Sun’s sibling spotted

One of the sun’s long lost siblings could be just 110 light-years away.

Astronomers have identified a star that might have formed in the same nebula as the sun and at the same time. The star, HD 162826, is in the constellation Hercules and is a bit warmer, brighter and more massive than the sun. Its discovery is part of an experiment to learn how to identify solar siblings efficiently. The experiment’s method could help researchers figure out where the solar system was born.

One sibling doesn’t reveal much. But having a good way to find many siblings should help scientists understand why the solar system is the way it is, says experiment leader Iván Ramírez, an astronomer at the University of Texas at Austin. Over the 4.57-billion-year lifetime of the sun, its nebular kin drifted away as the motions of the stars and relentless tug of the galaxy’s gravity tore the family apart.

A solar sibling should have the same chemical composition as the sun, Ramírez and colleagues reasoned, since it was born in the same gas cloud. Based on spectroscopic observations of 30 candidates, reported May 8 on arXiv.org in a paper to be published in the Astrophysical Journal, HD 162826 was one of a handful whose compositions matched the sun.

Ramírez’s team narrowed the field to candidates that have followed the sun’s motion around the galaxy. Looking back in time, Ramírez says, “most stars separate really quickly and keep going away from the sun.” This star, however, seems on a more parallel track.

The star appears to be the right age as well. Based on calculations that use a star’s color and brightness to estimate its age, HD 162826 is nearly 4.6 billion years old.

“The fact that these guys found one is not crazy,” says Fred Adams, an astrophysicist at the University of Michigan in Ann Arbor. While nudges from other stars and clouds over billions of years have smeared the birth cluster in a ring around the galaxy, he says, theorists predict that researchers should find a few sunlike siblings in the neighborhood. “It’s not a guarantee,” he adds, “but it’s an interesting possibility.”

Astronomers think the sun formed in a cluster of several thousand stars, which is roughly the number needed to explain the solar system’s existence. “The fact that we’re here,” Ramírez says, limits the size of the cluster. Too big, and radiation from too many massive stars would have destroyed the infant solar system. Too small, and there wouldn’t have been enough mass to produce the supernova that provided the planets with heavy elements (SN Online: 11/11/11). The nearest analog to the sun’s birth cluster, Adams says, is the Orion Nebula Cluster, about 1,600 light-years from Earth.

Finding solar siblings may reveal how solar systems form. In the last two decades, astronomers have discovered over 1,000 stars with planets of varying orbits, sizes and masses. Since birth clusters sculpt solar systems, Adams says, studying such clusters “lets us understand the grander question of how do you produce all the diversity in the solar systems that we see?”

Answering that question will require finding more of the sun’s family — and that will take work. The European Space Agency mission Gaia, which is measuring positions and motions of a billion stars, will create a database from which to find candidates. And another project, the Gaia-ESO survey, will get the detailed spectra needed for accurate chemical tagging of over 100,000 of those stars using the Very Large Telescope in Chile. HD 162826 is just the start. “We’re creating a road map to find them in the future,” Ramírez says. “The fact that we found one was a bonus.”