Earth’s Magnetosphere Appears Designed for Habitability
Without Earth’s longstanding, magnificent magnetosphere, we wouldn’t be here nor would any life. The Sun would have sputtered away the entirety of the planet’s atmosphere and surface water and made Earth permanently lifeless. Earth stands out among the known exoplanets that have been studied as candidates for life, but why?
Ongoing research has revealed many features of Earth’s magnetosphere, including its size, strength, and longevity, that exhibit extraordinary fine-tuning. I’ll explain the details so you can appreciate the forethought and care that went into our home’s design.
A magnetosphere is a region of space that surrounds an astronomical body in which magnetic field lines generated by the body’s magnetic field guide charged particles. It takes an active interior dynamo in the astronomical body to produce the magnetic field.
Planets and moons possessing magnetospheres can mitigate or block out, at least to some degree, stellar or cosmic radiation that would otherwise exterminate life and prevent life from forming. For surface life to be possible on a planet or a moon, that planet or moon must possess a magnetosphere. Figure 1 shows a diagram of Earth’s magnetosphere.
Figure 1: Earth’s Magnetosphere
Circulating liquid iron in Earth’s core generates strong magnetic field lines (red lines) that shield Earth from deadly solar and cosmic radiation. In this diagram, solar particle radiation streaming in from the left as it engages Earth’s magnetosphere generates a bow shock that guides deadly radiation away from Earth. Credit: NASA
For a magnetosphere to form around a planet, the planet must possess a magnetic field. For a planet to possess a magnetic field, it must have an active interior dynamo. An interior dynamo requires the circulation of liquid ferrous metals in the planet’s core (see figure 2), akin to the rotor in an electric motor. The ferrous metals most easily magnetized are iron, nickel, and cobalt. All three are present in Earth’s core, but iron is by far the most abundant.
Figure 2: Planet Interior Structure Requirement for a Magnetosphere
Ferrous metals in a liquid state must be circulated in the planet’s interior for the planet to possess a strong enough magnetic field to generate a magnetosphere. Diagram credit: Hugh Ross
It takes a lot of interior heat for iron, nickel, and cobalt in a planet’s core to be liquefied. The core temperature must be at least 1,540°C (2,800°F). It is not enough for the iron, nickel, and cobalt to be liquefied. The liquid iron, nickel, and cobalt must also be circulated and the circulation must be continually sustained.
Geophysicists have identified four sources of convection currents in the liquid core sufficient to drive liquid core circulation:
- temperature difference between the liquid core bottom and the liquid core top
- tidal forces exerted by a nearby massive moon
- release of latent heat from the cooling and growth of a solid inner core
- squeezing of light elements out from a solid inner core and their rise upward through a liquid outer core
Earth’s Marvelous Magnetosphere
For life to survive on the surface of a planet, the magnetic field generated by circulation of iron, nickel, and cobalt in the planet’s liquid core must be strong enough and sufficiently stable. A rare feature of Earth is that all four of these sources of liquid core convection currents currently operate.
Not until 1.3 billion years ago did Earth’s interior cool sufficiently for a solid inner core to begin to form. However, before that time the Moon was close enough that the tidal forces it exerted on Earth made a substantial contribution to driving convection currents in Earth’s liquid core.
Previous to 4.0 billion years ago, the Moon’s interior was above the melting point of iron. Its distance from Earth was less than a third of what it is now. This proximity to Earth resulted in powerful tidal forces exerted by Earth, which generated strong convection currents in the Moon’s liquid iron core. Thus, previous to 4.0 billion years ago the Moon had a strong magnetic field and a magnetosphere. Its proximity to Earth at that time caused its magnetosphere to couple with Earth’s magnetosphere.1 Nothing less than this coupled magnetosphere would have prevented the Sun’s intense particle and radiation intensity from sputtering away all Earth’s atmosphere and surface water and making Earth permanently lifeless.2
Figure 3 shows the Sun’s particle, radiation, and flaring activity throughout its history. Thanks to the Moon and Earth’s hot origin that resulted from the proto-Earth’s collision and merger with the planet Theia, the Moon’s early proximity to Earth, and the timing and growth of Earth’s solid inner core, Earth has always had a strong and big enough magnetosphere to prevent the Sun from sputtering away most or all its atmosphere and surface water.
