Kuipergordel
De Kuipergordel is een gordel van vele miljarden komeetachtige, uit steen en ijs bestaande objecten, transneptunische objecten genoemd, voorbij de baan van de achtste planeet van ons zonnestelsel, Neptunus. De gordel bevindt zich op 30 tot 50 AE van de zon.
Ontdekking[bewerken | brontekst bewerken]
Het bestaan van de Kuipergordel werd in 1951 gesuggereerd door de Nederlands-Amerikaanse astronoom Gerard Kuiper. In 1950 had Jan Oort de theorie geponeerd dat zich op een afstand van 100.000 AE (1 AE is ca. 150 miljoen km) van de zon een sfeer moet bevinden vanwaaruit kometen op ons zonnestelsel afkomen. Dat gebied wordt de Oortwolk genoemd. Kuiper meende dat er op relatief korte afstand eveneens zo'n gordel moest zijn. Bovendien bood het een verklaring voor het ontstaan van Pluto, een van de dwergplaneten van ons zonnestelsel, die zich bevindt aan de binnenste rand van de Kuipergordel. Deze wijkt als rotsachtige ijsdwerg sterk af van de grote gasreuzen en is relatief klein, waardoor men lang bleef twijfelen of ze wel of niet als planeet bestempeld moest worden. Kuiper veronderstelde dat Pluto ooit werd gevormd door samenklonterende brokken in dat gebied, waardoor ze nu een van de grootste van de familie van ijsdwergen in de Kuipergordel is. Pas in 1992 werd het bestaan van een tweede object in de gordel aangetoond, en sindsdien zijn er duizenden objecten bij gevonden.
Objecten en hun afmetingen[bewerken | brontekst bewerken]
De meeste Kuipergordelobjecten weerkaatsen vanwege hun grote afstand tot de zon maar weinig licht en zijn daardoor moeilijk waar te nemen. Hun afmetingen variëren van ca. 100 m tot meer dan 1000 km. Het grootste object dat tot nog toe is ontdekt is Eris, ontdekt in 2005. Met een diameter van ongeveer 2400 km is Eris groter dan de grootste planetoïde Ceres en ook groter dan Pluto. Daarvoor was het grootst bekende Kuipergordelobject Quaoar. De in 2004 ontdekte planetoïde Orcus is mogelijk nog groter dan Quaoar.
Een ander relatief groot object is 2000 EB173, ontdekt op 27 oktober 2000. Uit de mate van lichtweerkaatsing leidde men af dat het object ongeveer 600 km in diameter is, hoewel dat een vertekend beeld zou kunnen zijn. Later zijn nog veel meer vergelijkbare objecten ontdekt met diameters tussen de 400 en 800 km. Varuna en Ixion zijn nog twee voorbeelden van grote Kuipergordelobjecten. Sommige van deze objecten volgen de baan van Pluto en worden plutino's genoemd. Isaac Asimov stelde voor dergelijke objecten mesoplaneten te noemen.
Kuiper-klif[bewerken | brontekst bewerken]
De Kuipergordel lijkt aan de buitenzijde bij 50 AE vrij plotseling op te houden. Er wordt gesproken van de 'Kuiper-klif'. Een sluitende verklaring daarvoor is nog niet gevonden. Er wordt gespeculeerd dat er zich buiten de Kuipergordel een nog onontdekte planeet bevindt die dit gebied heeft schoongeveegd.[1]
Het gebied buiten deze Kuiper-klif is echter niet geheel leeg: in 2003 werd een object ontdekt dat op dat moment 86 AE van de zon verwijderd was, dus buiten de Kuiper-klif, gecatalogiseerd als 2003 VB12 en officieel Sedna genoemd. Het is kleiner dan Pluto (Pluto: 2274 km, Sedna: 1770 km), maar, na Eris, het grootste object in ons zonnestelsel buiten de baan van Pluto. Het is echter denkbaar dat Sedna een uitzondering is en oorspronkelijk van buiten het zonnestelsel afkomstig is.[2]
Naamgeving[bewerken | brontekst bewerken]
Het systeem in de naamgeving van deze objecten is enigszins ingewikkeld. Als een Kuipergordelobject wordt ontdekt, krijgt het eerst een voorlopig nummer dat het jaartal van de ontdekking aangeeft, evenals een volgnummer. Een voorbeeld is het object 2000 WR106. Als de precieze baan is berekend krijgt het object een catalogusnummer van het Minor Planet Center. Zowel planetoïden als Kuipergordelobjecten krijgen zo'n catalogusnummer. Het object uit ons voorbeeld kreeg het mooie ronde catalogusnummer van 20000. Tot slot mag een ontdekker het object ook een naam geven, hoewel dat meestal niet gebeurt. Het object 20000 werd (20000) Varuna gedoopt.
