Red dwarf star’s super-flares strip away planet’s atmosphere

In the chaotic realm of a fiery red dwarf star, life for newborn planets is anything but serene. Consider it a trial by fire, as these planets must endure the wrath of their ill-tempered host. The red dwarf’s magnetic fields, intricately entwined, give rise to colossal “super-flares” dwarfing those witnessed in our own sun by a factor of 100 to 1,000. Moreover, the star’s relentless outpouring of blistering ultraviolet radiation necessitates extreme protective measures, with the star system’s inhabitants resorting to a formidable “sunscreen 5,000.”

One such tempestuous star, AU Microscopii, stands out as a nearby and violently active exemplar. This petulant celestial body is merely a mere 1% the age of our sun. Yet, its proximity of 32 light-years is a mere eightfold greater than that of Proxima Centauri, our sun’s closest neighbor, another notorious red dwarf.

The star’s wrath is particularly harsh on the innermost planet in its system, AU Microscopii b. Clocking in at about four times the diameter of Earth, this unfortunate planet revolves a mere 6 million miles from the malevolent star’s searing “dragon’s breath.” Consequently, its predominantly hydrogen atmosphere endures constant stripping, an unsettling spectacle witnessed by the unblinking gaze of the Hubble Space Telescope. However, this atmospheric erosion exhibits a perplexing pattern of fits and starts.

During one of the planet’s transits across the star’s face, the Hubble Telescope detected a dramatic occurrence: hydrogen boiling off the planet, giving rise to an expansive cloud ahead of its path. Such erratic behavior provides tangible evidence that the interplay between this planet and its red dwarf host’s fiery fireworks is far more intricate and capricious than previously imagined.

With each orbit, this young planet, dancing around its petulant red dwarf star, undergoes unpredictable metamorphoses. Placed so proximally to its parent star, it endures a relentless onslaught of energy, causing its hydrogen atmosphere to evaporate and puff away.

However, intriguingly, during a subsequent orbit observed by the Hubble Space Telescope, the planet appeared remarkably unaffected, exhibiting no signs of atmospheric loss. Only an orbit and a half later did clear indications of atmospheric erosion reemerge.

A young planet whirling around a petulant red dwarf star is changing in unpredictable ways orbit-by-orbit. It is so close to its parent star that it experiences a consistent, torrential blast of energy, which evaporates its hydrogen atmosphere – causing it to puff off the planet. But during one orbit observed with the Hubble Space Telescope, the planet looked like it wasn’t losing any material at all, while an orbit observed with Hubble a year and a half later showed clear signs of atmospheric loss. Credit: NASA Goddard Space Flight Center, Lead Producer: Paul Morris

Astronomers were left utterly astounded by the striking unpredictability observed between orbits. Keighley Rockcliffe from Dartmouth College in Hanover, New Hampshire, expressed her surprise, stating, “We’ve never seen atmospheric escape go from completely undetectable to highly detectable over such a short period when a planet passes in front of its star. We were fully anticipating a more predictable and repeatable pattern. But what we found was simply bizarre. My initial reaction was, ‘That can’t be right.'”

The enigmatic behavior continued to bewilder researchers as they observed the planet’s detectable atmosphere puffing out in front of it, resembling the headlights of a swiftly moving train. Rockcliffe found this peculiar observation to be an extraordinary test case for planetary evolution models and physics. She emphasized the uniqueness of the situation, stating, “This observation is so fascinating because it allows us to explore the most extreme interplay between a star and a planet.”

The host of this intriguing phenomenon is the parent star AU Microscopii (AU Mic), located 32 light-years away from Earth. Remarkably, AU Mic boasts one of the youngest known planetary systems, being less than 100 million years old—a mere fraction of our sun’s age, which stands at 4.6 billion years. The innermost planet, AU Mic b, maintains a short orbital period of 8.46 days, orbiting a mere 6 million miles away from the star—roughly one-tenth of Mercury’s distance from our sun. This planet, characterized by its bloated, gaseous nature, is approximately four times the diameter of Earth.

The discovery of AU Mic b took place in 2020, thanks to NASA’s Spitzer and TESS (Transiting Exoplanet Survey Satellite) space telescopes. Employing the transit method, scientists detected a slight reduction in the star’s brightness as the planet crossed in front of it.

A panel of exoplanet demographics information from the NASA Exoplanet Archive as of 2022 August. Nonspecified, field-age planets are represented by the light-blue contours and light-blue triangles. Specified planets have been searched for atmospheric escape. Filled circles indicate at least one published detection of hydrogen and/or helium escape. The crosses indicate hydrogen/helium escape nondetections. Among the detections/nondetections, field-age planets are dark blue, whereas young planets are red. The black star represents AU Mic b, with its range in bulk density showing its spread in mass measurements. The left panel shows the distribution of exoplanets in radius and period. The middle panel shows exoplanet radius vs. its X-ray-to-ultraviolet irradiation (Foster et al. 2022). The right panel shows exoplanet radius vs. bulk density limited to planets with σρ < 0.1ρ. Credit: The Astronomical Journal (2023). DOI: 10.3847/1538-3881/ace536

Red dwarfs, such as AU Microscopii, dominate the population of stars in our Milky Way galaxy, and it is likely that they host a large number of planets as well. However, the habitability of planets orbiting these red dwarfs, like AU Mic b, poses significant challenges due to the ferocious stellar flares emitting intense radiation. Unlike stars like our sun, young red dwarfs experience extended periods of high activity, with stellar flares powered by tangled magnetic fields in their turbulent atmospheres. These flares release astonishingly massive amounts of energy, surpassing our sun’s outbursts by 100 to 1,000 times.

The resulting spectacle is a blistering fireworks display of torrential winds, X-rays, and flares, making life inhospitable for planets in close proximity to the star. Keighley Rockcliffe describes this as an “unconstrained and frankly, scary” stellar wind environment that impacts the planet’s atmosphere.

Planets forming within the first 100 million years of the star’s life are expected to undergo the most significant atmospheric escape, potentially leading to the complete stripping of their atmospheres. Scientists are eager to understand which types of planets can survive such harsh conditions and how they might evolve when the star eventually settles down. The potential for habitability is uncertain, and there is a possibility that planets could end up scorched or transformed into super-Earths with surviving cores.

Observations of AU Mic b’s atmospheric outflow, facilitated by the Hubble Space Telescope, reveal unprecedented changes in response to the host red dwarf’s outbursts. The variability in the star’s magnetic field lines contributes to this unpredictability. There are two possible explanations for the disappearance of hydrogen during one of the planet’s transits: a powerful stellar flare may have photoionized the escaping hydrogen, rendering it transparent to light and undetectable, or the stellar wind itself may be shaping the planetary outflow, causing observable fluctuations.

Researchers, including lead author Keighley Rockcliffe, plan to conduct further Hubble observations during more AU Mic b transits to gain additional insights into the star and planet’s peculiar behavior. These observations will help test scientific models of exoplanetary atmospheric escape and evolution.

Published in The Astronomical Journal, this study sheds light on the complexities of planetary survival and evolution around red dwarfs, providing valuable evidence for researchers studying exoplanets and their diverse environments.

Source: ESA/Hubble Information Centre

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