The Life of Super-Earths (21 page)

8
Martin Nowak,
Evolutionary Dynamics
(Cambridge: Harvard University Press, 2006).

9
This is especially true when lateral gene transfer and symbiosis are added to the paradigm. Lateral gene transfer, also known as horizontal gene transfer, is the sharing of genes between unrelated species. Ancient lineages of microbes show evidence for such sharing (Carl Woese, “A New Biology for a New Century,”
Microbiology and Molecular Biology Reviews,
June 2004); lateral gene transfer and endosymbiosis seem to have been critical for creating complex genomes in the distant past.

10
J. Baross et al.,
The Limits of Organic Life in Planetary Systems
(Washington, DC: National Academies Press, 2007).

11
The cosmic microwave background radiation that permeates the entire observable Universe is today at 2.7 K and sets a common lower bound for most of the gas and dust in the vast spaces between stars and galaxies. This radiation cools with time, so it was hotter, but not by much, a few billion years in the past.

1
The HMS
Challenger
expedition is considered to have opened a new field—oceanography. It mapped the minerals of the ocean floor, discovered a large number of new species, studied
global currents, and so on. The Apollo 17 lunar module and the second space shuttle were both named after the HMS
Challenger
.

2
Narrative of the Cruise of
HMS
Challenger with a General Account of the Scientific Results of the Expedition by Staff-Commander T. H. Tizard, R.N.; Professor H. N. Moseley, F.R.S.; Mr. J. Y. Buchanan, M.A.; and Mr. John Murray, Ph.D.; Members of the Expedition. Partly Illustrated by Dr. J. J. Wild, Artist to the Expedition.
Parts First and Second, 1885.

3
In his very entertaining book,
Life on a Young Planet
(Princeton: Princeton University Press, 2003), my colleague Andrew Knoll lays out the different threads of evidence found in Earth's ancient rocks of microbial communities surviving, adapting, and even influencing dynamic environmental change on a global planetary scale.

4
NASA's Office of Planetary Protection (
http://planetaryprotection.nasa.gov/pp
) develops the protocols for sterilizing spacecraft before they are launched, but the methods and tolerances are only as good as our knowledge of the limits. As we discover more extremophiles, they continue to break previous records.

5
S. Basak and H. S. Ramaswamy, “Ultra High Pressure Treatment of Orange Juice: A Kinetic Study on Inactivation of Pectin Methyl Esterase,”
Food Research International
29 (1996): 601.

6
Alan T. Bull, ed.,
Microbial Diversity and Bioprospecting
(Washington, DC: ASM Press, 2004), 154.

7
E. Trimarco et al., “In Situ Enrichment of a Diverse Community of Bacteria from a 4–5 Km Deep Fault Zone in South Africa,”
Geomicrobiology Journal
23 (2006): 463.

8
A. Pearson, “Who Lives in the Sea Floor?”
Nature
454 (2008): 952–953, and references. R. John Parke did pioneering studies of the deep biosphere in the sediments and rocks below the ocean floor in the 1990s.

9
A 2009 expedition to the middle north Atlantic reports metabolically active microbes in 111-million-year-old sedimentary
rocks at 1,600 meters below the seabed. A. L. Mascarelli, “Geomicrobiology: Low Life,”
Nature
459 (2009): 770.

10
W. B. Whitman, D. Coleman, and W. Wiebe, “Prokaryotes: The Unseen Majority,”
Proceedings of the National Academy of Sciences
95 (1998): 6578; J. S. Lipp et al., “Significant Contribution of Archaea to Extant Biomass in Marine Subsurface Sediments,”
Nature
454 (2008): 991; A. Pearson, “Who Lives in the Sea Floor?”
Nature
454 (2008): 952–953. Lipp and colleagues showed that most of the microbes in the sub–ocean floor sediments belong to the domain Archaea, not Bacteria.

11
For decades it was assumed that the combination of high temperature, oxygen constraints, and lack of food and energy sources would prevent any multicellular organism from surviving deep inside the crust. In 2011 a team of international researchers (Borgonie et al., “Nematoda from the Terrestrial Deep Subsurface of South Africa,”
Nature
474 [2011]: 79) discovered a nematode,
Halicephalobus mephisto
(a new species), at depths where only ex-tremophilic microbes were known to live, surprising everyone and showing that the deep biosphere is complex.

