Hubble sees evaporating planet getting the hiccups
IMAGE: THIS ARTIST'S ILLUSTRATION SHOWS A PLANET (DARK SILHOUETTE) PASSING IN FRONT OF THE RED DWARF STAR AU MICROSCOPII. THE PLANET IS SO CLOSE TO THE ERUPTIVE STAR A FEROCIOUS BLAST OF STELLAR WIND AND BLISTERING ULTRAVIOLET RADIATION IS HEATING THE PLANET'S HYDROGEN ATMOSPHERE, CAUSING IT TO ESCAPE INTO SPACE. FOUR TIMES EARTH'S DIAMETER, THE PLANET IS SLOWLY EVAPORATING ITS ATMOSPHERE, WHICH STRETCHES OUT LINEARLY ALONG ITS ORBITAL PATH. THIS PROCESS MAY EVENTUALLY LEAVE BEHIND A ROCKY CORE. THE ILLUSTRATION IS BASED ON MEASUREMENTS MADE BY THE HUBBLE SPACE TELESCOPE. view more
CREDIT: NASA, ESA, JOSEPH OLMSTED (STSCI)
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 NASA's 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.
This extreme variability between orbits shocked astronomers. "We've never seen atmospheric escape go from completely not detectable to very detectable over such a short period when a planet passes in front of its star," said Keighley Rockcliffe of Dartmouth College in Hanover, New Hampshire. "We were really expecting something very predictable, repeatable. But it turned out to be weird. When I first saw this, I thought 'That can't be right.'"
Rockcliffe was equally puzzled to see, when it was detectable, the planet's atmosphere puffing out in front of the planet, like a headlight on a fast-bound train. "This frankly strange observation is kind of a stress-test case for the modeling and the physics about planetary evolution. This observation is so cool because we're getting to probe this interplay between the star and the planet that is really at the most extreme," she said.
Located 32 light-years from Earth, the parent star AU Microscopii (AU Mic) hosts one of the youngest planetary systems ever observed. The star is less than 100 million years old (a tiny fraction of the age of our Sun, which is 4.6 billion years old). The innermost planet, AU Mic b, has an orbital period of 8.46 days and is just 6 million miles from the star (about 1/10th the planet Mercury's distance from our Sun). The bloated, gaseous world is about four times Earth's diameter.
AU Mic b was discovered by NASA’s Spitzer and TESS (Transiting Exoplanet Survey Satellite) space telescopes in 2020. It was spotted with the transit method, meaning telescopes can observe a slight dip in the star's brightness when the planet crosses in front of it.
Red dwarfs like AU Microscopii are the most abundant stars in our Milky Way galaxy. They therefore should host the majority of planets in our galaxy. But can planets orbiting red dwarf stars like AU Mic b be hospitable to life? A key challenge is that young red dwarfs have ferocious stellar flares blasting out withering radiation. This period of high activity lasts a lot longer than that of stars like our Sun.
The flares are powered by intense magnetic fields that get tangled by the roiling motions of the stellar atmosphere. When the tangling gets too intense, the fields break and reconnect, unleashing tremendous amounts of energy that are 100 to 1,000 times more energetic than our Sun unleashes in its outbursts. It's a blistering fireworks show of torrential winds, flares, and X-rays blasting any planets orbiting close to the star. "This creates a really unconstrained and frankly, scary, stellar wind environment that's impacting the planet's atmosphere," said Rockcliffe.
Under these torrid conditions, planets forming within the first 100 million years of the star's birth should experience the most amount of atmospheric escape. This might end up completely stripping a planet of its atmosphere.
"We want to find out what kinds of planets can survive these environments. What will they finally look like when the star settles down? And would there be any chance of habitability eventually, or will they wind up just being scorched planets?" said Rockcliffe. "Do they eventually lose most of their atmospheres and their surviving cores become super-Earths? We don't really know what those final compositions look like because we don't have anything like that in our solar system."
While the star's glare prevents Hubble from directly seeing the planet, the telescope can measure changes in the star's apparent brightness caused by hydrogen bleeding off the planet and dimming the starlight when the planet transits the star. That atmospheric hydrogen has been heated to the point where it escapes the planet's gravity.
