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The Sun and Earth in the distant future  Introduction  Evolution of the Sun  Fate of the planets (and us)  The end-game 3-D simulation of a pulsating.

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Presentation on theme: "The Sun and Earth in the distant future  Introduction  Evolution of the Sun  Fate of the planets (and us)  The end-game 3-D simulation of a pulsating."— Presentation transcript:

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

2 The Sun and Earth in the distant future Introduction  The Sun’s luminosity is slowly increasing – what will that do to us?  On ZAMS, L sun ~ 70% L sun (now) – but geological evidence suggests T earth ~ 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?  Global warming climate change

3 The Sun and Earth in the distant future What happened in the past?  Early atmosphere was rich in CO 2 – 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  CO 2 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 CO 2 production as vegetation dies back – so feedback probably won’t help in future.

4 The Sun and Earth in the distant future Evolution of the Sun (schematic) The Sun now Core H exhausted Sun expands up red giant branch He flash Detailed calculations made by Peter Schröder, using modified Eggleton code Core He burning Expansion up Asymptotic Giant Branch

5 The Sun and Earth in the distant future Evolution of the Sun: No mass loss Log R sun and Log D planets (Units: R sun (now)). Sun Mercury Venus Mars All planets swallowed at RGB or AGB stage Earth

6 The Sun and Earth in the distant future Evolution of the Sun: With mass loss  Solar wind mass-loss is negligible (~ M sun /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 A&A, 465, 593, 2007 ): where  = 8  solar masses/year. This leads to a loss of M sun by the tip RGB.

7 The Sun and Earth in the distant future Evolution 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:  At the tip RGB, the Sun reaches a radius of 1.2 AU, but the Earth’s 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: where r E is the radius of the Earth’s orbit at time t and  E is the (constant) orbital angular momentum.

8 The Sun and Earth in the distant future Evolution of the Sun: With mass loss Earth’s orbit Sun: RGB AGB

9 The Sun and Earth in the distant future Fate 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:

10 The Sun and Earth in the distant future Fate of the Earth – doomsday! Sun’s radius Earth’s orbit Effect of mass loss Effect of tides and drag

11 The Sun and Earth in the distant future What 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 CO 2, 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

12 The Sun and Earth in the distant future Can 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, Ap&SpSci, 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

13 The Sun and Earth in the distant future What 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 M sun ) 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 M sun 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 M sun

14 The Sun and Earth in the distant future Conclusions  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

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

16 The Sun and Earth in the distant future Conclusions  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 doesn’t even produce a proper planetary nebula!


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