2012 Astrophysics Prize Explanatory Notes
Ever since humans first looked up at the night sky, we’ve been fascinated by the planets. So much so that we invented human characters for them: seductive Venus, war-like Mars, majestic Jupiter. Although there have been a few additions to the family in recent centuries, these wanderers among the stars have remained a constant in our lives. Then, two decades ago, everything started to get more, well, complicated. The Kavli Prize for Astrophysics this year honours the individuals who showed that there is much more to the solar system and whose work prompted astronomers to re-evaluate what it means to be a planet.
Kuiper Belt Objects. Artwork of two icy dwarf planets orbiting within the Kuiper Belt of the outer solar system. The Sun is at upper left. The Kuiper Belt consists of a numerous collection of small icy bodies that mostly orbit beyond the planets, but in the same plane as them. The Kuiper Belt extends outwards from the orbit of Neptune at 30 AU (an AU is the Earth-Sun distance) past the orbit of Pluto out to about 50 AU. Credit: Mark Garlick/Science Photo Library
Prior to their discoveries, the outer solar system beyond the orbit of Pluto had seemed a vast empty void out of which a comet would occasionally swoop down for a brief visit. In the 1980s David Jewitt, an astronomer specialising in the primitive bodies of the solar system such as comets, wondered why the outer solar system seemed so empty. He figured that either the strong gravitational fields of the giant planets had cleared all small bodies from the vicinity or that anything out there was too small, distant and dark to be seen. So in 1986 Jewitt, then working at the Massachusetts Institute of Technology, resolved to find out and enlisted the help of Jane Luu, a graduate student in his department who was looking for a new research project. Searching for slow-moving, nearly invisible objects in the outer solar system was a laborious and thankless task that most astronomers didn’t think was worth the effort. CCD detectors existed at the time but only had a narrow field of view. So Jewitt and Luu carried out two parallel surveys: they used the Palomar Observatory’s Schmidt telescope equipped with conventional glass photographic plates to scan large areas of the sky for the very faintest objects, while also watching a narrow field of view in the plane of the planets for rare but slightly brighter objects using MIT’s 1.3-metre telescope fitted with a CCD.
To find moving objects against a background of stars astronomers take two or more pictures of the same patch of sky at different times. They then study pairs of images in a device called a blink comparator that allows them to quickly switch their view from one image to the next, known as “blinking.” Stars in the pictures do not move so will appear unchanged after each blink, but a solar system object, which will have shifted in the time between the two images being taken, will appear to hop from one position to another with each blink making it easy to spot. Clyde Tombaugh used this technique to discover Pluto in 1930.
By the end of 1987, Jewitt and Luu had studied a huge number of plates and found nothing. Exhausted by the process, they abandoned the plates and focused their efforts on using CCDs. In the years that followed they won time on a variety of telescopes, including one at Kitt Peak National Observatory in Arizona, the Cerro Tololo Inter-American Observatory in Chile, and the University of Hawaii’s 2.24 m telescope at Mauna Kea, and got access to better and better CCDs that increased their probability of finding something. They never won observing time on the largest telescopes because their quest was considered so hopeless. Colleagues often asked when they were going to give up their futile search. In 1992 the University of Hawaii’s telescope was fitted with a new CCD that was twice as sensitive and had four times the field of view. Jewitt won some time on it and on only their second night, 30 August, they spotted something. Jewitt was blinking two images (now done on a computer rather than a comparator) when he spotted a faint dot that appeared to move. It was going at around the right speed, in the right direction, and didn’t look like a blip caused by something like a cosmic ray hitting the camera. They waited anxiously for third and fourth images of the same area to arrive and sure enough the dot continued on its path across the sky.
Given the official name of 1992 QB1, Jewitt and Luu dubbed it “Smiley” after the enigmatic spy in John Le Carré’s novels. Calculations showed that Smiley was distant from the Sun by 44 astronomical units (an AU is the distance from the Sun to the Earth)—far beyond Neptune’s orbit at 30 AU—and was 280 kilometres across, one-eighth the diameter of Pluto. Six months later Jewitt and Luu found another object they named “Karla,” after Smiley’s Soviet nemesis. Others soon followed, and before long they ran out of Le Carré-themed names.
Jewitt and Luu’s success caused others to join the hunt and won them access to bigger telescopes. Soon the trickle of new objects became a flood and a new region of the solar system began to take shape: the Kuiper belt, named after Gerard Kuiper who in 1951 had suggested that such a collection of small bodies might have existed early in the history of the solar system. Today there are more than a thousand known Kuiper belt objects (KBOs) and astronomers estimate that there may be more than 70,000 KBOs with diameters greater than 100 kilometres.
