Astronomers have discovered a new kind of alien world – the first known “ultra-hot Neptune,” a giant planet that orbits its star 60 times closer than Earth or the sun, according to a new study.
Previous research found that about 1 in 200 Sun-like stars has a planet orbiting its star so tightly that it orbits Earth in less than a day. Astronomers call these worlds “ultra-short-period planets”. (In contrast, it takes the earth a little over 365 days to orbit the sun. Mercury, the closest planet to our sun, takes less than 88 days to complete its year.)
All previously known ultra-short period planets were either rocky planets less than twice the width of the earth or so-called hot Jupiter, gas giants whose size is more than ten times the diameter of the earth. Mysteriously, scientists have not found many medium-sized ultra-short period planets, a phenomenon known as the “hot Neptune desert,”
Now scientists have discovered an exoplanet that lies right in the hot Neptune desert. But this planet is much hotter than any hot Neptune discovered so far, enough for researchers to call “ultra-hot”.
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Astronomers used NASA’s Transiting Exoplanet Survey Satellite (TESS) to analyze the bright, Sun-like star LTT 9779, located about 260 light years from Earth. Follow-up observations with other telescopes confirmed the existence of an exoplanet around this star. This newly discovered world, called LTT 9779 b, is about 4.7 times the diameter of the earth and 29 times the earth’s mass.
LTT 9779 b orbits its star approximately every 19 hours and orbits its star approximately 23 times closer than Mercury to the Sun, with the star heating it to temperatures of approximately 1,700 degrees Celsius. In an article on the finding, scientists argued that this exoplanet was not just a hot Neptune, but an ultra-hot Neptune. (A normal hot Neptune may only get up to 1,225 degrees Celsius, study director James Jenkins, an astronomer at the University of Chile at Las Condes, told Space.com.)
The Ultrahot temperatures on LTT 9779 b can break down molecules into their constituent parts and ionize metals in its atmosphere. This means that its atmosphere “can be very different from just ‘hot’ planets,” said Jenkins, “which makes them exciting laboratories for studying the chemistry of planets.”
The average density of the newly discovered exoplanet is similar to that of Neptune. The researchers suggested that LTT 9779b was unlikely to be composed of pure rock or pure water. Instead, they proposed a solid core surrounded by a hydrogen and helium atmosphere.
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Mysteriously, LTT 9779b still appears to have a thick atmosphere that is nearly 10% its mass, or roughly 2.6 times the mass of the earth, Jenkins said. One would expect ultra-high Neptunes “to lose their atmosphere very early due to the star’s high-energy radiation. So for this planet we need to think about more exotic types of formation scenarios,” Jenkins said.
One possible explanation for this ultra-hot Neptune’s thick atmosphere “is that the planet was originally much larger, perhaps a gas giant like Jupiter that migrated too close to the star,” Jenkins said. In this scenario, the star’s gravity would have stripped off much of the exoplanet’s atmosphere. After losing much of its mass, the world may have moved a little further from the star and acquired a Neptune-like mass, he said.
In the future, Jenkins and his colleagues will analyze the light falling through this exoplanet’s atmosphere “to find out what elements are in the atmosphere, what the temperature is around the planet and whether the planet has clouds,” he said.
The relatively bright nature of its star “means we can use ground-based and space-based instruments to study the planet in extreme detail,” added Jenkins. “This means we will hear a lot more about this planet in the near future.”
The scientists published their results online on September 21 in the journal Nature Astronomy.
Follow Charles Q. Choi on Twitter @cqchoi. Follow us on Twitter @Spacedotcom and on Facebook.