Figure 3: Sun’s Historical Particle, X- and UV-Radiation, and Flaring Intensity Levels
The y-axis is logarithmic. The intensity levels during the first half-billion years of the Sun’s history were more than 10,000 times higher than its present levels. Furthermore, during the Sun’s first half-billion years it was rotating at least five times faster, which means Earth’s surface was nearly always directly exposed to the Sun’s coronal mass ejections.3 Diagram credit: Hugh Ross
Earth’s magnetic field and magnetosphere have been sufficiently strong and stable to ensure that life could be abundant and diverse throughout the past 3.8 billion years. Previous to 4 billion years ago, the Earth-Moon coupled magnetosphere prevented the Sun from sputtering away into interplanetary space all Earth’s atmosphere and surface water, but it did not block out the Sun’s deadly x-ray and ultraviolet radiation. Consequently, life could not survive on Earth during Earth’s first 700 million years. Thereafter, however, as the Moon’s contribution to convection currents in Earth’s liquid core declined as the Moon slowly migrated away from Earth, the cooling of Earth’s core and especially the formation and subsequent growth of Earth’s solid inner core were perfectly timed to ensure that Earth always had a strong and big enough magnetosphere to prevent the loss of Earth’s oceans and atmosphere and to protect Earth’s life from deadly radiation.
Clearly, for a planet to be habitable it must possess a magnetosphere. Furthermore, the planet’s magnetosphere must be big, strong, and stable. Astrobiologists have considered the probability that another planet in our galaxy or in nearby galaxies would possess a magnetosphere that would enable life to exist on that planet. Consequently, a number of research efforts have focused on the characteristics of known exoplanets (planets orbiting stars other than the Sun) to determine how many of them might possess such a magnetosphere.
Currently, astronomers have detected and measured the characteristics of 5,333 exoplanets.4 Several teams of astronomers and astrobiologists have determined the planetary features that are most critical for assuring that a planet possesses a magnetosphere permitting the planet to host life.
metallic core size: First, both astronomers and astrobiologists agree that for a planet to possess other than a remote possibility of hosting ephemeral primitive microbes it must be a rocky planet. For the rocky planet to have a magnetosphere it must possess a sizeable core predominantly composed of iron, nickel, and cobalt. For rocky planets, the planet’s density and the mass fraction of its metallic core decreases with the distance from the host star.5 For example, Mercury’s metallic core comprises about 3/4 of Mercury’s total mass. For Venus and Earth the fraction is 1/3 and for Mars it is 1/5.
More than the core mass fraction, what determines the potential strength of a planet’s magnetic field is the actual size of its metallic core. By this measure, Earth has the largest metallic core of the Sun’s rocky planets. Its metallic core is 5.14 times larger than Mercury’s.
Given its distance from the Sun, Earth has an anomalously high density and large metallic core. These anomalies are explained by its unique origin. The primordial Earth formed like the Sun’s other rocky planets—from condensation and accretion within the Sun’s protoplanetary disk. Some 50–95 million years later, primordial Earth collided and merged with Theia, a rocky planet about twice the mass of Mars. This collision/merger increased Earth’s mass, density, and size of its metallic core and resulted in the formation of the Moon.6
plate tectonics: The strength of a planet’s interior dynamo strongly depends on the heat transport and temperature contrast across the planet’s core-mantle boundary.7 The more pervasive and stronger a planet’s plate tectonic activity the greater the cooling of the planet’s mantle and core.8 The absence of an interior dynamo in Venus can be attributed largely to its lack of plate tectonic activity.
