The Helmi stream is a stellar stream of the Milky Way galaxy. It started as a dwarf galaxy, now absorbed by the Milky Way as a stream. It was discovered in 1999, is formed of old stars deficient in heavy elements and has a mass of 10 to 100 million solar masses. It was absorbed by the Milky Way some 6 to 9 billion years ago.
The Helmi stream was home to the first planet purportedly of extragalactic origin, orbiting the star HIP 13044. Unfortunately, further analysis of radial velocity data failed to confirm the discovery.
- Scientific American, "Extragalactic Expat: Newfound Exoplanet Likely Came from Another Galaxy", John Matson , 18 November 2010
- Jones, M. I.; Jenkins, J. S. (2014). "No evidence of the planet orbiting the extremely metal-poor extragalactic star HIP 13044". Astronomy & Astrophysics. 562: id.A129. arXiv: . Bibcode:2014A&A...562A.129J. doi:10.1051/0004-6361/201322132.