The Life of Super-Earths (20 page)

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 now 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 M
_E
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)SiO
₃
means that the mineral is a mixture of MgSiO
₃
and FeSiO
₃
; for example, (Mg
_0.6
,Fe
_0.4
)SiO
₃
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.

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 MgSiO
₃
, discovered by M. Murakami et al., “Post-Perovskite Phase Transition in MgSiO
₃
,”
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.

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.

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.

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.