New research on flares from a hot-tempered star could inform the search for habitable planets

Like a toddler right before naptime, TRAPPIST-1 is a small yet moody star. This little star, which sits in the constellation Aquarius about 40 light-years from Earth, spits out bursts of energy known as “flares” about six times a day. 

New research led by the University of Colorado Boulder takes the deepest look yet at the physics behind TRAPPIST-1’s celestial temper tantrums. The team’s findings could help scientists search for habitable planets beyond Earth’s solar system.

The researchers used observations from NASA’s James Webb Space Telescope and computer simulations, or “models,” to understand how TRAPPIST-1 produces its flares—first building up magnetic energy, then releasing it to kick off a chain of events that launches radiation deep into space. The results could help scientists unravel how the star has shaped its nearby planets, potentially in drastic ways.  

The team published its findings Nov. 20 in “The Astrophysical Journal Letters.”

“We think that the innermost TRAPPIST-1 planets are just bare, denuded rocks because the star has blown away their atmospheres,” said Ward Howard, lead author of the new study and a NASA Sagan Fellow in the Department of Astrophysical and Planetary Sciences (APS) at CU Boulder. 

It’s a highlight for the little star, which has attracted a lot of attention from scientists in recent years.

TRAPPIST-1 has less than 10% the mass of the sun and is only a bit larger than the planet Jupiter. But it also hosts seven Earth-sized planets, three of which lie in what researchers call the “habitable zone”—a region of space that may have just the right temperatures for liquid water to form on the surface of a planet.

There’s just one big problem: Scientists have struggled to get a good look at those planets because of the star’s volatile activity.

“When scientists had just started observing TRAPPIST-1, we hadn’t anticipated the majority of our transits would be obstructed by these large flares,” Howard said. 

The challenge of studying flares

Studying a flare is a bit like investigating the scene of a crime. Scientists can see the aftermath of a flare—in this case, a big flash. The Webb telescope, for example, records how much infrared radiation, or heat, a star releases during a flare.

But that same space telescope can’t tell you whodunnit.

Howard said that all stars, from TRAPPIST-1 to our own sun, are surrounded by magnetic fields. These magnetic fields twist and bend, forming something that looks like a bowl of noodles. They also shape the plasma, an ultra-hot gas made up of charged particles, in a star’s outer atmosphere.

Sometimes those magnetic fields can get a little too twisted. When that happens, the fields snap, and a beam of electrons hurtles through the star’s atmosphere. That beam is the culprit behind a flare.

“Those beams will continue down into the stellar atmosphere where they smack into the plasma and heat it up,” Howard said. “And once you have a nice hot plasma, it glows.”

To solve the mystery of TRAPPIST-1’s flares, Howard and his colleagues analyzed data from six flares collected by the Webb telescope in 2022 and 2023.

The researchers turned to a new grid of models that describe the physics of flares developed by Adam Kowalski, an associate professor in APS who is also a co-author of the current study. 

The models use a series of complex equations to, essentially, wind back time for these flares. If the researchers spot a flare coming from TRAPPIST-1, they can use the models to predict what kind of electron beam kicked off that flare in the first place.

Wimpy flares

Knowing about those electron beams may open up a range of opportunities for scientists studying TRAPPIST-1, Howard said.

For a start, his team discovered that TRAPPIST-1’s flares seem to be surprisingly weak. Most flares from similar stars, by comparison, are produced by electron beams about 10 times stronger.

“These flares were a little wimpier than we expected,” Howard said.

He added that the same electron beams that produce the infrared light seen by Webb also generate a wide range of other kind of radiation—from visible light to ultraviolet radiation and powerful X-rays. The group’s research will allow scientists to explore that full range of radiation coming from TRAPPIST-1’s flares. This information could help researchers understand how these events might alter the atmospheric chemistry of nearby planets. 

Scientists suspect that one of the planets in TRAPPIST-1’s habitable zone, named TRAPPIST-1e, may carry a hint of an Earthlike atmosphere—a possible sign of habitability.

“If we can simulate these events using a computer model, we can reverse engineer how a flare might influence the radiation environment around each of these planets,” Howard said.

Co-authors of the new study include researchers at the University of Chicago; Johns Hopkins University; Max Planck Institute for Solar System Research; Massachusetts Institute of Technology; University of Oxford; and Université de Montréal.