Neptune-Sized Exoplanet Found Orbiting A-Type Star HD 56414

by johnsmith

With an effective temperature of 8,500 K, the A-type star HD 56414 is one of the hottest stars known to host a planet smaller than Jupiter.

An artist’s impression of a Neptune-sized planet around an A-type star. Image credit: Steven Giacalone, University of California, Berkeley.

An artist’s impression of a Neptune-sized planet around an A-type star. Image credit: Steven Giacalone, University of California, Berkeley.

HD 56414 is located approximately 873 light-years away in the constellation of Volans.

Also known as TOI-1228, TIC 300038935 or SAO 249773, the star is roughly 420 million years old, much younger than our Sun’s 4.5-billion-year age.

The newly-found exoplanet orbits HD 56414 every 29 days at a distance equal to about one-quarter the distance between Earth and the Sun.

The alien world has a radius 3.7 times that of Earth and belongs to the class of warm Neptunes.

Named HD 56414b (TOI-1228b), it was detected by NASA’s Transiting Exoplanet Survey Satellite (TESS) as it transited its parent A-type star.

“Current exoplanet detection methods most easily find planets with short, rapid orbital periods around their stars, but this planet has a longer orbital period than most discovered to date,” said University of California, Berkeley astronomer Steven Giacalone and his colleagues.

“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 was unknown because of the dearth of planets known around some of the Galaxy’s brightest stars.”

The discovery of HD 56414b just outside the zone where the planet would have been stripped of its gas suggests that bright, A-type stars may have numerous unseen cores within the hot Neptune zone that are waiting to be discovered through more sensitive techniques.

“Determining whether the hot Neptune desert also extends to A-type stars provides insight into the importance of near-ultraviolet radiation in governing atmospheric escape,” said University of California, Berkeley astronomer Dr. Courtney Dressing.

“This result is important for understanding the physics of atmospheric mass loss and investigating the formation and evolution of small planets.”

In their study, the astronomers also modeled the effect that radiation from the star would have on the planet and concluded that, while the star may be slowly whittling away at its atmosphere, it would likely survive for a billion years — beyond the point at which the star is expected to burn out and collapse, producing a supernova.

“Jupiter-sized planets are less susceptible to photoevaporation because their cores are massive enough to hold onto their hydrogen gas,” Giacalone said.

“There’s this balance between the central mass of the planet and how puffy the atmosphere is.”

“For planets the size of Jupiter or larger, the planet is massive enough to gravitationally hold on to its puffy atmosphere.”

“As you move down to planets the size of Neptune, the atmosphere is still puffy, but the planet is not as massive, so they can lose their atmospheres more easily.”

The results appear in the Astrophysical Journal Letters.


Steven Giacalone et al. 2022. HD 56414b: A Warm Neptune Transiting an A-type Star. ApJL 935, L10; doi: 10.3847/2041-8213/ac80f4

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