Some 5 billion years ago, our Sun was born in a cloud of dust and gas, probably along with about 1000 other stars. These stars must now have a similar age and composition to Sol.

A study of these stellar siblings could answer important questions about the origin of the Solar System and the Sun. Their distribution would tell us where in the Milky Way the Sun has travelled during the last 5 billion years, how this journey might have effected the Earth’s climate and the life it supports and their composition might explain why the Sun appears to have a higher metal content than expected for an object in this part of the galaxy.

So where are these siblings? Today, Anthony Brown at Missouri State University and a couple of pals publish the results of the first serious search for the Sun’s siblings. And the results are disappointing.

The problem is the sheer scale of the task. In recent years, astronomers have learnt that star formation is a dynamic and chaotic process in which gravitational interactions eject stars from the gas cloud in which they have formed, as if sprayed from some kind of interstellar garden hose.

That means the Sun’s 1000 or so siblings will now be spread over huge distances stretching some 3000 light years from here. Brown and co say this volume of space contains 100 million stars.

Unfortunately, astronomers have accurate information on only 100,000 stars, most of this gathered by the Hipparcos space mission in the early 1990s. Most of these stars are within just a few hundred light years of here and it by no means represents a complete map of the local environment.

Given the numbers, the likelihood is that only one or possibly none of these nearby stars is a solar relative .

Despite these odds, Brown and co have been carefully combing the Hipparcos catalogue for any sign of long lost relatives.

The search turned up various candidates. But only one of these is the same age of the Sun and has a velocity consistent with a common origin. This is HIP 21158, a white, magnitude 7 star in the constellation of Taurus.

But Brown and co say that even HIP 21158 is unlikely to be a long lost sibling because it’s velocity is a little on the high side.

Their conclusion is that: “This means we have not found a single convincing solar sibling within 100 pc from the Sun.”

That’s depressing, not least because the next significant update to the star catalogue isn’t due for a decade. Hipparcos’s successor, Gaia, is due for launch in 2012 and will catalogue about a billion stars, creating a decent 3D map of the MIlky Way for the first time. However, this catalogue will not be ready until 2020.

There is plenty do in the meantime, however. To narrow the search, astronomers will need to better understand the way that stars disperse from their birth cluster. That will mean better simulating the effect of gravitational interactions between the stars, taking the asymmetric gravitational pull of the Milky Way’s spiral arms into account and understanding how collisions with molecular dust clouds effect the dynamics.

None of that will be easy. The words needle and haystack come to mind.

Ref: arxiv.org/abs/1004.4284:The Quest for the Sun’s Siblings: an Exploratory Search in the Hipparcos Catalogue