Milky Way galaxy, the home galaxy of the Solar System, and of Earth, belongs to a rare subset among the billions that populate the cosmos and just four percent of galaxies are similar to our Milky Way, according to a new research.

We are interested in how the Milky Way fits into the broader context of the universe, said Stanford University astrophysicist Risa Wechsler. This research helps us understand whether our galaxy is typical or not, and may provide clues to its formation history.

The research team compared the Milky Way to similar galaxies in terms of luminosity--a measure of how much light is emitted--and distance to other bright galaxies. They found galaxies that have two satellites that are as bright and close by as the Milky Way's two closest satellites, the Large and Small Magellanic Clouds, are rare.

For this analysis, Wechsler's group studied more than 20,000 galaxies with properties similar to the Milky Way and investigated the galaxies surrounding these Milky Way twins, to create a census of galaxies similar to the Milky Way in the universe.

The research showed that just four percent of the simulated galaxies had two satellites like the Magellanic Clouds.

This is an excellent example of data-enabled science, said Nigel Sharp, of National Science Foundation's Division of Astronomical Sciences. Comparing the 'fake' and 'real' Universes is how we discriminate between successful and unsuccessful theories. This work interconnects three of the four legs of science: theory, observation and simulation, for a powerful scientific result.

The latest results also lend support to a leading theory of galaxy formation called the Cold Dark Matter (CDM) theory that many consider to be the simplest explanation for the arrangement of galaxies throughout the universe following the Big Bang.

The theory assumes that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation (dark) and whose constituent particles move slowly (cold). Dark matter, an invisible and exotic material of unknown composition, is believed to influence the distribution of galaxies in space and the overall expansion of the universe. The rareness of this aspect of the Milky Way may provide clues to its formation history.

Because the presence of two galaxies like the Magellanic Clouds is unusual, we can use them to learn more about our own galaxy, said Wechsler. Using their simulation, the team identified a sample of simulated galaxies that had satellites matching the Milky Way's in terms of their locations and speeds.

The combination of large surveys of the sky like the SDSS and large samples of simulated galaxies provides a new opportunity to learn about the place of our galaxy in the Universe, said Wechsler. Future surveys will allow us to extend this study to even dimmer satellite galaxies, to build a full picture of the formation of our galaxy.

The study was published in the May 20 issue of the Astrophysical Journal and the findings are based on analyses of data collected from the Sloan Digital Sky Survey (SDSS). The work is the first of three papers that study the properties of the Milky Way's two most massive satellites.

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