TRAPPIST-1 Has Flares. What Does This Mean for its Planets?

The TRAPPIST-1 system continues to fascinate astronomers, astrobiologists, and exoplanet hunters alike. In 2017, NASA announced that this red dwarf star (located 39 light-years away) was orbited by no less than seven rocky planets – three of which were within the star’s habitable zone (HZ). Since then, scientists have attempted to learn more about this system of planets to determine whether they could support life. Of particular concern is the way TRAPPIST-1 – like all M-type (red dwarf) stars – is prone to flare-ups, which could have a detrimental effect on planetary atmospheres.

Using the James Webb Space Telescope (JWST), an international team of astrophysicists led by the University of Colorado Boulder (CU Boulder) took a closer look at this volatile star. As they describe in their paper (which recently appeared online), the Webb data was used to perform a detailed spectroscopic investigation of four solar flares bursting around TRAPPIST-1. Their findings could help scientists characterize planetary environments around red dwarf stars and measure how flare activity can affect planetary habitability.

The research was led by Ward S. Howard, a NASA Sagan Fellow in the Department of Astrophysical and Planetary Sciences (APS) at CU Boulder. He was joined by colleagues from the National Solar Observatory (NSO), the Center for Astrophysics and Space Astronomy (CASA), and the Laboratory for Atmospheric and Space Physics (LAPS) at CU Boulder. They were joined by members of the NIRISS Exploration of the Atmospheric diversity of Transiting exoplanets (NEAT) Collaboration, led by Olivia Lim and David Lafrenière at the University of Montreal.

Other team members included researchers from the Carl Sagan Institute, Johns Hopkins University, the Trottier Institute of Exoplanet Research, the Observatoire du Mont-Mégantic, the Herzberg Astronomy and Astrophysics Research Center, the Space Telescope Science Institute (STScI), and multiple universities. The preprint of their paper, “Characterizing the near-infrared spectra of flares from TRAPPIST-1 during JWST transit spectroscopy observations,” appeared on arXiv and was recently accepted for publication by The Astrophysical Journal.

Until recently, it has been very difficult to resolve smaller planets that orbit closer to their stars, where terrestrial (rocky) “Earth-like” planets are believed to reside. Thanks to JWST’s advanced infrared optics, astronomers can get a closer look at these planets and obtain spectra from their atmospheres, thus providing data on their chemical composition. Based on recent findings, astronomers have also determined that M-type red dwarf stars are a very likely place to find terrestrial planets that orbit within their parent star’s HZs.

This is particularly exciting since red dwarfs are the most common type of star in the Universe, accounting for 75% of stars in the Milky Way alone. However, these stars are also prone to flare activity, which raises doubts about whether orbiting planets can maintain their atmospheres for long. Consider TRAPPIST-1, a red dwarf less than 12% the size of our Sun (and less than 9% its mass), making it slightly larger and more massive than Jupiter. This same star hosts seven known terrestrial planets ranging in size from 0.77 to 1.13 Earth radii and 0.41 to 1.38 Earth masses. Said Howards in a recent CU Boulder Today press release:

“Because of JWST, it is the first time in history that we’ve been able to look for planets around other stars that have the sorts of secondary atmospheres you could find around, say, Earth, Venus, or Mars. If we want to learn more about exoplanets, it’s really important to understand their stars. With TRAPPIST-1, we have a really great opportunity to see what an Earth-sized planet around a red dwarf would look like.”

TRAPPIST-1 also produces powerful flares several times a day, whereas our Sun experiences similar flares only about once a month. In their new research, Howard and his colleagues recorded a series of flares bursting from TRAPPIST-1 over roughly 27 hours using Webb‘s Near-infrared Spectrometer (NIRSpec) and Near Infrared Imager and Slitless Spectrograph (NIRISS) instruments. This is the first time astronomers have observed flares in near-infrared wavelengths, allowing them to track the evolution of those four flares in exquisite detail.

Their observations coincided with TRAPPIST-1b, f, and g passing in front of their star (aka. transiting), which allowed them to study the interactions between the flares and the planets’ atmospheres. The researchers also developed a mathematical method to separate about 80% of the light produced by these flares from the star’s normal radiation. While this did not correct for 100% of TRAPPIST-1’s flare activity, the team’s results demonstrate how astrophysicists could collect clearer and more accurate data on TRAPPIST-1’s seven planets – and other red dwarfs nearer to our Solar System.

“There are only a handful of stellar systems where we have the opportunity to look for these sorts of atmospheres,” said Howards. “Each one of these planets is truly precious. If you don’t account for flares, you could detect molecules in the atmosphere that aren’t really there, or get the amount of material in the atmosphere wrong.”

These results are not the only ones provided by the JWST, which include a recent study conducted by Olivia Lim and an international team of TRAPPIST-1b. For this study, Lim and her colleagues examined the first spectra obtained by Webb of the system, which found no traces of an atmosphere around TRAPPIST-1b. Soon enough, astronomers will likely have spectra to share from Proxima b and other exoplanets in nearby red dwarf systems. With these results, scientists will be one step closer to answering the question that’s been on everybody’s mind since these exoplanets were first discovered: “Could there be life?”

Further Reading: CU Boulder Today