Astronomers have made a groundbreaking effort to search for exoplanets in the remnants of dwarf galaxies that merged with the Milky Way billions of years ago. This research, published in the Publications of the Astronomical Society of the Pacific, represents a major step forward in our understanding of planet formation in diverse cosmic environments.
Exploring Exoplanets Beyond the Milky Way
While the search for exoplanets has been highly successful within the Milky Way, with over 6,000 planets detected so far, little is known about planets that formed outside our galaxy. The Milky Way has at least 61 confirmed satellite galaxies, many of which have experienced dramatic transformations through interactions with the Milky Way's halo.
To understand how these environments affect exoplanet formation and characteristics, astronomers have launched a new survey called VOYAGERS (Views Of Yore - Ancient Gaia-enceladus Exoplanet Revealing Survey). This survey aims to find exoplanets in the remnant stars of ancient dwarf galaxies that merged with the Milky Way.
"Observations over the past few decades have found that planets are common around nearby stars in our Galaxy, but little is known about planets that formed outside the Milky Way," said lead author Robert Aloisi, a grad student at the University of Wisconsin-Madison.
Focusing on the Gaia-Enceladus Remnant
VOYAGERS is specifically focused on the Gaia-Enceladus (also known as the Gaia Sausage), which is the remnant of a dwarf galaxy that merged with the Milky Way between 8 and 11 billion years ago. Astronomers have identified seven globular clusters in the Milky Way that used to be part of this ancient galaxy.
The survey aims to address the limitations of our current exoplanet sample, which is mostly comprised of planets orbiting main sequence stars in the Milky Way's disk with metallicities similar to the Sun. Metallicity, which refers to the concentration of elements heavier than hydrogen and helium, is a critical factor in the formation of both stars and planets.
Metallicity and Exoplanet Formation
Studies have revealed intriguing patterns connecting stellar metallicity and exoplanet characteristics. For example, there are fewer massive planets around low-metallicity stars, while the occurrence rates for smaller planets seem less dependent on metallicity. Additionally, sub-Neptune mass planets tend to have lower densities when formed around low-metallicity stars.
VOYAGERS seeks to investigate how these patterns relate to stars and exoplanets in remnant satellite galaxies like Gaia-Enceladus. As lead author Aloisi explains, Searching for planets in GES presents an intriguing opportunity, as it remains unclear how planets form in environments outside the Milky Way and how low-metallicity conditions influence these processes.
Survey Methodology and Progress
The VOYAGERS survey employs the radial velocity (RV) method to study main sequence and slightly evolved stars in the Gaia-Enceladus remnant. Starting with over 47,000 identified GES stars, the researchers carefully filtered and screened them based on brightness and suitability for RV observations, ultimately selecting 22 target stars.
The survey aims to obtain 160 observations for each target star, for a total of 3,520 observations. As of the publication of this research, 778 observations have been completed, representing approximately 22% of the survey. The team plans to focus future observations on 10 main sequence stars to expedite results while continuing to observe the remaining targets during less optimal conditions.
Implications and Future Directions
The VOYAGERS survey has the potential to significantly expand our understanding of exoplanet formation and characteristics in diverse cosmic environments. If the survey detects no planets around the GES targets, it will provide strong evidence that occurrence rates for Neptune-mass exoplanets are significantly lower in these ancient, low-metallicity systems compared to the Milky Way.
On the other hand, discovering one or more planets orbiting these old, metal-poor stars would extend our knowledge of when and where planets and potentially life can evolve in the Universe. As the authors conclude, If we discover one or more planets orbiting these ancient, low-metallicity stars, the survey results will extend our understanding of when and where planets and potentially life can evolve in the Universe.
The findings from VOYAGERS will contribute to our growing understanding of the complex interplay between star formation, metallicity, and exoplanet formation. As these pieces fall into place, we move closer to unraveling the mysteries of habitability and the potential for life beyond Earth.
