HIGH in the sky, giant ribbons of colour dance and flicker. It is a good night for aurora watchers. The strips of red turn cyan and back again as the clouds roll by. Stars twinkle in the night sky beyond. A full moon is rising above the horizon, while the crescents of another two moons arch majestically.
Welcome to one of the many planets outside our solar system. More than , most by watching for wobbles in starlight or a brief dimming of their parent star. Such techniques can tell us about a planet’s mass, radius and orbit. Other details can be gleaned from the handful of planets that are big enough and close enough for molecules in their atmospheres to be detected.
Now space scientists think that alien auroras could further enhance our understanding of exoplanets. No one has ever seen the auroral light from an exoplanet – even from those orbiting the nearest of the stars outside our solar system. The light is simply too faint to travel the huge distances involved. But auroras have another trait that researchers think could be detected: they emit radio waves.
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Detecting such radio emissions from just one exoplanet would give us a wealth of information that is not available from other techniques. As well as revealing previously hidden worlds, different aspects of the emission could also allow us to calculate the length of an exoplanet’s day and the strength of its magnetic field. It could even provide clues about the internal processes that drive the magnetic field, as well as how the planet interacts with its parent star and, possibly, whether it has any moons. “All of this is about understanding more about exoplanets than what we can determine from present methods,” says Jonathan Nichols of the .
Here on Earth, the northern and southern lights are the result of electrons accelerated by the solar wind colliding with gas molecules in the upper atmosphere, causing them to radiate light at a characteristic wavelength. Oxygen emits the most familiar greenish-yellow light, and nitrogen molecules glow red or blue. But before they collide, the electrons gyrate around the planet’s magnetic field lines and in the process emit radio waves.
The sky-lighting effect isn’t just confined to Earth. Auroras have been spotted on , , . And there are good reasons to expect that at least some exoplanets will have them too, based on a of solar-like flares thought to be caused by the magnetic field of the host star entangling with the magnetic field of a planetary companion.
“There are good reasons to expect that at least some exoplanets will have auroras like Earth’s”
Of all the auroras in our solar system, Jupiter has the brightest. Twice as massive as all the other planets combined, Jupiter is the king of the planets. It also has the strongest magnetic field and the most beautiful rings of auroras at its poles. We can’t observe them using ground-based telescopes because the light from Jovian auroras is mostly ultraviolet, which cannot penetrate Earth’s atmosphere.
The first photographs were beamed back by NASA’s Voyager 1 spacecraft when it flew by the planet in 1979. Today we rely on ultraviolet images from the Hubble Space Telescope and X-ray images from the Chandra Observatory as they orbit Earth.
But it was, in fact, low frequency radio signals from Jupiter that first revealed that it has auroras (see diagram). The signals allowed scientists to calculate the planet’s magnetic field strength long before we sent Voyager 1 there for a direct measurement.
Radio is still a promising means of finding and studying exoplanets, because if a planet has a magnetic field, it is potentially capable of emitting radio signals that are stronger than those of its parent star.
“If you want to observe a planet far away and you are looking in optical or infrared wavelengths, the planet is much fainter than its parent star, so it’s very difficult to detect,” says Philippe Zarka of the Paris Observatory in France. The reason is that stars are very hot and bright at optical wavelengths and planets are not, so our view of them is swamped by starlight. “By contrast, the radio emission is related to the magnetic field strength of the planet and the electrons moving along it, not its temperature,” he adds.
To be able to predict and interpret these observations, however, our understanding of Jupiter is crucial. “A lot of the exoplanets found so far resemble Jupiter in some respects,” says Nichols. “Jupiter provides a link to other bodies that we can’t get to.”
Nichols is a dedicated Jupiter enthusiast, having focused on the planet and its auroras for his PhD and stayed with it ever since, alongside study of the auroras on Saturn. “Jupiter is a fascinating system, it’s hard not to evangelise. My wife just rolls her eyes these days,” he says.
In the low frequency range, at a few tens of megahertz, Jupiter’s radio emissions show up as brightly as those of our sun, but even this would not be detectable if Jupiter were as far away as the nearest stars. Not all is lost, though. “We expect that some planets will produce radio emissions much stronger than Jupiter,” says Zarka, who is simulating how an exoplanet could emit a sizable radio signal, and what that signal could reveal.
For example, the frequency with which an aurora emits radio waves depends on the strength of the magnetic field. Helpfully, this radio emission is beamed outwards from the magnetic field in conical beams, which sweep around as the planet rotates, like a lighthouse. This sweep would appear as a pulse of radio waves at a telescope on Earth, allowing us to calculate the planet’s rotation period.
The signal is also circularly polarised, meaning that its electric field rotates as the signal travels. This allows us to differentiate between planetary signals and radiation from the star, which is unpolarised because it is produced by bursts of electrons rushing through its outer atmosphere. “If you measure strong circular or elliptical polarisation, it’s very likely to have come from the planet rather than the star,” says Zarka.
Tune in to other worlds
The first group to search for exoplanets using radio was led by William Erickson, professor emeritus at the University of Maryland. Inspired by the success of Jupiter’s auroral radio signals, in 1977 his team looked for planets around nearby stars by searching for pulsed radio emissions from alien auroras. Using the near Borrego Springs, California, the group looked at 22 stars, estimating that their telescope could detect a burst if it were 1000 times stronger than Jupiter’s strongest bursts. Nothing came of it. Radio telescopes at the time were not sensitive enough to pick up signals from such distant sources.