In the solar system the only planet known to have strong, pervasive, enduring plate tectonic activity is Earth. Earth’s plate tectonic level is mostly due to the heat it possesses—owing to the radioactive decay from its superabundance of uranium and thorium and to its high accretional heat that it gained from its collision and merger with Theia. A team of geophysicists led by Francis Nimmo concluded that in spite of Earth’s extraordinarily high abundance of uranium and thorium (340 and 610 times the average for the Milky Way Galaxy), “Earth had just enough radiogenic heating to maintain a persistent dynamo.”9 Earth indeed may be alone among rocky planets in possessing both strong, pervasive, enduring plate tectonics and a strong, enduring magnetosphere.
planet mass: The more massive a planet the lesser the cooling rate of its metallic core and, consequently, the more anemic its dynamo and the weaker its magnetic field.10 For planets more massive than Earth the dynamo lifetime becomes shorter with increasing planet mass. Also, the larger a planet’s diameter the lower its tectonic-driving forces.11
For planets less massive than Earth the mantle cools more rapidly. As the mantle cools, it becomes more viscous. Eventually, the viscosity reaches a level where it shuts down plate tectonics. Thus, for multiple reasons any planet that possesses a sufficiently strong and enduring magnetosphere so that diverse life can persist on its surface must have a mass equivalent to Earth’s.
rotation rate: Astronomers Sarah McIntyre, Charles Lineweaver, and Michael Ireland determined that the slower a planet rotates the weaker the planet’s magnetic field.12 In particular, they calculated that a tidally locked planet (a planet orbiting its host star at a close enough distance where the star’s tidal forces exerted on the planet result in the planet’s rotation rate being slowed to a large fraction of its revolution rate), at best, will have a magnetic field strength only 1/3 of that of a nontidally locked planet. Their calculations, in part, explain why tidally locked Mercury’s magnetic field is only 1% as strong as Earth’s.
The problem that the correlation of a planet’s rotation rate with the strength of its magnetic field poses for rocky exoplanets is that at least 99% of known rocky planets are tidally locked. Only 4 of the more than 457 known rocky exoplanets orbit their host stars more distantly than Mercury orbits the Sun. These 4 are Kepler 442b with an orbital distance of 0.409 AU (1 Astronomical Unit or AU = Earth’s average orbital distance from the Sun: 149,597,871 kilometers or 92,955,807 miles); Kepler 186f with an orbital distance of 0.432 AU; KOI-4878b with an orbital distance of 1.125 AU; and KIC 5522786b with an orbital distance of 1.98 AU.13 For comparison, Mercury orbits the Sun at a distance of 0.387 AU.
Neither KOI 4878b or KIC 5522786b are considered potentially habitable planets since both orbit stars brighter, faster burning, more metal-poor, and more luminosity unstable than the Sun. In neither case is the planet’s mass or atmospheric composition known.
The tidal interaction between Kepler 186f and its host star is about 14 times stronger than it is between Earth and the Sun. For Kepler 442b the tidal interaction is about 22 times stronger. Consequently, both planets will have rotation periods longer than five Earth days. Such long rotation periods not only ensure that the magnetic fields of both planets will be much weaker than Earth’s, they also imply a day-night temperature difference far too high for life to survive on the planet.
That all or nearly all known rocky exoplanets orbit their host stars at less than half the distance Earth orbits the Sun means that they are exposed to much more intense particle, x-ray, and short ultraviolet radiation and flaring activity from their host stars. Surprisingly, stars less massive than the Sun exhibit much more frequent and intense flaring activity and greater emission of x-rays and short ultraviolet radiation.14 Consequently, planets orbiting their host stars at less than half the distance Earth orbits the Sun will require magnetic field strengths at least twice as strong as Earth’s.
Could it be that astronomers simply have not yet discovered rocky planets orbiting their host stars at greater than half the distance Earth orbits the Sun? Astronomers certainly have the telescope power to discover such planets, as evidenced by KOI 4878b or KIC 5522786b. As I explained in a previous article and in my book Designed to the Core, the Sun possesses an elemental abundance that astronomers have yet to detect in any other star.15
The only possible explanation for the Sun’s unique elemental abundance is that early in its history it transferred an anomalously large amount of its mass, refractory elements, and angular momentum to its rocky planets. Such a transfer explains why the solar system stands alone among the 3,932 known planetary systems in that its rocky planets are so massive, dense, and orbitally distant. Such a transfer, in part, explains why Earth alone—among known rocky planets—possesses a magnetosphere capable of shielding surface life from the particles, radiation, and flares of its host star.
