The Life of Super-Earths (22 page)

11
These estimates are based on the observed high correlation between current discoveries of giant extrasolar planets and the metallicity (enrichment in heavy elements) of their parent stars. D. Fischer and J. Valenti, “The Planet-Metallicity Correlation,”
Astrophysical Journal
622 (2005): 1102. Planet formation seems to go much more efficiently once the metallicity reaches at least a tenth of that in the Solar System. Continued searches for planets in old environments poor in heavy elements (the globular cluster 47 Tuc, halo stars) have failed to find planets. Gilliland et al., “A Lack of Planets in 47 Tucanae from a Hubble Space Telescope Search,”
Astrophysical Journal
545 (2000): 47; Sozzetti et al., “A Keck.” Evidence relevant to rocky planets will emerge from the Kepler mission, when the mission tallies its findings and statistical analysis, in a few years. For the time being, preliminary results summarized in papers by W. Borucki et al., “Characteristics of Planetary Candidates Observed by Kepler. II. Analysis of the First Four Months of Data,”
Astrophysical Journal
736 (2011): 19, and A. Howard et al., “Planet Occurrence Within 0.25 AU of Solar-type Stars from Kepler,”
Astrophysical Journal,
preprint, ArXiv: 1103.2541 (2011), show that the trend with stellar metallicity is
still there, though it appears less pronounced than the trend for giant hot Jupiters.

12
The literature on the Fermi paradox is vast, but Paul Davies offers a thorough and very thoughtful discussion in his excellent new book
The Eerie Silence
(New York: Houghton Mifflin Harcourt, 2010).

13
The estimate was introduced in Bennett et al.,
The Cosmic Perspective
(Boston: Addison-Wesley, 2007).

14
Our Kepler team made this estimate in a paper (Howard et al., “Planet Occurrence”), but only for candidate super-Earth-size planets discovered by Kepler close to their stars (within 0.25 Astronomical Unit).

15
Madau et al., “The Star Formation History of Field Galaxies,”
Astrophysical Journal
498 (1998): 106, plotted the star formation rate versus red shift.

1
H. J. Melosh, “Exchange of Meteorites (and Life?) Between Stellar Systems,”
Astrobiology
3 (2003): 207, explores the issue in detail and concludes that interstellar trips of meteorites are very unlikely, while exchange between planets in the same system is common.

2
For example, A. Foster and G. Church, “Towards Synthesis of a Minimal Cell,”
Molecular Systems Biology
2 (2006): 1, describe the “recipe” for making a living cell ab initio. A press release by the J. C. Venter Institute dated January 24, 2008, describes the completion of the full synthetic genome of a microbe provisionally named
Mycoplasma genitalium
JCVI-1.0; published in D. G. Gibson et al., “Complete Chemical Synthesis, Assembly, and Cloning of a
M. genitalium
Genome,”
Science
319 (2008): 1215. This was a major step that eventually led the same team to the
creation of a bacterial cell controlled by a chemically synthesized genome—
Mycoplasma mycoides
JCVI-syn1.0 (Gibson et al., “Creation of a Bacterial Cell Controlled by a Chemically Synthesized Genome,”
Science
329 [2010]: 53). See also P. Berry, “Life from Scratch,”
Science News,
January 12, 2008.

3
Pier Luigi Luisi, “The Synthetic Approach in Biology: Epistemological Notes for Synthetic Biology,” in
Chemical Synthetic Biology,
ed. P. L. Luisi and C. Chiarabelli (Hoboken, NJ: John Wiley, 2011), 343; John Brockman, ed.,
Life: What a Concept!
(New York:
Edge.org
, 2008).

