Some extrasolar planets are truly out of this world.

Astronomers have for the first time discovered a planet in the Milky Way that came from another galaxy. The planet, which has a mass of at least 1.25 Jupiters, orbits an elderly star that was ripped from a small satellite galaxy some 6 to 9 billion years ago.

Johny Setiawan and Rainer Klement of the Max Planck Institute for Astronomy in Heidelberg, Germany, describe the finding online November 18 in Science.

“The coolness factor is definitely that the planet and star came from another galaxy,” says Sara Seager of MIT, who was not part of the study. “The planet almost certainly formed during the time the star was in the other galaxy.”

In hunting for extrasolar planets, Setiawan and his colleagues homed in on HIP 13044, about 2,000 light-years from Earth, because it’s part of a stream of stars called Helmi, believed to have originated in another galaxy. The star’s motion could also be monitored for many months each year with a spectrograph at the European Southern Observatory’s La Silla site in Chile, looking for telltale wobbles that would indicate the tiny tug of an unseen, orbiting planet.

HIP 13044 and the other stars in the Helmi stream stand out in the solar neighborhood because they have elongated orbits that take them about 42,000 light-years above and below the plane of the Milky Way’s disk. Such orbits strongly suggest the stars were part of a group torn from a satellite galaxy and stretched out by gravitational tidal forces into a filament or stream.

The discovery, notes planet hunter Scott Gaudi of Ohio State University in Columbus, “is doubly weird: It is a weird planet around a weird star.” The star is unusual because it has the lowest abundance of metals — about 1 percent of the sun’s — of any star known to have a planet. (In astronomical parlance, a metal refers to any element heavier than helium.) The vast majority of the roughly 500 extrasolar planets known are found around stars with a much higher metal abundance, and the leading theory of planet formation suggests that stars with high metal contents are those that form giant, Jupiter-like planets.

Also unusual is that HIP 13044 is old enough to have exhausted its supply of hydrogen fuel and passed through the red giant phase of evolution, in which it mushroomed in size. Since then the star contracted to a diameter about seven times that of the sun and is now burning helium at its core. A star in this phase of evolution, known as the red horizontal branch, has never before been found to have a planet.

In part, that’s because the enhanced activity of old, evolved stars, including the presence of magnetically driven disturbances known as starspots, makes it more difficult to discern a stellar wobble, says Setiawan. In addition, “there is a high risk that you will not find any planets because they have been engulfed by the star during the [red giant] evolutionary phase,” he adds.

In order to survive, HIP 13044’s planet, which now resides much closer to the star than Mercury does to the sun, must have originally orbited at a much greater distance, the researchers say. That’s the only way it could have escaped being swallowed during the time the star was a red giant. (In several billion years, the sun will also become a puffed-up red giant and is likely to engulf Earth and the other inner planets.)

Other planets that resided closer to HIP 13044 would not have been so lucky. One explanation for the star’s relatively rapid rate of rotation is that it has been spun up by the angular momentum of planets it swallowed. Other rapidly rotating, elderly stars that have evolved to the red horizontal branch may have had similar dining habits, researchers have previously noted.

Even though the newfound planet has dodged one bullet, it will soon face another. In a few million years, when the star exhausts all the helium forged at its core, it will undergo a more rapid and larger expansion in which the planet is likely to be destroyed.

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