There is something strangely reassuring about knowing that the sun, in its cosmic youth, had company. Someone to share the journey, or at least the direction, with. Between 4 and 6 billion years ago, our sun left the innermost regions of the Milky Way together with thousands of stars almost identical to him: same temperature, same chemical composition, same history. A silent mass migration, occurring over hundreds of millions of years, that changed everything. For us too.
The discovery comes from a team of Japanese researchers led by Daisuke Taniguchi of Tokyo Metropolitan University and from Takuji Tsujimoto of the National Astronomical Observatory of Japan, who published their results on Astronomy and Astrophysics. And what they found is, to put it simply, an answer to a question that scientists have been wondering about for decades.
How does a star escape from the center of the galaxy?
The sun was born about 4.6 billion years ago in a much deeper part of the Milky Way than it is now — at least 10,000 light-years closer to the galactic heart. So far, nothing surprising: the chemical composition of our sun betrays this, and astronomers have known this for some time. The problem is another, and it is that at the center of the galaxy there is a colossal structure in the shape of a rotating bar, a sort of enormous cosmic rotor, which generates a gravitational barrier capable of trapping stars in the central regions.
Specialists call it a “corotation barrier”, and in theory it should make such a long stellar escape practically impossible. Yet the sun is here. Far away, quiet, with us around.
To understand how it got there, researchers built the largest and most precise catalog ever made of solar twins – stars with surface temperature, gravity and composition almost identical to ours – using data from the European Space Agency’s Gaia mission, an astronomical database containing observations on around two billion celestial objects. The catalog obtained includes 6,594 solar twinsthirty times richer than any previous collection. An enormous leap in scale, which finally allowed us to see patterns invisible in smaller investigations.
The galactic bar was still forming
By analyzing the age of all these stars, the team found something precise: a very sharp peak in the range between 4 and 6 billion years. Too many stars of the same age, all positioned at the same distance from the galactic center, all with sun-like characteristics. A coincidence of this kind, in astrophysics, is not a coincidence. That concentration tells a real event: a collective migrationa moment when something gave way in the gravitational barrier and a huge number of stars were able to move outward.
The explanation most consistent with the data is that, at that time, the central bar of the Milky Way was still under construction. The corotation barrier depends directly on the presence of that structure, and if the structure was still taking shape, then there would have been a window – temporary, unrepeatable – during which stars could move freely towards more external regions. The sun and its twins passed through that window together, and then the door closed.
This detail goes double, because the ages of the solar twins don’t just tell the story of our sun, they provide a Precise time stamp on when the galactic bar formed. Information that astronomers have been looking for for a long time, and which now emerges from a catalog of stars that are almost familiar to us.
Then there is the other half of the story, the one about life. The central areas of a galaxy like the Milky Way are brutal environments: intense radiation, closely spaced stars, frequent stellar explosions, instabilities of all kinds. The external regions, where the sun has landed thanks to this migration, are much quieter, much more stable.
Conditions in which planets can form without being destroyed every few million years, in which water can remain liquid, in which life finds the necessary time to appear and transform. If the sun had remained where it was born, the Earth would probably be a very different place, if it existed in the form we know it.
There’s something beautiful, and a little dizzying, about the idea that our existence depends in part on a migration that occurred billions of years ago, at a time when the galaxy was still deciding what shape to take. The sun has found its right position in the universe, together with the others. And we are here because he succeeded.