The never-before-seen changes in atmospheric outflow from AU Mic b may indicate swift and extreme variability in the host red dwarf's outbursts. There is so much variability because the star has a lot of roiling magnetic field lines. One possible explanation for the missing hydrogen during one of the planet's transits is that a powerful stellar flare, seen seven hours prior, may have photoionized the escaping hydrogen to the point where it became transparent to light, and so was not detectable.
Another explanation is that the stellar wind itself is shaping the planetary outflow, making it observable at some times and not observable at other times, even causing some of the outflow to "hiccup" ahead of the planet itself. This is predicted in some models, like those of John McCann and Ruth Murray-Clay from the University of California at Santa Cruz, but this is the first kind of observational evidence of it happening and to such an extreme degree, say researchers.
Hubble follow-up observations of more AU Mic b transits should offer additional clues to the star and planet's odd variability, further testing scientific models of exoplanetary atmospheric escape and evolution.
Rockcliffe is lead author on the science paper accepted for publication in The Astronomical Journal.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
JOURNAL
The Astronomical Journal
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b
ARTICLE PUBLICATION DATE
27-Jul-2023
Violent atmosphere allows rare look at the early life of a planet
Neptune-sized planet weirdly and sporadically burps hydrogen as it circles its star
Peer-Reviewed PublicationIMAGE: AN ILLUSTRATION OF THE PLANET AU MIC B SHEDDING ITS ATMOSPHERE AS IT ORBITS ITS SUN, AU MICROSCOPII. view more
CREDIT: IMAGE BY NASA, ESA, JOSEPH OLMSTED, STSCI.
Trillions of miles from Earth, the violent and erratic shedding of a young planet's atmosphere could provide a rare glimpse into the tumultuous early life that besets most of the planets in our galaxy.
A new study led by Dartmouth researchers found that a Neptune-sized gas planet known as AU Mic b exhibited some bizarre behavior—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.
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.
"This is the first time we've seen a planet’s atmospheric escape go from unobservable to very, very observable," said first author Keighley Rockcliffe, a Ph.D. candidate in physics and astronomy at Dartmouth. "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," Rockcliffe said. "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."
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.
The young age and atmospheric behavior of AU Mic b and its sun suggest that the researchers have captured the early stages of planetary evolution, said co-author Elisabeth Newton, an assistant professor of physics and astronomy at Dartmouth. Most studies on planets outside—and even within—Earth's solar system pertain to very old worlds.
Older planets have already experienced a wide range of evolutionary processes that make it difficult to extrapolate to planetary evolution at large, Newton said. They're like trying to study developmental psychology by only observing adults, she said.
"This planet is like observing a totally generic toddler," Newton said. "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."
The planet's sun is a common type of small, low-intensity star known as a red dwarf. Seventy percent of all stars are red dwarfs, Newton said, including Proxima Centauri, the closest star to our sun.
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.
"I'm trying to observe that loss of mass as it's happening to get a snapshot of how planets develop before they get to that smaller, potentially rockier endpoint," she said.
Young planets such as AU Mic b also provide scientists with opportunities to examine the tempestuous early years of their young stars. These observations can be used to fine-tune computer models of how planets evolve and interact with their stellar environment, Rockcliffe said.
"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," Rockcliffe said.
"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," she said. "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, Newton said. But even if they don't, red dwarf systems are currently the best places to find habitable, Earth-like planets.
"Stars like AU Mic are potential hunting grounds for an Earth 2.0," Newton said. "By understanding this system, we can answer questions about what an Earth-like planet orbiting a red dwarf star would have to contend with early in its evolution."
"The systems we're looking at are extremely different from our solar system. We can't really extrapolate from our own experience of atmospheric mass loss," Rockcliffe added. But "knowing how atmospheres evolve and which planets will have stable atmospheres is important for finding life on other planets. Atmospheres are essential in understanding how life can form and persist."
The paper "The variable detection of atmospheric escape around the young, hot Neptune AU Mic b" was published in The Astronomical Journal on July 27, 2023.
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JOURNAL
The Astronomical Journal
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU Mic b
ARTICLE PUBLICATION DATE
27-Jul-2023
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