10 Things A Planet Needs to Make it Habitable

artist rendition of Kepler 186f

Using the latest Kepler telescope, scientists have recently discovered close to 140 Earth-like planets among thousands of exotic exoplanets in the galaxy. They claim that many more could harbor the right conditions for life. Astrophysicist Natalie Batalha, mission scientist for the Kepplar Space telescope (NASA Ames Research Centre as part of the NASA Discovery Program): “to determine the fraction of stars in our galaxy that harbor potentially habitable Earth-size planets.”… The one common ingredient that makes a planet habitable, Batalha tells us, is the need for liquid water. They are looking for planets “with rocky services where water could pool, that are receiving the right amount of energy from the star where the water wouldn’t be locked up in a frozen state because the planet is too cold, nor would it be evaporated away because it’s too hot. We call it the Goldilocks Zone where liquid water can exist.”

Exoplanet Kepler 186f, located 490 light-years from Earth and nicknamed Earth’s cousin was discovered by the Kepler telescope earlier this year. Orbiting star Kepler 186, it is the first validated Earth-like planet to orbit a distant star in its habitable zone. It could support oceans and alien life. 

I’m an ecologist and, like my mother who is a master baker, I love to create ecosystems from scratch. So the research to world-build has been a fun part of writing my science fiction novels to date.

• The duology Darwin’s Paradox and Angel of Chaos, are set on Earth in 2095 in a climate-changed Greater Toronto Area (now called Icaria-5); Icaria-5 is an enclosed community, nested in a wild heathland, abandoned by a fearful society governed by a Technocratic government of ecologists. 

• The Splintered Universe Trilogy explores several potentially habitable worlds that I researched through NASA files.  Each world portrayed in the three books is a realizable and habitable world—OK, in some cases with the help of a little bio-geo alteration by the Eosian race…

Most of the places I researched and used for our intrepid hero, galactic detective Rhea Hawke of the Splintered Universe Trilogy, appeared on NASA’s Terrestrial Planet Finder top 100 list:

Iota Horologii: Neon City is Rhea Hawke’s hometown and where the main precinct of the Galactic Guardian force, for which she works, is headquartered. It’s on Iota Hor-2, which orbits Iota Horologii b (a Jupitor-like gas giant) in the Iota Horologii system (a Class G0Vp, yellow-orange main sequence dwarf star. The moon was tidally influenced by the jovian giant: it had an atmosphere in danger of being periodically sucked away, and an eccentric orbit that swung from a tropical summer to a Siberian winter. That was bio-geo altered by the Eosians. The Horologium Constellation is located near Taurus and Orion. Iota Horologii became one of the top 100 target stars for NASA’s Terrestrial Planet Finder (TPF). 

70 Virginis: Rhea visits Virgil City on the moon Virgil 9, orbiting 70 Virginis b (Goldilocks), a jovian planet. The star is a G4V class yellow-orange main sequence dwarf. Virgil city suffers from periodic intense heat and drought to long nights of intense flooding. The natives, an amoeba-like colony have adapted to these severe conditions. 

47 Ursae Majoris: Rhea first visits Pyramid City on 47 Ursae Majoris b (47 Uma b), a volcanic  planet called Horus by its bird-like inhabitants. She then visits Paradise City on Uma 1, an icy moon that orbits the planet and used as a spiritual retreat by the Schiss, a Gnostic religious sect. 47 Ursae Majoris is a solar analog, yellow dwarf star that is listed as one of the top 100 target stars in NASA’s TPF. 

Pleiades Nabula: this open star cluster in the Taurus Constellation is the home for the planet Eos, where the Eosians, who run the Galactic Guardian force come from.

Other systems Rhea visits include: HD 177830, HD 168443, HD 70642, HD 222582, HD 28185, 55 Cancri, Gliese 876, Fomalhaut, and the M103 star cluster.

Ten Criteria for Habitable Worlds

Here are ten criteria identified by NASA scientists for a habitable planet:

1. Habitable Goldilocks Neighbourhood

The habitable zone (HZ) is the distance from a star where an Earth-like planet can maintain liquid water on its surface and Earth-like life. The habitable zone is not the same as “planetary habitability”. While planetary habitability describes the planetary conditions needed to maintain carbon-based life, the habitable zone describes the stellar conditions required to maintain carbon-based life.

A ” Goldilocks planet ” is a planet that falls within a star’s habitable zone, and the name is often specifically used for planets close to the size of Earth. The name comes from the story of Goldilocks and the Three Bears, in which the little girl, Goldilocks, rules out extreme choices (large or small, hot or cold, etc.), In the same way, a planet following the Goldilocks Principle is one that is neither too close nor too far from a star to rule out liquid water on its surface and life (as humans understand it).