4
The term “synthetic biology” seems to have been introduced by W. Szybalski in 1974 in
Control of Gene Expression,
ed. A. Kohn and A. Shatkay (New York: Plenum, 1974), according to S. Benner et al., “Synthetic Biology, Tinkering Biology, and Artificial Biology: A Perspective from Chemistry,” in
Chemical Synthetic Biology,
ed. P. L. Luisi and C. Chiarabelli (Hoboken, NJ: John Wiley, 2011), 69. It was reused by Barbara Hobom in “Surgery of Genes: At the Doorstep of Synthetic Biology,”
Medizinische Klinik
75 (1980): 14, and then reutilized by Eric Kool (Stanford) and others in 2000, though with somewhat different connotations. A comprehensive technical review of synthetic biology in all its different forms appears in
Nature Review's Genetics
6 (2005): 533, “Synthetic Biology,” by S. Benner and A. M. Sismour. A nontechnical review by Ed Regis appears in his nice book on the subject,
What Is Life? Investigating the Nature of Life in the Age of Synthetic Biology
(New York: Farrar, Straus & Giroux, 2008). My definition of “synthetic biology” is not the widely used one as of the time of this writing, though it is essentially the same as “chemical synthetic biology” discussed by Pier Luigi Luisi and by Steven Benner in their essays in the recent compilation
Chemical Synthetic Biology,
ed. P. L. Luisi and C. Chiarabelli (Hoboken, NJ: John Wiley, 2011). The field and its language remain largely in flux. We need a new vocabulary for the novel concepts that are being introduced with its rapid development.

5
P. Luigi Luisi, “Chemical Aspects of Synthetic Biology,”
Chemistry and Biodiversity
4 (2007): 603.

6
For a detailed, accessible discussion of minimal cell definitions and the rich history of the concept, see Regis,
What Is Life?
My colleagues Jack Szostak and George Church are working on different approaches, but they encompass the conceptual framework of the synthesis. J. Szostak, D. Bartel, and P. Luigi Luisi, “Synthesizing Life,”
Nature,
January 18, 2001; and A. Forster and G. Church, “Towards Synthesis of a Minimal Cell,” 1.

7
There is some tantalizing evidence that cosmic and planetary environments might influence the choice of symmetry of some biomolecules; see D. Glavin and J. Dworkin, “Enrichment of the Amino Acid L-isovaline by Aqueous Alteration on CI and CM Meteorite Parent Bodies,”
Proceedings of the National Academy of Science USA
106 (2009): 5487.

8
A mirror system might allow molecular biology experiments that suffer from less contamination and are easier to perform to high fidelity. If minimal cells can be maintained, their accelerated evolution might teach us the basics of designing a minimal genome, akin to what Jack Szostak refers to as a “protocells arms race.” J. Szostak, “Learning About the Origin of Life from Efforts to Design an Artificial Cell,” Konrad Bloch Lecture, Harvard University, November 23, 2010. Such a genome might be central to the transition from prebiotic chemistry to biochemistry.

9
Examples of this approach that are relevant to origins of life research are the pioneering work of the A. Eschenmoser Group, for example, A. Eschenmoser, “Searching for Nucleic Acid Alternatives,” in
Chemical Synthetic Biology,
ed. P. L. Luisi and C. Chiarabelli, (Hoboken, NJ: John Wiley, 2011), 12. Breakthrough work on nucleotides synthesis in prebiotically plausible planetary conditions was done by the J. Sutherland Group. M. Powner, B. Gerland, and J. Sutherland, “Synthesis of Activated Pyrimidine Ribonucleotides in Prebiotically Plausible Conditions,”
Nature,
May 14, 2009.

10
Generation II life is different from Generation I evolving (via cultural evolution) to a postbiological state, as discussed by Steven Dick, “Cultural Evolution, the Postbiological Universe, and SETI,”
International Journal of Astrobiology
2 (2003): 65, and references; or the “closer to us humans who transcend biology” of Ray Kurzweil or the Homo evolutis of Juan Enriquez.

11
The story follows the research done after the discovery of well preserved mummies in burials dating 3,000–4,000 years ago in the Tarim basin, around several ancient cities that later became an essential part of the Silk Road, as described in J. Mallory and V. Mair,
The Tarim Mummies
(London: Thames & Hudson, 2000).

