Exoplanet habitability depends on a whole host of factors, with liquid water at the top of the list. It also needs a stable atmosphere, the right chemistry, and possibly even things like plate tectonics or other geological activity. Planetary magnetic fields are a critical part of the formula, too, but detecting them from Earth’s surface is difficult.
The problem is Earth’s ionosphere. This atmospheric layer creates a barrier or ceiling that blocks some radio frequencies from reaching Earth’s surface. Astronomers think that the most detectable signals from exoplanet atmospheres are below 10 MHz, but this is below the barrier created by the ionosphere.
The Moon has no such ionospheric barrier, and a new white paper shows how a radio array on the Moon could be the tool needed to study exoplanet magnetic fields. It’s titled “Studying Exoplanets in the Radio from the Moon,” and the lead author is Dr. Jake Turner. Turner is from the Department of Astronomy at Cornell University.
Earth’s ionosphere is made up of ions, electrically charged atoms and molecules. The Sun’s UV and X-ray emissions have stripped electrons from them , meaning they’re not electrically neutral. The ionosphere isn’t a single layer; instead it’s made of layers with different ion densities. It also makes up the inner portion of Earth’s magnetosphere.
The ionosphere is an abundant layer of electrons and ionized atoms and molecules that stretches from about 48 kilometers (30 miles) above the surface to the edge of space at about 965 km (600 mi), overlapping into the mesosphere and thermosphere. This dynamic region grows and shrinks based on solar conditions and divides further into the sub-regions: D, E and F; based on what wavelength of solar radiation is absorbed. Image Credit: By NASA Goddard – https://www.nccs.nasa.gov/news-events/nccs-highlights/global-ozone-profile, Public Domain, https://commons.wikimedia.org/w/index.php?curid=147614046
The ionosphere is used to propagate radio signals across large distances on Earth, and it’s that property that makes it a barrier to studying exoplanet magnetic fields.
The Moon is barren, and has neither a thick atmosphere nor an ionosphere. It has only an extremely thin atmosphere called an exosphere, which is made up of sparse molecules held in place by the Moon’s gravity. This is why Turner and his colleagues have their sights set on the Moon.
“Despite decades of searching, there is still no conclusive detection of an exoplanet’s magnetic field,” the authors write, while noting that some promising hints have begun to emerge. In 2021, Turner and co-researchers detected hints of a magnetosphere at Tau Bootis b, an exoplanet about 51 light-years away.
The search for exoplanet magnetic fields comes down to auroral emissions. “Observing planetary auroral radio emission is one of the most promising methods to detect exoplanetary magnetic fields,” the researchers explain in their white paper. “An exoplanet’s magnetic field can be detected through radio emission from the planet generated by the electron-cyclotron maser instability (CMI).” In our Solar System, all of the magnetized planets and moons emit radio emissions through the same mechanism.
This figure shows the planetary magnetic fields of some Solar System planets and also shows the frequency cutoff for radio observations from space versus from the ground. Image Credit: Turner et al. 2025.
Most of these emissions were detected by satellite; only Jupiter’s were strong enough to detect from the ground. This is simply not possible when it comes to exoplanets, since they’re much further away and their emissions are much weaker when they reach us. An array of radio antennae on the Moon is what’s needed to study exoplanet magnetic fields according to the authors.
There are already so-called pathfinder missions in this regard. The Lunar Surface Electromagnetics Experiment (LuSEE-Night) is a robotic radio telescope in the planning stages. If completed, it will land on the lunar far side as early as 2026, by Commercial Lunar Payload Services (CLPS). Lu-SEE Night will make all-sky radio observations from the far side by using the Moon to block Earth’s interfering radio emissions.
The Radiowave Observations on the Lunar Surface of the photo-Electron Sheath (ROLSES-1) instrument travelled to the Moon on Intuitive Machines’ Odysseus lunar lander in February 2024. Unfortunately, the lander tipped over on its side after landing. However, ROLSES-1 was able to collect a small amount of data from the lunar surface, though deployment was not optimal. Though short-lived, the instrument’s experience and limited data were invaluable for future lunar radio telescopes.
In their white paper, the authors explain how two proposed missions could advance the study of exoplanet magnetic fields. The FarView Observatory is a proposed radio telescope array comprising 100,000 individual dipole antennae covering about 200 square km. It would be manufactured in-situ on the lunar farside by extracting metals from lunar regolith.
FARSIDE (Farside Array for Radio Science Investigations of the Dark Ages and Exoplanets) is another concept for a lunar farside radio telescope. It would consist of an array of 128 dual-polarization antennas covering 10 square km. It would image the available sky every minute.
“FarView and FARSIDE5 will revolutionize the study of exoplanetary magnetospheres,” the authors write. FarView can detect weaker signals, so should be able to study the magnetic fields of everything from Earth-size terrestrial planets to gas giants like Jupiter. “Interestingly, the available target-list for FarView within the 5-10 MHz frequency range includes a handful of super-Earths and Neptune-like planets,” the researchers explain. That means that FarView will refine the dynamo modelling for habitable planets in preparation for FARSIDE’s eventual operation.
FARSIDE will be able to study magnetic fields around dozens of exoplanets, including some of the nearest candidate habitable exoplanets. It will also work in conjunction with other observatories that study exoplanet atmospheres. “FARSIDE will be extremely complementary to the atmospheric studies by JWST, HWO, and ground-based 30-m class telescopes for understanding the habitability of nearby terrestrial exoplanets,” the authors explain.
The concept of the habitable zone is useful even though it’s a rather ill-defined concept. Liquid water is almost certainly necessary for life, and the habitable zone as defined by an exoplanet’s distance from its star is a primary consideration. But Earth shows us there’s more to habitability than water. Our planet’s magnetic shield makes life possible, as do things like plate tectonics and geological activity, most likely.
While sensing geological activity from light-years away is beyond our grasp, sensing magnetic fields is almost within it.
“In summary, FarView and FARSIDE will open up a whole new regime in statistical exoplanetary radio science and comparative planetology,” the authors conclude.