Habitable zones and the Search for Life on Planets

Around Other StarsJames Kasting

Department of GeosciencesPenn State University

Talk Outline

• Part 1—Basic requirements for life• Part 2—Definition and boundaries of the

habitable zone• Part 3—Future space missions to search for

habitable planets (and life)

• Part 1—What are the requirements for life?

First requirement for life: a liquid or solid surface

• It is difficult, or impossible, to imagine how life could get started on a gas giant planet– Need a liquid or solid

surface to provide a stable P/T environment

• This requirement is arguably universal

Second requirement for life: carbon

• Carbon is unique among the elements in forming long, complex chains

• Something like 95% of known chemical compounds are composed of organic carbon

• Silicon, which is located right beneath carbon in the Periodic Table, forms strong bonds with oxygen, creating rocks, not life

Proteins

Third requirement for life (as we know it) : Liquid water

• Life on Earth requires liquid water during at least part of its life cycle

• So, our first choice is to look for other planets like Earth

• Subsurface water is not relevant for remote life detection because it is unlikely that a subsurface biota could modify a planetary atmosphere in a way that could be observed (at modest spectral resolution)

• Part 2—Definition and boundaries of the habitable zone

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)

Michael Hart’s calculations(Icarus, 1978, 1979)

• 4.6-Gyr CHZ around our own Sun is quite narrow– 0.95 AU: Runaway greenhouse– 1.01 AU: Runaway glaciation

• CHZ’s around other spectral types are even narrower– Corollary: Earth may be the only

habitable planet in our galaxy

Problems with Hart’s model

• He concluded, incorrectly, that a fully glaciated planet could never deglaciate by building up CO2

• There is strong evidence that this actually happened on Earth several times in the aftermath of Snowball Earth glaciations

• Furthermore, there is a well-known negative feedback that causes CO2 to accumulate on frozen planets…

The carbonate-silicate cycle

• The habitable zone is relatively wide because of negative feedbacks in the carbonate-silicate cycle: Atmospheric CO2 should build up as the planet cools• Higher CO2 is also at least part of the problem to the faint young Sun problem on Earth

The ZAMS habitable zone

http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/

• With this in mind, one gets a habitable zone that is fairly wide compared to the mean planetary spacing• Figure applies to zero-age-main-sequence stars; the HZ moves outward with time because all main sequence stars brighten as they age

Outer edge of the HZ

• There is a limit, though, at which the CO2 starts to condense. This reduces the greenhouse effect by lowering the tropospheric lapse rate

• Rayleigh scattering by CO2 raises the planet’s albedo and therefore also competes against the greenhouse effect

• This leads to something termed the ‘maximum greenhouse limit’, which one can calculate using a 1-D climate model

Maximum greenhouse limit

• The minimum solar flux at which you can warm the mean surface temperature of Mars to >OoC is about 82% of the present solar flux at Mars’ orbit, or ~36% of the present solar flux at Earth

S/S0 = 0.75 at 3.8. b.y. ago, when most of the valleys formed

Mars

R. Ramirez et al., Nature Geosci. (2014)

Early Mars limit

• But our climate calculations are probably overly pessimistic, as there is plenty of evidence that Mars was warm and wet at 3.8 Ga when the Sun was only 75% as bright as today

• This gives us an empirical ‘early Mars’ limit on the outer edge of the habitable zone

Warrego Vallis (from Viking)

~200 km

Inner edge of the HZ

• Near the inner edge of the HZ, the problem is just the opposite: a planet can develop either a runaway or moist greenhouse– Runaway greenhouse: The oceans evaporate

entirely– Moist greenhouse: The ocean remains liquid, but

the stratosphere becomes wet, leading to rapid photodissociation of water and escape of hydrogen to space

Runaway greenhouse thresholds: old and new

New model(Kopparapu et al., Ap.J., 2013)

Old model(Kasting et al., 1988)

• Our own calculations using updated absorption coefficients for both H2O and CO2 suggest that the runaway greenhouse threshold is much closer than previously believed (runaway: 0.97 AU, moist greenhouse: 0.99 AU)

Runaway greenhouse thresholds: old and new

New model(Kopparapu et al., Ap.J., 2013)

Old model(Kasting et al., 1988)

• That said, we are well aware that these inner edge limits are probably too conservative, as our models are cloud-free and fully saturated with water vapor. The HZ needs to be recalculated with 3-D climate models…

3-D modeling of habitable zone boundaries

• Fortunately, new studies using 3-D climate models predict that the runaway greenhouse threshold is increased by ~10% because the tropical Hadley cells act like radiator fins– This behavior was pointed

out 20 years ago by Ray Pierrehumbert (JAS, 1995) in a paper dealing with Earth’s tropics

• We have adjusted our (1-D) HZ inner edge inward to account for this behavior

Leconte et al., Nature (2013)