12
Sven Hedin,
Der Wanderde See,
2nd ed. (Leipzig: Brockhaus, 1941).

13
J. Mallory and V. Mair,
The Tarim Mummies
.

INDEX

Absorption lines, spectrum

Amino acid

Ammonia

Arrhenius, Svante

Artificial cells

Astronomical unit

Atmosphere (planetary)

Bakos, Gaspar

Basalt

Beaulieu, J.-P.

Biochemistry

Bonds,

chemical

weak

Borucki, William

Brown dwarf

Brown, Tim

Brownlee, Donald

Butler, Paul

Camera

Carbonate-silicate cycle

See also
CO
₂
cycle

Carbon dioxide

Cells

Charbonneau, David

Chemical landscape

Church, George

Civilizations

Columbus, Christopher

Convection

Cook, James

Copernican revolution

Core-mantle boundary

D” layer

Core (planetary)

CoRoT mission

CoRoT-7

Cosmic microwave background (CMB)

Crust (planetary)

Darwin, Charles

Darwin project (ESA)

Davies, Paul

Deep biosphere

Dick, Steven

Differentiation (planetary)

Disk (proto-planetary)

DNA

Doppler effect

Doppler shift,
See
Method, Doppler shift

Doppler wobble,
See
Method, Doppler shift

Earthquakes

Eclipse (planets)

Eclipse (stars)

Einstein, Albert

Electromagnetic waves

Electrons

Elements, chemical

origin

Enriquez, Juan

Enzymes

Eratosthenes

Evolution,

Darwinian

of the atmosphere

stellar

theory of

Extremophiles

False positives

Fermi, Enrico

Fermi paradox

51 Peg

Force(s)

electromagnetic

gravitational

Frail, Dale

Gassendi, Pierre

Generation I and II life

Genetic molecules

Geochemical cycle

Geological timescale

GJ1214

Gibson, D.G.

Gliese

Gliese

Gliese

Goldilocks hypothesis

Granite

Greenhouse gas

HAT (project)

HD

HD

HMS
Challenger

HR8799

Habitable planets

Habitable potential

Habitable zone

Handedness

Hart, Michael

Horowitz, Paul

Horrocks, Jeremiah

Howard, Andrew

Hoyle, Fred

Humboldt, Alexander von

Hydrogen

Hydrogen sulfide

Jha, Saurabh

Joyce, Gerald

Jupiter

hot

-like

super-

Initial conditions

Kaltenegger, Lisa

Kant-Laplace model

Kasting, James

Kepler, Johannes

Kepler's laws of planetary motion

Kepler's law of planetary volumes

Kepler mission (NASA)

Kepler-11

Knoll, Andrew

Konacki, Maciej

Kulkarni, Shri

Kurzweil, Ray

Latham, David

Life,

definition

Light

infrared

visible

UV

Lissauer, Jack

Mantle (planetary)

Marcy, Geoffrey

Mars

Martian meteorites

Mason, Charles and Dixon, Jeremiah (Mason-Dixon Line)

Mayor, Michel

Mercury

transit of

MEarth (project)

Meteorites

Methane

Method (for planet discovery)

astrometry

direct imaging

Doppler shift

gravitational lensing

timing

transit

wobble.
See
Method, astrometry, Doppler shift

Microbes

Minimal artificial cell

Mirror life

Miller, Stanley

Moravec, Hans

Mycoplasma mycoides JCVI-syn1.0

Neptune

hot

mini-

-like

Newton, Isaac

Nisenson, Peter

O'Connell, Richard

OGLE (team)

OGLE-TR-33

OGLE-TR-56

OGLE-2005-BLG-390Lb

Oganov, A.

Olivine

Ono, S.

Orbital eccentricity

Orbital inclination

Orbital period

Orbital speed

Origins of life

Paczynski, Bohdan

Panspermia

Papaliolios, Costas

Perovskite

Photometry

Photons

Planet-metallicity trend

Planets

Earth-like

carbon

dwarf

exo

extrasolar

gas giant

habitable

ocean

pulsar

rocky

terrestrial

transiting

with habitable potential