Scientists make a positive ID: Nearby star is a 'sibling' of our sun

Astronomers say they have positively identified a relatively nearby star as coming from the same vast cloud of dust and gas that gave birth to the sun.

If the analysis holds up to further scrutiny, it marks the first time researchers have identified a solar sibling, paving the way for a family reunion of sorts – at least on the pages of star catalogs.

Efforts to find the sun's cosmic womb-mates are important; a head count holds clues to the wider environment in which they and the sun formed. That environment is thought to wield influence on the composition, distribution, and orbits of any planets that form around the sibling stars – or whether planets form at all.

Beyond its role in providing information about the sun and its nursery, "identifying groups of stars that were born together is very important for understanding not just star formation or cluster formation, but even galaxy formation and evolution," says Ivan Ramirez, an astronomer at the University of Texas at Austin and the lead author of a paper on the results set to appear in the Astrophysical Journal.

Over time, he explains, stars born in so-called open clusters, as the sun is thought to have been, spread out. Their mutual gravity is too weak to corral them. The degree of spreading depends on the properties of the galaxy. By identifying sibling stars and using models to work backward to find out where they originated, researchers can better pin down the extent of clusters' dispersion and their influence on the galaxy's evolution.

An appreciation for the sun's wider environment is relatively new, notes Freed Adams, an astrophysicist at the University of Michigan at Ann Arbor. Much of the theoretical groundwork has emerged since about 2000, centered around the concept that, like other stars at birth, the sun most likely was part of a cluster of stars that emerged from the same cloud.

Hints as to conditions in the sun's cosmic cocoon have come from observations of the solar system.

If a cluster has too many stars packed too closely, their radiation can disrupt the circumstellar disks of dust and gas from which planets form. But the solar system clearly has planets, so the stellar nursery couldn't have been excessively star-rich.

At the same time, the presence of radioactive elements in meteorites indicates that the stars were packed closely enough to allow a nearby supernova to seed our sun's vicinity with radioactive elements that would get caught up in the planet-formation process.

Moreover, Sedna, a dwarf planet beyond Pluto, has an elongated orbit that takes 11,400 years to complete. And at its closest approach to the sun, it comes no closer than 75 astronomical units, or 75 times the distance between Earth and the sun. This suggests that early in the solar system's history, a close encounter from a passing star tweaked its orbit.

The current best estimate for the number of siblings who shared a primordeal cloud with the sun ranges from 10,000 to 100,000 stars.

"If we were born in a cluster and there were a few thousand stars in the cluster, it's a natural thing to say: Well, where are they?" says the University of Michigan's Dr. Adams, who was not involved in the new study.

The newly identified sibling doesn't quite qualify as long-lost. The lone star, 110 light-years away in the constellation Hercules, has been under observation for more than 15 years as part of a hunt for extrasolar planets, says Dr. Ramirez.

It had been included in a list of 30 solar-sibling candidates analyzed by Ramirez and colleagues from Russia and Australia. The list had been published in 2010 by another group of astronomers.

Analyses of the stars' chemical compositions narrowed the field of candidates, as did modeling efforts that reconstructed the motions of the 30 stars over the past 4.6 billion years. Only one star remained: HD 162826. It boasts 1.15 times the sun's mass.

In the process of analyzing the stars, the team found that the ratio of barium to iron in a star's spectra is very sensitive to the star's place of origin, indicating that establishing same-cluster relationships may best be achieved by using that ratio, rather than a more inclusive set of elements.

Noting astronomers' keen interest in studying populations of related stars in general, Adams says the approach the team devised for its analysis appears to be "a real step forward." 

Given efforts to take the measure of ever-larger numbers of stars using space- and ground-based telescopes, Ramirez and colleagues say they expect researchers to find more solar siblings "in the very near future" using this approach.

After more than 4 billion years of dispersal, only a few solar siblings with a mass comparable to the sun's are likely to still be in the neighborhood.