AU Mic b, a “hot Neptune”: Observing Planet Evolution

A new study led by Dartmouth researchers found that a Neptune-sized gas planet known as AU Mic b has exhibited some bizarre behaviour—it showed no atmospheric shedding during one orbit around its sun then spewed its hydrogen-rich atmosphere into the cosmos on its next go-round.

An illustration of the planet AU Mic b shedding its atmosphere as it orbits its sun, AU Microscopii. Image credit: NASA, ESA, Joseph Olmsted, STScI.

The planet—which is more than four times the diameter of Earth, orbits a star called AU Microscopii that is 32 light years (roughly 192 trillion miles) from Earth. In star terms, AU Microscopii is a youthful 23 million years old; our sun is roughly 4.6 billion years old.

The planet AU Mic b is only 6 million miles from its sun—or one-tenth of the distance Mercury is from our sun. Despite its size, AU Mic b completes a full orbit in less than nine Earth days. It’s discovery by NASA’s Spitzer and TESS space telescopes was published in the journal Nature in 2020. The latest study is based on data from the Hubble Space Telescope.

 Keighley Rockcliffe, a Ph.D. candidate in physics and astronomy at Dartmouth explained:

“This is the first time we’ve seen a planet’s atmospheric escape go from unobservable to very, very observable.

 “In addition, the hydrogen cloud was not a tail behind the planet like we normally see, but like a puff in front of the planet as it orbited. We don’t usually think of planets as burping hydrogen as they go around a star.

“We are directly probing an essential evolutionary mechanism that the most common planets in our galaxy go through. We think our work captures the early stages of this extremely typical process, and we want to use our observations of this system to understand the most common experiences of planets beyond our solar system.”

All planets with an atmosphere lose some gas as they orbit their suns—a process known as atmospheric escape—either subtly like Earth or in dramatic plumes like AU Mic b. But scientists have never before seen atmospheric escape stop and start between orbits, the researchers report in The Astronomical Journal.

 Elisabeth Newton, an assistant professor of physics and astronomy at Dartmouth commented:

“Systems like AU Mic are our insight into the broader planetary-evolution process. Keighley is making very challenging observations, and there are limited opportunities to even attempt them.”

AU Mic b itself is a type of planet known as a “hot Neptune,” a world similar in size to Neptune that orbits close to its parent star. The evolution of hot Neptunes is thought to be broadly applicable to other gas planets in the galaxy. Scientists think the planets quickly burn off their large gaseous layer and evolve into smaller planets, Rockcliffe said. Only one other young hot Neptune has been observed undergoing atmospheric escape.

Keighley Rockcliffe continued:

“There’s an enormous difference between a 23-million-year-old star and a 5-billion-year-old star. The very young stars are going to be throwing out lots of flares and very high-energy radiation. Because we’re looking at young planets, we can see this very extreme but typical interaction happen and use our observations to see if we’re understanding the physics correctly.

“I’m becoming more convinced that AU Mic b is this nice example of a planet undergoing all these violent but typical processes at once. It can hit the corners of many different models and ensure we’re making the most accurate models possible when we’re talking about planet evolution.”

With every new planet discovered, the question arises: Could it be another Earth? Some rocky planets experience an early stage similar to what AU Mic b is going through now. But even if they don’t, red dwarf systems are currently the best places to find habitable, Earth-like planets.

Click on this link to access, The variable detection of atmospheric escape around the young, hot Neptune AU Mic b, published in The Astronomical Journal on July 27, 2023.

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