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the key:
Auroras on exoplanets might be one of the first indicators
that we should look a little closer for signs of life.

from
https://www.space.com/do-all-planets-have-magnetic-fields

Space mysteries: Do all planets have magnetic fields?
News
By Conor Feehly published 3 hours ago
Scientists are learning more about how common magnetic fields are around planets and moons.

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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.

red rays radiating out from the sun are deflected by blue rays emanating
from Earth

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.

RELATED STORIES:
 — Ancient rocks hold proof of Earth's magnetic field. Here's why that's puzzling

— Why do Earth's magnetic poles flip?

— Debris from burning satellites could be affecting Earth's magnetic field

"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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night sky and more! And if you have a news tip, correction or comment,
let us know at: [email protected].

Conor Feehly
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.

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