Ancient galactic collision may have birthed our Solar System

Around 70,000 light years away from Earth, a dwarf galaxy is spiraling in a polar orbit. The galaxy, known as Sagittarius, has crashed through the Milky Way's disc at least three times in the past six billion years. And one of those times may have resulted in the formation of the Solar System.

A new study, published in the journal Nature Astronomy, has traced the influence these galactic crashes have had on the Milky Way, and found that one of them coincided with the the birth of our Sun around 4.7 billion years ago.

The study is based on data gathered by Gaia, a space observatory launched by the European Space Agency (ESA) in 2013 to survey the stars of the Milky Way. As it looked onto the stars, Gaia discovered that the ripples caused by the Sagittarius galaxy crashing into the Milky Way triggered an uptick in star formation.

Sagittarius was first discovered in 1994 as a satellite galaxy orbiting the Milky Way. As it orbits the Milky Way, Sagittarius is pulled in tighter due to the galaxy's gravitational force, and repeatedly smashes into the disk of stars, gas and dust that surrounds the Milky Way. Each time, Sagittarius loses a bit of its own gas and dust, making it smaller and smaller after every passage.

Astronomers have known of three incidents in which Sagittarius swerved straight into the Milky Way's disc. The first time taking place around five or six billion years ago, then around two billion years ago, and lastly around one billion years ago.

“When we looked into the Gaia data about the Milky Way, we found three periods of increased star formation that peaked 5.7 billion years ago, 1.9 billion years ago and 1 billion years ago, corresponding with the time when Sagittarius is believed to have passed through the disc of the Milky Way," Tomás Ruiz-Lara, a researcher in Astrophysics at the Instituto de Astrofísica de Canarias (IAC) in Tenerife, Spain, and lead author of the new study, said in a statement.

The researchers used models of star evolution, and compared them with the luminosity, distances and colors of stars that are around 6,500 lightyears around the Sun.

The team behind the new study believes that as Sagittarius crashed into the Milky Way, it disrupted its steady flow of star formation by causing the gas and dust of the galaxy to "to slosh around like ripples on the water," as Ruiz-Lara described it.

These ripples would result in a higher concentration of gas and dust in certain areas of the Milky Way, triggering the formation of new stars.

The Solar System is believed to have formed around 4.6 billion years ago. This means that the first time Sagittarius crashed into the Milky Way, it may have resulted in the birth of our Sun, which later formed the planetary bodies around it, including Earth.

“The Sun formed at the time when stars were forming in the Milky Way because of the first passage of Sagittarius,” says Carme Gallart, a researcher at IAC and co-author of the new paper, said in a statement. “We don’t know if the particular cloud of gas and dust that turned into the Sun collapsed because of the effects of Sagittarius or not. But it is a possible scenario because the age of the Sun is consistent with a star formed as a result of the Sagittarius effect.”

The recent study also found evidence of a recent burst of star formation in the Milky Way, as existing data suggest that Sagittarius may have crashed in the Milky Way's disc for the fourth time quite recently, within the past hundred million years or so.

Therefore, it is quite possible that Sagittarius may have caused the birth of thousands of new stars, and possible new worlds orbiting around them.

Abstract: Satellites orbiting disk galaxies can induce phase space features such as spirality, vertical heating and phase-mixing in their disks. Such features have also been observed in our own Galaxy, but the complexity of the Milky Way disk has only recently been fully mapped by Gaia Data Release 2 (DR2) data. This complex behaviour is mainly ascribed to repeated perturbations induced by the Sagittarius dwarf galaxy (Sgr) along its orbit, pointing to this satellite as the main dynamical architect of the Milky Way disk. Here, we model Gaia DR2-observed colour–magnitude diagrams to obtain a detailed star formation history of the ~2 kpc bubble around the Sun. It reveals three conspicuous and narrow episodes of enhanced star formation that we can precisely date as having occurred 5.7, 1.9 and 1.0 Gyr ago. The timing of these episodes coincides with proposed Sgr pericentre passages according to (1) orbit simulations, (2) phase space features in the Galactic disk and (3) Sgr stellar content. These findings most probably suggest that Sgr has also been an important actor in the build-up of the stellar mass of the Milky Way disk, with the perturbations from Sgr repeatedly triggering major episodes of star formation.