The Life of Super-Earths

by Dimitar Sasselov

  3 More precisely, an ordered network of chemical reactions.

  4 More precisely, an energy-dissipating, out-of-equilibrium system.

  5 To be more complete, life is adaptive, self-optimizing, fed back, forward, and stable to perturbations.

  6 H. Moravec, Mind Children: The Future of Robot and Human Intelligence (Cambridge: Harvard University Press, 1988). See F. Dyson, A Many-Colored Glass (Charlottesville: University of Virginia Press, 2007); R. Kurzweil, The Singularity Is Near: When Humans Transcend Biology (New York: Viking, 2005); S. J. Dick, “Cultural Evolution, the Postbiological Universe, and SETI,” International Journal of Astrobiology 2 (2003): 65.

  7 G. Joyce et al., in Origins of Life: The Central Concepts, ed. D. Deamer and G. Fleischaker (Boston: Jones & Bartlett, 1994). Joyce, as well as Jack Szostak of Harvard and David Bartel of MIT, pioneered the understanding and practical application of Darwinian evolution at the molecular level: molecules capable of self-catalyzing their own replication.

  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.

  CHAPTER NINE

  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.

  CHAPTER TEN

  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 de
tailed 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 CO2 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.

  CHAPTER ELEVEN

  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.

  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.

  CHAPTER TWELVE

  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 d
ated 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.