12
See J. Annis, “An Astrophysical Explanation for the ‘Great Silence,'”
Journal of the British Interplanetary Society
52 (1999): 19; J. Scalo and C. Wheeler, “Astrophysical and Astrobiological Implications of Gamma-Ray Burst Properties,”
Astrophysical Journal
566 (2002): 723; B. Thomas et al., “Gamma-Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects,”
Astrophysical Journal
622 (2005): L153, regarding ozone loss.

13
A. Knoll,
Life on a Young Planet
(Princeton: Princeton University Press, 2004).

14
See J. Laskar and M. Gastineau, “Existence of Collisional Trajectories of Mercury, Mars, and Venus with the Earth,”
Nature
459 (2009): 817.

15
This is averaged from measurements over the entire planet and is very difficult to do with high precision; it is also difficult to account for all the heat precisely. See Geoffrey F. Davis,
Dynamic Earth: Plates, Plumes, and Mantle Convection
(Cambridge: Cambridge University Press, 1999). The difference between measured Earth heat loss and the theoretical estimates (which predict lower values) could be due to the abundance of radioactive elements with increasing depth, or peculiar slow motions inside the mantle. J. Labrosse and C. Joupart, “A Critical Analysis of Earth's Heat Loss and Secular Cooling,” American Geophysical Union, abstract T41H-03, December 2004; Lenardic et al., “Continental Growth, the Archean Paradox, and the Global Heat Flow Paradox,” American Geophysical Union, abstract V32A-01, December 2004.

16
David Stevenson, “Life Sustaining Planets in Interstellar Space?”
Nature
400 (1999): 32.

17
Knoll,
Life on a Young Planet.

1
How this runaway greenhouse really works was explained thirty years ago by James Kasting, who has written a wonderful book on the subject:
How to Find a Habitable Planet
(Princeton: Princeton University Press, 2010).

2
The study of habitable zones in our Solar System and around other stars goes back to the 1960s and the pioneering work of Carl Sagan. The concept has been refined since then to involve changes of the Sun in time (M. Hart, “The Evolution of the Atmosphere of the Earth,”
Icarus
33 [1978]: 23) and to account for the response and evolution of the atmosphere (J. Kasting, “Runaway and Moist Greenhouse Atmospheres and the Evolution of Earth and Venus,”
Icarus
74 [1988]: 472). The concept of a habitable zone has been broadened to the Galaxy (e.g., Peter Ward and Donald Brownlee,
Rare Earth
[New York: Copernicus, 2000]) and beyond. Since there
are many factors that contribute to making a given planet habitable, it is best to talk about habitable potential instead. See Selsis et al., “Habitable Planets Around the Star Gliese 581?”
Astronomy and Astrophysics
476 (2007): 1373.

3
The Lick-Carnegie team added a fourth Uranus-mass planet to the Gliese 876 system—876e, which orbits farther out at a period of 127 days. E. J. Rivera et al., “A 7.5 M Planet.”

4
Not surprisingly, the super-Earth planet Gliese 581d has been the subject of detailed work trying to establish its habitability, from models of a possible atmosphere and its warming effect (most recently by Wordsworth et al., “Gliese 581d Is the First Discovered Terrestrial-mass Exoplanet in the Habitable Zone,”
Astrophysical Journal
733 [2011]: 48) to spectral signatures (L. Kaltenegger et al., “Model Spectra of the First Potentially Habitable Super-Earth—Gl581d,”
Astrophysical Journal
733 [2011]: 35). However, since none of these planets are transiting we know precious little about their size and, hence, mean density. The recently discovered planetary system Kepler 11 is a cautionary tale. Though five of the Kepler 11 planets have masses smaller than Gliese 581d, none of them is rocky or has a solid surface. They are all gas rich, like Neptune.

5
D. Valencia, R. O'Connell, and D. Sasselov, “Inevitability of Plate Tectonics on Super-Earths,”
Astrophysical Journal
670 (2007).

6
P. D. Ward and D. Brownlee devote a chapter to “The Surprising Importance of Plate Tectonics” in their book
Rare Earth: Why Complex Life Is Rare in the Universe
(New York: Copernicus, 2004). That chapter is an eloquent and detailed account of all aspects of the phenomenon as it applies to Earth, as well as to animal life.

7
Ward and Brownlee,
Rare Earth,
203.

8
See J. Kasting,
How to Find a Habitable Planet,
for detailed descriptions of the organic carbon cycle and the inorganic carbon cycle (a.k.a. carbonate-silicate cycle) and their properties as a thermostat.

