The Sun and Earth in the distant future

Introduction

Evolution of the Sun

Fate of the planets (and

us)

The end-game3-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:

The Sun and Earth in the distant futureIntroduction

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?

Global warming climate change

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 won’t help in future.

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

The Sun and Earth in the distant futureEvolution of the Sun: No mass loss

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

12

,10

0

12

,12

5

12

,15

0

12

,17

5

12

,20

0

12

,22

5

12

,25

0

12

,27

5

12

,30

0

Age/10^6 yrs

Log Rsun and Log Dplanets (Units: Rsun(now)).

Sun

Mercury

Venus

Mars

All planets swallowed at RGB or AGB stage

Earth

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 A&A, 465, 593, 2007):

3.5

* *

* *

14000 4300

eff sunT gL R

MM K g

where = 810-14 solar masses/year. This leads to a loss of 0.332 Msun by the tip RGB.

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:

2 / ( )E E sunr M t

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 rE is the radius of the Earth’s orbit at time t and E is the (constant) orbital angular momentum.

The Sun and Earth in the distant future Evolution of the Sun: With mass loss

Earth’s orbit

Sun: RGB AGB

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:

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

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

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, 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

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

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

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

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 doesn’t even produce a proper planetary nebula!