Astronomers have confirmed the first discovery of an alien planet in our Milky Way that came from another galaxy. The planet, which has a mass of at least 1.25 that of Jupiter’s, orbits an elderly star that was ripped from a small satellite galaxy some 6 to 9 billion years ago researchers said Thursday. “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.” The find may also force astronomers to rethink their ideas about planet formation and survival, researchers said, since it’s the first planet ever discovered to be circling a star that is both very old and extremely metal-poor. Metal-poor stars typically lack elements heavier than hydrogen and helium.
The newfound planet, called HIP 13044b, survived through its star’s red-giant phase, which our own sun will enter in about 5 billion years. So studying it could offer clues about the fate of our solar system as well, researchers said. 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. An 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.
The newfound planet “likely formed when the star was not yet a part of the Milky Way. It’s traveled with the star all this time,” said study leader Johny Setiawan, an astronomer at the Max-Planck Institute for Astronomy in Germany. HIP 13044b sits extremely close to its parent star, which has now contracted again. The planet completes an orbit every 16.2 days, and it comes within about 5 million miles of its parent star at closest approach — just 5.5 percent of the distance between Earth and the sun. Alan Boss, a planet-formation theorist at the Carnegie Institution for Science in Washington, D.C., said HIP 13044b is “big news,” because it’s such an anomaly in terms of its origins. “This object … is unlikely to have formed by the conventional mechanism of first building a massive core of rock and ice and then pulling on enough gas to form a true gas giant planet,” said Boss, who was not part of the study team. Study leader Setiawan agrees: “Now we have this finding, and it suggests maybe there are other mechanisms of planet formation around metal-poor stars that we don’t know about.”