The Life of Super-Earths

by Dimitar Sasselov

  5 Stars orbiting each other, as well as spinning on their axis, are randomly oriented in the Galaxy, as studied for many decades in orbits of binary stars.

  6 The argument is purely geometrical: if the distribution of inclinations is random, then the probability of transit is (Rs/2a).

  7 The Doppler shift method discovery was described in a paper by T. Mazeh et al., “The Spectroscopic Orbit of the Planetary Companion Transiting HD 209458,” Astrophysical Journal 352 (2000): L55, while the photometric detection of the transit was announced in the circulars of the International Astronomical Union by David Charbonneau et al. (IAU Circular 7315, 1999) and G. Henry et al. (IAU Circular 7307, 1999).

  8 Based on the single known transiting planet HD 209458b, with its fairly deep transits and very “quiet” star, and a simple extrapolation ignoring many subtleties of the simultaneous photometric measurement of many stars (rather than a single one, moreover—with a known phase of the planet’s orbit), estimates of the number of transiting planets that would be detected within a year run into the hundreds!

  9 A. Udalski et al., “The Optical Gravitational Lensing Experiment. Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign,” Acta Astronomica 52 (2002): 1 and supplement on page 115.

  10 The star is the bright star Beta Persei, an eclipsing binary star named by the Arab astronomer Al Gul (a.k.a. Algol), meaning “The Ghul’s Head” and also referred to as the “Devil’s Star,” most likely because Arab astronomers noticed its regular “blinks.” Italian astronomer Geminiano Montanari (1633–1687) noted its variability in 1670. John Goodricke (1764–1786), a celebrated English astronomer in the field of variable stars, rediscovered its variability, determined that it is strictly regular, and understood the nature of the dimming as eclipses.

  11 We described the procedure, as it is now generally applied to all transit searches, in a series of papers. G. Torres, M. Konacki, D. Sasselov, and S. Jha, “Testing Blend Scenarios for Extrasolar Transiting Planet Candidates. I. OGLE-TR-33: A False Positive,” Astrophysical Journal 614 (2004): 979; and “New Data and Improved Parameters for the Extrasolar Transiting Planet OGLE-TR-56b,” Astrophysical Journal 609 (2004): 1071.

  12 G. Torres et al., “Testing Blend Scenarios,” 979.

  13 M. Konacki, G. Torres, S. Jha, and D. Sasselov, “An Extrasolar Planet That Transits the Disk of Its Parent Star,” Nature 421 (2003): 507.

  14 An excellent account of this and many other stories can be found in The Taste of Conquest by Michael Krondl (New York: Ballantine, 2007), a well-researched and entertaining account of one of the main motivations behind the European age of exploration—the spice trade.

  15 “CCD” stands for “charged coupled device,” a silicon chip with 10-micron-size pixels arranged in rows and columns, detecting light and registering its brightness as an electrical charge at each pixel.

  16 One station is the Fred Lawrence Whipple Observatory (FLWO) of the Smithsonian Astrophysical Observatory (SAO) on Mount Hopkins in Arizona with four telescopes, and the other is the rooftop of the Submillimeter Array Hangar (SMA) of SAO atop Mauna Kea, Hawaii. These telescopes are modest 0.11m diameter f/1.8 focal ratio telephoto lenses that use front-illuminated CCDs at five-minute integration times.

  17 A transit lasts a couple of hours and recurs every few days (for a hot Jupiter), so it is easy to miss them if you have gaps in coverage. Even worse, you often end up with many partial transits (starts or ends) that can be completely useless because the photometry during start and end of night often has systematic errors.

  18 Our proposal to NASA did not go through, but the drive to discover and study super-Earths grew stronger. The term “super-Earth” (as well as “super-Venus”) seems to have appeared first in print in our NASA proposal—we called the imaging/spectroscopic mission ESPI; Melnick et al., “The Extra-Solar Planet Imager (ESPI): A Proposed MIDEX Mission,” Bulletin of the American Astronomical Society 34 (2001). The context was spectral differences, namely, that we could distinguish between a gas giant, an ice giant, and a super-Earth spectrophotometrically in reflected light. Our ESPI team did not pay attention to refining the criteria for what we called a super-Earth, as the case was marginal for any detection anyway. It was a convenient shorthand and sounded better than “fat-Earth,” a short-lived suggestion in an email written by Tim Brown. In the Kepler proposal, which was written at about the same time as ESPI, we never gave a name to the 2 and 10 Earth-mass planets. I joined the NASA Kepler team in 2000, a NASA mission that was powerful enough to deliver super-Earths and real analogs to Earth.

