# Could a planet have a core of anything other than iron?

Specifically, I was wondering if a core of magnitite or some such other magnetic metal could be a planet's core. This is in relation to my other question, found here:

Can airborne floating/flying islands be scientifically possible?

• Are you including gas giants in this, or are you just concerned with terrestrial planets? – HDE 226868 Jan 17 '16 at 22:15
• Thank you everyone for answering and questioning. This is phenomenal help! ^^ – Mr. Question Jan 19 '16 at 5:27

The answer depends on what your question really is.

Could you theoretically construct a magnetic planet using a core materially different than iron? Yes

Nickel and cobalt have are similar to iron in terms of their electrical, magnetic, and physical properties and both are a little denser than iron.

Could such terrestrial planets form naturally? No

Based on the standard models of stellar nuclear synthesis and planetary formation, neither cobalt nor nickel exist in sufficient quantity to become the dominant material in a planetary core. Although nickel is formed in considerable abundance in supergiant stars, the nickel is unstable and decays into iron. Not all of the nickel in stellar syntheses decays to iron, though much if not most nickel is thought to be derived from supernova debris. Supernovas are the accepted source for essentially all of the heavy elements past iron. But the mass fractions are all relatively small.

Note that Earth's core is considered to consist of iron primarily, but with a significant fraction of nickel (perhaps 5%).

I know that at normal temperature an electromagnet typically uses a ferromagnetic core to intensify the magnetic field. I also know that the temperature of the Earth's core is expected too far exceed the Curie temperature of iron, nickel and cobalt so this is not a conventional option. But it is also reported that Curie temperature is pressure dependent, and that it increases with pressure - but I was not able to find Curie temperature for material under core pressures. Given that the Earth's magnetic field is thought to derive from the flow of liquid iron outside the inner core, I would expect the inner core to still be well above the Curie temperature in any configuration able to generate a magnetic field, but I could not actually confirm this.

I don't assume the terrestrial planet must have core temperatures that match Earth's (it could have less radioactives, etc.) but I would expect that pressure would be similar given a similar planet size. It should also be observed that the details of the Earth's magnetic field are poorly understood as should not be surprising considering that we cannot actually observe the conditions at the core.

We only know of three types of planets, rocky, gas giant, and ice planets. Earth has the strongest magnetic field of the rocky planets. The gas giants all have much stronger fields in terms of total field strength, whereas icy Pluto (former planet) has no magnetic field.

The question referenced a previous question that clearly restricts the interest to earth-like planets as it is inhabited by humans. But Jupiter is thought to have a magnetic core due to the flow of liquid metallic hydrogen.

But for earth-like planets, the only reasonable natural magnetic core is iron; it has the abundance and density requirements. Magnesium (mostly as oxides) is thought to the second most abundant metal on earth - mostly in the mantle - but it is largely excluded in the core by the considerably more dense iron. The also relatively common metals aluminum and calcium are also too light to exist in abundance in the core.

The problem with having magnetite as the core of an Earth-like planet is the pressure and associated temperature that the core would reach. Magnetite ($\text{Fe}_{3}\text{O}_{4}$) breaks down at very high pressures and temperatures (800°C and 10-11GPa) to a different phase, $\text{Fe}_{4}\text{O}_{5}$, which is similarly magnetic to magnetite. However, the pressure in the core of the Earth is more than 300GPa and temperatures can reach as high as 5000°C. I haven't seen any proper research on magnetite in these conditions, but it could be safe to assume that pushing it to those temperatures would cause either completely new phases with differing magnetic properties to form, or it would simply cause the oxide to melt.

The reason that the core of the Earth is iron (and nickel, gold, and platinum group elements) is that these elements are among the heaviest elements present in the formation of the planet, and preferentially dissolve into molten iron (siderophile elements). As such, when the planet was freshly accreting from the disc of dust around the Sun, these elements sunk into the core because they were denser than the surrounding material. So to answer your first question, whatever the core of your planet is has to be the densest thing that was there when it formed, assuming your planet formed in the same way as Earth did.

• The fact that oxygen is common in rocks and thus available but not in the core is a good sign that it's excluded and floats at this temperature and pressure. – JDługosz Jan 18 '16 at 12:07

Possible candidates for a magnetic planetary core are

• Those materials that are heavy enough to sink to the center of a planet during formation.
• Those materials that are reasonably abundant (probably those that can be produced through ordinary nucleosynthesis in stars).
• Those that can generate a magnetic field (meaning either Ferromagnetic, or electrically conductive).

It should be noted that iron is the heaviest element that can be produced by ordinary nuclear fusion in stars. Fusion that creates heavier elements does not release energy and therefore stars are not likely to produce them. Therefore likely core materials for a planet are probably below iron on the periodic table.

Also that elements with odd numbered atomic masses are typically much less abundant in the universe.

Based on their abundance in the universe C, Si, S are the most likely to form the bulk of a planet, but none of them are magnetic.

Nickel is another possible choice for a core and it is magnetic, but much less abundant than Iron.

Most materials will not retain any magnetism once they pass their curie temperature.
For iron the curie temperature is 1043 Kelvin
For Nickel the curie temperature is 627 Kelvin

If there is magnetic field generated in a molten metal core it is most likely due to circulating electric currents in the metal rather than permanent magnetism. In that case, any conductive metal would probably do. Aluminum is the third most abundant element on Earth, and it is very conductive, so it is a reasonable choice.

• Aluminum is the 3rd most abundant element in the Earth's crust, not the whole planet. Its abundance in the crust is attributed at least in part due to the fact that it is comparatively light. – Gary Walker Jan 18 '16 at 4:37
• Ferromagnetism is not going to happen in liquid or high temperature in general. The magnetic dynamo simply requires a conductive liquid. – JDługosz Jan 18 '16 at 12:09