新发现的行星HD 131399Ab有三个太阳

If you thought Luke Skywalker's home planet, Tatooine, was a strange world with its two suns in the sky, imagine this: a planet where you'd either experience constant daylight or enjoy triple sunrises and sunsets each day, depending on the seasons, which happen to last longer than human lifetimes. Such a world has been discovered by a team of astronomers led by the University of Arizona in Tucson, Arizona, using direct imaging. The planet, HD 131399Ab, is unlike any other known world - on by far the widest known orbit within a multi-star system. The discovery will be published online by the journal Science on Thursday, 7 July, 2016. 

Located about 340 light years from Earth in the constellation Centaurus, HD 131399Ab is believed to be about 16 million years old, making it one of the youngest exoplanets discovered to date, and one of very few directly imaged planets. With a temperature of 850 Kelvin (about 1,070 degrees Fahrenheit or 580 degrees Celsius) and weighing in at an estimated four Jupiter masses, it is also one of the coldest and least massive directly imaged exoplanets. 

"HD 131399Ab is one of the few exoplanets that have been directly imaged, and it's the first one in such an interesting dynamical configuration," said Daniel Apai, an assistant professor of Astronomy and Planetary Sciences who leads a research group dedicated to finding and observing exoplanets at the UA. 

"For about half of the planet's orbit, which lasts 550 Earth-years, three stars are visible in the sky, the fainter two always much closer together, and changing in apparent separation from the brightest star throughout the year," said Kevin Wagner, a first-year PhD student in Apai's research group and the paper's first author, who discovered HD 131399Ab. "For much of the planet's year the stars appear close together, giving it a familiar night-side and day-side with a unique triple-sunset and sunrise each day. As the planet orbits and the stars grow further apart each day, they reach a point where the setting of one coincides with the rising of the other - at which point the planet is in near-constant daytime for about one-quarter of its orbit, or roughly 140 Earth-years."