16 mins read 18 Nov 2021

Distant Aurorae Hint At Possible Exoplanet Detections

Using low-frequency radio telescopes, along with a space-based observatory, Australia’s Dr Joseph Callingham and Dr Benjamin Pope have been able to achieve something that has been sought after for decades - finding potential exoplanets at radio frequencies. And it might not all be good news for the exoplanets.

Artist concept of an exoplanet interacting with its host star’s strong magnetic field. Credit: ASTRON.

Over the last few weeks, thousands of people in the higher northern and southern latitudes have been treated to an astronomical feast, with night skies lighting up in a dazzling dance of giant-curtain-like structures flaring in bright neon greens, electric purple and shimmering reds. 

These events aren’t anything new and have been occurring around Earth’s polar regions for, well as long as the Earth has practically been around. But as humans, we are drawn to the sky, and the spectacular display of the aurora is nothing short of stunning. In particular, the recent aurora events have been particularly strong, even being observed as far north as Perth, or as far south as Glasgow. The reason for this comes back to a special interaction that occurs between the Sun and the Earth. 

Aurorae are generated when charged particles are streamed off the Sun, sometimes as a result of massive eruptions known as coronal mass ejections, and traverse outwards across the Solar system. Normally they pass by us or are heading in a variety of directions, but in some cases - the Earth is in the firing line of these high-energy particles.

When they arrive here, our magnetic field - which acts as a sort of force field around us - takes the brunt of the hit. These charged particles become caught in the polar regions of our magnetosphere, and spiral down, colliding with different molecules in our atmosphere. As this happens, it causes these molecules of oxygen, nitrogen and sometimes hydrogen and helium to vibrate and become excited - giving off different colours. 

The process highlights a dynamic connection between the Sun and the planets - where the Sun reaches out and kisses the atmospheres of these bodies and the planets respond by producing aurora. 

The Earth is not alone in our Solar system in experiencing these phenomena, with aurorae observed on all of the large planets. Whilst Saturn’s aurorae are driven by the solar wind (much like Earth), Jupiter presents an interesting scenario - one of Jupiter’s moons, called Io, happens to also be the most volcanic body in our Solar system. As eruptions eject particles and material from Io’s interior into space (namely sulfur and oxygen), they get caught in and accelerated by Jupiter’s powerful magnetosphere - facilitating the aurora effects on the Gas Giant. 

Across all planets in our system, these auroral effects often emit in a multitude of wavelengths, and in particular, at radio frequencies along with the visual bands. Even on Earth, our aurora often emits powerful radio emissions around the 150 kHz band, whilst Jupiter happens to be the brightest object in the Solar system at these wavelengths. 

Hubble captured images of aurora on Jupiter. Credit: NASA, ESA, and J. Nichols (University of Leicester).

Well, it now turns out that, after decades of trying, we can also use this type of observation method to detect potential exoplanets around distant stars. Remarkably, this is achieved through radio telescopes, rather than optical telescopes. 

The new paper (recently published in the journal Nature Astronomy) outlines not only this achievement of detecting auroral activity in distant stars using radio telescopes but also highlights the connection that exoplanets have with their host stars, through the interaction of the planet’s magnetic field. 

To make these observations, Australian scientists like Dr Joseph Callingham from Leiden University (and ASTRON), and Dr Benjamin Pope from the University of Queensland, along with their teams, used an odd-looking telescope, even by radio telescope standards. 

“The radio emission from these stellar systems are incredibly bright but have the same polarisation characteristics as the emission we see from the Jupiter-Io interaction,” said Dr Callingham. 

“This makes sense if we scale up the system - replacing Jupiter with a star and Io with an exoplanet.” 

Located primarily in the northwest corner of the Netherlands, the Low-Frequency Array (LOFAR - one of the world’s most powerful radio instruments) was used to scan the cosmos at low-frequency bands, and unexpectedly came across a series of stars that appeared to be blasting out radio waves - potentially as a tell-tale sign of magnetically-induced, radio-frequency producing interaction with an orbiting planet. 

