O BROTHERS, WHERE ART THOU? Finding the sun's siblings, the stars it was formed with, could help illuminate the history of the solar system.
SOHO (ESA & NASA)

For decades astronomers have been on the hunt for so-called "solar twins"—stars with the same ages, masses, temperatures, luminosities and chemical abundances as our own sun.

But seeking out solar twins in this way is akin to looking for people on the street wearing a coat like yours and calling them family, says Simon Portegies Zwart, a computational astrophysicist at the University of Amsterdam in the Netherlands.

Instead, Portegies Zwart wants to search for our star's true siblings—those that actually formed alongside our own star and still have the stellar bloodlines to prove it.

Until now, mainstream astronomers have dismissed the idea of actually finding any of our wayward kin, some 4.3 billion years after our stellar birth cluster is believed to have dissipated. But Portegies Zwart, in a new paper (being reviewed for publication in Astrophysical Journal Letters), offers up what may be a credible way to find 50 or 60 of our own sun's original siblings, that small fraction of the estimated 1,000 to 6,000 original cluster members Portegies Zwart thinks is located within some 300 light-years of Earth.

Portegies Zwart proposes identifying stellar relatives via their proper motion, or apparent movement across our line of sight, their position in the sky, and their chemical signatures, which he likens to their stellar DNA. Such stars, he says, would be roughly one solar mass (the size of the sun) or less and have chemical abundances similar to the sun.

"Several months ago," the astrophysicist says, "I would have said looking for the sun's siblings would be absolutely useless."

But Portegies Zwart changed his mind after calculating the sun's velocity backward over the past 4.6 billion years. From that putative starting point in the Centaurus constellation, he then projected how stellar members of our birth cluster would actually disperse forward in time.

"To my surprise," he says, "these stars all sort of stayed in the neighborhood with their proper motions and positions pretty much intact. Whether we can really recognize these stars is still not completely clear. Out of about a million stars within some 300 light-years, the expectation is that about 50 of them are siblings."

Leslie Looney, an astrophysicist at the University of Illinois at Urbana–Champaign, praises the idea in principle but says that astronomers will never know for sure if any given star is a sibling. He believes researchers will, however, be able to narrow down a group of stars that are "likely" kin.

And if observational astronomers were able to identify a few dozen of the sun's relatives, Portegies Zwart says, it would bring a sea change in our understanding of how the solar system formed and evolved.

One of his graduate students is already combing stellar catalogues to find candidates on which to try out the methodology. Portegies Zwart says the best place to look is in the plane of the galaxy, in the constellations of Vela and Cygnus, along the sun's trajectory around the galactic center.

The European Space Agency's 2011 Gaia mission, due to measure the position and motions of a billion stars in our part of the galaxy, should give the effort a boost.

But Eric Mamajek, a University of Rochester astronomer who specializes in stellar dynamics, likens Portegies Zwart's proposal to searching for members of his own 1993 high school class from a random sampling of Manhattan thirtysomethings.

"You might run into a few after an exhaustive search," Mamajek says, "but trying to extrapolate from whence they came based on their current trajectories in life would only lead to frustration."