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The following passage is adapted from an article entitled “What (Maybe) Didn’t Kill the Dinosaurs” (© 2009 by John Matson).

  1. The chunks of ice and dust
  2. that make their home in the
  3. Oort cloud, far beyond the
  4. orbit of Pluto, sometimes
  5. become dislodged and head
  6. into the solar system as
  7. streaky comets. Some
  8. disruptions, caused by
  9. passing stars and other
  10. interactions with the Milky
  11. Way galaxy, are severe enough
  12. to send Oort comets into orbits
  13. that buzz or even collide with
  14. Earth. New simulations
  15. have revealed novel
  16. mechanism for their entry into
  17. our part of the solar system, a
  18. method that also suggests that
  19. comet showers may not have
  20. been strongly involved in
  21. major extinctions on Earth.
  22. Comet dynamics depend heavily
  23. on Jupiter and Saturn: their
  24. huge gravitational fields tend to
  25. keep objects away from Earth.
  26. Comets that manage to skirt
  27. Jupiter and Saturn, the
  28. conventional thinking goes,
  29. had to have originated in the
  30. outer reaches of the Oort cloud,
  31. where perturbations from
  32. outside the solar system can be
  33. felt most strongly and are writ
  34. large across vast cometary orbits
  35. that take hundreds of years to
  36. complete. Only during comet
  37. showers caused by close stellar
  38. passages, the theory holds, have
  39. extreme gravitational disruptions
  40. brought inner Oort cloud comets
  41. into the mix.
  42. A computer model by Nathan Kaib
  43. and Thomas Quinn has upended
  44. this thinking. They have found
  45. that the comets that manage to
  46. cross the Jupiter-Saturn barrier
  47. do in fact originate in large
  48. numbers in the inner Oort cloud,
  49. even in the absence of a large
  50. disruption causing a comet
  51. shower. Specifically, they found
  52. that the relatively nearby objects
  53. of the inner Oort cloud can be
  54. kicked into the reaches of the
  55. outer cloud via interactions
  56. with the massive planets. Those
  57. newly far-flung comets,
  58. suddenly enjoying a longer orbit
  59. and greater gravitational
  60. perturbations from interstellar
  61. space, can find their orbits so
  62. changed that, by the time they
  63. pass through the planetary
  64. region again, they slip past the
  65. gas giants. “They can basically
  66. hop over the Jupiter-Saturn
  67. barrier,” Kaib says.
  68. Kaib and Quinn estimate that
  69. more than half of the comets we
  70. observe streaking in from the
  71. Oort cloud reach our
  72. neighborhood via this route,
  73. and other researchers agree the
  74. simulation appears valid. “This
  75. mechanism, this dynamical
  76. path, as we call it, could work
  77. and could be a significant
  78. contributor,” says Paul
  79. Weissman, a senior research
  80. scientist at the NASA Jet
  81. Propulsion Laboratory.
  82. The new research presents a
  83. route for comet production “that
  84. goes some way” toward resolving
  85. discrepancies between the
  86. standard model and the
  87. observations, says Scott
  88. Tremaine, an astrophysicist at
  89. the Institute for Advanced Study
  90. in Princeton, N.J. “One of the
  91. issues is that the conventional
  92. view of the cometary formation
  93. process is so inefficient; in order
  94. to produce the number of comets
  95. that we see, you’d need a really
  96. massive protoplanetary disk, one
  97. that appears to be incompatible
  98. with our best estimates from
  99. other sources.”
  100. Kaib and Quinn used their
  101. newfound mechanism, as well as
  102. the number of observed comets,
  103. to estimate an upper limit on
  104. how much material could be in
  105. the inner Oort cloud. They then
  106. produced a statistical model of
  107. how many comets would have
  108. hit Earth in comet showers in
  109. the past several hundred million
  110. years. Their conclusion: large
  111. cometary showers were few and
  112. hence probably did not cause
  113. more than one extinction event.
  114. Using cometary dynamics to
  115. unwind the extinction history on
  116. Earth will likely meet with some
  117. controversy. Weissman notes that
  118. the extinction implications of Kaib
  119. and Quinn’s analysis would
  120. involve comet showers, not comets
  121. in general, and that even a
  122. diminished profile of showers
  123. does not rule out the role of comets
  124. in extinctions. One big strike is all
  125. that’s needed to, he points out.