright objects with the photographic plates and abundant, faint objects with the CCD. Or that was what we were hoping. We would distinguish distant objects from nearby objects (e.g., ordinary asteroids) by their speed, since nearby objects moved faster than distant objects. Based on this speed limitation, we named our project the Slow-Moving Object (SMO) Survey. Every sky survey is now done with CCDs, because astronomers now have access to cameras with as many as 1 billion pixels, but back then, CCDs were quite small; ours had 512 x 512 pixels, and due to vignetting (obscuration at the edges), only the central part was usable. The idea of surveying the sky with a tiny CCD, without guaranteed telescope time, was thus radical. I remember asking Dave, Isn t it crazy to do a large survey with a CCD? His answer: If we don t do it, who will? Whether with photographic plates or CCD images, the technique to find moving objects is the same: one simply looks at a sequence of images, consisting of 2 or more images, very rapidly, so that any object that moved between image would stand out. This process is called blinking.

4 4 With photographic plates the entire image analysis process was especially painful, since everything was done manually. The plates first had to be developed in a dark room. Then, to analyze them, we had to return to Kitt Peak Observatory for extended visits to use their plate comparator. This was a machine that would let you compare 2 photographic plates in quick succession, the process called blinking. The user put two photographic plates of the same star field on the left and right side of the comparator, then looked through an eyepiece in the middle. The comparator switched back and forth between the two plates, allowing the user to see the plates in quick succession. Stationary things like stars and galaxies remained in the same spot on both plates, but solar system objects moved so they would appear to jump from one spot to another when comparing the two plates. This blinking process is the traditional way to identify solar system objects. We would do this for hours until every dot on the plates was examined. The whole ordeal was so painful that after completing the first part of our survey, we decided to use only CCDs for the rest of our search. In contrast, blinking was much easier with a CCD, as you will shortly see. At this point I have to digress and give you a bit of history about what people thought of the trans-neptunian solar system at the time. The truth was, hardly anybody was thinking about it at all. Forty years earlier, Kenneth Edgeworth (1949), and Gerard Kuiper (1951) speculated that, since the solar nebula most likely did not end abruptly at Neptune or Pluto, there might still be a swarm of small bodies beyond Neptune, the leftover from the planet formation epoch, so to speak. These objects could not be planet-size because the collision time at such large distances from the Sun would be too long to form big things, so they would be smaller, perhaps much like comets. However, Kuiper also thought that this trans-neptunian population would no longer exist, having been depleted by the gravitational influence of Pluto, which he erroneously thought to be a massive body. For both Edgeworth and Kuiper, these speculations were not based on any calculation or model; they were little more than educated guesses. Both Edgeworth s and Kuiper s speculations went largely unnoticed but theorists sporadically revisited them in connection to the problem of the origin of the short-period comets (Whipple 1964, Safronov 1972, Fernandez 1980, Bailey 1983, Duncan, Quinn, and Tremaine 1988). The problem is the following. Since 1950, it was known that long-period comets (those taking longer than 200 yrs to go around the Sun) originated in the Oort cloud, a large, spherical cloud located at roughly 50,000 AU (Oort 1950). Based on the long-period comets large, randomly oriented orbits, Oort inferred that they comets must come from a spherical cloud of comets about 10 5 AU away. At this kind of distance, the Sun s gravity is so weak that these comets are susceptible to the gravitational influence of passing stars and the Milky Way tides, and these perturbations slowly deflect the comet orbits and send them toward the Sun. It was thought the short-period comets (orbital period < 200 years) might have originally been long-period comets that were captured into smaller orbits by the giant planets, primarily Jupiter, but this was never proven. These theoretical works had little effect on us; we were unaware of them when we started our search. Furthermore, they yielded no testable predictions: no prediction as to how many comets there might be beyond Neptune, whether they were bright enough for detection, etc. What motivated us was not any theory, but actually an anti-theory: we did not quite believe the accepted view that the outer solar system was empty. We detected no SMO after the first year of searching, and we published our null results (Luu and Jewitt 1988). But we were not about to stop

