Is the Earth Rare?

James KastingDepartment of Geosciences

Penn State University

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

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

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

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

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

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

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

The (liquid water) habitable zone

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

• 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)

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

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 0.5-2.0 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 0.4-0.6 (Kasting et al., PNAS, 2014, and refs. therein)

Petigura et al., PNAS (2013)

AU

0.5 1.0 2.0

Conservative HZ

• 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

NewPreviouslyknown

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

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)

• 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)

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

Leconte et al., Nature (2013)

Outgoing IR radiation

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

Most recent habitable zone

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

Conservative HZ

Credit: Sonny Harman

Most recent habitable zone

Optimistic HZ

Credit: Sonny Harman

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

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

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

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..

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

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

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

‘’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)

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

• Backup slides

Thermal-IRspectra

Source:R. Hanel, Goddard Space Flight Center