Hot Neptune Desert — Harsh Radiation From Bright Stars May Be Stripping Off Gas From Their Planets

The brightest stars of the sky such as Sirius and Vega have one thing in common - they are both A-type stars, meaning they are about 1.5 to 2 times hotter than the sun and are orbited by Jupiter-like large planets. A new study has renewed interest in these A-type stars and the planets that orbit them.

Astronomers from the University of California, Berkeley have discovered a new Neptune-sized planet — called HD 56414 b — around one of these hot, but short-lived A-type stars. Their study, published in the Astrophysical Journal Letters, clues us in on why there are fewer planets smaller than Jupiter revolving around the brightest stars in our galaxy.

Talking about the newly discovered planet, co-author and UC Berkeley graduate student Steven Giacalone said, "It's one of the smallest planets that we know of around these really massive stars. 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."

Researchers suggest that we don't find more of these Neptune-sized planets close to the A-type star because of the stripping of their planet's gas by the strong stellar radiation emitted by the star, resulting in an undetectable core.

Their theory propounds that the concept of so-called hot Neptune deserts - a region of mass-radius space that contains few planets - found near redder stars can be extended to A-type stars.

"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 co-author Courtney Dressing, UC Berkeley assistant professor of astronomy. "This result is important for understanding the physics of atmospheric mass loss and investigating the formation and evolution of small planets."

So what makes the Jupiter-sized planets evade photoevaporation?

There is a "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," Giacalone explained.

The planet HD 56414 b was found by NASA's TESS mission as it transited its star, HD 56414. The planet's radius is 3.7 times that of Earth and it orbits its A-type star every 29 days. The distance between the star and the planet is equivalent to one-fourth the distance between Earth and the sun. This system is much younger than our sun's 4.5-billion-year age, being only about 420 million years old.

Based on their study, researchers concluded the planet might be slowly losing its atmosphere, though it would likely survive for a billion years. It is expected to collapse into a supernova after that.