5 Oslo lect 1 5 searching, because we were moving to Hawaii. In 1988, Dave moved to the University of Hawaii, and I followed him there, while remaining a graduate student at MIT. Our stomping ground was now the summit of Mauna Kea, the tallest mountain in the world (although most of it is underwater). We continued our search for SMOs at the University of Hawaii 2.2m telescope, a bigger telescope than what we used to have. The observing conditions of Mauna Kea were so good that a 2m telescope there was as powerful as a 4m telescope at most other sites on the mainland. We worked on many different projects, but continued the SMO Survey. For those of you who think a life of an astronomer is glamorous, it is not. A more accurate representation of our life is the many long cold nights in a control room on top of a mountain in the middle of nowhere. Because of its altitude (13,796 ft, or 4205 m) and location in the middle of the ocean, the air above Mauna Kea is exceptionally stable and dry, creating a wonderful place for observing, but observing on Mauna Kea was not pleasant. The oxygen content of the air at the summit is 40% lower than at sea level. Besides being deprived of oxygen and becoming steadily dehydrated, there was also a lot of sitting and waiting. There was waiting for the weather to clear up contrary to the typical image of Hawaii, it snows on Mauna Kea, hence its name, which means white mountain or waiting for the long exposures to end. We were glad to have access to Mauna Kea, but we were also very happy to leave the mountain at the end of each observing run. Our search was not popular. Telescope time committees and funding agencies did not look favorably upon searches for things that were not predicted by any theory. Even our friends and colleagues often asked us when we were going to quit, to which we usually answered, We will quit when we have covered 1 square degree. One square degree is about the size of 4 full moons, which is not very big compared to the whole sky, but was still a considerable area to cover with our tiny detectors. So we procured telescope time for our search any way we could. We would write telescope proposals saying we would do work on another project, then use some of the time for our search. This went on for years. In the meantime, CCDs slowly got bigger, and each time we had a new, larger CCD, we would hope that this time we would find something. Late summer of 1992, there was a new camera on the University of Hawaii telescope: a CCD camera with 2048x2048 pixels, 20 times more pixels than our first device, built by Gerry Luppino, a graduate student we had known at MIT. The first time we went observing with this camera, we succeeded in finding a Kuiper Belt object. It was on August 30th, 1992, almost exactly 20 yrs ago. Figure 1 shows the four discovery images from that night, the time of each image is indicated. As was our strategy, we took four images of the same area of the sky, spaced out over a few hours. We needed at least 3 or 4 images to confirm any detection, because there were many possibilities for image artifacts, such as cosmic rays, that could mimic SMOs. We had taken the first two of the sequence and Dave was blinking them on a computer while the third exposure was underway. I did not like blinking with just two images because of the number of false alarms, but Dave liked doing it. A candidate quickly caught Dave s eye, and he alerted me. The candidate was moving slowly, in

