Astronomers have detected what they believe to be a planet at least the size of Jupiter that came from another galaxy. If true, the world is the first planetary immigrant ever detected in the Milky Way. The find would also violate the leading hypothesis of how and where planets form.
The planet lives 2200 light-years away inside the Helmi stream, a ring of ancient stars that cuts through the plane of the Milky Way. Astronomers believe the stream formed 6 billion to 9 billion years ago, when the Milky Way ripped another galaxy to shreds, swallowing some of its stars in the process. Astronomer Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, likes looking at these stars because they tend to have unusual properties. But even by these standards, one star in particular caught his eye: HIP 13044.
Setiawan and colleagues at Max Planck and the European Space Agency noticed that the star didn't move at a constant velocity relative to our sun. Their instruments picked up a regular 16-day pattern, where sometimes the star was closer and sometimes farther away. That could be caused by spots on the star, which could confuse velocity measurements, or by the star expanding and contracting (known as stellar pulsations). But the researchers didn't detect either. That left only one possibility: a planet gravitationally pulling on the star.
If that's the case, the world is the first foreign planet detected in the Milky Way, the team reports  online today in Science. But that's not the only unusual thing about it. It shouldn't have formed in the first place.
That's because HIP 13044 is a very ancient, very metal-poor star. It's about 3 billion years older than our own sun and has only 1% as much metal. Until now, the prevailing hypothesis has said that as stars evolve, metals (astronomers' term for any chemical elements heavier than hydrogen and helium) in the swirling disk around them form tiny "seeds" that attract other matter and slowly grow into planets. So far most surveys have backed up the theory: Stars rich in metals, such as our Sun, are much more likely to have planets than stars that don't. "The fact that you can form planets around a star that has so little of this material is a very surprising and unusual thing," says Christopher Johns-Krull, an astronomer at Rice University in Houston, Texas, who was not involved in the new work.
But there is a second, more violent, theory about how planets form. If the swirling disk of gas is massive enough—one-tenth the mass of its star or more—Johns-Krull says the gravitational power of the disk can make the disk unstable. In those cases, the star isn't big enough to keep the disk in check, so the disk starts to collapse under its own gravity and forms planets.
Although it's still not certain that a planet is behind the variations in the star's speed, all the findings so far point that way, says NASA astronomer Steven Pravdo. To be sure, astronomers will need to run other tests. Ideally, they would look for the planet to pass in front of the star, but that would work only if the planet orbits at just the right angle, he says. "The claim that it's extragalactic is kind of a guess," says Pravdo. It's possible the star was originally part of the Milky Way when it collided with the other galaxy billions of years ago.
Although an extragalactic planet is "a nice possibility," says Pravdo, the more exciting find is what this planet could do to the idea of tame, gradual planet formation. Johns-Krull agrees. "This planet says, maybe that's not right," he says. "Maybe it's this other, more dramatic process."