Most stars are formed in clusters, not as single suns. They usually begin their lives as dense clumps of dust within a giant cloud of gas and dust called a molecular cloud. Under the force of gravity these knots of material collapse, spin and heat up until their internal temperatures are great enough to fuse hydrogen into helium to produce energy. This is how a star is born.
The sun likely began to coalesce within such a cloud 4.57 billion years ago. Stars that formed near the sun — each a member of the “solar cluster” — formed from the same basic materials and would be expected to look very similar chemically. In other words, they’d be made of the same materials in similar amounts as what we see in the sun.
Once a star cluster forms it’s subject to getting pulled apart by the gravity of similar clouds as it orbits the Milky Way’s center. Gravitational interactions between the stars can also cause them to leave the cluster over time. The stars disperse, and its member scatter far and wide, making them very difficult to find. The life of a typical cluster is about 200 million years or less than 5% of the sun’s current age — plenty of time for solar siblings to stray.
Astronomers have looked far and wide for the sun’s former roommates by studying candidates with a spectrograph, an instrument that spreads out a star’s light to reveal its chemical “fingerprints.” And while they’ve come up with several possible candidates, there have been no perfect fits. Now, an international team led by researcher Vardan Adibekyan (Astrofísica e Ciências do Espaço) have found a near-match used a new method to detect solar siblings.
In collaboration with several other astronomers, Adibekyan obtained a sample of 230,000 spectra of stars from the AMBRE Project and selected those with chemical compositions that best match the sun’s. They then estimated the stars’ ages and movements across the galaxy using new, high-accuracy data from the GAIA Mission. GAIA mapped precise positions and movements in three dimensions of some 1 billion stars, creating the most massive census of the sun’s neighbors to date.
That’s how they arrived at HD 186302, a 9th magnitude star in the southern constellation Pavo the Peacock. From a dark sky, the star is visible in a pair of binoculars. Starting with 17,000 possible candidates, the team winnowed the list to 55, then 12 and finally down to four best-fit candidates. Of the four, HD 186302 came closest to matching the sun. Think of it like getting one of those DNA tests that are so popular nowadays. HD 186302 turns out to be an excellent match for the sun in type and age (for those who like technical details, it’s a G3-type main sequence star) and has a very similar chemical composition. Keep in mind that the scientists haven’t proved it grew up near the sun, only that it’s a great candidate.
We look for stellar siblings because we want know more our origins and where we grew up, the same reasons some of us seek long-lost brothers, sisters and cousins. Some of our old cluster companions may have had planets, too. Finding them could tell us more about planet formation in a shared environment.
Adibekyan wants to take it a step further to consider the possibility that life may have been transported between stars within our birth cluster: “Solar siblings can also be good candidates to search for planets with life, assuming that the life transportation between solar systems in the Sun’s birth cluster was efficient.”
For that reason, the team is now planning to search for planets around HD 186302. To learn more, see the scientific paper here. I think there’s something in all of us that responds to finding our “origin.” To whom are we related? Where do we come from? Seeking solar siblings is that same desire writ large.