There aren’t a lot of Hot Neptunes in the galaxy, and it’s not entirely clear why. A Hot Neptune is, as the name implies, a Neptune-sized planet orbiting within 1 AU of its host star. While we’ve definitely detected a few of them, there simply aren’t as many as we might have expected. There are, on the other hand, plenty of mini-Neptunes, at about 10 Earth masses (Uranus and Neptune are 14.5 Earth masses and 17 Earth masses respectively). It’s been theorized that the abundance of mini-Neptunes and lack of Hot Neptunes could collectively point to a different cause — planets that start off as Hot Neptunes often have their atmospheres boiled away, leaving them shrunken versions of themselves. Now, scientists have managed to actually see it in action.
“The question has been, where have the hot Neptunes gone?” said astronomer Vincent Bourrier of the University of Geneva in Sauverny, Switzerland, to HubbleSite. “If we plot planetary size and distance from the star, there’s a desert, a hole, in that distribution. That’s been a puzzle.”
Researchers observing a planet known as Gliese 3470 b, an exoplanet just under 14 Earth masses and currently thought to be a mini-Neptune. If so, it’s probably entered that phase only recently. They currently estimate that the planet has lost between 4-35 percent of its total mass over the past 2B years. The vast range in scope is driven by the fact that we’ve only been observing for such a short period of time. Compared with Gliese 436 b, another planet we know is undergoing atmospheric loss due to interactions with its host star, Gliese 3470 b is losing atmosphere at 100x the rate.
“This is the smoking gun that planets can lose a significant fraction of their entire mass,” said physicist and planetary scientist David Sing of Johns Hopkins University.
“GJ 3470b is losing more of its mass than any other planet we seen so far; in only a few billion years from now, half of the planet may be gone.”
It’s theorized that Hot Neptunes may be far less common than Hot Jupiters because they lack the gravity to hold their atmospheres against the onslaught of their host stars’ solar wind. Coronal mass ejection events and other solar storms are believed to be responsible for the loss of at least some of Mars’ atmosphere, for example. Orbiting at a fraction the distance between Mercury and the Sun, these smaller gas giants may eventually transform into the super-Earths we’ve found fairly frequently as well, helping to explain the prevalence of these types of bodies.
Feature Image Credits: NASA, ESA, and D. Player (STScI)
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