  The term was used again in Valencia, O’Connell, and Sasselov, “Internal Structure of Massive Terrestrial Planets,” Icarus 181 (2006): 545, a paper we wrote in 2004, though this time I made an effort in defining it—in terms of mass (1–10 Earth-mass). For the first super-Earth to be discovered (GJ 876d) by Rivera et al. (“A 7.5 M Planet Orbiting the Nearby Star, GJ 876,” Astrophysical Journal 634 [2005]: 625), the authors did not use a specific term. The discoveries to follow were all based on the Doppler method and hence mass became the defining parameter as the term “super-Earth” was adopted by the observers.

  We had an open discussion on the topic at a workshop in Nantes in June 2008. O. Grasset and I pushed for my view—to call all RV-DETECTED planets below 10 Earth-mass “super-Earths” for the time being, since we’d be unable to distinguish between the subclasses of rocky super-Earths, ocean planets, and mini-Neptunes, and sort things out later. There were all kinds of opinions. For example, Michel Mayor and S. Udry suggested we limit the lower mass bound at 2 Earth-mass. Some were suggesting higher upper mass limits (up to 20–30), and some French colleagues were suggesting alternative names. In the end, there was no particular consensus.

  19 Sunspots are temporary perturbations of the solar photosphere, the gaseous shiny surface of the sun, caused by the complex tangles of the solar magnetic field near the sun’s surface. A sunspot is a region of the photosphere that has a lower temperature than its surroundings and hence appears black against the shiny surface of the sun. Sunspots are often the size of the projected circle of Mercury or bigger and can change little over days and even weeks. They appear to move slowly as the sun rotates, which is in the same direction as the planets, and in almost the same plane.

  20 Eli Maor, Venus in Transit (Princeton: Princeton University Press, 2004), gives a detailed account of Gassendi’s life and transit observations.

  21 See Maor, Venus in Transit.

  22 Maor, Venus in Transit, 27.

  23 K. Chang, “Puzzling Puffy Planet, Less Dense Than Cork, Is Discovered,” New York Times, September 15, 2006.

  24 See Maor, Venus in Transit.

  CHAPTER FIVE

  1 Peter Ward and Donald Brownlee, Rare Earth (New York: Copernicus, 2000); see a more general view of the Universe as a whole in Paul Davies, The Cosmic Jackpot: Why Our Universe Is Just Right for Life (New York: Orion, 2007).

  2 I’ve been asked about this choice of name—super-Earth. The story goes back to 1999–2000, when I helped write an innovative proposal to NASA for a planet-finding space telescope with a square-shaped mirror. My colleagues Costas Papaliolios and Peter Nisenson had devised this unusual design to minimize stellar glare and allow glimpses of planets huddled close to their stars. Led by our experienced space missions scientist Gary Melnick, our team prepared a detailed scientific and engineering proposal. My job was to figure out what kind of planets our telescope might be able to discover. It seemed that planets smaller than Neptune (i.e., very large versions of Earth and Venus) were within its reach. I liked to call them super-Earths and super-Venuses for short, as it has been common in astronomy to use the adjective “super” for newly discovered or hypothesized objects that are larger in size or energy than known ones. The shorthand ended up in our publication, Melnick et al., “The Extra-Solar Planet Imager (ESPI): A Proposed MIDEX Mission,” Bulletin of the American Astronomical Society 34 (2001): 559. It is no
w widely used.

  3 The first super-Earth was discovered by E. Rivera et al. in 2005 and followed up by J. P. Beaulieu et al. in the same year. Many more followed.

  4 The Kepler mission measures only radius (a planet’s mass could be determined by separate observations in some cases), so our team has adopted a radius-based nomenclature currently. We call planets “super-Earth-size” when their radius is less than 2.0 Earths but larger than 1.25 Earths. The upper limit of 2.0 corresponds to a 10 Earth-mass planet with no bulk water and nominal range of Fe/Si ratios, similar to Earth.

  5 E. Rivera et al., “A 7.5 M Planet Orbiting the Nearby Star, GJ 876,” Astrophysical Journal 634 (2005). The name of the star is simply the consecutive number (876) in a catalog of nearby stars compiled by Gliese in 1969.

  6 S. Udry et al., “The HARPS Search for Southern Extra-Solar Planets. XI. Super-Earths (5 and 8 M) in a 3-planet System,” Astronomy and Astrophysics 469 (2007): 43. The Gliese 581 planetary system consists of a hot Neptune of at least 25 ME in a very short orbit discovered by the same team: Bonfils et al., “The HARPS Search for Southern Extra-Solar Planets. VI. A Neptune-Mass Planet Around the Nearby M Dwarf Gl 581,” Astronomy and Astrophysics 443 (2005): 15. The two super-Earths are farther out. Unfortunately no transits were seen, so we do not know their size or exact mass.

