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The Sun and Earth in the distant future

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1 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/)

2 The Sun and Earth in the distant future Introduction
Global warming climate change The Sun’s 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?

3 The Sun and Earth in the distant future What 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 won’t help in future.

4 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

5 The Sun and Earth in the distant future Evolution 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

6 The Sun and Earth in the distant future Evolution 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 A&A, 465, 593, 2007): where  = 810-14 solar masses/year. This leads to a loss of Msun by the tip RGB.

7 If angular momentum is conserved, then:
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: where rE is the radius of the Earth’s 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 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:

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

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!
Effect of mass loss Earth’s orbit Effect of tides and drag Sun’s radius

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 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

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 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 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 Msun

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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