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<em>Science</em>: Exploring an Unlikely Planet

Ever wonder what other planets are lurking beyond our galaxy? Well, a new planet has been discovered near a star of extragalactic origin, implying that it comes from outside the Milky Way, reports a new study appearing in the 18 November issue of the journal Science Express.

The finding challenges our current understanding of planet formation and survival, since it’s the first time astronomers have found a planet around an extremely metal-poor, ancient star.

So far, few planets have been detected around metal-poor stars (stars that contain minuscule amounts of elements other than hydrogen and helium) or very old stars.


This artist’s impression shows HIP 13044 b, an exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy. This planet of extragalactic origin was detected by a European team of astronomers using the MPG/ESO 2.2-meter telescope at ESO’s La Silla Observatory in Chile. The Jupiter-like planet is particularly unusual, as it is orbiting a star nearing the end of its life and could be about to be engulfed by it, giving clues about the fate of our own planetary system in the distant future. | Credit: ESO/L. Calçada
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Now, Johny Setiawan and colleagues at the Max Planck Institute for Astronomy in Germany have found a giant planet around a metal-poor star that has gone past the red giant phase of stellar evolution, when stars like the Sun expand up to many times their original size.


“The most important thing in this case is the metal content of the star, not just the origin or evolutionary status,” said Setiawan. “This so called ‘metallicity’ is very important in the widely accepted planet formation model, known as the core accretion model.”

This newly discovered planet is at odds with expectations because it typically would have been swallowed up by its host star during the transition into the red giant phase, when the star burns through all the hydrogen in its core, swells up, and flashes bright red in color.

“According to the core accretion theory, it was probably impossible for this planet to form—but it did!” Setiawan continued. “That’s what we learned in this discovery. Thus, there must be an alternative to form a planet around such a metal-poor star.”

Furthermore, the planet likely originated outside of our galaxy because its host star belongs to a group of stars that formed in a tiny satellite galaxy of the Milky Way that was gravitationally disrupted several billion years ago. As the small galaxy passed close to the Milky Way, it was ripped apart by the Milky Way’s intense gravitational pull.

“Certainly, it is amazing to discover such a special planetary system,” the astronomer said. “Even if it is not an earth-like planet discovery, it has its own history. The fact that this evolved system comes from other place outside our galaxy is indeed very exciting.”


Read the abstract for “An Unlikely Planet.”

Visit Science Express.