Historically, the 4,500+ exoplanets discovered have been through a number of methods, such as measuring the radial velocity of a host star (the periodic back and forth in the star’s spectrum) or by the transit method (where observatories, especially those located in space, detect tiny variations in the star’s light-curve). There’s also been direct imaging of exoplanets with powerful optical telescopes. 

But by looking at radio frequencies emitted by stars, scientists can now also determine if they are seeing the kinds of radio emissions we would expect from aurorae created by an orbiting planet’s strong magnetic field as it orbits around its host star. The ability to only do this now is because LOFAR finally affords us the sensitivity to detect this effect which has eluded astronomers for several decades. 

These types of low-frequency radio emissions are generated in these circumstances through a process known as electron-cyclotron maser instability (ECMI - a mechanism in which non-thermal energetic electrons trapped in a magnetic field amplify electromagnetic radiation) and the key features are that this is highly polarised, and extremely bright, with brightness temperatures exceeding 1012 Kelvin. 

Polarised light can be thought of as the orientation of the plane of light as it traverses in the longitudinal direction. It comes in a couple of flavours - linear polarisation, circular polarisation and elliptical polarisation. If you’ve ever held a hose or skipping rope and moved your hand up and down, the waves that oscillate in the up-down motion as they traverse down the rope are linearly polarised. Repeat the process but instead move your arm in a circle, and the spiral oscillations that traverse down the rope, are circularly polarised. Light behaves exactly the same way, especially when it encounters surfaces or magnetic fields. 

Historically, however, many searches of this nature have focused on more active stars - because these are thought to be the most interactive with any orbiting planets. 

“We focussed on them [bright/active stars] because of sensitivity issues and detection rates,” said Dr Callingham.

“Also, before this work, we did not expect the stars we detected to be radio bright - so you would have a hard time convincing a telescope allocation committee - such as the Jansky Very Large Array to point at these stars.”

Covering 20% of the northern sky at 144 MHz (which is about 30% higher than Triple J’s broadcast frequency), this new LOFAR study searched for these types of events from a particular type of star, known as M dwarfs (also known as red dwarfs). These are the smallest, and coolest types of stars in our Galaxy, and also the most common. Because of their low surface temperatures (roughly 2,000 - 3,900 Kelvin), they are much more challenging to detect from Earth. But they also happen to be some of the most common kinds of stars and are almost always orbited by exoplanets, with many of the already-discovered 4,500+ systems containing a host red dwarf. 

As a part of this study, 19 new low-frequency detections of  M dwarfs were made, increasing the population of these types of stars (through this discovery method) by a significant factor. All of these detections by LOFAR outlined the long-lived low-frequency radio emission mechanism to likely be caused by ECMI processes (which would produce a high brightness temperature and circularly polarised signal) - possibly pointing to the first observations of exoplanets that interact with their stellar host’s magnetic spheres. 

In short-hand: an exoplanet, in low-frequency radio waves. 

Using LOFAR’s Sensitivity

Aerial view of the LOFAR telescope. Credit: LOFAR/ASTRON.

LOFAR is a similar instrument to Australia’s Murchison Wide-Field Array (MWA) telescope - it is made up of 20,000 antennas all bunched into approximately 52 stations (a station is a collection of antennas). Stations are separated by distance, with some located relatively close to each other, whilst some extend off to distances as large as 1,000-kilometres away. The core of the telescope (made up of 38 stations) is located in the Netherlands, whilst other stations reside in Germany, Poland, France, Great Britain, Ireland, Latvia, Sweden and Italy. 

The instrument acts as an interferometer - so the largest distance between two antennas (known as a ‘baseline’) effectively gives the telescope a simulated aperture of this size. This technique is utilised across radio astronomy because it would be both impossible and abhorrently expensive to try and build one giant telescope. This method also introduces a higher resolving power for the telescope, ensuring that the finer detail of distant structures can be determined. 

Thanks to the two types of antennas available that form part of LOFAR, the telescope makes observations in the 10 MHz - 240 MHz range, with signals from antennas transported to a central digital processing unit before being combined to form images or produce data. 

