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← Origins of Life: Astrobiology & General Theories of Life - Exoplanets - The Habitable Zone

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  1. Welcome to
    "Hunting for habitable worlds."
  2. This lecture takes us away
    from our own planet
  3. to look at what we currently know about
    planets orbiting around other stars.
  4. Before the early 1990s, the only planets
    we knew for sure that existed
  5. were the worlds that orbited
    around our own Sun,
  6. but as our instruments
    became sensitive enough to spot
  7. the dim whisper of a planet
    around other stars in our galaxy,
  8. we discovered our planetary system
    was one of multitudes.
  9. We now know of thousands of
    extrasolar planets or exoplanets -
  10. planets that orbit stars
    other than our Sun.
  11. This results in an obvious question:
  12. could any of these newly discovered
    worlds be habitable?
  13. The problem with that question is,
    while we have discovered many worlds,
  14. we actually know very little
    about each planet.
  15. The majority of planets
    we have discovered so far
  16. have been found by one
    of two techniques:
  17. the radial velocity technique
    used by ground-based telescopes,
  18. such as the ESO Observatory in Chile,
  19. or the transit technique
    used by instruments such as
  20. the Kepler space telescope
    and its successor - TESS.
  21. The radial velocity technique,
  22. sometimes known as
    the "Doppler wobble,"
  23. detects a planet via the tiny wobble
    it excites in the star.
  24. While we normally think of the star
    as stationary and the planet in orbit,
  25. in truth, both the star and planet
    orbit their common center of mass.
  26. As a star is so much bigger
    than the planet,
  27. this center of mass lies very close
    to the star's own center,
  28. causing its orbit to be
    just a tiny wobble
  29. in comparison to
    the planet's wide circuit.
  30. This wobble causes the star
    to move periodically
  31. slightly further away
    and then closer to the Earth.
  32. As the star moves slightly
    from the Earth,
  33. its light waves stretch out
    and redden slightly.
  34. Conversely, as a star moves
    back towards us,
  35. the light waves compress
    and become bluer.
  36. This regular shift from red to blue
  37. is what astronomers can measure
    to detect a planet.
  38. The second main method
    for planet detection
  39. is the transit technique.
  40. Here, a slight dip
    in the star's brightness
  41. is detected as the planet passes
    in front of the star
  42. as seen from Earth.
  43. These two methods give you
    just two properties about the planet.
  44. The transit technique gives you
    an estimate of the planet's radius
  45. while the radial velocity technique
  46. tells you about the planet's
    minimum mass.
  47. This may be significantly less
    than the true mass of the planets
  48. as the radial velocity technique
    only measures the wobble of the star
  49. directly towards the Earth.
  50. If the planet's orbit is tilted
    with respect to us,
  51. then part of the star's motion
    will be directed away from us.
  52. We won't measure this and so
    underestimate the planet mass.
  53. Both techniques also tell you about
    the amount of radiation
  54. the planet receives from the star.
  55. But, this can be very different
    from the surface temperature
  56. as it does not allow for
    the heat trapping effects
  57. of the different atmospheric gases.
  58. The challenge we're trying to determine -
  59. if a plant is habitable -
  60. is therefore that we can only measure
    two or three properties
  61. and none of these actually tell us
    what it's like on the planet surface.
  62. This will change as
    the next generation of telescopes
  63. will be able to detect light that passes
    through the planet's atmosphere.
  64. Different molecules in the atmosphere
    absorb different wavelengths of light,
  65. providing a fingerprint
    of missing wavelengths
  66. that indicate atmospheric composition -
  67. our first hint at what is happening
    on the planet's surface.
  68. But, this brings us to a new problem:
  69. such atmospheric spectroscopy
    for rocky, temperate planets
  70. is time-consuming and difficult.
  71. We therefore need a way
    of selecting planets
  72. most likely to reveal interesting results.
  73. But how do we select planets
    best suited for habitability
  74. without knowing any surface properties?
