Extra-galactic planet discovered

For the first time, scientists have discovered a planet that originated in another galaxy, but currently resides in the Milky Way. The findings, described by astronomers in Heidelberg, Germany in November, describe a planet that is unusual in many ways.

The planet, called HIP 13044, orbits a star that was absorbed into the Milky Way from a satellite galaxy six to nine billion years ago. In fact, a whole stream of stars known as Helmi is believed to have originated in another galaxy. It is likely that the planet, which has a mass of about 1.25 times that of Jupiter, was formed while the star was still in the other galaxy.

Johny Setiawan and Rainer Klement, both from the Max Planck Institute for Astronomy, published their findings in the November 18th edition of Science. Their research has also piqued the interest of other scientists.

“The coolness factor is definitely that the planet and star came from another galaxy,” Sara Seager of MIT said. Ohio State University’s Scott Gaudi comments, “It’s a weird planet around a weird star.”

HIP 13044 is an old planet — old enough for the star it orbits to have passed through the red giant phase of its evolution. In this phase, it mushrooms in size before contracting. In fact, the star has exhausted its supply of hydrogen fuel and is now burning helium at its core. Never before has a star in this phase been known to have a planet orbiting it.

The survival of HIP 13044 may be attributed to its having originally orbited the star at a much greater distance; it now orbits it at a distance shorter than that between Mercury and our Sun. Otherwise, it would have been swallowed up by the star when it was a red giant, just as the Earth and our solar system’s inner planets are expected to when our Sun reaches that stage.

The rapid rate of the star’s rotation, however, suggests that it may have swallowed a few other planets and been affected by their angular momentum. In a few million years, however, HIP 13044 is also likely to be swallowed up when the star undergoes another expansion — an even more rapid and larger one — once all the helium at its core has been exhausted.

The star’s composition is also unique; it has about one percent of the Sun’s minerals, and in fact has the lowest abundance of metals of any known star. Furthermore, planets in the Helmi stream, which is about 2,000 light years away, have unusually long orbits that take them 42,000 light years above and below the Milky Way’s plane.

The star that HIP 13044 orbits can be observed for many months out of each year at the La Silla site in Chile, run by the European Southern Observatory. A spectrograph looks for wobbles that indicate that an unseen planet is orbiting around it. The nature of these elongated orbits supports the theory that the stars were torn from a satellite galaxy and thereafter stretched out into a stream by gravitational tidal forces.