  7 D. Valencia, R. O’Connell, and D. Sasselov, “Internal Structure of Massive Terrestrial Planets,” Icarus 181 (2006): 545; D. Valencia, D. Sasselov, and R. O’Connell, “Radius and Structure Models of the First Super-Earth Planet,” Astrophysical Journal 656 (2007): 545.

  8 D. Valencia, D. Sasselov, and R. O’Connell, “Detailed Models of Super-Earths: How Well Can We Infer Bulk Properties?” Astrophysical Journal 665 (2007): 1413.

  9 Common materials like honey and peanut butter are very viscous. The latter is about 100 times more viscous; amorphous solids like glass are another 10 times more viscous, but still far below the 1018 times more viscous mantle.

  10 “Convection” is the physics term for a large-scale motion of fluid or gas up and down in a gravitational field due to a heat source and density differences. An example is air thermals that rise due to the Sun heating the ground, especially in the summer. The hot air near the surface rises up and is replaced by colder air coming down, and so on.

  11 The notation for mantle perovskite (Mg,Fe)SiO3 means that the mineral is a mixture of MgSiO3 and FeSiO3; for example, (Mg0.6,Fe0.4)SiO3 means that 60 percent is in the former, and 40 percent is in the latter.

  12 Ice VII is a cubic crystal with two interpenetrating lattices; it has a mean density of 1.65 grams per cubic centimeter (g/cc) at room temperature and exists at pressures higher than 2.5 GPa. Ice X forms after further compression of Ice VII and is denser at 2.5 g/cc; in Ice X the hydrogens are equally spaced between the oxygens. M. Choukroun and O. Grasset, “Thermodynamic Model for Water and High-Pressure Ices up to 2.2 GPa and down to the Metastable Domain,” Journal of Chemical Physics 127 (2007): 124506.

  13 For super-Earths that are relatively young and large, the temperature in the interior might reach thousands of degrees. Under such high temperature water might be in an even more exotic form known as superionic water phase: the oxygens are still “frozen” in place, just as in ice VII and X, but the hydrogens (protons) can move around.

  14 Water is common in the Universe because both hydrogen and oxygen are very abundant.

  15 For early work on ocean planets, see M. Kuchner, “Volatile-rich Earth-Mass Planets in the Habitable Zone,” Astrophysical Journal 596 (2003): 105; A. Leger et al., “A New Family of Planets? Ocean-Planets,” Icarus 169 (2004): 499.

  16 Steven D. Jacobsen and Suzan Van Den Lee, Earth’s Deep Water Cycle (American Geophysical Union, 2006).

  17 See Valencia, O’Connell, and Sasselov, “Internal Structure of Massive Terrestrial Planets.”

  18 See L. Elkins-Tanton and S. Seager, “Coreless Terrestrial Exoplanets,” Astrophysical Journal 688 (2008): 628; Valencia, Sasselov, and O’Connell, “Detailed Models of Super-Earths”: 1413.

  19 Carbon will be mostly in the form of carbon monoxide gas, CO, and largely inaccessible to newly forming planets, while the excess oxygen will be in water, silicates, and other oxides with different metals. In any case, all the silicon will end up bonding with oxygen, not carbon.

  20 E. Gaidos in 2000 and M. Kuchner in 2005 described the properties of carbon planets. Overabundance of carbon in a planetary system can be inferred from analysis of the spectrum of the parent star, but such stars are extremely rare and not “normal” in many ways.

  21 The planet Gliese 436b was discovered by Butler et al., “A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436,” Astrophysical Journal 617 (2004): 580, using Doppler shifts. In 2007 the team of Gillon et al., “Detection of Transits of the Nearby Hot Neptune GJ 436b,” Astronomy and Astrophysics 472 (2007): 13, found that the planet is actually transiting its parent star, which allowed the determination of its size. A careful study by G. Torres refined the mass and radius of Gliese 436b, derived by Butler et al., “A Neptune-Mass Planet.” From its mean density it appears to be a Neptune-like planet, yet a very hot one, orbiting its star just seven stellar radii away every three days.

  22 Computed near-infrared spectra of mini-Neptunes are markedly different from those of super-Earths due to the very different pressure scale heights in a hydrogen envelope/atmosphere: E. Miller-Ricci, S. Seager, and D. Sasselov, “The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres,” Astrophysical Journal 690 (2009): 1056.

  23 See simulations by R. Marcus, D. Sasselov, L. Hernquist, and S. Stewart, “Minimum Radii of Super-Earths: Constraints from Giant Impacts,” Astrophysical Journal 712 (2010): 73.