“End of the day LOFAR just has the sensitivity to make this science possible - it was not possible with the MWA for example,” said Dr Callingham. 

“Furthermore, we had the idea to look at circularly polarised light, which gets a big problem of chance associations - that is if you have too many optical sources and don’t know your radio positions well enough, you can get chance alignments between a red dwarf and quasar. But, because quasars are not circularly polarised, you can get around this.”

These recently published results are part of the LOFAR Two-meter Sky Survey (LoTSS) - the deepest wide-field, low-frequency radio survey of the sky conducted so far. This project aims to survey the entire northern sky at 6-arcsecond resolution and is expected to detect over 10 million radio sources - mostly from active galactic nuclei at the centre of distant galaxies, or star-forming galaxies. 

Historically, radio telescopes could only detect the very nearest stars with most objects found in radio surveys being attributed to distant supermassive black holes, AGN, and interstellar gas. Stars, whilst emitting across the entire electromagnetic frequency, are not often the target of radio telescope projects. What makes this instrument special to do this kind of work is its sensitivity, and the ability to spend 8 hours (instead of minutes) on the field of sky. 

But now - even stars are within reach of LOFAR’s radio observations with this technique, and from these observations, astronomers have the opportunity to search for exoplanets orbiting them. 

Graphical image of the TESS spacecraft, currently in orbit on its mission in discovering new exoplanets. Credit: NASA Goddard Space Flight Centre/C. Meaney.

To complement the results presented by the LOFAR study, Dr Pope and his team also assessed if these findings could have been associated with stellar activity, as opposed to exoplanet activity, in a second paper (published in The Astrophysical Journal Letters). To do so, they used the orbiting space-based observatory, TESS, to obtain optical data on 15 of LOFAR’s detected candidates. 

What they found was that it was unlikely that the bright, circular polarised radio emissions coming from these systems was associated with stellar activity, and instead, support the hypothesis that the observations are made from processes occurring as the stellar M dwarf host interacts (magnetically) with an orbiting exoplanet. In most cases, flares generated by M dwarf stars could explain these results - but for four of the cases reported, the team looked at data from TESS and found them to be extremely quiet. As such, in these circumstances, the flares would not be able to explain the detected radio emissions. 

“TESS is a real game-changer for characterizing stellar variability, providing 2-minute-resolution data on stars across nearly the whole sky,” said Dr Pope.  

“The standard picture of stellar radio emission is that the radio waves come from electrons accelerated in flares and that these flares also heat a corona of plasma that gives off X-rays in turn - so the radio and X-rays are correlated.” 

We already had a clue that some of our sources were unusually faint in X-rays compared to what you’d expect from radio, but TESS allows us to skip that middle step and look for the flares directly. Some of the stars were bright in radio and X-rays and flaring all the time, but some were dead flat in TESS doing nothing at all - some of the quietest of all M dwarfs seen by TESS. It’s hard to imagine powering the radio emission with flares when we see none in the independent optical data,” he said.

That’s not to say that this is 100% confirmation that exoplanets have been found - but rather that the observations and results from LOFAR, complimented by the TESS results, indirectly suggest that the presence of an exoplanet is a good model to explain the detected low-frequency radio emissions.

What this tells us about Exoplanets

An artist’s depiction of a coronal mass ejection event on Proxima Centauri. Red dwarfs like Proxima Centauri give off enormous flares, causing them to double in brightness in just minutes. This also generates a lot of radio signals that can be detected from Earth. Credit: Mark Myers / OZGrav.

This new data now offers an opportunity to use a powerful tool, in further assessing the population of exoplanets in our galaxy, complimenting other methods, such as radial velocity and transiting techniques. Much like our own Sun being connected to and interacting with the planets in the Solar system, this new methodology is the first way to detect a magnetic link between an exoplanet’s host star and the orbiting planet field which in turn provides an opportunity to explore these types of systems at radio frequencies. 

