Free Astronomy Magazine March-April 2019

52 MARCH-APRIL 2019 SPACE CHRONICLES found that one of the warmest known Neptunes (GJ 436b) is losing its atmosphere. The planet isn’t ex- pected to evaporate away, but hot- ter Neptunes might not have been so lucky. Now, astronomers have used Hub- ble to nab a second “very warm” Neptune (GJ 3470b) that is losing its atmosphere at a rate 100 times faster than that of GJ 436b. Both planets reside about 3.7 million miles from their star. That’s one- tenth the distance between our solar system’s innermost planet, Mercury, and the Sun. “I think this is the first case where this is so dra- matic in terms of planetary evolu- tion,” said lead researcher Vincent Bourrier of the University of Geneva in Sauverny, Switzerland. “It’s one of the most extreme examples of a planet undergoing a major mass- loss over its lifetime. This sizable mass loss has major consequences for its evolution, and it impacts our understanding of the origin and fate of the population of exoplan- ets close to their stars.” As with the previously discovered evaporating planets, the star’s in- tense radiation heats the atmos- phere to a point where it escapes the planet’s gravitational pull like an un- tethered hot air balloon. The escap- ing gas forms a giant cloud around the planet that dissipates into space. One reason why GJ 3470b may be evaporating faster than GJ 436b is that it is not as dense, so it is less able to gravitationally hang on to the heated atmosphere. Hubble finds a fast evaporating exoplanet by NASA/ESA F ishermen would be puzzled if they netted only big and little fish, but few medium-sized fish. Astronomers likewise have been per- plexed in conducting a census of star- hugging extrasolar planets. They have found hot Jupiter-sized planets and hot super-Earths (planets not less than 1.5 times Earth’s diameter). These planets are scorching hot be- cause they orbit very close to their star. But so-called “hot Neptunes,” whose atmospheres are heated to more than 1,700 degrees Fahrenheit, have been much harder to find. In fact, only about a handful of hot Neptunes have been found so far. In fact, most of the known Neptune- sized exoplanets are merely “warm,” because they orbit farther away from their star than those in the re- gion where astronomers would ex- pect to find hot Neptunes. The mysterious hot-Neptune deficit suggests that such alien worlds are rare, or, they were plentiful at one time, but have since disappeared. A few years ago astronomers using NASA’s Hubble Space Telescope T his artist’s illustration shows a giant cloud of hydrogen stream- ing off a warm, Neptune-sized planet just 97 light-years from Earth. The exoplanet is tiny compared to its star, a red dwarf named GJ 3470. The star’s intense radiation is heating the hydrogen in the planet’s upper at- mosphere to a point where it es- capes into space. The alien world is losing hydrogen at a rate 100 times faster than a previously observed warm Neptune whose atmosphere is also evaporating away. [NASA, ESA, and D. Player (STScI)]