Outgoing IR radiation

Updated habitable zone(Kopparapu et al., 2013, 2014)

• Note the change in the x-axis from distance units to stellar flux units. This makes it easier to compare where different objects lie

Credit: Sonny Harman

Updated habitable zone(Kopparapu et al., 2013, 2014)

• Also note that there is still a lot of uncertainty regarding the location of the inner edge

Conservative HZ

Credit: Sonny Harman

Updated habitable zone(Kopparapu et al., 2013, 2014)

Optimistic HZ

Credit: Sonny Harman

• Also note that there is still a lot of uncertainty regarding the location of the inner edge

• The effect of going to 3-D is even bigger for tidally locked (synchronously rotating) planets orbiting late-K and M stars

ZAMS habitable zones

Kasting et al., Icarus (1993)

• Older diagram: Not quite as pretty as the other one, but it illustrates the tidal locking problem for planets around late K and M stars

3-D climate model calculations for M- and K-

star planets• Clouds dominate the

sunny side of tidally locked planets orbiting M and late-K stars, raising their albedos

• The inner edge of the HZ is therefore pushed way in– Seff 2 for a

synchronously rotating planet around a K star (dark blue curves)

Yang et al., ApJ Lett (2013)

Most recent habitable zone

• Thus, our current estimate of the habitable zone looks something like this. The inner edge is still highly uncertain

Kopparapu et al., ApJ Lett (2014)

Part 3: Ongoing searches for Earth-like planets and determinations of Earth

Definition of Earth

• Earth—the fraction of stars that have at least one rocky planet in their habitable zone– This is what we need to

know in order to design a space telescope to look for such planets around nearby stars

– We should be conservative when calculating Earth for this purpose, because we don’t want to undersize the telescope

Kepler Mission

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

• This space-based telescope will point at a patch of the Milky Way and monitor the brightness of ~160,000 stars, looking for transits of Earth- sized (and other) planets• 105 precision photometry• 0.95-m aperture capable of detecting Earths• Launched: March 5, 2009• Died (mostly): April, 2013

Kepler target field: The stars in this field range from a fewhundred to a few thousand light years in distance

Source: Christopher Burke, AAS presentation, Long Beach, CA, Jan. 7, 2013

Earth for FGK stars lookslike it’s about 15% (Batalhaet al., work in progress)

JWST and TESS• NASA’s James Webb Space

Telescope, scheduled for launch in 2018, could in principle characterize Earth-size planets using transit spectroscopy– NASA’s TESS mission, which

launches in 2017, will look for transiting habitable zone planets around nearby K and M stars in hopes of providing targets for JWST

• In practice, however, this is considered to be a scientific long-shot

• We want instead to look for non-transiting planets using direct imaging..

NASA’s James Webb Space Telescope

6.5 m

• Direct imaging means looking for the light reflected or emitted by a planet when it is beside its parent star (which is hard to do because stars are very bright, and planets are dim)• Such missions have been studied previously under the name of TPF (Terrestrial Planet Finder)• With such a telescope, we could also look for spectroscopic biomarkers (O2, O3, CH4) and try to infer whether life is present on such planets

TPF-I/Darwin

TPF-C

TPF-O

Conclusions• Detectable life requires, at a minimum, a planet with a

solid (or liquid) surface, sufficient availability of carbon, and surface liquid water

• Habitable zones should be defined conservatively if they are being used to generate design parameters for future space-based telescopes– 3-D climate models are needed to further refine the boundaries of

the HZ• Earth for FGK stars appears to be ~15%, based on results

from Kepler• JWST may be able to get transit spectra of an Earth-like

planet around an M star, but only if TESS finds some good nearby candidates

• In the long run, NASA wants to do direct imaging of exoplanets. This is our best chance for finding Earth-like planets and for looking for signs of life

Terrestrial Planet Finder (TPF)

Visible or thermal-IR?

• Contrast ratio: 1010 in the visible 107 in the thermal-IR• Resolution: /D• Required aperture: ~8 m in the visible 80 m in the IR• NASA’s current plan is to do the visible mission first, maybe by the early 2030’s

≈ 1010

TPF-CTPF-I

≈ 107

Courtesy: Chas Beichman, JPL

Looking for life via the by-products of metabolism

• Green plants and algae (and cyanobacteria) produce oxgyen from photosynthesis: CO2 + H2O CH2O + O2

• Methanogenic bacteria produce methane CO2 + 4 H2 CH4 + 2 H2O

• CH4 and O2 are out of thermodynamic equilibrium by 20 orders of magnitude!* Hence, their simultaneous presence is strong evidence for life

O2

CH4

*As first pointed out by James Lovelock (Nature, 1965)

Visible spectrum of Earth

Integrated light of Earth, reflected from dark side of moon: Rayleigh

scattering, chlorophyll, O2, O3, H2O

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