The brightest stars in the night sky can strip planets of their rocky cores

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Artist’s impression of a Neptune-sized planet, left, around a blue A-type star. UC Berkeley astronomers have discovered an elusive gas giant around one of these bright but short-lived stars, right in the edge of Neptune. hot desert where the star’s strong radiation is likely to strip any giant planets of their gas. (Image credit: Steven Giacalone, UC Berkeley)

In the last 25 years, astronomers have found thousands of exoplanets around stars in our galaxy, but more than 99% of them orbit smaller stars, from red dwarfs to stars slightly more massive than the Sun, which is considered a star of medium size. .

Few have been discovered around even more massive stars, such as A-type stars, bright blue stars that are twice the size of the sun, and most of the exoplanets that have been observed are Jupiter-sized or larger. Some of the brightest stars in the night sky, such as Sirius and Vega, are A-type stars.

Astronomers at the University of California, Berkeley now report a new Neptune-sized planet, called HD 56414 b, around one of these hot-but-short-lived A-type stars, hinting at why is there so little gas. Giants smaller than Jupiter have been observed around the brightest 1% of stars in our galaxy.

Current exoplanet detection methods more easily find planets with short, fast orbital periods around their stars, but this newly discovered planet has a longer orbital period than most discovered to date. The researchers suggest that a Neptune-sized planet, easier to find and closer to a bright A-type star, would be rapidly stripped of its gas by strong stellar radiation and reduced to an undetectable core.

Although this theory has been proposed to explain Neptune’s so-called hot deserts around redder stars, it was unclear whether this extended to hotter stars (A-type stars are 1.5 to 2 times hotter than the Sun). ) due to the scarcity of known planets. around some of the brightest stars in the galaxy.

“It’s one of the smallest planets we know of around these really massive stars,” said UC Berkeley graduate student Steven Giacalone. “It’s actually the hottest star we know of with a planet smaller than Jupiter. This planet is interesting first and foremost because these types of planets are really hard to find, and we probably won’t find many like them in the foreseeable future.” future.

Neptune’s hot desert

The discovery of what researchers are calling a ‘hot Neptune’ just outside the area where the planet would have been stripped of its gas suggests that bright A-type stars may have many unseen cores in Neptune’s hot zone, waiting to be discovered in the near future. through more sensitive sensors. techniques

a plot of all exposure networks found so far, showing a gap where hot Neptunes should be

Astronomers have found thousands of exoplanets (black spots) around stars in the Milky Way galaxy, but few Neptune-sized planets have been discovered in short-period orbits around their stars, creating what astronomers call a desert. hot Neptune (pink region, representing planets with radii 3-10 times greater than Earth’s with orbital periods less than 3 days). A new Neptune-sized planet (yellow star) suggests they don’t survive long enough to be detected. The planets on this map were discovered when they passed or transited through their star, dimming their light. Current techniques are limited to finding planets in close, short-period orbits of less than 100 days. (Graphic by Steven Giacalone, using data courtesy of NASA)

“We might expect to see a stack of remaining Neptunian cores in short orbital periods” around these stars, the researchers concluded in their paper.

The discovery also adds to our understanding of the evolution of planetary atmospheres, he said. Courtney Dressingassistant professor of astronomy at UC Berkeley.

“There’s a big question about how planets retain their atmospheres over time,” Dressing said. “When we look at smaller planets, are we looking at the atmosphere they formed with when they originally formed from an accretion disk? Are we in the presence of an atmosphere that has degassed from the planet over time? If we’re able to look at planets that get different amounts of light from their star, specifically different wavelengths of light, which A stars allow us to do, that allows us to change the ratio of X-rays to ultraviolet light, then we can try to see exactly how a planet retains its atmosphere over time.

Giacalone and Dressing reported their discovery in a paper accepted by The Astrophysical Journal Letters and published online today (Aug. 12).

According to Dressing, it is well established that highly irradiated Neptune-sized planets orbiting less massive Sun-like stars are rarer than expected. But it is not known whether this is true for planets orbiting A-type stars because these planets are difficult to detect.

And an A-type star is a different animal from small F, G, K, and M dwarfs. Nearby planets orbiting Sun-type stars receive large amounts of X-rays and ultraviolet radiation, but nearby planets orbiting Sun-type stars A experience much more near ultraviolet radiation than X-rays or extreme ultraviolet radiation.

“Determining whether Neptune’s hot desert also extends to A-type stars provides insight into the importance of near-ultraviolet radiation in managing atmospheric escape,” he said. “This result is important for understanding the physics of atmospheric mass loss and studying the formation and evolution of minor planets.”

The planet HD 56414 b was detected by NASA’s TESS mission as it transited its star, HD 56414. Dressing, Giacalone, and their colleagues confirmed that HD 56414 was an A-type star by obtaining spectra with the 1.5-meter telescope operated by the Small and SMARTS (Moderate Aperture Research Telescopic System) Consortium at Cerro Tololo in Chile.

The planet has a radius 3.7 times that of Earth and orbits the star every 29 days at a distance equivalent to about a quarter of the distance between Earth and the sun. The system is about 420 million years old, much younger than our sun’s 4.5 billion years.

The researchers modeled the effect that the star’s radiation would have on the planet and concluded that although the star could slowly eat away its atmosphere, it would likely survive for a billion years, beyond the point at which the star would burn up and collapse. producing a supernova.

Giacalone said Jupiter-sized planets are less susceptible to photoevaporation because their cores are massive enough to retain their hydrogen gas.

“There is this balance between the central mass of the planet and the expansion of the atmosphere,” he said. “For planets the size of Jupiter or larger, the planet is massive enough to gravitationally cling to its bloated atmosphere. As you go towards Neptune-sized planets, the atmosphere is always puffy, but the planet isn’t that massive, so they can lose their atmosphere more easily.

Giacalone and Dressing continue to search for other Neptune-sized exoplanets around A-type stars, hoping to find others in or near Neptune’s hot desert, to find out where these planets form in the accretion disk during star formation. whether they move or not. inward or inward. outward over time, and how their atmospheres change.

The work was supported by a FINESST award from NASA (80NSSC20K1549) and the David and Lucile Packard Foundation (2019-69648).

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