Astronomers from the University of California, Berkeley have made a fascinating new discovery in deep space. They found a Neptune-sized planet around a hot-burning, but short-lived, A-type star.
A-type stars are bright blue stars which are twice as large as the sun and two times hotter. Few exoplanets have been located around these massive stars, while most of the planets that have been observed are the size of Jupiter or larger.
The new planet, called HD 56414 b, has a radius 3.7 times that of Earth and orbits the star every 29 days at a distance equal to nearly one-quarter the distance between the Earth and sun. Astronomers say the system is 420 million years old.
“It’s one of the smallest planets that we know of around these really massive stars,” says Steven Giacalone, a graduate student at UC Berkeley, in a media release. “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.”
This newly discovered planet has a longer orbital period than most discovered to date, as other planets have short, rapid orbital periods around their stars. This suggests an easier-to-find Neptune-sized planet sitting closer to an A-type star would be rapidly stripped of its gas by the severe stellar radiation and reduced to an undetectable core.
Researchers believe these A-type stars may have several unseen cores within the hot Neptune zone that are waiting to be discovered through more sensitive techniques. That’s because “warm Neptune” is just outside the zone where the planet would have been stripped of its gas. Courtney Dressing, an assistant professor of astronomy at UC Berkeley, says their discovery adds to their understanding of how planetary atmospheres evolve.
“There’s a big question about just how do planets retain their atmospheres over time,” explains Dressing. “When we’re looking at smaller planets, are we looking at the atmosphere that it was formed with when it originally formed from an accretion disk? Are we looking at an atmosphere that was outgassed from the planet over time? If we’re able to look at planets receiving different amounts of light from their star, especially different wavelengths of light, which is what the A stars allow us to do — it allows us to change the ratio of X-ray to ultraviolet light — then we can try to see how exactly a planet keeps its atmosphere over time.”
Dressing says it’s well-known that highly-irradiated, Neptune-sized planets orbiting less massive, sun-like stars are rarer than expected. However, it’s not known whether this holds for planets orbiting A-type stars since they are hard to detect. Planets orbiting A-type stars experience more near-ultraviolet radiation than X-ray radiation or extreme ultraviolet radiation.
“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,” notes Dressing. “This result is important for understanding the physics of atmospheric mass loss and investigating the formation and evolution of small planets.”
HD 56414 b was located by NASA’s TESS mission as it transited its star, HD 56414, an A-type star. Astronomers modeled the effect that radiation from the star would have on the planet and found that it would likely survive for a billion years, despite the star paring down its atmosphere. Its expected survival is beyond the point at which the star is expected to burn out and collapse, producing a supernova.
The study is published in The Astrophysical Journal Letters.