Now, 35 years on, interest in auroral radio emissions has been reignited by the completion of a new, highly sensitive radio telescope called the Low Frequency Array. is the largest and most sensitive radio telescope below 250 megahertz ever constructed.
Ten years in the making, it is a vast array of over 45,000 small antennas, with its heart in the Netherlands – an army of tethered poles rising above a grassland nature reserve in the quiet north east of the country. Other antennas are installed across France, Germany, Sweden and the UK. Scientific operations began last month and Nichols hopes to be searching for exoplanets in the very near future.
In the meantime, his team and Zarka’s have been calculating what sort of giant planets could emit radio signals detectable by LOFAR, and what the signals could reveal about the underlying planet. This is where expert knowledge of auroral processes in our own backyard comes into its own.
Both teams have considered a Jupiter-like planet, because most exoplanets found so far have a mass greater than or equal to Jupiter. There are two possible scenarios for strong radio emissions.
First, the planet orbits close to its parent star and is strongly buffeted by the solar wind, which reconfigures the planet’s magnetic field. This drives flows of charged particles and aurora-producing currents, characteristic of auroral processes on Earth.
Zarka and Sebastien Hess, who is based at the Atmosphere, Environment and Space Observation Laboratory () in Guyancourt near Paris, France, considered this “hot Jupiter” scenario, developing a the radio signals that could arise depending on how the planet interacts with its parent star and where the auroral emission occurs on the planet.
“We found the model very successful at accounting for the radio emissions from planets in our own solar system,” says Zarka. What’s more, they found that they could analyse a radio signal from an exoplanet and use it to deduce physical quantities that cannot be found any other way. These include the inclination of a planet’s orbital plane, the tilt of the magnetic field relative to the rotation axis, as well as rotation period, orbital period and magnetic field strength. This will help us to understand the evolution of exoplanets as well as the way they behave today.
The second option is that the radio emissions are mainly associated with an orbiting moon, as is the case for Jupiter. Its third largest moon Io is a volcanic world, with eruptions spewing ionised gas towards Jupiter at a rate of 1000 kilograms per second. Unlike the northern and southern lights on Earth, which are due to the solar wind, it is this ionised gas that is mostly responsible for Jupiter’s auroras. “It is reasonable to assume that such active moons may be relatively prevalent amongst Jupiter-like exoplanets,” says Nichols.
Nichols tackled this second scenario, examining how a planet’s rotation rate, rate of outflow of ionised gas from a moon, orbital distance and the ultraviolet brightness of its parent star would affect radio emissions. He found that massive, fast-rotating planets could produce bright radio emissions detectable from 150 light-years away.
So far there are no confirmed detections of auroral signals from exoplanets in the ongoing searches at other telescopes. Walid Majid of NASA’s Jet Propulsion Laboratory in Pasadena, California, and colleagues have looked at half a dozen exoplanets using the located 80 kilometres north of Pune in India. They believe the main reason for not detecting any radio signals from these planets is that the instrument can’t observe at a low enough frequency.
For example, Jupiter doesn’t emit intense radio waves above 40 megahertz, a cut-off frequency that depends on the planet’s magnetic field strength. So if you don’t look below this cut-off frequency for an exoplanet, you wouldn’t see anything. The lowest frequency GMRT can detect is 50 megahertz.
LOFAR will be able to detect radio signals down to 10 megahertz, which is more promising. However Earth’s atmosphere blocks frequencies below 10 megahertz, so you would need a space-based antenna to search below this limit. Majid suggests the surface of the moon would be a good location for such a telescope.
Telescope sensitivity also influences our ability to observe auroral radio signals. This can be improved by adding more antennas or by identifying and removing noise in the signal caused by other sources of radio waves. Majid is optimistic that radio astronomers are up to the task, with LOFAR starting up and another huge radio telescope called the which is under construction in South Africa and Australia.
“If we don’t detect anything with LOFAR after a few years, it won’t be because we are missing the beams of radio waves from exoplanets,” says Zarka. “It will be because there is no emission. I have a fair hope that we will detect something. But of course, this is research, so you’re never sure.”
Space physicists hail low frequency radio as ““. Certainly as the sensitivity of LOFAR increases, and our detection of exoplanets using other techniques improves, being able to define the magnetic field characteristics of a planet will reveal more and more. “The existence of a magnetic field is interesting as we try to understand planetary evolution, but also because it has effects that are favourable for the habitability of a planet,” says Zarka. Earth’s magnetic field produces a large protective shield called the magnetosphere that deflects energetic charged particles, such as cosmic rays that can damage DNA and potentially prevent life from evolving. It also preserves our atmosphere by trapping the ionised upper layers of the atmosphere.
“The existence of a magnetic field is favourable for the habitability of a planet or moon”
The importance of such a magnetosphere for the habitability of a planet is partly what has motivated the European Space Agency’s mission to Jupiter and its moons in 2022 – dubbed the , or JUICE – which will eventually go into orbit around Jupiter’s largest moon Ganymede.
Ganymede is made of rock and ice and has its own magnetic field and auroras. “It’s one of the best places in the solar system to look for extraterrestrial life,” says Nichols. He’s excited by the prospect of sending a spacecraft directly to the action, something you could never do for exoplanets. For this reason, and because Jupiter has already revealed so many interesting features, “Jupiter will always be my favourite planet”, says Nichols.