Banen[bewerken | brontekst bewerken]
De objecten wentelen traag rond de zon, in dezelfde richting als de planeten en planetoïden. Het duurt honderden jaren voor ze één omwenteling hebben gemaakt. Sommige objecten lijken echter zo te worden beïnvloed door de zwaartekracht van Neptunus en Uranus dat ze sterk afwijkende, elliptische banen beschrijven, soms zelfs tot in het centrum van ons zonnestelsel. Pluto en zijn maan Charon beschrijven eveneens een elliptische baan, waardoor ze soms binnen de baan van Neptunus komen.
Afkomstig van Jupiter?[bewerken | brontekst bewerken]
Een theorie is dat de objecten afkomstig zijn uit de invloedssfeer van de grote planeet Jupiter, die door zijn enorme massa objecten kan aantrekken en wegslingeren. Dat is bijvoorbeeld het geval bij kometen die afwisselend door de zon en Jupiter in hun loop worden beïnvloed. Deze kometen leggen daardoor een zeer elliptische baan af, die 3 tot 10 jaar duurt.
Ook van Triton (maan van Neptunus) wordt gedacht dat het ooit een object uit de Kuipergordel was. Het zou door de grote massa van Neptunus in een baan om deze planeet zijn getrokken.
TNO's[bewerken | brontekst bewerken]
De objecten van de Kuipergordel worden thans geschaard onder de transneptunische objecten (objecten buiten de baan van Neptunus).
New Horizons ruimtesonde[bewerken | brontekst bewerken]
De NASA heeft op 19 januari 2006 de ruimtesonde New Horizons gelanceerd naar Pluto en zijn maan Charon. Na het bezoek aan Pluto is de sonde doorgevlogen naar andere Kuipergordelobjecten. Een ontmoeting met Kuipergordelobject 2014MU69 is gebeurd op 1 januari 2019. New Horizons zou zijn tweede Kuipergordelobject kunnen bezoeken na 2020. Het is de eerste keer dat een sonde de Kuipergordel onderzoekt.
Externe link[bewerken | brontekst bewerken]
- (en) New Horizons
The mystery of Planet X
New Scientist
Something weird is going on beyond Neptune, but Pluto is too puny to blame. Time to call in the planet hunters, says Govert Schilling
Two years ago the solar system lost a planet. Pluto was deemed too insignificant to rank alongside Mars, Jupiter and the rest, and was demoted to dwarf planet status. Pluto’s fall from favour left us with only eight bona fide planets. But what the solar system has lost, Patryk Lykawka now hopes to replace.
Lykawka, an astronomer at Kobe University in Japan, suspects a ninth planet as large as Earth is hiding beyond Pluto. So far, this frigid "super-Pluto" has escaped detection. But not for much longer, Lykawka hopes. "Within five years or so, we will know for sure if it exists."
Lykawka has become convinced of the existence of this planet thanks to a number of puzzling features in the Kuiper belt, a disc of icy debris in the outer solar system, of which Pluto is one of the largest members.
He is not alone in thinking there is another planet out there. "There are similar proposals in the literature," says Renu Malhotra at the University of Arizona in Tucson, "but Lykawka has done a more comprehensive job. I think his idea should be given fair attention."
Conjuring up a new planet is a tried-andtested way of explaining puzzling observations. In the 19th century, Neptune’s existence was predicted on the basis of irregularities in the orbit of Uranus. Much later, American astronomer Percival Lowell thought that some further glitches in the orbits of Uranus and Neptune might be caused by what he dubbed Planet X. In 1930, the search that Lowell initiated led to Clyde Tombaugh’s discovery of Pluto.
When Pluto turned out to be much too small to tug strongly enough at the two giant planets, the search began for other unknown planets roaming the pitch-black outer regions of the solar system. Increasingly sophisticated observations have now revealed that there is in fact nothing anomalous in the orbits of Uranus and Neptune, but notions of distant, unseen planets still seem to have proved irresistible. Larger telescopes, better detectors and more comprehensive surveys have so far come up empty-handed, but none has actually ruled out the possibility that there is an elusive planet just beyond observational reach. The hunt for Planet X is still on.
The evidence for Planet X lies in the region just beyond Neptune, which orbits the sun about 30 times as far away as Earth. This is the beginning of the Kuiper belt, named after planetary scientist Gerard Kuiper, who speculated in 1950 that this region ought to contain a belt of debris left over from the formation of the solar system. The first object in this region was discovered in 1992 by Dave Jewitt and Jane Luu, working at the Mauna Kea Observatory in Hawaii, and well over 1000 have been spotted since.
