chapter 19: future climate evolution/habitable planets around other stars

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Chapter 19: Future climate evolution/Habitable planets around other stars

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Chapter 19: Future climate evolution/Habitable planets

around other stars

Future climate evolution

• The Sun continues to get brighter at a rate of ~ 1 percent every hundred million years

• This should increase surface temperatures, which in turn should cause faster silicate weathering and a corresponding decrease in atmospheric CO2

Long-term implications for habitability of Earth

Solar luminosity

0 0.4 1.2 1.6

Kump et al., The Earth System (2002), Fig. 19-1

C4 photosynthesis

• 500 m.y: CO2 falls below 150 ppmv C3 plants should become extinct

• 900 m.y.: CO2 falls below 10 ppmv C4 plants become extinct

Long-term implications

• 1.2 b.y.: The rapid rise in surface temperature causes the stratosphere to become wet Earth’s oceans should be lost over the next few hundred million years, and all life will go extinct

Is there any way to counteract these effects?Yes, one could do this by building a solar shield!

Solar luminosity

The solar shield

• This probably isn’t a good solution to the problem of global warming, as it doesn’t solve the related problem of ocean acidification

• As CO2 goes down in the more distant future, however, this problem goes away

uanews.org (Univ. of Arizona)

• We are also interested in the possibility of finding habitable planets around other stars

• As a first step, we need to figure out where such planets might reside…

Liquid water is essential for life

(as we know it)• Clever biochemists have

suggested that non-carbon-based, non-water-dependent life could possibly exist

• Nonetheless, the best place to begin the search for life is on planets like the Earth that have liquid water on their surfaces

• This means that we should look within the conventional habitable zone around nearby stars

Definitions(from Michael Hart, Icarus, 1978)

• Habitable zone (HZ) -- the region around a star in which an Earth-like planet could maintain liquid water on its surface at some instant in time

• Continuously habitable zone (CHZ) -- the region in which a planet could remain habitable for some specified period of time (e.g., 4.6 billion years)

Finding the boundaries of the habitable zone

• Inner edge determined by loss of water via runaway or moist greenhouse effect

• Venus is a case in point…

Venus

• 93-bar, CO2-rich atmosphere• Practically no water (10-5

times Earth)• D/H ratio = 150 times that on Earth

What went wrong with it?

Positive feedback loops(destabilizing)

Surfacetemperature

AtmosphericH2O

Greenhouseeffect

Water vapor feedback

(+)

• This feedback loop appears to have gotten out of control on Venus because of its position closer to the Sun

Finding the boundaries of the habitable zone

• Outer edge depends on how large a planet’s greenhouse effect might be

• Mars, at 1.52 AU, is cold and dry today but looks as if it may have been habitable in the distant past…

Evidence for water on early Mars

• The ancient, heavily cratered terrain on Mars is cut through by fluvial channels

• So, Mars was probably inside the habitable zone early in its history

• What might have kept early Mars warm?

Warrego Vallis (image courtesy ofNASA)

The carbonate-silicate cycle

• CO2 builds up in a planet’s atmosphere as its climate cools• Planets located farther from their parent star should therefore build up dense CO2 atmospheres and large greenhouse effects

metamorphism

Negative feedback loops(stabilizing)

The carbonate-silicate cycle feedback

(−)

Surfacetemperature

Rainfall

Silicateweathering

rate

AtmosphericCO2

Greenhouseeffect

• The carbonate-silicate cycle feedback loop ensures that the habitable zone is relatively wide

• We can also calculate HZs and CHZs for other types of stars…

Hertzsprung-Russell (HR) Diagram

http://www.physics.howard.edu/students/Beth/bh_stellar.html

See also The Earth System,p. 191

• Our Sun is a normal, hydrogen burning star along the stellar main sequence

G stars

O and Bstars

Main sequence

M-stars

Sun

ZAMS habitable zones

Kasting et al., Icarus (1993)

• Planets orbiting late K and M stars may be tidally locked• Early F and A stars have short lifetimes and give off lots of UV radiation• Habitable zones around solar-type stars appear to be relatively wide

• Intriguingly, astronomers are now beginning to find planets around other stars

• Most of these so far have been detected using the radial velocity (or Doppler) method

Radial velocity (Doppler) method

http://www.eso.org/public/videos/eso1035g/

• The pull of the planet on its host star makes the star wobble back and forth in the observer’s line of sight

Kepler Mission

http://www.nmm.ac.uk/uploads/jpg/kepler.jpg

• This space-based telescope points at a patch of the Milky Way and monitors the brightness of ~150,000 stars, looking for transits of Earth- sized (and other) planets• 105 precision photometry can find Earths• Launched: March 7, 2009• 2,326 “planet candidates” found so far (Dec, 2011)

Transit Method

December 2011 data release

Candidate label

Candidate size (RE)

Number of candidates

Earth-size Rp < 1.25 207

Super-Earths 1.25 < Rp < 2.0

680

Neptune-size 2.0 < Rp < 6.0

1181

Jupiter-size 6.0 < Rp < 15 203

Very-large-size

15 < Rp < 22.4

55

TOTAL 2326

Known extrasolar planets

• 708 extrasolar planets identified as of Dec. 09, 2011

• Few, if any, of these planets are very interesting, however, from an astrobiological standpoint– Gliese 581g (the “Goldilocks

planet”) is probably not real

Howard et al.(2010)

704

~ 2300 more “candidate” planets from Kepler mission !!

Kepler-22b

• 600 l.y. distant

• 2.4 RE

• 290-day orbit, late G star

• Not sure whether this is a rocky planet or a Neptune (RNeptune = 3.9 RE)

http://www.nasa.gov/mission_pages/kepler/news/kepscicon-briefing.html

Direct imaging

• The real payoff will come from observing Earth-like planets directly, i.e., separating their light from that of the star, and taking spectra of their atmospheres

• This will require large, space-based telescopes– Earth-sized planets could

conceivably be detected by future 30 m-class ground-based telescopes; however, looking for biomarker gases through Earth’s atmosphere is probably impossible

• What we’d really like to do is to build a big TPF (Terrestrial Planet Finder) telescope and search directly for Earth-like planets• This can be done either in the thermal-IR (TPF-I) or in the visible/near-IR (TPF-C or –O)

TPF-I

TPF-C

TPF-O

Visible Spectrum of Earth

Integrated light of Earth, reflected from dark side of moon; Rayleigh, chlorophyll, O2, O3, H2O

Ref.: Woolf, Smith, Traub, & Jucks, ApJ 2002; also Arnold et al. 2002

10 Å = 1 nm

O2 = life?

Thermal-IRspectra

Source:R. Hanel, Goddard Space Flight Center

O3 = life?

Take-home lessons from this class

• We need to preserve our environment, as Earth is the only habitable planet that we know of

• Global warming is a real problem with which we will someday have to deal

• There may well be other Earth-like planets around other stars. Looking for them, and looking for signs of life on them, is a scientific endeavor that is well worth undertaking