Design and Habitability Implications
For a planet to be habitable it is not enough that it simultaneously resides in all eleven of the known circumstellar planetary habitable zones.16 It must also possess an Earth-like electromagnetic shield of sufficient size, strength, and longevity to protect its surface life from the particles, radiation, and flares of its host star and from cosmic radiation. Numerous features of the planet must be exquisitely fine-tuned for such a magnetosphere to possibly envelop the planet. In addition, the planet must experience a collision/merger event early in its history virtually identical to the collision/merger that occurred between the primordial Earth and Theia. Furthermore, it must possess a moon with the same physical and orbital characteristics and the same origin as the Moon.
Such an extraordinary level of fine-tuning testifies of the existence and operation of a Fine-Tuner. That Fine-Tuner must be orders of magnitude more intelligent, knowledgeable, capable, powerful, and caring than we humans. It must be the God of the Bible.
- James Green et al., “When the Moon Had a Magnetosphere,” Science Advances 6, no. 42 (October 14, 2020): id. eabc0865, doi:10.1126/sciadv.abc0865.
- Hugh Ross, “Moon’s Early Magnetic Field Made Human Existence Possible,” Today’s New Reason to Believe (blog), Reasons to Believe, November 16, 2020; Hugh Ross, Designed to the Core (Covina, CA: RTB Press, 2020), 178–181.
- J. Varela et al., “MHD Study of the Planetary Magnetospheric Response during Extreme Solar Wind Conditions: Earth and Exoplanet Magnetospheres Applications,” Astronomy & Astrophysics 659 (March 2022): id. A10, doi:10.1051/0004-6361/202141181.
- Exoplanet TEAM, Extrasolar Planet Encyclopaedia: Exoplanet Catalog (March 16, 2023), exoplanet.eu/catalog/.
- William F. McDonough and Takashi Yoshizaki, “Terrestrial Planet Compositions Controlled by Accretion Disk Magnetic Field,” Progress in Earth and Planetary Science 8 (July 2, 2021): id. 39, doi:10.1186/s40645-021-00429-4.
- Robin M. Canup, “Forming a Moon with an Earth-Like Composition via a Giant Impact,” Science 338, no. 6110 (October 17, 2012): 1052–1055, doi:10.1126/science.1226073; Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids, MI: Baker Books, 2016), 48–58.
- Eric Gaidos et al., “Thermodynamic Limits on Magnetodynamos in Rocky Exoplanets,” Astrophysical Journal 718, no. 2 (July 2, 2010): 596–609, doi:10.1088/0004-637X/718/2/596.
- Francis Nimmo, “Why Does Venus Lack a Magnetic Field?,” Geology 30, no. 11 (November 1, 2002): 987–990, doi:10.1130/0091-7613(2002)030<0987:WDVLAM>2.0.CO;2.
- Francis Nimmo et al., “Radiogenic Heating and Its Influence on Rocky Planet Dynamos and Habitability,” Astrophysical Journal Letters 903, no. 2 (November 10, 2020): id. L37, page 1, doi:10.3847/2041-8213/abc251.
- Gaidos et al., “Thermodynamic Limits.”
- C. O’Neill and A. Lenardic, “Geological Consequences of Super-Sized Earths,” Geophysical Research Letters 34, no. 19 (October 11, 2007): id. L19204, doi:10.1029/2007GL030598.
- Sarah R. N. McIntyre, Charles H. Lineweaver, and Michael J. Ireland, “Planetary Magnetism as a Parameter in Exoplanet Habitability,” Monthly Notices of the Royal Astronomical Society 485, no. 3 (May 2019): 3999–4012, doi:10.1093/mnras/stz667.
- Exoplanet TEAM, “Extrasolar Planet Encyclopaedia: Exoplanet Catalog.”
- Allison Youngblood et al., “The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca II K, and Energetic Particle Fluxes from M Dwarfs.” Astrophysical Journal 843, no. 1 (June 28, 2017): id. 31, doi:10.3847/1538-4357/aa76dd.
- Hugh Ross, “How Did the Sun End Up with Its Unique Rocky Planets?” Today’s New Reason to Believe (blog), Reasons to Believe, May 17, 2021; Ross, Designed to the Core, 225–228.
- Hugh Ross, “Tiny Habitable Zones for Complex Life,” Today’s New Reason to Believe (blog), Reasons to Believe, March 4, 2019.