The Sun and Earth in the distant future

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The Sun and Earth in the distant future
(K-P Schröder R C Smith, MNRAS, submitted)
Will the Sun look like this?
Outline

• Introduction
• Evolution of the Sun
• Fate of the planets (and us)
• The end-game

3-D simulation of a pulsating red
giant (http//www.lcse.umn.edu/research/RedGiant/)
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The Sun and Earth in the distant
futureIntroduction

• Global warming climate change

• The Suns luminosity is slowly increasing what
  will that do to us?
• On ZAMS, Lsun 70 Lsun(now) but geological
  evidence suggests Tearth constant for last 3-4
  billion years
• Can the feedback mechanism that kept the
  temperature constant in the past also do so in
  the future, and for how long?

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The Sun and Earth in the distant futureWhat
happened in the past?

• Early atmosphere was rich in CO2 kept Earth
  warm by strong greenhouse effect
• Clouds may also matter some evidence that CR
  encourage cloud cover at low altitudes, leading
  to higher reflection and lower temperature
  strong early solar wind may have excluded
  galactic CR, leading to lower cloud cover and
  higher temperature
• CO2 gradually locked up in carbonates and plants
  (limestone deposits contain about 70 atmospheres
  of it!) so greenhouse effect decreased as solar
  irradiation increased (the Gaia effect)
• Current climate models suggest that including
  biospheric (Gaia-type) effects may actually
  increase CO2 production as vegetation dies back
  so feedback probably wont help in future.

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The Sun and Earth in the distant future
Expansion up Asymptotic Giant Branch
He flash
Evolution of the Sun (schematic)
Core He burning
Sun expands up red giant branch
Core H exhausted
Detailed calculations made by Peter Schröder,
using modified Eggleton code
The Sun now
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The Sun and Earth in the distant futureEvolution
of the Sun No mass loss
Log Rsun and Log Dplanets (Units Rsun(now)).
Mars
Earth
Venus
Mercury
All planets swallowed at RGB or AGB stage
Sun
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The Sun and Earth in the distant futureEvolution
of the Sun With mass loss

• Solar wind mass-loss is negligible (10-14
  Msun/year).
• Mass loss is much greater at RGB and AGB stages.
  We use a new semi-empirical formula, calibrated
  from globular cluster giants and nearby galactic
  giants (Schröder Cuntz, ApJ, 630, L73, 2005 and
  AA, 465, 593, 2007)

where ? 8?10-14 solar masses/year. This leads
to a loss of 0.332 Msun by the tip RGB.
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The Sun and Earth in the distant futureEvolution
of the Sun With mass loss

• Mass loss ? weaker gravitational pull so the
  Sun expands a bit more, but also the planetary
  orbits expand.
• If angular momentum is conserved, then

where rE is the radius of the Earths orbit at
time t and ?E is the (constant) orbital angular
momentum.

• At the tip RGB, the Sun reaches a radius of 1.2
  AU, but the Earths orbit has moved out to 1.5
  AU.
• By the time it gets to the AGB, the Sun has lost
  so much mass from its envelope that it expands
  less far, only to 0.7 AU, so the Earth escapes

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The Sun and Earth in the distant future
Evolution of the Sun With mass loss
Earths orbit
AGB
Sun RGB
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The Sun and Earth in the distant futureFate of
the planets

• So it appears that Mercury and Venus get
  swallowed, but the other planets escape.
• Is that the whole story?
• No orbital angular momentum is NOT conserved
  tidal interaction and dynamical friction act to
  decrease it.
• Assuming the Sun is non-rotating on the RGB
  (conservation of its AM), the orbital motion of
  the Earth raises a tidal bulge on the Sun that
  pulls the Earth back in its orbit.
• In addition, the Earth is orbiting through the
  extended chromosphere of the Sun, giving rise to
  drag.
• Detailed computations give

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The Sun and Earth in the distant future Fate of
the Earth doomsday!
Effect of mass loss
Earths orbit
Effect of tides and drag
Suns radius
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The Sun and Earth in the distant futureWhat
happens to life, and when?

• The Earth is swallowed 0.5 million years before
  the RGB tip, or about 7.59 Gy in the future
• But increased solar irradiation acts much faster
  even without increased CO2, the rise in
  temperature will cause evaporation of the oceans
  to start and water vapour is another greenhouse
  gas
• A moist greenhouse effect will continue until
  the oceans have boiled dry (Laughlin, Sky
  Telescope, June 2007, p.32)
• Solar UV will then dissociate the water
  molecules, and the hydrogen will escape, leaving
  the Earth a lifeless dust-bowl
• The subsequent dry greenhouse effect will raise
  the temperature further until the Earth is
  essentially a molten ball
• Timescale is 1 Gy so life will disappear long
  before the Earth does

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The Sun and Earth in the distant futureCan we
postpone the extinction of life?

• Options terra-forming Mars? Space stations
  drifting out through the solar system? Colonise
  the Galaxy? None would save more than a tiny
  fraction of life on Earth
• Move the Earth itself outwards?!
• Serious proposal (Korycansky et al, ApSpSci,
  275, 349, 2001) to use Kuiper Belt objects in
  close fly-by to nudge the Earth every 6000 years
  so that it moves out at just the right rate
• Energetically possible, and could be technically
  possible in the near future (a few centuries)
• Very dangerous! But could extend habitability of
  Earth for whole MS lifetime of Sun (6 Gy) a
  big gain

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The Sun and Earth in the distant futureWhat
happens to the Sun?

• Textbooks say Sun ends as white dwarf after
  ejecting planetary nebula (PN)
• Our calculations show mass loss on AGB is
  relatively low (0.116 Msun) because most of
  envelope lost on RGB
• PN usually emitted by superwind as part of last
  thermal pulse on AGB but our calculations show
  only 0.0075 Msun is lost in final pulse much
  less than a normal PN mass. So any PN would be
  very tenuous and faint
• A final thermal pulse after leaving the AGB
  causes final mass loss, and the expected final WD
  mass is 0.5405 Msun

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The Sun and Earth in the distant
futureConclusions

• The Earth is lost eventually engulfed by Sun
  7.59 Gy in the future
• Life might hang on for 6 Gy, but the ultimate
  future of the human race (if it survives) would
  need to be elsewhere
• If the Kuiper Belt Object nudging scheme worked
  and then in fact the Earth itself might avoid
  engulfment

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The Sun and Earth in the distant futureMight the
Earth be saved after all?
A planet with initial orbital radius 1.15 AU or
more will escape engulfment
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The Sun and Earth in the distant
futureConclusions

• The Earth is lost eventually engulfed by Sun
  7.59 Gy in the future (probably certainly true
  in absence of human intervention)
• Life might hang on for 6 Gy, but the ultimate
  future of the human race (if it survives) would
  need to be elsewhere

• The Sun doesnt even produce a proper planetary
  nebula!