There are not many hot Neptunes in the galaxy, and it is not clear why. A hot Neptune, as the name implies, is a Neptune-sized planet that rotates within 1 AU of its host star. We've definitely spotted some of them, there just are not as many as we might have expected There are many mini-Neptune, at about 10 Earth masses (Uranus and Neptune are 14.5 Earth masses or 17 Earth masses). Theoretically, it was thought that the abundance of mini-Neptunes and the lack of Hot Neptunes could all together point to another cause – planets that start as Hot Neptunes have often supplanted their atmospheres, causing them to shrink themselves. Now scientists have actually managed to see it in action.
"The question was, where did the hot Neptunes go?" Said astronomer Vincent Bourrier from the University of Geneva in Sauverny, Switzerland, to HubbleSite. "If we plot the size and distance of the planet from the star, there is a desert, a hole in that distribution. That was a mystery.
Researchers are observing a planet called Gliese 3470 b, an exoplanet just below 14 Earth masses that is currently considered a mini Neptune. If so, it is likely to have entered this phase recently. They currently estimate that the planet has lost between 4 and 35 percent of its total mass in the last 2 years. The wide bandwidth is due to the fact that we have been watching for so short a time. Compared to Gliese 436 b, another planet we know suffers an atmospheric loss due to interactions with its host star, Gliese 3470 b loses 100 times the atmosphere.
"This is the smoking weapon that can significantly lose planets their entire mass," said physicist and planetary scientist David Sing of Johns Hopkins University.
"GJ 3470b loses more in mass than any other planet we have have seen so far; In just a few billion years, half of the planet could have disappeared.
It is believed that Hot Neptunes are far less common than Hot Jupiters because they lack the gravity to absorb their atmospheres against the onslaught of their host stars' solar wind hold. For example, it is believed that coronary mass ejection events and other solar storms are responsible for the loss of at least part of the Martian atmosphere. These smaller gas giants, orbiting the distance between Mercury and the Sun in a fraction, can eventually turn into the super-earths that we've found quite frequently, helping explain the prevalence of these types of bodies.
Feature Image Credits: NASA, ESA, and D. Player (STScI)
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