How Did the Sun End Up with Its Unique Rocky Planets?

It’s easy to take the solar system for granted, but our present, life-friendly configuration of planets and the Sun has not always been this way. Astronomers continue to discover anomalous properties of the solar system and how they reveal design.

Our solar system began with five rocky planets: Mercury, Venus, Theia, Earth, and Mars; and five gaseous planets: Jupiter, Saturn, Uranus, Neptune, and an unnamed planet slightly smaller than Uranus. Theia merged with the proto-Earth to form the present-day Earth and Moon.1 The Sun’s unnamed gaseous planet either was ejected from the solar system or placed in an orbit about fifty times more distant from the Sun than Neptune.2

In their quest to find planets like ours, so far astronomers have discovered and confirmed 4,715 planets (exoplanets) beyond the solar system.3 Most of these planets are gaseous or what astronomers call super-Earths. While super-Earths may not be predominantly comprised of gases, nevertheless, their atmospheres are far too thick for any of them to be considered possible candidates to host life.

Based on what measurements are available, astronomers have developed two distinct definitions for a rocky planet. NASA defines rocky planets, also known as terrestrial or telluric planets, as planets with diameters less than 125 percent of Earth’s diameter where the masses, if known, are less than 1.4 times Earth’s mass and the orbital distances are less than 2.0 astronomical units from their host stars. (1 astronomical unit = Earth’s average orbital distance from the Sun: 149,597,871 kilometers or 92,955,807 miles.) Where astronomers know the measurements of the mass and diameter of a planet, a rocky planet is defined as a planet up to 110 percent Earth’s mass with a density greater than 3.0 grams/cubic centimeter. So far, astronomers have detected and confirmed 409 such extrasolar rocky planets.

The Sun’s Anomalous Rocky Planets
The Sun’s rocky planets orbit at greater distances than do rocky planets orbiting other stars. Mercury, Venus, Earth, and Mars orbit the Sun at distances of 0.387, 0.728, 1.00, and 1.524 astronomical units (AU), respectively. Their densities are 5.427, 5.243, 5.514, and 3.934 grams/cubic centimeter. Of the 409 detected and confirmed rocky planets orbiting other nuclear burning stars, the one that orbits its star most distantly, Kepler 367-c, orbits at a distance of only 0.253 AU. Of the 197 exoplanets with accurately measured orbital distances that have been classified as rocky planets, 94.4% orbit their host stars more closely than 0.10 AU.4 This statistic (of short orbital distances) can perhaps be partially, but surely not entirely, blamed on observational bias. Astronomers have detected small planets orbiting their host stars as distantly as 1.98 AU.

Astronomers expect the densities of rocky planets to decline with orbital distance. Greater orbital distance means there is less heat from the host star to drive off light gases and light elements. The solar system, with the exception of Earth, illustrates this correlation well. Earth is exceptional in that its collision/merger with Theia produced heat that stripped it of its gases, liquids, and many light elements.

Given how closely rocky exoplanets orbit their host stars, astronomers expected that their densities would far exceed those of the Sun’s rocky planets. Surprisingly, the average density of rocky exoplanets with measured densities is only 4.472 grams per cubic centimeter.5

Clearly, the Sun’s rocky planets stand radically apart from known rocky exoplanets. The big question is why? I’ll review some of the recent discoveries that explain how we got our anomalous planets. Feel free to skim and move to “Significance of the Sun’s Anomalous Elements and Rocky Planets” if you don’t need all the details.

Explanation of the Sun’s Anomalous Rocky Planets
In their search for a star that is a twin of the Sun, astronomers have recognized that the Sun is anomalous.6 Could it be that the Sun’s anomalous nature among stars may also explain the anomalous nature of its rocky planets?

The Sun’s relative abundance of elements is unlike that of any other known star. Several astronomical studies, when considered together, now explain the link between the Sun’s unique abundance of elements and unique configuration of rocky planets.

In one study, a team of astronomers led by Jorge Meléndez compared the Sun’s elemental abundance with that of 21 stars closely resembling the Sun.7 They found that the Sun is 20 percent more depleted than is typical in its ratio of refractory elements (those with high boiling points) to volatile elements (those with low boiling points). A follow-up study of 79 Sun-like stars affirmed this finding.8

In particular, the quantity of lithium observed on a star’s surface (photosphere) holds special importance. It serves as a sensitive indicator of the star’s interior characteristics and composition. Stars like the Sun are unable to manufacture lithium in their nuclear cores. What lithium they acquired in their formation process came from the nucleosynthesis that occurred during the first few minutes after the cosmic origin event. However, reactions operating within the stellar core and radiative zone act to destroy the lithium.

Elemental analysis of primitive solar system meteorites indicates lithium was relatively abundant in the gas cloud out of which the Sun and its planetary system formed. However, the amount of lithium on the Sun’s surface9 is far below that of the primordial solar system—roughly 170 times below that level.10 This large difference is exceptional, given the Sun’s relative youth—just 4.567 billion years of age.11 (Stars close to the Sun’s mass can sustain nuclear fusion for 9–10 billion years.)

Astronomical observations have established a strong inverse correlation between stellar rotation rate and lithium depletion. Stellar rotation affects the mixing efficiency of stellar matter at the boundary between the convective and the deeper radiative zones. The higher the rotation rate, the more effectively stellar material in the convection zone, in the form of penetrating plumes, is blocked from entering the inner radiative zone.

