A planet has been found sitting in an orbit where it should have been consumed when its host star expanded into a red giant, the same fate that may one day befall Earth.
Called 8 Ursae Minoris b, the planet is more than 500 light years from Earth and is a gas giant聽slightly larger than Jupiter. It orbits its star at about half the distance between Earth and the sun, completing this journey every 93 days. Though the star has returned to a smaller size, it appears to have previously expanded into a red giant as it ran out of fuel 鈥� which should have destroyed the planet.
鈥淥nce low to intermediate-mass stars exhaust their hydrogen, they become red giants,鈥� at the University of Hawaii told the in early January. 鈥淭he star grows really big, and if you have a close-in planet, it鈥檚 doomed. The star will engulf the planet.鈥�
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While 8 Ursae Minoris b was discovered in 2015, follow-up work by Hon and his colleagues shows that the host star had already been a red giant and is now shrinking, a phase in which it is called a red clump star. It would have expanded to reach about 70 per cent of the distance between Earth and the sun, which should have consumed 8 Ursae Minoris b in the process. 鈥淭his planet is in a forbidden place,鈥� he says.
How the planet ended up there is a puzzle. One possibility is that it was originally further out in the system before being pulled in by the red giant as it expanded, but it is unclear why the planet would have stopped at its current location rather than continuing towards the star. 鈥淲e disfavour such a process,鈥� says Hon.
Another possibility is that the current star is the result of a merger between a pair of closely orbiting stars. This could have prevented the red giant from expanding to the orbit of the planet, or might have created a debris disc that formed the planet following the red giant phase, making 8 Ursae Minoris b a rare example of a second-generation planet. 鈥淲e are very caught up with the idea of this being a binary system,鈥� says Hon. The merger would perhaps have been between a red giant and a white dwarf, which is the remnant core left behind after a star has expanded into a red giant.
The finding is an 鈥渆xciting result鈥� that 鈥渞aises questions about how the planet survived鈥�, says at the Space Telescope Science Institute in Baltimore, Maryland. While planets have been found orbiting red giants before, they have been seen during the first stage, when the star is still growing in size. 鈥淚f the planet gets too deep into the star, you expect it to be pulled apart,鈥� says Carlberg.
The existence of 8 Ursae Minoris b could have implications for Earth, says at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. While our planet could 鈥渇all into the sun鈥� as the star expands, he says, it is also possible that it could survive, perhaps being pushed outwards as the solar wind removes mass from the expanding sun. 鈥淲e may have a shot,鈥� says Vissapragada. 鈥淲e all want to know how the story ends for our solar system.鈥�
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