An article published in “The Astrophysical Journal Letters” reports a study on the exoplanet HD 56414 b, a warm Neptune discovered orbiting an A-type star that has a mass almost twice the Sun’s. A team of researchers used detections conducted by NASA’s TESS space telescope to find a candidate exoplanet that was subsequently verified thanks to follow-up observations conducted with other instruments. This is a rare combination, as these massive stars usually have giant planets like Jupiter or even more massive ones.
Gas giant exoplanets are much easier to discover and exoplanets closer to a star are also easier to discover. The problem with looking for exoplanets around massive stars is that they have a relatively short lifespan and a rocky planet orbiting close to such a star may be difficult to spot due to the very bright starlight. Neptune-like exoplanets are relatively easy to find but are rare in certain areas close to their star. This is another clue that hot Neptunes near their star turn into rocky planets after their atmosphere is stripped by the stellar wind.
Most of the exoplanets discovered orbit around low to medium mass stars. They make the vast majority of the stars in the universe and therefore also in the cosmic neighborhood where it’s possible to search for planets that orbit around them. The rarity of Neptune-like exoplanets with an orbit close to a small or medium star makes astronomers think that this type of planet, called hot Neptune in jargon, loses its atmosphere due to the stellar wind and turns into a rocky planet. The discovery of a warm Neptune orbiting a massive star but not too close to it may offer more clues to this transformation.
The exoplanet HD 56414 b has a year that lasts almost exactly 29 Earth days with a distance from its star that is about a quarter of that of the Earth from the Sun. It has a radius that is about 3.71 times the Earth’s, and that makes it one of the smallest planets discovered around stars that are much more massive than the Sun.
The position of HD 56414 b is close to the so-called Neptune desert, the area around a star where it’s rare to find that type of planet. It may be close enough to the star to suffer an atmosphere loss effect. However, such a massive star could consume its hydrogen and die in a supernova before its planet is transformed from Neptune-like to rocky. The star is young in astronomical terms with an age estimated at less than half a billion years but researchers estimate that it would take about a billion years to strip the atmosphere of exoplanet HD 56414 b, longer than the star has yet to live.
It’s possible that near massive stars, rocky planets are orbiting that were born as Neptune-like, sometimes mini-Neptunes, too close to their star to maintain their atmosphere for long. The authors of the study on exoplanet HD 56414 b intend to look for more hot Neptune-sized exoplanets orbiting massive stars. Understanding if there’s a Neptune desert just like for small and medium stars would help to better understand the evolution of certain exoplanets. These are also useful studies to better understand the possibilities of planetary migrations due to interactions between planets of the same system.
