AU Mic b: Insights into Exoplanet Atmospheric Evolution

 Situated a mere 32 light-years away from our planet, the red dwarf star AU Microscopii (AU Mic) is home to one of the most youthful planetary systems we've ever seen. This star is under 100 million years old, barely a fraction of the age of our own Sun (approximately 4.6 billion years). The Hubble Space Telescope has observed the youngest planet, AU Mic b, with an 8.46 day orbit and only 6 million miles from its star, which is around a tenth of the distance between Mercury and our Sun. This gaseous exoplanet, four times the diameter of Earth, is so close to its star that it's continually bombarded with energy that causes it to lose its hydrogen atmosphere.

NASA's Spitzer and TESS (Transiting Exoplanet Survey Satellite) discovered AU Mic b in 2020 through the transit method, where a slight dimming in the star's brightness is observed as the planet passes in front of it.

Red Dwarfs and Potential for Life

Red dwarfs, such as AU Microscopii, are the most common stars in our galaxy, the Milky Way, and hence, likely to host many exoplanets. However, can these planets, like AU Mic b, orbiting red dwarfs truly support life? Young red dwarfs emit lethal radiation from powerful stellar flares, a period of high activity that lasts longer than that of stars like our Sun.

Stellar flares are caused by intense magnetic fields that are twisted by the chaotic movements in the star's atmosphere. When this twisting becomes too intense, the fields rupture and reconnect, releasing energy bursts 100 to 1,000 times stronger than our Sun's explosions.

This creates a spectacular display of intense winds, flares, and X-rays that heavily impact any nearby planet. The flare-induced stellar wind has a devastating effect on the planet's atmosphere. Planets formed within the first 100 million years of the star's birth are expected to lose most of their atmosphere, potentially stripping the exoplanet of its protective layer.

Hubble's inability to view the planet directly due to the star's light was offset by observing changes in brightness caused by hydrogen emission from the planet and the dimming of the star's light during the planet's transit. The atmospheric hydrogen heated to an extreme temperature, escaping the planet's gravity during these passages.

Hydrogen Escape

Hubble has witnessed dramatic changes in AU Mic b's atmospheric outflow, ranging from significant hydrogen clouds to complete absence, pointing to the extreme variability in red dwarf flares. 

This variability is influenced by the star's shifting magnetic field lines. One theory for the disappearance of hydrogen during the planet's transit is that a strong stellar flare, observed seven hours earlier, may have ionized the escaping hydrogen to the point of becoming transparent to light, hence undetectable.

Alternatively, the stellar wind could be shaping the planetary outflow, making it visible at certain times and invisible at others, causing a sort of "hiccup" in the outflow in front of the planet. This is a theory some models suggest, but it's the first time observational evidence supports it to such an extent.

Further observations of AU Mic b transits by Hubble should shed more light on the peculiar variability of the star and planet, providing additional insights into the scientific models of exoplanet escape and atmospheric evolution.

Rif: NASA, The Astronomical Journal