“You can get direct measurements of the magnetic field and topology [of exoplanets] - which is super important for assessing their habitability,” said Dr Callingham. 

“Furthermore, we expect ~7,500 detected with SKA-Low, which makes this system as competitive as some of the next generation transit instruments.”

As M dwarfs are the most common type of star to host exoplanets, this research has important implications for estimates of the amount of habitable real estate in the universe. M dwarfs, whilst small and cool, are known to have violent stellar flares, which can lash the atmospheres of orbiting planets. Whilst these events might produce auroral activity, they also do have a detrimental impact by damaging the atmosphere - especially if the planet doesn’t have a strong enough magnetic field (remember, the Earth’s magnetic field protects us from the Sun’s wind). 

This has downstream considerations for the field of astrobiology - could the most common type of star in the Galaxy, which hosts a large number of confirmed exoplanets, also be the worst-case scenario for any opportunity for life to form elsewhere? 

A lot of research has so far pointed to this being the case - for a number of factors, not just that M dwarfs lash their planetary bodies with intense radiation and particles, but also because many are also tidally locked to their host star. This means that as they orbit, only one face remains fixed facing the star - a perpetual daytime, whilst on the opposite side of the planet, perpetual night. This creates unfavourable temperature conditions for life to emerge or evolve. 

But on the other hand, many exoplanets in M dwarf systems orbit in the habitable zone - a belt region that is just the right distance from a star for liquid water to exist on the surface - an essential for life (as we know it) to emerge. So the discussions and debates are still open and thriving amongst the astronomers. 

The closest exoplanet to Earth happens to reside in a system of the closest star to Earth - the Proxima Centauri system, located roughly 4.3 light-years away. Proxima Centauri b orbits within the habitable zone of the M dwarf star, every 11 or so days, with some studies suggesting it is also tidally locked. The planet features a strong magnetic field, but the host star Proxima Centauri is known to give off powerful flares as well. 

Future Exoplanet Surveys in Radio

20-second exposure showing Milky Way above the SKA-Low antennas. Credit: Michael Goh/ICRAR/Curtin.

The exciting outcome of this new LOFAR study is that astronomers are only just starting to scratch the surface on this technique, with potential for further enhancements (in both methodology and sensitivity) down the track.

In fact, one of LOFAR’s limiting factors is that it can only catch these types of events and stars from the relatively nearby cosmic neighbourhood, out to approximately 165 light-years. To see further out. As well as in lower frequencies, we’re going to need a much more powerful telescope. 

And here is where LOFAR leads into Australia’s upcoming mega-science project - the SKA. 

LOFAR (much like Australia’s ASKAP and the MWA telescope) is an SKA pathfinder project - meaning it is engaged in developing and demonstrating SKA-related technology and science cases so that when the mega-telescope finally starts collecting its light from the Universe, a number of proven techniques are hitting the ground running and collecting vast volumes of data. Once the SKA starts collecting data from the cosmos, it will be an order of magnitude more powerful than current technologies, allowing astronomers to peer deeper, and at more sensitivity, into the sky. 

For Dr Callingham, once the SKA comes online he expects that with the greater sensitivity, many more new sources and many fainter sources closer to home will be able to come to light. 

“If we want to understand habitability in the universe, the saying goes ‘know thy star, know thy planet’, and right now the magnetic effects of stars on planets are almost completely unknown,” said Dr Pope. 

“LOFAR has given us the first direct glimpse of star-planet magnetic interactions, and this will become routine and quantitative with the SKA.” 

“Exoplanet and astrobiology researchers are going to be blown away by SKA’s potential not just for probing this magnetic connection, but planetary aurorae more generally, and even SETI searches for direct communication,” he said. 

In a way, we are seeing the first results of these early pioneering steps, which will accumulate and result in the largest and most powerful radio telescope in the world, hunting down and finding the radio signature of distant exoplanets, as they connect and interact with their host stars. 

Auroras and radio waves that leap into the void of space, from worlds, far, far away. 

The paper is available in the journal, Nature Astronomy