While most of the known Kuiper belt objects (KBOs) are little more than icy clumps a few hundred kilometres across, some are as big as 1000 kilometres. The largest identified so far, called Eris, is 2400 kilometres across and is 27 per cent more massive than Pluto. It was its discovery in 2003 by Mike Brown, Chad Trujillo and David Rabinowitz working at the Palomar Observatory that prompted the International Astronomical Union to define the term "planet" – a move that in turn led to Pluto’s demotion.
Unspectacular as they are, it was clues from this multitude of frozen chunks that put planet hunters back on the trail. The first clue comes from the unexpectedly sharp outer edge of the Kuiper belt, some 50 astronomical units from the sun (1 AU being the distance between the sun and the Earth, about 150 million kilometres). At this point, known as the Kuiper cliff, the number of KBOs drops dramatically. Second, the belt itself contains different populations of icy rocks with at least three very distinct orbits. Something must have sculpted the belt, says Lykawka, and that something might well be Planet X.
We know from Saturn’s rings that when a ring of small orbiting objects has a sharp, well- gravitational effect of a large object orbiting further out. Could a similar phenomenon, on a larger scale, have created the Kuiper cliff? Mario Melita at Queen Mary University of London and Adrian Brunini at the National University of La Plata in Argentina argued in 2002 that it could. They proposed the existence of a Planet X at least as massive as Mars and some 60 AU from the sun (New Scientist, 14 December 2002, p 30). But two years later, when Melita looked more closely, he discovered a problem. Working with astronomers at the University of London and Queen’s University Belfast, he found that his proposed Planet X could not explain all of the Kuiper belt’s intricate features.
Lykawka has now followed this up with a computer simulation which shows that a massive planet so close to the Kuiper cliff would create more of a disturbance amongst many other objects in the Kuiper belt than is in fact the case.
"My simulations ruled out many other Planet X proposals too," says Lykawka, who finished his PhD research at Kobe University last year. "None of them is compatible with what we know about the dynamics of the Kuiper belt."
Nevertheless, the lure of an unseen outer planet proved too strong for Lykawka to abandon the chase. A more distant Planet X might yet explain other strange features in the Kuiper belt, including a group of objects within the main part of the belt that have highly elongated orbits and loop round the sun at an unruly angle.
The odd behaviour of some bodies even further out than the main Kuiper belt also need explaining. Take Sedna, an object over 1000 kilometres across whose stretched-out orbit takes it 975 AU from the sun before swooping back in to 76 AU. Sedna is not the only "detached" object – one that never comes close to Neptune at all – so could a single Planet X have set them on their peculiar paths, and also account for the other Kuiper belt oddities?
Lykawka teamed up with his colleague Tadashi Mukai to find out. "I thought it would be easy," he says, "but it wasn’t." Using largescale computer simulations, the pair worked out the path that Planet X would need to have taken to produce all the known properties of the Kuiper belt. Our best theories of the early days of the solar system suggest that dozens of embryonic planets formed much closer to the sun, from the colliding and clumping of many smaller bodies. Most of these Mars or Earthsized objects further coalesced into the giant planets Jupiter, Saturn, Uranus and Neptune, which ultimately migrated away from their birthplace around the sun. Gravitational interactions with the fledgling giants would have flung others – including Lykawka and Mukai’s Planet X – into distant orbits.
According to their model, Planet X was ejected by a young Neptune into an elongated orbit in the outer reaches of the solar system. There its gravity stirred up the Kuiper belt and swept part of it clean of debris, creating the Kuiper cliff. The second stage in the orbital history that Lykawka and Mukai propose for their Planet X also has it roots in established theories of planetary migration. Tens of millions of years after Neptune formed, its gravitational interaction with debris in the outer solar system caused the planet to drift slowly outwards. As it migrated, it captured and swept up KBOs into resonant orbits. This mechanism is generally considered to be the best explanation for the existence of large populations of KBOs, including Pluto, that are resonant with Neptune.
According to Lykawka, Neptune’s migration pushed Planet X into a distant, resonant orbit. By settling into an average orbit of between 100 and 170 AU from the sun, Planet X was far enough away to leave most other objects in resonant orbits undisturbed, yet close enough for its gravity to create the detached population of objects like Sedna.
Finally, Lykawka and Mukai believe the same subtle gravitational interactions that shape the orbits of small moons around planets played a big part in the evolution of Planet X’s orbit. These interactions were found in 1962 by the Japanese astronomer Yoshihide Kozai when he was looking at the orbits of asteroids. He showed that a group of large objects all orbiting in the same plane can tilt the path of a smaller object and make it more circular. This would explain why Planet X no longer approaches the Kuiper belt.
Today Lykawka’s Planet X would take anywhere between 1000 and 2500 years to complete one orbit of the sun, compared to Pluto’s 248 years. It would never get any closer to the sun than 80 AU, and its orbital inclination could be as much as 40 degrees from the plane occupied by the major planets.