The influence of the Sun’s changing rotation rate on plume penetration makes perfect sense of the helioseismology data.12 It also explains observations of the Sun’s magnetic field and the Sun’s magnetic activity.13

Stellar rotation rates decrease as stars age. For stars of similar mass to the Sun, the observed rate of rotation accurately reveals the age of the star. The older the star and the more its rotation has slowed, the less lithium that remains in the star’s photosphere (surface).

The Sun differs, however, from the typical relationship between a star’s age and surface lithium abundance. A team of astronomers led by Marília Carlos measured the surface lithium abundance in 77 stars that most closely matched the Sun’s effective temperature, surface gravity, and metallicity.14 Their research affirmed a strong linear correlation between stellar age and surface lithium depletion, but the Sun’s surface lithium abundance proved far lower than that of any star in its age range, 4.1–5.1 billion years.

Carlos’s team noted that the most lithium-depleted stars in their sample of solar-like stars also had the fewest refractory elements. In this respect they appeared similar to the Sun. But this finding led to a question: How can such relatively metal-rich stars be so poor in terms of refractory elements?

In 2004, another team of astronomers, led by Garik Israelian, compared the surface lithium abundance of 79 stars that host planets with 38 stars that host none.15 They noticed greater lithium depletion in planet-hosting stars with effective temperatures between 5,600 and 5,850 Kelvin, closely matching the Sun’s effective temperature of 5,772 Kelvin. This severe lithium depletion, they explained, is the expected outcome of the transfer of angular momentum from a star to its protoplanetary disk and eventual planets. Such angular momentum transfer will have a braking effect on rotation in the planet-hosting star’s convective zone, thus deepening the zone and leading to greater penetration of plumes into the radiative zone.

In 2006, in a parallel study, astronomers Yu-Qin Chen and Gang Zhao affirmed that planet-hosting solar-type stars manifest greater lithium depletion than stars without planets.16 In 2009, in a follow-up study of 451 stars with effective temperatures similar to the Sun’s, Israelian and his colleagues found that 50% of solar-analog stars with no detected planets have, on average, 10 times more surface lithium than solar-analog, planet-bearing stars.17

In 2019, Carlos’s team clarified that the key difference maker is not just whether a star hosts planets but, rather, what kinds of planets it hosts.18 In particular, what matters is the total mass of rocky planets a star hosts and how far they orbit from the host star.

The solar system has the second-highest total mass of rocky planets of any known planetary system, exceeded only by the TRAPPIST-1 system.19 The seven TRAPPIST-1 planets, however, orbit their host star at distances ranging from only 0.01–0.06 AU, where the most massive planet in the system is only 15% more massive than Earth. Furthermore, five of the seven TRAPPIST-1 planets are not actually rocky. They possess very thick “envelopes of volatiles in the form of thick atmospheres, oceans, or ice.”20 Therefore, the solar system differs from other known planetary systems in both the quantity of refractory elements and of angular momentum transferred from the host star to its system of rocky planets. (The angular momentum transfer is roughly determined by the masses and orbital distances of the planets.)

Astronomers now understand that the Sun’s exceptionally low abundance of lithium and refractory elements likely explains the Sun’s unique system of rocky planets. One team of astronomers concluded their research paper by saying the peculiar solar elemental composition “would imply that solar-like stars with planetary systems similar to our own are a relatively rare occurrence.”21

Significance of the Sun’s Anomalous Elements and Rocky Planets
What do all these atypical features tell us? The Sun’s anomalously low abundance of lithium and refractory elements together with the Sun’s mass and age help keep the Sun’s flaring activity level extremely and uniquely low.22 These solar features help account for the Sun’s exceptionally low levels of short ultraviolet and x-ray radiation, at present. Only because these levels and intensities of flares and of short ultraviolet and x-ray radiation are extremely low is global, high-technology civilization on Earth possible.

Theoretically, the enormous quantity of refractory elements and angular momentum transferred from the Sun to its rocky planets should allow for a high-mass, high-density rocky planet, orbiting as distantly as 1.0 AU. Such a transfer of elements and angular momentum does not guarantee that this planet would not also possess a relatively thick atmosphere, one at least twice as thick as Venus’s, and a thick hydrosphere. In reality, the Sun’s transfer of angular momentum and refractory elements to its rocky planets produced five distantly orbiting rocky planets, two of which orbited at a distance of approximately 1.0 AU.

The early collision between proto-Earth and the Sun’s fifth rocky planet, Theia, guaranteed that Earth would become massive enough, dense enough, and sufficiently stripped of gases and water that it could become a home for advanced life. This early collision also produced the Moon. It was the early coupling of the Moon’s magnetosphere with Earth’s magnetosphere that prevented Earth from losing all its atmosphere and hydrosphere.23

Earth’s orbit from the Sun is barely distant enough to prevent Earth from becoming tidally locked to the Sun and ending up with a long rotation rate like that of Mercury or Venus. Earth’s companion rocky planets play important roles in stabilizing the orbital architecture of Earth and the other solar system planets.

These discoveries about the Sun’s anomalous elements and anomalous rocky planets add to the weight of evidence for the rare Earth, rare Sun, and rare planetary system doctrines. These doctrines state that Earth, the Sun, and the solar system all are amazingly and uniquely designed to make it possible for humans and global high-technology civilization to exist and thrive on Earth.


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