Nearly everything we can see in the night sky without the aid of a telescope is in Earth’s cosmic neighborhood, the Milky Way Galaxy. And the hundreds of planets that have been discovered outside our solar system all orbit stars within the Milky Way; their relative proximity permits the kind of careful look needed to identify an orbital companion.
Now a new study has located yet another extrasolar planet, or exoplanet, within our galaxy. But this one seems to have started out in another galaxy that was then consumed by, and incorporated into, the Milky Way several billion years ago.
The newfound planet, which is somewhat more massive than Jupiter, was announced online November 18 in Science by a group of researchers from the Max Planck Institute for Astronomy (M.P.I.A.) in Heidelberg, Germany, and the European Space Agency. The team used a telescope on a Chilean mountaintop to observe HIP 13044, an aging star about 2,000 light-years from our solar system.
The star showed a periodic wobble on a timescale of 16.2 days that the researchers concluded was most plausibly explained by the gravitational tug of a massive planet orbiting very close by. Astronomers have found the vast majority of known exoplanets by tracking these stellar wobbles through Doppler shifts in the star’s light. But the discovery of the newfound HIP 13044 b, named by convention for its host star, is surprising for a few reasons.
One is that HIP 13044 b is the first verifiable exoplanet of extragalactic origin. (A tentative detection of a possible planet in Andromeda was announced in 2009, but the kind of one-off observation used in that study is not confirmable.) HIP 13044 belongs to the Helmi stream, a population of stars that stretches through the Milky Way with similar, unusual orbits and compositions. The Helmi stream was determined in 1999 to have originated in a small galaxy, similar to the Sagittarius dwarf galaxy, that was cannibalized by the larger Milky Way. Later work found that the ingested galaxy must have been devoured six billion to nine billion years ago.