6 6 the right direction: 2.6 arcsec/s West and 1.1 arcsec/s South. It was faint (roughly 23rd magnitude), so it was unlikely it had been seen before, yet it was bright enough that we knew it was not due to noise in the image. Still, it could have been almost anything. And after so many years of searching, we were cautious. The third image came out, and the object was still there, still moving in the same direction, at the same speed. Then came the fourth image: the slow, westward moving object was there again. The object was real. Figure 1. Discovery images of Kuiper Belt object 1992 QB1 on 30 Aug The times of observation are indicated. Figure 2. Announcement of the discovery of 1992 QB1. The object s speed allowed us to calculate that its distance was 41 AU, far beyond Neptune. Based on this distance and the brightness of the object, we estimated its size to be about 250 km. We knew we had covered 0.7 sq deg before finding this object, and this allowed us to estimate how many similar objects there were in total, if these things occupied a belt in the plane of the solar system. There were thousands more like it awaiting discovery. The new object came to be known as 1992 QB1, a name determined by the order of its discovery in 1992 (Jewitt and Luu 1993). Fig. 2 shows the announcement of its discovery. The trans-neptunian population to which QB1 belongs is now called the Kuiper belt, even though Kuiper anti-predicted it. Our excitement was not shared by everyone: A lot of people were skeptical about our discoveries: Brian Marsden, then the director of the IAU Minor Planet Center, made a $500 bet with us saying that QB1 was most likely a long-period comet; with a size of 250 km, it would be the comet of the century! He subsequently lost his bet and graciously paid up four years later. Other people told us that maybe 1992 QB1 was the only one of its kind, and that we were lucky. We did not think so: it was unlikely that we had found the only existing Kuiper Belt object

7 Oslo lect 1 7 (KBO) in the solar system after covering an area not much bigger than the size of the Moon. These criticisms did not bother us much because we were confident in our ability to discover more objects like 1992 QB1. We had broken through a perception barrier; finding other KBOs would be easy. Sure enough, we found the next KBO on the next observing run, and from then on, more and more at every subsequent observing run. By now more than 1500 KBOs have been identified as a result of many searches by many people. From their orbits, we know that the Kuiper Belt is a thick ring (true thickness 25 o 30 o ) of bodies extending roughly from Neptune s orbit at 30 AU to at least 1000 AU (Jewitt, Luu, and Chen 1996). We estimated about 70,000 KBOs larger than 100 km in size, and hundreds of millions KBOs down to 1 km in size. Several Pluto-size bodies had been discovered (e.g., Brown 2008). The Kuiper Belt has emerged as a new frontier in astronomy, scientifically important on several different levels, not the least of which was solving the mystery of the origin of the shortperiod comets. What if we had not found anything? We would have been a bit disappointed, certainly, but the time we put into our survey would not have been wasted. That is because the original goal of our survey was NOT to look for the Kuiper Belt itself, but really to verify whether the outer Solar System was as empty as it seemed. This is an important point, because we had no preconception of what the outer solar system might contain. If there was nothing beyond Neptune (besides Pluto), we would have found this out. And this data point would have been useful, because then someone would have to explain why the outer solar system was so empty. But it is always more fun to find something rather than nothing, not to mention receiving awards like the Kavli prize. Figure 3. Semimajor axis vs. eccentricity for Kuiper Belt objects known in Figure from Jewitt and

8 8 Luu (1997). As astronomers continued to probe the Belt, they found plenty of surprises. I will not go into these surprises that is Dave s talk but I will mention the finding that probably was most influential in Pluto s demotion from planethood. Fig. 3 is a plot from one of our older papers (Jewitt and Luu 1997), using data from 1996, only 4 yrs after our first discovery. At this point in time, we had discovered about 30 objects, and the Figure plotted their semimajor axis vs. eccentricity. (Semimajor axis indicates how large the orbit is, and eccentricity how elliptical the orbit is). Pluto is indicated by the red cross. The long, bracketed regions on the plot are the locations of the resonances, and the names of the resonances are indicated: 3:2, 2:1, and so on. Pluto is in the 3:2 resonance. Objects in the resonance have a special dynamical relationship with Neptune. In Pluto s case, it completes two orbits around the Sun for every three orbits that Neptune completes around the Sun. Similarly, objects in the 4:3 resonance complete 3 orbits around the Sun for every 4 orbits of Neptune, and so on. These are very precise mathematical configurations and they have special consequences. The orbit of Pluto actually crosses Neptune s