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

2. 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 CO 2 

3. 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 C 4 photosynthesis 500 m.y: CO 2 falls below 150 ppmv  C 3 plants should become extinct 900 m.y.: CO 2 falls below 10 ppmv  C 4 plants become extinct

4. 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

5. 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 CO 2 goes down in the more distant future, however, this problem goes away uanews.org (Univ. of Arizona)

6. 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…

7. 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

8. 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)

9. 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…

10. Venus 93-bar, CO 2 -rich atmosphere Practically no water (10 -5 times Earth) D/H ratio = 150 times that on Earth What went wrong with it?

11. Positive feedback loops (destabilizing) Surface temperature Atmospheric H 2 O Greenhouse effect Water vapor feedback (+) This feedback loop appears to have gotten out of control on Venus because of its position closer to the Sun

12. 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…

13. 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 of NASA)

14. The carbonate-silicate cycle CO 2 builds up in a planet’s atmosphere as its climate cools Planets located farther from their parent star should therefore build up dense CO 2 atmospheres and large greenhouse effects metamorphism

15. Negative feedback loops (stabilizing) The carbonate-silicate cycle feedback (−)(−) Surface temperature Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect

16. 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…

17. 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 B stars Main sequence M-stars Sun

18. 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

19. 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

20. 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

21. 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 10  5 precision photometry  can find Earths Launched: March 7, 2009 2,326 “planet candidates” found so far (Dec, 2011) Transit Method

22. December 2011 data release Candidate label Candidate size (R E ) Number of candidates Earth-sizeR p < 1.25207 Super-Earths1.25 < R p < 2.0680 Neptune-size2.0 < R p < 6.01181 Jupiter-size6.0 < R p < 15203 Very-large-size15 < R p < 22.455 TOTAL2326

23. 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 !!

24. Kepler-22b 600 l.y. distant 2.4 R E 290-day orbit, late G star Not sure whether this is a rocky planet or a Neptune (R Neptune = 3.9 R E ) http://www.nasa.gov/mission_pages/ kepler/news/kepscicon-briefing.html

25. 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

26. 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

27. Visible Spectrum of Earth Integrated light of Earth, reflected from dark side of moon; Rayleigh, chlorophyll, O 2, O 3, H 2 O Ref.: Woolf, Smith, Traub, & Jucks, ApJ 2002; also Arnold et al. 2002 10 Å = 1 nm O 2 = life?

28. Thermal-IR spectra Source: R. Hanel, Goddard Space Flight Center O 3 = life?

29. 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