I still remember the excitement I experienced as a kid each time someone gave me a box of Cracker Jacks. Not only were the caramel-coated popcorn and peanuts delicious, each box contained a surprise. Astronomers just found a similar surprise as they studied the stars that comprise the Milky Way Galaxy (MWG), namely a planet from another galaxy.
The Milky Way and Andromeda dominate the local group of galaxies’ mass budget. However, more than 30 dwarf galaxies currently belong to the local group as well. Throughout its history, the MWG (and Andromeda) occasionally “eats” these smaller galaxies. This galactic snacking helps fuel the ongoing star formation necessary for the Milky Way’s habitability. The Helmi stream is the group of stars left over from one dwarf galaxy that the MWG devoured around seven billion years ago. A team of astronomers observed one of the stars (HIP 13044) in this stream to look for planets and found the telltale signature of a planet roughly 25 percent more massive than Jupiter. The star in question, however, is no ordinary star.
HIP 13044 formed in a galaxy other than the Milky Way, but that is not the reason that astronomers searched around this star for planets. Stars spend the majority of their lifetime burning hydrogen in their cores. After exhausting the supply of hydrogen (the Sun will run out of hydrogen around 10 billion years from its formation), the subsequent evolution of a star depends on its mass. The most massive stars will quickly fuse helium into heavier and heavier elements (up to iron) and then go supernova. For stars similar in mass to the Sun, core hydrogen exhaustion leads to a red-giant phase where the star swells to the size of Earth’s orbit or larger. However, hydrogen fusion in a shell around the star’s core continues to dump helium into the core, which causes the star’s temperature to rise. Eventually, the helium begins to fuse into carbon and the star enters what astronomers refer to as the horizontal branch phase. HIP 13044 is a horizontal branch star.
- HIP 13044b (the gas giant planet) has a mass 25 percent larger than Jupiter and orbits with a period of 16.2 Earth days, giving the planet a semi-major axis of 0.116 AU (one AU is the distance between Earth and the Sun). This mass and orbit mean are similar to those of other hot-Jupiters.
- This is the first planet known to have survived the red-giant phase of its host star.
- HIP 13044 exhibits a high rotation velocity for a horizontal branch star. One possible explanation for this rotation rate is that the red-giant phase of the star engulfed other planets that existed in the system originally. The process of “eating” a planet can make a star rotate faster.
- HIP 13044b represents the first definitive evidence that planets don’t require lots of metals to form. Astronomers possess evidence that planets form more easily in metal-rich environments (those more like the environment where the solar system formed). However, HIP 13044 and its planets formed in a very metal-poor galaxy (remember, astronomers consider anything except hydrogen and helium a metal).
- For completeness, HIP 13044b is the first planet of extragalactic origin.
Aside from the “coolness” factor of this planet, its unique characteristics will help astronomers better understand the process of planet formation as well as what factors contribute to a planetary system’s habitability. So far, the data indicates that Earth remains unique in its capacity to support life. I expect that future planetary discoveries will strengthen this conclusion.
- Johny Setiawan et al., “A Giant Planet around a Metal-Poor Star of Extragalactic Origin,” Science Express, published online November 18, 2010, https://www.sciencemag.org/content/early/2010/11/17/science.1193342.