20 November 2010

It Came from Another Galaxy

Posted by Ryan Anderson

It came from hundreds of thousands of light years away. Trapped in a dance of death with an ancient, doomed star, this behemoth interloper roves through our galaxy, thirsty for blood!

Okay, maybe not that last part. But believe it or not, I’m talking about a recent press release from the European Southern Observatory, not a B-movie from the 50s. Earlier this week, ESO announced that a team of astronomers have discovered a planet orbiting a star from another galaxy! It’s a little confusing because the star is now part of our own galaxy, but it’s part of the “Helmi stream”, which is an elongated swarm of stars left over from when the Milky Way devoured a small galaxy about nine billion years ago.

The star that the planet orbits is an old red giant, so the planet probably formed back when it was part of a separate galaxy. The planet is at least 1.25 times as massive as Jupiter and it is scorchingly close to its star: it’s year is only 16.2 earth days! There’s circumstantial evidence that the big planet was not the only planet in its system. The star is spinning unusually fast, which could be explained if it consumed some inner planets and absorbed their angular momentum.

So, does the fact that this planet formed in a different galaxy make it physically different from our home-grown planets? Not really. One of the fundamental results of modern astronomy is that all the stuff out in the universe is made of the same atoms we have here at home. But just because this new planet is made of regular old hydrogen and helium doesn’t change the very cool fact that we now can say in all honesty that we have discovered hundreds of planets outside of our solar system, and at least one of them is from another galaxy. That’s just cool.