Coreless Terrestrial Planets

Coreless Terrestrial Planets

It is now obvious to all planetary scientists that Earth possesses many apparently designed features that have enabled it to support life for billions of years, and to support advanced life in particular.

As I described in last week’s Today’s New Reasons To Believe, two MIT planetary scientists added to the list of these features. Their modeling of the formation history of planets the size of, and larger than, Earth led to this conclusion: barring any subsequent extraordinary collision events, such planets will end up with at least a hundred times more water and carbon than what our home planet presently contains.1Earth’s extreme lack of water and carbon makes possible its thin atmosphere and exposed continental landmasses—both essential features for advanced life.

Now, the same two scientists have extended their modeling of terrestrial planets (rocky, Earth-like planets) to test whether or not the pervasive assumption that all such planets will form a metallic core is really correct.2 This assumption arises from the fact that all the rocky planets in the solar system possess metallic iron-dominated cores. These cores developed very early in the formation of the solar system. Driven by the heat arising from the kinetic energy of accretion (growth), the melting of the metal and silicate portion of the planet caused the denser metallic iron to fall toward the planet’s core.

The reseachers suggest two different accretionary paths for the formation of a rocky planet that produces a coreless planet. In the first case, the planet forms from material in the star’s protoplanetary disk that is already fully oxidized before the accretion process begins. Support for this proposal comes from the existence of chondrite meteorites (the remnants of the solar system’s protoplanetary disk) that contain no metallic iron but rather iron oxides bound into silicate mineral crystals. Cooler accretion temperatures caused by either a later planetary accretion time, or a planetary accretion more distant from its star or about a cooler star would favor this scenario.

In the second case, the planet forms from both water-rich and metal-rich material. (Protoplanetary material contains three dominant components: water, silicate rock, and iron metal.) Instead of sinking into the core, the metal iron reacts with the water to form iron oxides. This oxidized iron becomes trapped in the mantle, unable to form a core. The planet will end up coreless, providing the oxidation rate for the iron is faster than the sinking of metallic iron to form a core.

The planet formation pathways the MIT team describe appear to be just as likely, perhaps even more, than the means by which the solar system’s array of rocky planets formed. One way to find out for sure would be to measure the density of rocky planets orbiting other stars. A planet with a metallic core will exhibit a higher density than a planet without such a core. Further, the larger the metallic core relative to the remainder of the planet, the higher the density. Earth with its huge metallic core manifests an extremely high density (even more remarkable given how far away Earth is from the Sun).

Calculating the density of extrasolar planets requires measurements of the planet crossing in front of the image of its star (or transit measures). Such measures provide the diameter of the planet, which when combined with the mass determinations that arise from calculating the planet’s orbital parameters, yield its density. (Density = mass divided by volume.)

So far, astronomers lack the instrumental power necessary to accurately measure the diameters of extrasolar planets (see here and here) as small as Earth. However, such instrumental power is soon to arrive. Then, astronomers will be able to test the conclusions of the MIT team.

No one can argue, however, that the researchers’ conclusions are unreasonable. No longer can astronomers presume that all rocky planets possess metallic cores and certainly not metallic cores as enormous as Earth’s.

Earth is only the second densest planet in the solar system. Mercury is slightly denser. However, Earth’s density is truly gigantic when one considers both its mass and its distance from the Sun. The more massive a protoplanet, the stronger will be its gravitational capacity to accrete lightweight material from the protoplanetary disk. The more distant a protoplanet from its star the cooler will be the protoplanetary disk material in its immediate vicinity. Cooler temperatures permit the accretion of lighter-weight material.

The table below shows just how outstanding the Earth is among all the Sun’s rocky planets. For each planet its density is multiplied by its distance squared from the Sun (relative to Earth’s distance) and by its mass (relative to Earth’s mass).

– Mercury 0.048
– Venus 2.176
– Earth 5.517
– Mars 1.033

No accretion pathway exists by which Earth could have ended up with such a high density and huge metallic core. It attained these features thanks to an exquisitely designed collision event early in its history, an event that also led to the formation of the Moon.3

Our planet’s gigantic metallic core is just one of many features that must be fined tuned in order for it to sustain advanced life. Without this particular core, Earth could not maintain a long lasting, powerful dynamo in its core. It is this dynamo that is responsible for the strong, enduring magnetic field that so ably protects advanced life on Earth from deadly solar and cosmic radiation.

As the MIT team’s analysis demonstrates, the more we learn about the physics of extrasolar planetary systems, the more evidence we accumulate for the supernatural, super-intelligent design of the Milky Way Galaxy, the solar system, and Earth for the benefit of all life, both simple and complex.

  1. Linda T. Elkins-Tanton and Sara Seager, “Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets,” Astrophysical Journal 685 (October 1, 2008): 1237-46.
  2. Linda T. Elkins-Tanton and Sara Seager, “Coreless Terrestrial Exoplanets,” Astrophysical Journal 688 (November 20, 2008): 628-35.
  3. Hugh Ross, Creation As Science (Colorado Springs: NavPress, 2006), 111-15.