Space mysteries: Do all planets have magnetic fields?

red and blue lines arc out of the north pole of Earth
On the day side of Earth, magnetic reconnection funnels material and energy from the sun into Earth's magnetic environment. (Image credit: NASA's Goddard Space Flight Center)

One of Earth's defining features is its magnetic field. It forms a protective shield against high-energy particles ejected by the Sun and thus arguably has provided life with a safer place to grow into the complex array of organisms we see today. 

The most stunning indications of Earth's magnetic field are auroras, dancing curtains of colorful light that appear near the North and South poles during times of high solar activity. Another sign that Earth has a magnetic field is that a compass points north wherever you are on the planet. 

But how can we tell if other planets or bodies in the solar system have magnetic fields? And is it possible to know if distant exoplanets have magnetic fields? 

 Related: Why do Earth's magnetic poles flip? 

We know that the solar system's gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune) have strong intrinsic magnetic fields. However, it's a little more complicated for the terrestrial planets and moons, according to Joseph G. O'Rourke, a planetary scientist at Arizona State University. 

Earth, Mercury and Jupiter's moon Ganymede all have internally generated magnetic fields today. Mars and Earth's moon have old crustal rocks that preserve remnants of magnetization from magnetic fields that existed early in their history, O'Rourke said.  

As for Earth's other neighbor, "No intrinsic magnetism has been detected at Venus, but we haven't delivered instruments close enough to the surface to search for magnetized crust," he added.

For a magnetic field to exist on a planet or moon, a large volume of conductive liquid has to be in motion inside that body. A body could lose its magnetic field if those materials were to stop moving, or if there weren't enough of a temperature difference between the heating and cooling of materials to drive the convection of fluids inside a planet or moon, in which case the fluids would move too slowly, O'Rourke said. 

In the case of Venus' apparent lack of a magnetosphere, there are four possibilities, according to O'Rourke. 

The generally accepted idea is that Venus has an Earth-like core that is cooling too slowly. Because Venus lacks plate tectonics, its interior could be cooling more slowly than Earth's.

An illustration shows a coronal mass ejection bursting out of the sun, then striking Earth's magnetosphere. (Image credit: ESA/NASA - SOHO/LASCO/EIT)

However, an alternative possibility is that Venus' interior is completely solid. This would require the planet's core to be far colder than Earth's, which O'Rourke thinks is unlikely. NASA's Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy mission planned for 2031 and the European Space Agency's EnVision mission will attempt to figure out if Venus' core is at least partially liquid. 

Alternatively, Venus may lack an inner core. Earth's inner core helps generate our planet's magnetic field. As it crystallizes, it expels impurities (elements lighter than iron), which creates chemical buoyancy that drives fluid motions. Maybe Venus has not yet nucleated an inner core, so it's missing that extra power source. 

The fourth possibility, O'Rourke said, is that Venus' core might be chemically stratified. The moon-forming impact might have stirred up ancient Earth's core, letting it generate a magnetic field when it started cooling. However, Venus has no moons, which might mean that nothing ever mixed up its core. 

The best way to determine whether bodies in the solar system have magnetic fields is to have a spacecraft travel to the object to measure the magnetic field's intensity with a magnetometer. However, scientists were able to remotely detect Jupiter's magnetic field back in the 1950s by picking up radio emissions from the planet's auroras. 

O'Rourke said magnetic fields are one of the best ways to learn about planets' interiors. The presence of a strong magnetic field tells scientists that the planet has a large reservoir of electrically conductive fluid that can move around. 

"A dynamo is the process by which the energy of fluid motion is transformed into a magnetic field," O'Rouke explained. "In terrestrial planets, metallic cores can host dynamos, as in Earth today. However, liquid silicates (molten rocks, basically) are also electrically conductive at extreme pressures and temperatures. Hydrogen becomes metallic deep in the interiors of gas giants like Jupiter and Saturn, which enables their strong magnetic fields." 

Do any exoplanets have magnetic fields? 

When it comes to exoplanets — planets outside the solar system — planetary scientists have not unambiguously detected the presence of a magnetic field yet. However, O'Rourke thinks we aren't too far off. Astronomers have detected auroras, which arise from magnetic fields, in small stars known as brown dwarfs and low-mass M dwarfs. 

"I would guess that the next generation of instrumentation will be able to detect magnetic fields from Jupiter-like exoplanets," O'Rourke said. "Detections of magnetic fields from Earth-like planets are on a more distant horizon, but hopefully achievable in the next several decades. In general, we can detect exoplanet magnetic fields directly (e.g., by observing aurorae or radiation belts) or indirectly (e.g., by observing the interactions of planetary magnetic fields with their parent stars)." 

