Intense auroral emissions from the universe’s tiniest stars may offer a new technique to search for rocky planets that would otherwise go unnoticed.
Bursts of radio waves can be produced as a planet passes through its star’s magnetic field.
The event is similar to one that scientists have been studying attentively right here in our solar system: periodic radio emissions caused by Jupiter’s moon Io’s interactions.
Researchers have found numerous stars generating the telltale activity using a sophisticated radio telescope.
They claim that each one might be home to a small world.
The magnetic field of a star sweeps through space as it rotates, interacting with charged particles ejected from the stellar surface and driven away by the stellar wind.
If a planet passes close enough to the rotating star, the particles are accelerated, even more, resulting in a dazzling flash in low-frequency radio waves.
The Low Frequency Array (LOFAR), a European radio telescope network operating at the lowest frequencies that can be viewed from Earth, has detected such flashes.
LOFAR is now conducting a wide-field, low-frequency radio survey to look for sources.
Researchers identified strange radio flashes from 19 red dwarf stars in the first data release from 2019, which covered nearly a sixth of the Northern Hemisphere sky.
Five of the stars’ flashes were first discovered as closely resembling estimates for how a planet’s auroral fireworks should appear from light-years away.
Those findings were published in Nature Astronomy, and a subsequent preprint paper limited the field to four choices.
“We observe no trends that we would expect if the emission was driven by stellar activity,” says Joseph Callingham, a radio astronomer at Leiden University in the Netherlands, who led the Nature Astronomy study.
Because all four stars are rather quiet, they are unlikely to release huge flares regularly, which could be mistaken for an auroral signal from a nearby planet.
Astronomers have been looking for signals of planets interacting with their stars’ magnetic fields for years, focusing on certain subsets of suns that are thought to be the most favorable to the search.
Rather than focusing on individual stars, Callingham and his colleagues used LOFAR’s blind, catch-all sky survey, which allowed for a more objective search.
“This is a cool result,” says Gregg Hallinan, a California Institute of Technology astronomer who was not part of the project. “No one has ever been able to do it objectively before.”
Despite their small size, many red dwarfs punch considerably above their weight in terms of stellar activity, whacking any orbiting planets with massive flares.
The more quickly a red dwarf spins, the more flares it produces. Slow-moving stars, like those found in the LOFAR survey, can occasionally burp them out.
The researchers conducted a follow-up analysis to rule out common flaring as the cause of the flashes seen in LOFAR’s radio survey.
The scientists double-checked the activity levels of their target stars using optical data from NASA’s Transiting Exoplanet Survey Satellite (TESS).
This research has been published in the Astrophysical Journal Letters and may be found on the preprint service arXiv.org.
While one of the five previously reported quiet stars was discovered to be actively flaring in TESS data, the other four remained silent, bolstering the case for whirling planets as the source of their audible radio flashes.
“We can kill [flares as a cause] for the least active stars since they don’t flare at all,” says Benjamin Pope, an astronomer at the University of Queensland in Australia and co-author of the Nature Astronomy study as well as the primary author of the second publication.
However, scientists cannot claim with certainty that the signals are linked to secret realms.
For each of the four stars, other, more advanced planet-detecting techniques have come up empty. “I’ve tried and tried to establish they’re planets,” Pope says.
The majority of the efforts to find the potential planets began last year when researchers revealed the finding of GJ 1151, one of the four quiet stars, as the first possibility for star-planet interaction.
Two separate teams tried and failed to find periodic wobbles in GJ 1151’s motions that should have resulted from the LOFAR-suggested companion—a one-Earth-mass world orbiting the star every few days, gently pushing it to and fro.
This isn’t great news for scientists hoping to discover new techniques to locate and study worlds outside our solar system.
Although planets may be able to betray their presence by auroral flashes, Suvrath Mahadevan, an astronomer at Pennsylvania State University who assisted in the search for GJ 1151’s putative planet but was not involved in the two new studies, believes that independent confirmation of the technique is necessary.
“You want to see numerous lines of evidence converging the first time,” he explains.
The periodic radio flashes seen by LOFAR or other comparable observatories should coincide with data from stronger planet-hunting techniques, with each echoing the other to conclusively indicate a world’s presence.
Mahadevan says, “Then I feel like you open up the field.” “It’ll be our next discovery tool,” says the researcher.
For the time being, Callingham and his colleagues are doubling down on their quest, reserving more time on LOFAR for GJ 1151 follow-up observations and continuing their deep dive into the observatory’s sky survey data.
Upgrades to LOFAR, as well as the launch of an even more powerful facility known as the Square Kilometer Array, will provide even more prospects for discovery in the future years. It appears that more auroral planet candidates will be announced in the near future.
SATELLITES WITH STEPPING STONES
There’s more to their activities than scholarly curiosity fueling them. Red dwarfs (or M dwarfs, as scientists prefer to call them) are the tiniest stars in the universe, as well as the most plentiful and longest-lived.
According to some estimations, M dwarfs account for up to 75% of all stars in the universe, and each one can light for hundreds of billions, if not trillions, of years.
Most crucially, extrapolations based on several surveys suggest that nearly every M dwarf harbors at least one planet.
M dwarf worlds appear to make up the majority of the universe’s planetary real estate based on raw numbers alone.
It’s unclear if any of those locations might support life, but studies like Callingham’s can help settle the debate.
No one expects any planet buried in the magnetic field of an M dwarf to be habitable.
The nearest star would sear such worlds to the point where liquid water, the foundation of life as we know it, would not be able to survive on their surface.
Instead, they can assist scientists in answering more fundamental concerns about how M dwarfs affect their planetary broods.
The tendency of these stars to have large outbursts, for example, may wipe out the atmospheres of normally habitable planets—but a planet with a strong magnetic field might be enough insulated to keep its precious air.
In a few select systems, astronomers can already distinguish between atmospheric and airless planets, but there are currently no effective methods for measuring a small world’s magnetic field.
Observations of auroral flashes, according to work by Leiden University Ph.D. student Robert Kavanagh and associate professor Aline Vidotto, could do just that, if the strength of a flash is proportionate to the intensity of a planet’s magnetism.
According to Vidotto, studies of auroral M dwarf planets could reveal the density and speed of a host star’s stellar wind.
(The two new investigations did not include Vidotto or Kavanagh.) Such data could aid scientists in determining how frequently M dwarfs encounter coronal mass ejections, which are massive belches of particles that, like flares, can be harmful to surround planets.
“I believe [this technique] will allow us to learn a lot more about the star,” Vidotto says.
All of this, of course, adds to the unresolved riddle of M dwarf habitability, as well as the larger puzzle of where life-bearing planets are most likely to be found in the cosmos.
“Planets can’t live in a vacuum. “They thrive in the vicinity of their star,” Mahadevan explains.
“I believe that properly knowing the magnetic activity and magnetosphere of these stars is the key to [understanding M dwarf habitability].”
About the author:
Nola Taylor Tillman is a science writer with a focus on space and astronomy.