  CHAPTER SIX

  1 Some amusing stories, reported originally by the Times Berlin correspondent, appeared in the New York Times on April 4, 1874.

  2 John Sinkakas, ed., Humboldt’s Travels in Siberia, 1837–1842: The Gemstones by Gustav Rose (Tucson, AZ: Geoscience Press, 1994).

  3 A regular octahedron consists of eight equilateral triangles; it looks like two pyramids connected at their bases. An octahedron has six vertices. In perovskite there is an oxygen atom in each vertex, and the octahedrons are seen as connected at each vertex in each direction; the Si atom is in the middle of the octahedron.

  4 Superconductivity is a curious (and very useful) phenomenon courtesy of quantum physics. A superconductor is a material that conducts electricity with zero resistance. Superconductivity was discovered and commonly occurs at the lowest of low temperature, near absolute zero. The discovery by Muller and Bednorz in 1986 was a real breakthrough because it showed superconductivity at 36 K. That is still terribly cold by human standards, but very high compared to the near 0 K of yore.

  5 National Audubon Society Field Guide to Rocks and Minerals (New York: Knopf, 2000).

  6 Post-perovskite is a high pressure phase of perovskite MgSiO3, discovered by M. Murakami et al., “Post-Perovskite Phase Transition in MgSiO3,” Science, May 7, 2004; and A. Oganov and S. Ono, “Theoretical and Experimental Evidence for a Post-Perovskite Phase of MgSiO3 in Earth’s D Layer,” Nature 430 (2004): 445.

  7 The nanoscale corresponds to scales/distances measured in nanometers (10–9 m) and is typical of the distances between atoms in small molecules and crystals. By this token, “nano” has become a prefix used commonly for fabricated structures at that scale (e.g., nanolayers, nanowires, etc.), as well as for nanotechnology itself.

  8 See D. Sasselov, D. Valencia, and R. O’Connell, “Massive Terrestrial Planets (Super-Earths): Detailed Physics of Their Interiors,” Physica Scripta 130 (2008): 14035.

  9 Thermonuclear energy is the source of energy in the Sun and involves the transmutation of hydrogen into helium with no radioactive by-products. It is not to be confused with nuclear energy, which involves radioactive decay to unstable heavy elements, like uranium.

  10 T. Mashimo et al., “Transition
to Virtually Incompressible Oxide Phase at a Shock Pressure of 120 GPa: Gd3Ga5O12,” Physical Review Letters 96 (2006): 105504.

  CHAPTER SEVEN

  1 William Shakespeare, As You Like It, 2.7.

  2 The impending collision between the Andromeda galaxy and the Milky Way is described by T. J. Cox and A. Loeb, “The Collision Between the Milky Way and Andromeda,” Monthly Notices of the Royal Astronomical Society 386 (2007): 461. Currently the velocity of Andromeda is not known with enough accuracy for scientists to definitively predict the collision.

  3 Erwin Schroedinger, What Is Life? (Cambridge: Cambridge University Press, 1944).

  4 The quantum scale was discovered and explored in the first half of the twentieth century, when quantum mechanics—the part of physics that deals with the phenomena at this scale—was developed. The word “quantum” stands for the indivisible unit of energy, a concept that was introduced in order to explain the behavior of atoms, their electrons, their interaction with light, and the emission of light. Even at very low temperatures particles at the quantum scale (small molecules, atoms, electrons) are in constant motion and interaction with each other and with units of light (photons).

  5 William H. Press, “Man’s Size in Terms of Fundamental Constants,” American Journal of Physics 48 (1980): 597.

  CHAPTER EIGHT

  1 Tzvetan Todorov, Nous et les autres (Paris: Editions du Seuil, 1989).

  2 It is probably prudent to avoid the concept of a definition when it comes to life. We do not understand life as a phenomenon well enough to define it convincingly. The unity of biochemistry of all known life on Earth means that any definition will be based on a single example, and it will be very difficult to identify what features are essential. C. Cleland discusses the issue of defining life in Geology 29 (2001): 987. Others would argue that no clear threshold is crossed between inert and living matter, and hence life is not yet a precise scientific concept (see “Meanings of Life,” Nature, June 28, 2007, 447). An attempt to define life is described in a well researched and insightful report by the National Research Council, The Limits of Organic Life in Planetary Systems (Washington, DC: National Academies Press, 2007); D. Deamer, “Special Collection of Essays: What Is Life?” Astrobiology 10 (2010): 1001; George Whitesides, lecture before the Nobel Symposium on Origins of Life, 2006. For my purposes, a list of essential attributes is sufficient.