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

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

• 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 

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

• 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!

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

• 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

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

• 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…

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

• 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

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

• 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)

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

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

• Venus is a case in point…

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

• 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…

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

• 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?

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

• 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…

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

• 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

708 extrasolar planets identified as of Dec. 09, 2011

• 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)

600 l.y. distant

• 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)

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

• 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

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

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

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

• 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