Our search for exoplanets is focused on Milky Way stars. It’s been successful, with more than 6,000 detected so far. Scientists are even beginning to move beyond mere detections, and working on characterizing other characteristics of these planets, especially their atmospheres.

But the Milky Way (MW) has a confirmed 61 satellite galaxies, quite likely many more. Many of these smaller galaxies have had their star-forming hydrogen gas stripped away through interactions with the MW’s halo. In others, tidal interactions have created streams of stars stretching through space. Still other satellite galaxies are mere remnants, having lost most of their stars through merging with the MW. How do these environments affect exoplanets?

To understand that, astronomers have to find exoplanets in these satellite galaxies or their remnants. New research describes and defines an effort to find exoplanets in one of the MW’s remnant satellites. It’s titled “Searching for Exoplanets Born Outside the Milky Way: VOYAGERS Survey Design,” and it’s published in the Publications of the Astronomical Society of the Pacific. The lead author is Robert Aloisi, a grad student in the Department of Astronomy at the University of Wisconsin-Madison.

“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,” the authors write. “We describe the design and early implementation of a survey to test whether planets also exist orbiting the remnant stars of ancient dwarf galaxies that merged with the Milky Way, and if so, how they differ from their Milky Way counterparts.”

VOYAGERS stands for Views Of Yore - Ancient Gaia-enceladus Exoplanet Revealing Survey. It’s focused on Gaia-Enceladus (aka Gaia Sausage, Sausage Galaxy, Gaia-Enceladus-Sausage,) which is the remnant of a dwarf galaxy that merged with the MW between 8 and 11 billion years ago. It’s the last major merger in the MW’s history. Astronomers have identified seven of the MW’s globular clusters that used to be part of the Gaia Sausage. (The Sausage is not named for the shape of the remnant. It’s named for the sausage-like shape of the remnant stars when their velocities are plotted on a graph.)

6,000 exoplanets is a large sample, but it’s still a limited sample. Most of them are orbiting main sequence stars in the MW’s disk. These stars typically have metallicities very close to the Sun’s, and that’s a weakness in the sample. “However, this census of known exoplanet hosts does not fully represent the wider diversity of stars across the Universe, including metal-poor stars from the early Universe and stars found in dwarf galaxies,” the authors explain. The new survey is aimed to address this shortcoming.

Metallicity is a critical factor in both stars and planets, which both form form solar nebulae. Each nebula has a specific metallicity, which refers to its concentration of elements heavier than hydrogen and helium. Metallicity in the Universe increases as time goes and as generations of stars live and die, because stars create heavier elements via nucleosynthesis. When stars approach the end of their lives, these elements are spread back out into space to be taken up by the next generation of stars, and their planets.

“We speculate that some planets likely formed in the low-metallicity, high-alpha element environment (elements formed by fusion of He nuclei) of the early Universe, and this population may differ in occurrence rates and compositions compared to those found in more recently formed stars in the Milky Way disk,” the authors write. High-alpha elements include oxygen, neon, sulphur, and magnesium. They’re produced on shorter timescales by stars that explode as core-collapse supernovae in only millions of years.

Our 6,000 exoplanets have revealed some patterns in exoplanet formation. There are fewer planets with masses larger than Jupiter around low-metallicity stars, and in contrast, the occurrence rates for exoplanets of Neptune mass and smaller don’t seem to depend on metallicity at all. Exoplanet scientists also know that sub-Neptune mass planets have lower densities when formed around low-metallicity stars. In another connection between exoplanets and stellar metallicity, short-period super-Earths are relatively rare around low-metallicity stars.

This all factors into a greater understanding of exoplanet habitability. The question is, how do these patterns relate to stars and exoplanets in remnant satellite galaxies?

“Searching for planets in GES (Gaia-Enceladus Sausage) 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,” the researchers explain.

This figure shows the Gaia-Sausage structure detected by the ESA’s Gaia satellite in velocity space. (Credit: V. Belokurov et al. 2018, MNRAS, 478, 611).

VOYAGERS will use the radial velocity (RV) method to study main sequence stars and slightly evolved stars in the GES. The main goal is to find exoplanets formed in low-metallicity environments separate from the MW. There are more than 47,000 stars that are identified as GES stars, and the researchers started with them. Then they filtered them out and were left with a mere 156 stars that were suitable for exoplanet detection due to their brightness and other properties. The stars were further screened for their suitability for RV observations, and by the end of the rigorous assessment and screening, they were left with 22 stars in the GES.

“A key goal of our survey is not just to detect planets that were born outside the Milky Way, but also to be a first probe of their population and to see whether there are significant differences between GES planets and Milky Way planets,” the authors write. The survey is also focused on sub-Neptune mass exoplanets.

VOYAGERS is only partially complete. “Our goal is to obtain 160 observations for each GES target,” the authors write. “We have completed 778 observations.” With 22 targets and 160 observations each, that adds up to 3,520, so the survey is only 22% complete.

The research team say that their future observations will focus on 10 main sequence stars to expedite their results, while also observing the remaining targets during less optimal observing conditions.

“Further, the survey is designed such that if we detect no planets, we will be able to determine with confidence that occurrence rates for Neptune-mass exoplanets are significantly lower for GES targets than occurrence rates for stars born in the Milky Way,” the authors write.

If that turns out to be true, it supports the metallicity hypothesis in planet formation. One of the things the hypothesis states is that metal-rich stars are more likely to form giant planets because there’s more heavy elements for them to form their large cores.

Star formation, metallicity, and exoplanet formation are all pieces in a large, complex puzzle. As these pieces find their correct places in the puzzle, we’ll learn more about potential habitability and the prospects of life elsewhere. Finding Neptune mass planets in these low-metallicity environments would bring the puzzle one tiny step closer to completion.

“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 authors conclude.