1. Department of Geosciences
Is the Earth Rare?
James Kasting
Department of Geosciences
Penn State University

2. Talk outline Part 1: NASA’s Kepler Mission and the parameter Earth
Part 2: Update on the habitable zone and speculations on where Earth lies within it
Some of this is our work
Part 3: How can we look for Earth-like planets and life in the not-too-distant future?
This is what we’d like to do eventually, with NASA’s help. We can help them interpret the results

3. Kepler Mission 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
105 precision photometry
0.95-m aperture  capable
of detecting Earths
Launched: March 5, 2009
Died (mostly): April, 2013

4. Transit method
The light from the star dims if a planet passes in front of it
Jupiter’s diameter is 1/10th that of the Sun, so a Jupiter transit would diminish the sunlight by 1%
Earth’s diameter is 1% that of the Sun, so an Earth transit decreases sunlight by 1 part in 104
The plane of the planetary system must be favorably oriented
Transit probability is R*/a, where R* is the star’s radius and a is the planet’s orbital distance
Transit probability for our own Earth is 0.5%
Image from Wikipedia

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

6. December 2011 Kepler data 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
Classically, planets bigger than about 2 Earth radii (~10 Earth masses)
were expected to capture gas during their formation and turn into gas
or ice giants
We now suspect, based on planets whose masses have been determined,
that the upper radius for rocky planets is more like 1.4 REarth

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

8. We don’t just care about the sizes of the planets, though
We don’t just care about the sizes of the planets, though. We also care how far they are from their parent star 

9. The (liquid water) habitable zone
What our group is most interested in is planets that lie within the
habitable zone, where liquid water can exist on a planet’s surface
The blue strip represents the zero-age-main-sequence HZ (which
then moves outward as the stars age)

10. 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

11. Published Earth estimates
In November, 2013, Petigura et al. published an estimate of Earth for K stars and (one)
late-G star
They got 0.22, but they assumed a habitable zone of AU, which is too wide, so a conservative estimate might be ~0.1
By comparison, published estimates of Earth for M stars are of the order of (Kasting et al., PNAS, 2014, and refs. therein)
0.5
1.0
2.0 AU
Conservative HZ 
Petigura et al., PNAS (2013)

12. New
Previously
known
The estimates for Earth are still changing, however, as new analyses are performed on the Kepler data
Just this January, astronomers announced 8 new small (and maybe rocky) planets within the HZ

13. Part 2: Update on the habitable zone and speculations on where Earth lies within it

14. Habitable zone updates
Within the past two years, our group has recalculated HZ boundaries using updated 1-D climate models
The main thing that has changed is the development of a new HITEMP database for H2O absorption coefficients (replacing the older HITRAN database)
The new database gives more absorption of incoming sunlight at visible/near-IR wavelengths, thereby lowering a planet’s albedo
Goldblatt et al., Nature Geosciences (2013)

15. We derived new correlated k-coefficients for CO2 and H2O and put them into our existing 1-D climate model
The theoretical runaway greenhouse limit on the HZ inner edge moved farther out as a result
But there is significant uncertainty about the location of the inner edge because the empirical ‘recent Venus’ limit is much closer in
Kopparapu et al., Ap. J. (2013)

16. 3-D modeling of habitable zone boundaries
Our 1-D models are almost certainly too pessimistic near the HZ inner edge because we assume that the troposphere is fully saturated and we neglect cloud feedback (but not the clouds themselves)
3-D climate models predict that the runaway greenhouse threshold is significantly higher because of the ‘radiator-fin’ behavior of the tropical Hadley cells
Outgoing IR radiation
Leconte et al., Nature (2013)

17. Most recent habitable zone
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

18. Most recent habitable zone
Conservative HZ
Also note that there is still a lot of uncertainty regarding the location
of the inner edge
Credit: Sonny Harman

19. Most recent habitable zone
Optimistic HZ
Also note that there is still a lot of uncertainty regarding the location
of the inner edge
Credit: Sonny Harman

20. Part 3: How can we look for Earth-like planets and life in the not-too-distant future?

21. Identifying habitable planets
We can speculate until we’re blue in the face about whether of the Kepler planets are habitable, but we won’t know anything until we are able to look at them (or at their somewhat closer analogs)
The average distance to a Kepler target star is > 600 light years, so the planets found by Kepler will be difficult or impossible to characterize

22. 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 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..
6.5 m
NASA’s James Webb Space Telescope

23. Direct imaging means looking for the
TPF-C
TPF-I/Darwin
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-O

24. 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-C
TPF-I
≈ 107
Courtesy: Chas Beichman, JPL

25. 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)

26. 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

27. Visible spectrum of Earth
Note that one can see O2 but not CH4 or N2O. So, this leads to
an interesting question 
Ref.: Woolf, Smith, Traub, & Jucks, ApJ 2002; also Arnold et al. 2002

28. ‘’False positives’ for life
Can high atmospheric O2 concentrations build up abiotically on an exoplanet?
Recent calculations (at right) suggest that this might be especially easy on a planet orbiting an M star
Lower near-UV flux  less photolysis of H2O  less catalytic recombination of CO and O
Photochemical model calculations are underway to determine if and when one needs to worry about this problem
Sun
M star
F. Tian et al., EPSL (2014)

29. Conclusions
The Kepler dataset is a veritable gold mine and has clearly shown that Earth-sized planets exist in the habitable zones of many main sequence stars
Earth is still being evaluated for different types of stars, but is very unlikely to be below 0.1
Calculations of habitable zone boundaries using 3-D climate models are currently underway and are improving. More work to be done..
Ultimately, we need to fly some kind of TPF mission to directly image Earth-sized planets, to characterize their atmospheres, and to look for evidence of life

30. Backup slides

31. Thermal-IR
spectra
Source:
R. Hanel, Goddard
Space Flight Center