  75. Let's think about what we want to find.
  76. It's going to be easiest
    to recognize Earth-like life,
  77. that is, water and
    carbon-based chemistry.
  78. Also, this needs to be detectable,
  79. which means the water needs to be
    on the surface of the planet,
  80. not a subsurface system like Europa.
  81. Based on this, we can ask the question:
  82. how much insulation
    does an Earth-like planet need?
  83. The answer to this is
    a "Classical Habitable Zone."
  84. The Classical Habitable Zone
    is where an Earth-like planet,
  85. that is, a planet with
    our surface pressure,
  86. atmospheric gases
    and geological processes
  87. can support water on the surface.
  88. Often, in exoplanet literature,
  89. this is simply referred to
    as the "habitable zone"
  90. as we don't yet know about planets
    other than the Earth
  91. that can support life.
  92. At the inner edge of the habitable zone,
  93. it is too warm for surface water
    on the Earth and it evaporates.
  94. At the outer edge, carbon dioxide
    condenses into clouds
  95. and is no longer able to provide
    the thermal insulation
  96. of a greenhouse gas -
    so the planet freezes.
  97. Climate models predict that
    the habitable zone should stretch
  98. between 0.99 au and 1.67 au
  99. where 1 au is the average distance
    of the Earth from the Sun.
  100. Our planet, therefore,
    sits right on the inner edge.
  101. A slight extension to this is known as
    the "optimistic habitable zone,"
  102. which can broaden these limits
    based on the idea
  103. that Venus and Mars probably have
    supported surface water in their past.
  104. So, an earth-like planet
    could have a period of habitability
  105. just outside the habitable zone edges.
  106. The edges of the classical habitable zone
  107. are only calculated for the Earth.
  108. This is easily demonstrated as,
  109. while Venus sits outside
    the habitable zone,
  110. both the Moon and Mars orbit within it
  111. but neither are Earth-like enough
    to support liquid water in this region.
  112. Different planets might have different
    habitable zones at different locations,
  113. or they may not have
    a habitable zone at all.
  114. Of the planets we found so far orbiting
    in the classical habitable zone,
  115. almost 15 times as many
    are large enough
  116. to likely have thick,
    Neptune-like atmospheres
  117. compared to planets that might be rocky.
  118. We have discovered planets
    that are the right size to be rocky
  119. and orbit entirely within
    the habitable zone.
  120. Are these Earth-like enough
    to support liquid water in this region?
  121. We don't know.
  122. They may have very different
    atmospheric gases
  123. or geology that makes
    surface water impossible.
  124. The only thing we can say is that
  125. if another habitable,
    Earth-like planet is out there,
  126. it would be in the habitable zone,
    but being in the habitable zone
  127. does not mean you're
    Earth-like enough for life.
  128. So, in conclusion, we've discovered
    thousands of exoplanets,
  129. many of which are similar in size
    to the Earth.
  130. But, at the moment,
  131. we have no way of knowing
    what their surfaces are like.
  132. Note, in particular,
  133. that the Earth and Venus
    are both very similar in size -
  134. so, they are both Earth-sized planets.
  135. Our next generation of telescopes
  136. will be able to detect
    the atmosphere of these worlds
  137. and tell us something
    about their surfaces for the first time.
  138. The habitable zone is a useful concept
  139. for selecting planets
    for these new telescopes
  140. but it offers no guarantee
    that a planet is actually habitable.
  141. If you'd like to try playing with
  142. a simple climate model
    of an Earth-like planet,
  143. you can head over to "earthlike.world"
    or the associated Twitter feed.
  144. This website lets you see
  145. how different a planet might be
    from our own world today,
  146. even if it did have the same
    geological cycles as our own.
  147. The NASA NExSS "Many Worlds" blog
  148. covers the latest news for exoplanets
    and many origin of life stories.
  149. There's also a more technical overview
    of the search for biosignatures
  150. in a paper led by Yuka Fujii,
    published in "Astrobiology" last year.
  151. These are the references that were
    mentioned during the lecture.
  152. Thank you very much for listening.