So could Lykawka and Mukai’s planet explain away the Kuiper belt’s architecture? Maybe. "It is plausible, from the dynamical point of view," says Malhotra, an expert on planetary migration. "Their proposal is not entirely free of problems, but it has some significant strengths. I am very sympathetic to this idea."
Jewitt also thinks the idea is plausible, though he has some reservations. "The trouble is we are so ignorant of the outer solar system that many things seem plausible, even if they are not true."
Other dynamicists are more critical. When Lykawka presented his idea at the annual meeting of the American Astronomical Society’s Division of Planetary Sciences last October, Alessandro Morbidelli from the Côte d’Azur Observatory in Nice, France, dismissed the whole idea as contrived. "Lykawka didn’t mention that he is forcing the behaviour of the planet to match what he needs."
Morbidelli’s colleague Hal Levison from the Southwest Research Institute in Boulder, Colorado, agrees. "Lykawka sculpts the Kuiper belt by moving and pushing his planet around by hand. I don’t believe that it could have happened in the way he describes."
Morbidelli, Levison and two of their colleagues have developed a markedly different theory for the solar system’s early history. Called the Nice model, after the French town where it originated, it envisages Jupiter, Saturn, Uranus and Neptune being formed much closer together than they are today, in orbits that have changed over time, eventually triggering a series of dramatic disturbances and violent collisions that altered the orbits of the four large planets along with countless asteroids and ice dwarfs.
They claim it successfully explains the orbits of the giant planets, the existence of asteroids that share Jupiter’s path around the sun, and a violent epoch called the "late heavy bombardment" that afflicted the inner solar system some 700 million years after it formed (New Scientist, 28 November 2006, p 40 ).
In a new, as yet unpublished paper, Levison, Morbidelli and their colleagues also invoke the Nice model to explain the properties of the Kuiper belt without the need for a Planet X (www.arxiv.org/abs/0712.0553). According to their theory, the disc of gas and dust from which the first icy planetesimals formed was small and had a sharp outer edge, possibly the result of a passing star sweeping up material. The Kuiper belt formed and then later expanded as a whole thanks to the gravitational effects of the migrating giant planets, they say.
Levison and Morbidelli admit that their model has its own problems. It calculates that most objects in the Kuiper belt should have orbits stretched out far further than observations show, and it has difficulty accounting for known objects with extremely tilted orbits. The team consider these minor points, however. "We think that the list of successes of our model outweighs the problems that remain open," they say.
Levison dismisses as "unphysical" the idea that a planet half as massive as Earth sculpted the Kuiper belt, arguing that there would have been a back reaction from the giant ring of icy debris which would have pulled Planet X from the orbit that Lykawka and Mukai envisage. "The planet should end up in a circular orbit at less than 7 billion kilometres (50 AU) from the sun," Levison says.
Lykawka is undeterred by Levison and Morbidelli’s criticisms. "Several of their comments are prematurely too critical and unbalanced," he says. "I’m planning even more realistic simulations to study this in more detail."
So does Planet X really exist? Brown points out that his hunt for large KBOs is far from complete. "Could Lykawka’s planet be out there and have been missed? Easily," he says. Jewitt agrees. "Given the spottiness and the paucity of published deep sky surveys, one could put almost anything in the outer solar system and it would have escaped detection until now."
More comprehensive surveys on the horizon should eliminate these uncertainties. Sensitive, wide-field telescopes such as Pan- STARRS in Hawaii, the Discovery Channel Telescope in Arizona, and the Large Synoptic Survey Telescope in Chile will soon sweep the skies, leaving no star or space rock unturned.
"I’m always a big fan of theories like Lykawka’s," says Brown, "simply because they continue to provide the hope that we might find something big somewhere out there. But if big surveys like Pan-STARRS don’t find such a thing, I think it will have to be abandoned."
As ever, it will be astronomers who deliver the final verdict. "When we’ve done a suitable all-sky survey, we’ll know whether or not this Mars-like body exists in the type of orbit Lykawka describes," says Jewitt. "And that will be that – end of story."
Box --> Planet X: what to look forPatryk Lykawka and Tadashi Mukai’s Planet X is a "super-Pluto" made up of ice and rock. It has a mass between 30 and 70 per cent that of Earth, but will be much less dense, giving it a similar diameter to our own planet’s – probably between 10,000 and 15,000 kilometres.
At 100 to 170 AU from the sun, the amount of heat and light falling on the planet would be exceedingly small so the planet would be completely frozen. Life would be impossible on such a frigid world.
Would Planet X qualify as a bona fide planet, according to the rules of the International Astronomical Union? Probably. It orbits the sun, it would be big enough to be spherical under its own gravity, and it almost certainly has enough mass to have swept debris from the neighbourhood around its orbit. The solar system may have nine planets after all.
© Govert Schilling
