AU Microscopii

Red dwarfs are the most abundant type of stars in our galaxy. They are smaller and cooler than our Sun, but they are presumed to contain the bulk of the galaxy’s planet population, perhaps tens of billions of worlds. Surveys by the Kepler Space Telescope and other observatories have shown that rocky planets are common around these stars, and there are several such planets within the habitable zones of nearby red dwarfs. The temperate climates on such worlds could allow for oceans of water to exist on their surfaces, creating environments suitable for life. However, many of these rocky planets may not harbor water and organic material, the necessary ingredients for life as we know it. Earth formed as a “dry” planet and was later bombarded over hundreds of millions of years with icy material from comets and asteroids from the outer Solar System. If that’s the processes needed for planets to become possible habitats for life around, then planets around red dwarfs may be in trouble.

Observations of the nearby, young red dwarf AU Microscopii (AU Mic) using the Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope in Chile shows a rapidly eroding dust-and-gas disk encircling the star. Hubble took the image above in 2018 in visible light. The glare of the star, located at the center of the disk, has been blocked out. The box in the picture highlights one blob of material extending above and below the disk.

The disk is being cleaned out by fast-moving blobs of material which pushing small objects out of the system. (Astronomers aren’t sure what’s driving the process, but one theory is that powerful mass ejections from the star, a common phenomenon among young red dwarfs, may bee the cause.) If the debris disk around AU Mic continues to dissipate at the current pace, it wont last much more than 1.5 million years, the blink of an eye in cosmic time. Planets would be too large to be ejected, but smaller bodies, such as comets and asteroids, could be cleared out. Then with no later bombardment by watery bodies, the planets might end up dry, dusty, and lifeless.

Image Credits: NASA / ESA / J. Wisniewski (University of Oklahoma) / C. Grady (Eureka Scientific) / G. Schneider (Steward Observatory)

The Debris Disk Around a Red Dwarf Star

IDL TIFF fileThis visible-light image of the debris disk around the red dwarf star AU Microscopii hints that planets may be forming or might already exist within it. The disk glows in light reflected by tiny grains of dust resulting from  the collisions of asteroids and comets. This debris disk is more than 40 billion miles across. The star at the center is quite young, about 12 million years old. It is only 32 light-years from Earth which makes its disk the closest yet seen in reflected starlight. It is also the first disk imaged around an M-type red dwarf, the most common type of star in the stellar neighborhood around the Sun.The disk has been cleared of dust within about a billion miles of the star. Images taken by Hubble (including this one) confirm that the disk is warped and has small variations in density that may have been caused by the tugging of an unseen companion, perhaps a large planet. That would be consistent with presence of the inner gap as well.

This debris disk is unusual in that it is the only one known that appears bluer than the star it surrounds. This possibly could it having a greater proportion of very small grains of dust  than other such disks. Smaller grains scatter blue light better than red. The surplus of small grains may be caused by the star not being bright enough to blow away tiny particles. Brighter, hotter stars would produce sufficient radiation to push small dust grains out of the disk and out into interstellar space

Image Credit: NASA