9
The estimate for the CO
₂
cycle perturbation timescale is from Jeffrey O. Bennett et al.,
The Cosmic Perspective
(Boston: Addison-Wesley, 2007). See J. C. G. Walker, P. B. Hays, and J. F. Kasting, “A Negative Feedback Mechanism for the Long-Term Stabilization of Earth's Surface Temperature,”
Journal of Geophysical Research
86 (1981): 9776–9782.

10
P. Silver and M. Behn, “Intermittent Plate Tectonics?”
Science
319 (2008): 85.

11
Valencia, O'Connell, and Sasselov, “Inevitability of Plate Tectonics on Super-Earths,” 45. The detailed theory of plate tectonics is complex and remains largely unsolved today. V. Soloma-tov and L.-N. Moresi, “Scaling of Time-dependent Stagnant Lid Convection: Application to Small-scale Convection on Earth and Other Terrestrial Planets,”
Journal of Geophysical Research
105 (2000): 21795; C. O'Neill and A. Lenardic, “Geological Consequences of Super-sized Earths,”
Geophysical Research Letters
34 (2007): 19204; D. Valencia and R. O'Connell, “Convection Scaling and Subduction on Earth and Super-Earths,”
Earth and Planetary Science Letters
286 (2009): 492. This is likely due to the marginal efficiency of the process on small planets like Earth and Venus. Fortunately, these details are not important to the role plate tectonics plays for the habitable potential of a planet, and makes a super-Earth particularly favorable.

1
In typical bacteria about 1,000 nucleotides are replicated per second. The reaction is fast because of catalysis (enzymes reducing the activation energy for the reaction), not because of kinetics.

2
A synopsis of the recordings, which are preserved in the SETI Institute archives, is available on Paul Horowitz's website:
frank.harvard.edu/~paulh/unpublished/fermi.html
.

3
While it was Enrico Fermi who uttered the basic question of the paradox, it was Michael Hart who provided the formal description of the issue in a 1975 publication.

4
This is the same effect that you may have heard of already: the “red shift effect” seen in all distant galaxies. Their light appears progressively redder (shifted from blue color to red color) as they are farther away. Edwin Hubble and others (including Albert Einstein) observed the effect in the early twentieth century and correctly interpreted it as an indication that the entire three-dimensional space of the Universe is expanding—every galaxy is moving away from every other galaxy. The relic photons that we call the CMB are also traveling in this same ever expanding space; hence their gradual transformation from UV and optical light to microwave and radio radiation.

5
D. Spergel et al., “Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for Cosmology,”
Astrophysical Journal
170 (2007): 377, on the Wilkinson Microwave Anisotropy Probe mission. The discoverers of the CMB—E. Penzias and R. Wilson—received the 1979 Nobel Prize in physics, while the first mapping of the CMB with the space mission COBE brought the 2006 Nobel Prize in physics to George Smoot and John Mather. The new Planck mission by ESA was launched in 2009.

6
A. Loeb, “The Dark Ages of the Universe,”
Scientific American,
October 16, 2006.

7
The story of the stars as the crucibles of the elements is fascinating and has been told before, most notably by Carl Sagan in
Cosmos:
“We are made of star stuff.”

8
Edo Burger et al., “The ERO Host Galaxy of GRB 020127: Implications for the Metallicity of GRB Progenitors,”
Astrophysical Journal
660 (2007): 504, used GRBs at z = 4 to get [M/H] near 1.5, compared to DLAs, which give [M/H] <—2.0 but probe outskirts of
galaxies. Compare this to Sozzetti and colleagues, “A Keck HIRES Doppler Search for Planets Orbiting Metal-Poor Dwarfs. I. Testing Giant Planet Formation and Migration Scenarios,”
Astrophysical Journal
649 (2006): 428, for planet fraction versus [Fe/H].

9
F. Adams and G. Laughlin,
The Five Ages of the Universe: Inside the Physics of Eternity
(New York: Free Press, 1999).

10
Incidentally, about 9 billion years ago is also the time in the past when astronomers first begin to see a drop-off in the formation rate of stars; the rate has plummeted since then by a factor of 50 or more. See R. Bowens and G. Illingworth, “Rapid Evolution of the Most Luminous Galaxies During the First 900 Million Years,”
Nature,
September 14, 2006.