path, but like trains in a fancy train set, they never collide. This special property also holds for every object who shares the resonance with Pluto, so dynamically, these objects -- who are called Plutinos to emphasize their close relationship -- are identical to Pluto. Pluto s only difference from the other Plutinos is its large size. In my mind, this plot, more than any argument about size, shows very clearly that Pluto is more usefully viewed as a large KBO. As Pluto s planethood became more precarious with each new result from the Kuiper Belt, many people, including astronomers, became upset about the prospect of Pluto being stricken from the planet list. Why did people care so much about Pluto being a planet? Nostalgia. Some people felt betrayed by astronomers: they were once told that Pluto was a planet, and now they are told it isn t. Why should they believe astronomers now? The obvious answer is: It is our job as scientists to break down old ideas and replace them with new ideas that fit observations better. This is called scientific progress. When we find new ideas that fit observations better than the previous idea, there is nothing else to do but discard the old idea. Many people, including astronomers, said demoting Pluto would be disrespectful to Tombaugh, to the public, especially children, since kids like Pluto. We do not want to hurt anybody s feelings, especially children s, but science does not depend on the public s feeling. In short, all data clearly reveal that Pluto was misclassified as a planet: it is a large but otherwise unremarkable KBO. Calling Pluto a planet adds nothing to our understanding of its nature or origin, and misleads the public in its understanding of planet formation (Jewitt and Luu 2007). I hope I have conveyed the story of the Kuiper Belt as a story of exploration. The Vikings started the European tradition of exploring the world back in the 9 th century. In the 15th century, the rest of Europe caught on, sending out a series of explorers to map the world, establishing contact between different continents. I like to think that we, as 20 th - 21 st century astronomers, are following these explorers footsteps. In expanding the boundaries of our solar system, in searching for extrasolar planets, we are mapping the universe and making the connection between our own solar system and other planetary systems. I hope that we will find hospitable new worlds, maybe even soon establish contact between Earth and another world.

9 Oslo lect 1 9 References M. Bailey (1983). The structure and evolution of the solar system comet cloud. Monthly Notices of the Royal Astronomical Society, 204, Brown, M. E. (2008). The Largest Kuiper Belt Objects. In The Solar System Beyond Neptune, M. A. Barucci, H. Boehnhardt, D. P. Cruikshank, and A. Morbidelli (eds.), University of Arizona Press, Tucson, M. Duncan, T. Quinn, and S. Tremaine (1988). The origin of short-period comets. Astrophysical Journal Letters, 328, L69-L73. K. E. Edgeworth (1949). The origin and evolution of the Solar System. Monthly Notices of the Royal Astronomical Society, 109, J. A. Fernandez (1980). On the existence of a comet belt beyond Neptune. Monthly Notices of the Royal Astronomical Society, 192, D. C. Jewitt and J. X. Luu (1993). "Discovery of the Candidate Kuiper Belt Object 1992 QB1." Nature, 362, D. Jewitt, J. Luu and J. Chen (1996). "The Mauna Kea - Cerro Tololo (MKCT) Kuiper Belt and Centaur Survey." Astronomical Journal, 112, D. Jewitt and J. Luu (1997). "The Kuiper Belt." Invited review for the Stardust to Planetesimals Symposium, Santa Clara, California, Edited by Y. Pendleton and A. Tielens, San Francisco, pp D. Jewitt and J. Luu (2007). On Pluto, Perception and Planetary Politics. Daedalus, 136, P. C. Joss (1973). On the Origin of Short-period Comets. Astronomy and Astrophysics, 25, 271. G. P. Kuiper (1951). On the Origin of the Solar System. Proceedings of the National Academy of Sciences of the United States of America, Vol. 37, Issue 1, J. X. Luu and D. C. Jewitt (1988). "A Two Part Search for Slow Moving Objects." Astronomical Journal, 95, J. H. Oort (1950). The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin. Bulletin of the Astronomical Institutes of the Netherlands, Vol. 11, p V. S. Safronov (1972). In The Motion, Evolution of Orbits, and Origin of Comets, edited by C. A. Chebotarev and E. I. Kazimirchak-Polonskaya (Reidel, Dordrecht), pp F. L. Whipple (1964). Evidence for a Comet Belt beyond Neptune. Proceedings of the National Academy of Sciences of the United States of America, Vol. 51, Issue 5, pp