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AU Mic and planet
An artist’s conception shows the red dwarf star AU Microscopii with a hypothetical planet and moon in the foreground. (NASA / ESA Illustration / G. Bacon)

Red dwarf stars have been seen as the biggest potential frontier for alien life, in part because they’re the most common stars in our galaxy. But observations made using the Hubble Space Telescope suggest that the frontier might turn out to be a desert.

“We may have found the limit to habitable planets,” said Carol Grady, a co-investigator on the Hubble observations from Eureka Scientific in Oakland, Calif. She laid out the research team’s findings today at the American Astronomical Society’s winter meeting in Seattle.

Red dwarfs, or M-dwarf stars, are much smaller and dimmer than our sun — but they’re thought to account for 50 to 75 percent of the stars in our Milky Way galaxy. The focus of the Hubble observations is AU Microscopii, or AU Mic, a young red dwarf that’s about 32 light-years from Earth in the southern constellation Microscopium. It’s thought to be a mere 23 million years old.

Grady and her colleagues used Hubble as well as the European Southern Observatory’s Very Large Telescope in Chile and the XMM-Newton satellite to track what’s been happening to a broad disk of gas and dust surrounding AU Mic — exactly the kind of disk that gives birth to planets.

Over the course of several years, the astronomers watched the protoplanetary disk erode as fast-moving blobs of material pushed particles farther out into the void. The international research team, led by the University of Oklahoma’s John Wisniewski, calculated that the disk could be totally cleared out in 1.4 million years.

AU Mic
These two Hubble Space Telescope images, taken in 2011 and 2017, show fast-moving blobs of material sweeping outwardly through a debris disk around the red dwarf star AU Microscopii. (NASA / ESA / J. Wisniewski / C. Grady / G. Schneider)

Losing a protoplanetary disk that quickly would be bad news for life’s development, based on Earth’s example.

“The Earth, we know, formed ‘dry,’ with a hot, molten surface, and accreted atmospheric water and other volatiles for hundreds of millions of years, being enriched by icy material from comets and asteroids transported from the outer solar system,” another Hubble co-investigator, Glenn Schneider of Arizona’s Steward Observatory, said in a news release.

Grady said red dwarf seem likely to miss out on that part of the process.

“What this suggests is that processes which depend on disk survival may be inhibited in systems around young M-stars,” she said. “This includes the delivery of water and organics to terrestrial-mass planets in the habitable zone.”

The findings are in line with previous cautions that seemingly promising red-dwarf planets, including Proxima Centauri b and the TRAPPIST-1 planets, may not actually be habitable even though they’re in orbital zones where liquid water could theoretically exist.

Grady said more telescope time will be required to nail down the precise mechanism behind the disappearing disk. However, the likeliest scenario has to do with the radiation environment around red dwarfs.

“M-stars are extremely active,” she said. “In our week of XMM observations, we got a lot of flares.”

What’s more, the habitable zones for red-dwarf stars tend to be incredibly close in, which means potentially habitable planets are typically exposed to the “least benign radiation environment,” Grady said.

She said AU Mic isn’t an isolated example.

“We have a couple of other early M-stars where we think we’re looking at the same phenomenon, which has the distinctive feature that the outer portions of their disks look like Swiss cheese,” Grady said.

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