KBOs fascinate astronomers because their composition may be close to that of the primordial material that coalesced around the Sun during the solar system’s formation. Nearer to the Sun much material has been evaporated away by the Sun’s heat or gravitational pressure as planets formed. Out in the Kuiper belt it is so cold that most substances remain frozen and collisions between objects are rare. So studying KBOs will be akin to archaeology of the early solar system. The nature of the KBOs that astronomers were finding also provoked questions about Pluto: should this body really be considered a planet or just a very large KBO? That debate was accelerated by the discoveries of this year’s third Kavli astrophysics laureate, Michael Brown.
Brown was starting a new job at the California Institute of Technology in Pasadena in 1996 and, discovering that Caltech’s Schmidt telescope at Palomar—one of the ones Jewitt and Luu used—had a lot of free observing time available, set out to follow in their footsteps and survey the Kuiper belt. Only he had a more specific goal in mind: he wanted to find the tenth planet.
The first panel shows the orbits of the inner planets, including Earth, and the asteroid belt that lies between Mars and Jupiter. In the second panel, Sedna is shown well outside the orbits of the outer planets and the more distant Kuiper Belt objects. Sedna’s full orbit is illustrated in the third panel along with the object’s current location. Sedna is nearing its closest approach to the Sun; its around 11,400-year orbit typically takes it to far greater distances. The final panel zooms out much farther, showing that even this large elliptical orbit falls inside what was previously thought to be the inner edge of the Oort cloud. The Oort cloud is a spherical distribution of cold, icy bodies lying at the limits of the Sun’s gravitational pull. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
Brown and his colleagues went through the same routine: working nights, fixing photographic plates to the back of the camera and taking several exposures of each patch of sky. They didn’t use a blink comparator, instead they scanned the developed plates into a computer and blinked them that way, but it was still slow, laborious work. By 2001 they had been doing this for 3 years and had found nothing. To their relief, the telescope was then upgraded with a CCD camera and automated operation. After that the team could sleep easy in their beds while the telescope systematically scanned the skies. When he got to work in the morning, Brown simply had to download the previous night’s images and then use a computer algorithm he had developed to do the blinking for him and flag up potential candidates.
Now they could scan the heavens with prodigious speed, and in June 2002 they found something big. Later named Quaoar, after the creator god of the Tongva people native to the Los Angeles area, the KBO is roughly half the diameter of Pluto. Others quickly followed including Sedna, Orcus, Salacia and Makemake.
Sedna is by far the most intriguing of the bunch. When found it was a distance of 89.6 AU from the Sun—far beyond the normal boundaries of the Kuiper belt (30-70 AU). It’s not uncommon for KBOs to have eccentric orbits that take them far out of their normal realm—often the result of a close encounter with Neptune. But Sedna’s orbit was extreme: at closest approach it is 76 AU from the Sun and at its furthest point a huge 937 AU. One orbit takes around 11,400 years.
What could have set it on this path? When a planet such as Neptune kicks a small object into a far-reaching orbit, the kicked object will always return to the place it was kicked. But there are no known large planets between 76 AU and 937 AU. So what kicked Sedna? Candidate theories include another massive planet lurking out beyond the Kuiper belt, or that Sedna was knocked into its current orbit by another star passing nearby, or that it was orbiting another star and was captured by our solar system. The origin of Sedna remains perhaps the most perplexing mystery in this new territory beyond Neptune.
Such musings were banished from Brown’s thoughts when, on 5 January 2005, he hit the jackpot: a Pluto-sized object. He jokingly named it Xena, after the heroine of the TV series Xena, Warrior Princess, but it was later given the official name of Eris. Studies have shown that Eris is roughly the same size as Pluto, but has 27% more mass. Eris was hailed by many as the tenth planet in the solar system, but, with the prospect of more large KBOs being found, some astronomers were unhappy with the swelling roster of planets. The International Astronomical Union (IAU), which governs such matters, felt compelled to take action.
At the IAU’s next general assembly in August 2006 it proposed a new definition of a planet: any body circling the Sun that forms itself into a sphere under its own gravity. Such a definition would include Pluto, Eris and also Ceres, the largest asteroid. This was debated among the 2500 astronomers in attendance. Many argued that size and shape weren’t everything; a planet should also gravitationally dominate its region of the solar system, sweeping up or ejecting all other orbiting debris. This would exclude Eris and Ceres, and also Pluto. Delegates argued and debated the issue for more than a week in what was presented in the press as Pluto’s fight for survival as a planet. When the issue was to be put to a vote, the IAU came up with a new definition that included the requirement of gravitational dominance but also defined a new class of object—“dwarf planets”—which are spherical but don’t dominate.
With TV news crews from around the world in attendance at the assembly, and camped outside Michael Brown’s Pasadena office, the new definition was scrutinised in the debate among the IAU delegates and then put to the vote. The outcome was so convincing that a show of hands was enough: Pluto was demoted from the pantheon of planets and joined Eris and Ceres as dwarf planets. For a short time, an esoteric astronomical definition became a talking point the world over, discussed by young and old, astronomers and the public. Brown, who might have been known as “the man who found the tenth planet,” is happy instead to refer to himself as “the man who killed Pluto.”