Scientists describe areas they think are less suited to life than others:

• globular cluster in the midst of immense star densities with excessive radiation and gravitational disturbance.
• near an active gamma ray source.
• near the galactic center where a supermassive black hole is believed to lie (e.g., Gargantua in Interstellar)

2. Less Alterations in Luminosity of its Star

Changes in luminosity are common to all stars, but the severity of the fluctuations ranges broadly. A small number of variable stars experience sudden and intense increases in luminosity, making them poor candidates for hosting life-bearing planets. Life adapted to a specific temperature range would likely not survive great and variable temperature fluctuations. Upswings in luminosity create massive doses of gamma ray and X-ray radiation. Atmospheres mitigate such effects; however, planets orbiting variables may be periodically stripped of their atmosphere by the high-frequency energy buffeting them.

3. High Metallicity of its Star

A star’s metallicity results from the proportion of its matter made up of chemical elements other than hydrogen and helium. Since stars that make up most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use the blanket term “metal” to describe all other elements collectively. A low amount of metal hugely decreases the probability that planets of sufficient mass favorable for life would have formed.

4. Good Jupiters

These are gas giant planets, like our Jupiter, that orbit their stars in circular orbits far enough away from the habitable zone to not disturb it but close enough to “protect” terrestrial planets in closer orbit in two critical ways:

• they help stabilize the orbits, and climates, of the inner planets.
• they keep the inner solar system relatively free of comets and asteroids that could cause devastating impacts. Jupiter orbits the Sun at about five times the distance between the Earth and the Sun. This is the rough distance we should expect to find good Jupiters elsewhere. Jupiter’s “caretaker” role was illustrated in 1994 when Comet Shoemaker-Levy 9 impacted the giant; had Jovian gravity not captured the comet, it could have entered the inner solar system.

5. More Mass

Low-mass planets are poor candidates for life for two reasons:

• lesser gravity makes atmosphere retention difficult. Molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision.
• smaller planets have smaller diameters and higher surface-to-volume ratios than  larger planets. They will lose the energy left over from their formation too quickly, lacking the volcanoes, earthquakes and tectonic activity that supplies the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics recycle important chemicals and minerals; they also foster bio-diversity through continent creation and increased environmental complexity and help create the convective cells necessary to generate a magnetic field.

A larger mass will more likely retain a molten core as a heat engine, driving the diverse geology of the surface. A larger planet is also more likely to have a large iron core with a magnetic field to protect the planet from stellar wind and cosmic radiation.

6. Less Eccentric Orbit

Orbital eccentricity is the difference between a planet’s farthest and closest approach to its parent star divided by the sum of that distances. This ratio describes the shape of the elliptical orbit. The greater the eccentricity, the greater the temperature fluctuation on a planet’s surface. When the fluctuations overlap both the freezing point and boiling point of the planet’s main biotic solvent (e.g., water on Earth), life is severely compromised. The more complex the organism, the greater the temperature sensitivity. The Earth’s orbit is almost wholly circular, with an eccentricity of less than 0.02.

7. Axial Tilt

A planet’s movement around its rotational axis must also meet certain criteria for life to evolve. If little or no axial tilt (or obliquity) exists relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. Alternatively, if a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis.

8. Biomass & Long-Term Orbiting Bodies

The four elements most vital for life on Earth—carbon, hydrogen, oxygen, and nitrogen—are also the most common chemically reactive elements in the universe. Simple biogenic compounds, such as amino acids, were found in meteorites and in the interstellar medium. These four elements together comprise over 96% of Earth’s collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen (in the form of water) are the solvent in which biological processes take place and where the first reactions occurred that led to life’s emergence on Earth. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available through oxidizing organic compounds, is the fuel of all complex life-forms on Earth. Although these four “life elements” appear to be readily available elsewhere, a habitable system likely also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.

9. Microenvironment

Only a tiny portion of a planet needs to support life to make it habitable. The discovery of life in extreme conditions has complicated definitions of habitability, but also generated a lot of excitement in greatly broadening the known range of conditions under which life can persist. For example, a planet whose solar conditions would generally prohibit an atmosphere from forming, might nurture one within a deep shadowed rift or volcanic cave. Similarly, craterous terrain might offer a refuge for primitive life.

10. Different Metabolism Mechanism

Some scientists hypothesize that lifeforms evolving around a different metabolic mechanism may have arisen. In Evolving the Alien, biologist Jack Cohen and mathematician Ian Stewart suggest that Earth-like planets may be very rare, but that non-carbon-based complex life could emerge in other environments. The most frequently mentioned alternative to carbon is silicon-based life, while ammonia is sometimes suggested as an alternative solvent to water.

An April 17^th article on Space.com identifies 10 exoplanets that could host alien life. They include:

• Kepler 186f
• Gliese 581g
• Gliese 667Cc
• Kepler 22b
• HD 40307g
• HD 85512b
• Tau Ceti e
• Gliese 163c
• Gliese 581d
• Tau Ceti f

None were planets that I’d chosen. But that’s just 10 so far in a host of many more to come; it’s only a matter of time.

Nina Munteanu is an ecologist and internationally published author of novels, short stories and essays. She coaches writers and teaches writing at George Brown College and the University of Toronto. For more about Nina’s coaching & workshops visit www.ninamunteanu.me. Visitwww.ninamunteanu.ca for more about her writing.