Brightest stars in the night sky can strip Neptune-sized planets to their rocky cores: A rare find — a ‘warm Neptune’ around a bright blue star — offers clues to the dearth of Hot Neptunes

Over 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 our sun, which is considered an average-sized star.

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

University of California, Berkeley, astronomers now report a new, Neptune-sized planet — called HD 56414 b — around one of these hot-burning, but short-lived, A-type stars and provide a hint about why so few gas giants smaller than Jupiter have been seen around the brightest 1% of stars in our galaxy.

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

While this theory has been proposed to explain so-called hot Neptune deserts around redder stars, whether this extended to hotter stars — A-type stars are about 1.5 to 2 times hotter than the sun — was unknown because of the dearth of planets known around some of the galaxy’s brightest stars.

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

Hot Neptune desert

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