Planetary scientists are currently debating if magnetic fields overall protect planetary atmospheres. On one hand, magnetic fields can shield atmospheres from stellar winds, especially near the magnetic equator. On the flip side, magnetic fields can channel charged particles into polar regions, and a number of mechanisms that contribute to atmospheric escape are not strongly influenced by magnetic fields, O'Rourke explained. 

"Earth has maintained both a magnetic field and habitable surface for billions of years," O'Rourke said. "Mars lost most of its water to space roughly when its magnetic field died. Venus, the hell world, lacks a magnetic field. In our solar system, magnetism is correlated with habitability. However, correlation is not causation." 

As we get a larger sample size of exoplanets through observations with the James Webb Space Telescope, planetary scientists will start to reveal the relationship between magnetic fields and planetary habitability. Auroras might be one of the first indicators that we should look a little closer for signs of life. 

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Conor Feehly
Contributing Writer

Conor Feehly is a New Zealand-based science writer. He has earned a master's in science communication from the University of Otago, Dunedin. His writing has appeared in Cosmos Magazine, Discover Magazine and ScienceAlert. His writing largely covers topics relating to neuroscience and psychology, although he also enjoys writing about a number of scientific subjects ranging from astrophysics to archaeology.

  • pennyturtle
    Okay, so I may be putting my hand up for a "kook of the day" badge, but I have a different hypothesis for the generation of planetary magnetic fields. The use of seismology has given us insight into the makeup of the Earth's core, being a reasonably solid crystallising iron with some nickel under immense pressure. We also know that when pressure is applied to these crystals, an electrical charge is produced - piezoelectrics. Magma is known to behave as a plasma, providing a conduit for the flow of an electrical charge, which thanks to Maxwell we know produces both electrical and magnetic fields.

    On Earth, we have plate tectonics, which allows the movement of the charges (and fields) through to the surface. This may explain some of the lightning ubiquitous with active volcanic eruptions. Check out the research by Friedemann Freund if you would like to know more. The lack of tectonic plate movement on Venus may be due to less movement of magma deep in the planet which would otherwise facilitate the flow of charge (and hence fields) towards the surface of the planet, resulting in a vastly reduced manifestation of a magnetic field.

    If I may briefly transition from a badge for being a kook to a proper branding, this piezo model can be extended to the Sun, whereby a solid core of crystalised iron produces a plasma of charged particles on the surface which we see as positively charged protons (hydrogen ions) and negatively charged electrons providing the spectrographic signature for hydrogen.

    Another leap for the hypothesis, which to be honest is more speculation, is to make sense of observed magnetic fields which span not just galaxies, but the vast distances between them. The polarisation of light informs us of their presence, but I'm yet to hear of a plausible explanation for the electrical current producing them. It's important to keep in mind that thanks to old mate Maxwell, we know a magnetic field doesn't exist without an electrical charge or current. So what is the hypothesised generator of these magnetic fields measured in thousands of lightyears? Glad you asked...Piezo... Imagine the immense gravitational pressures toward the centres of galaxies. If we stretch our concept of stars to include iron and nickel etc crystalline cores, we have our mechanism. As for that being a stretch, to be honest I feel it takes more of a stretch to believe that a forming star would, under gravitational forces, see hydrogen collapsing into the centre to form the star, with heavy elements, such as iron, settling further out where planets are forming planets...
  • billslugg
    Piezo current only occurs where the crystal is squeezed in one direction and allowed to expand in the orthogonal direction in order to conserve volume. The Earth's core is indeed a crystal but it is surrounded by liquid thus there can only be isotropic forces, equal all around, thus piezo is not possible.
    Even if piezo occurred at the Earth's core, it is a one time shot. Once the forces reach equilibrium the piezo charges no longer form and dissipate.
    There is no solid core in the Sun. Helioseismology confirms this.
  • pennyturtle
    "Deeper still the rotation appears to be consistent with solid-body rotation." Michael J Thompson, Helioseismology and the Sun's interior, Astronomy & Geophysics, Volume 45, Issue 4, August 2004, Pages 4.21–4.25

    As far as the piezo part of my speculating, it might be worth your while checking out the work of the crystallographer I mentioned, Friedemann Freund, He worked for many years at SETI and NASA, with his research pivoting on attempting to identify early warning signals for earthquakes. His data in the video I've linked is based on testing the current flowing through granite, so yeah, it is pure speculation on my part to connect those processes with what may happen in the Earth's core, but it is, after all, speculation.