Recent studies have found that 80-90% of stars are born in groups of 100 or more, most likely including the sun. After a 100 million years or so these stellar siblings drift apart, losing each other in the crowd. Now however, University of Texas astronomers have identified a star they believe came from the same birth cloud as the sun.
For lead author Dr Ivan Ramirez the motivation is the same as for any orphan. "We want to know where we were born. If we can figure out in what part of the galaxy the sun formed, we can constrain conditions on the early solar system. That could help us understand why we are here."
Lost family can be disappointing, but in this case we stand to gain a fair inheritance, if only in knowledge. At 15% more massive than the sun it is the sort of place that might be a suitable candidate for life and at 110 light years-away it's not that distant, by galactic standards. The only disappointment is its boring name, HD 162826, and that you need binoculars to pick it out in the constellation Hercules, not far in the sky from Vega.
Ramirez investigated the chemical composition of 30 previously proposed siblings, looking for the same concentrations of rare earth elements. In the Astrophysical Journal he reveals two passed the test, and one of these turned out to have an orbit around the center of the galaxy that ruled it out. As a relatively nearby sunlike star HD 162826 has been a target for planet hunters for 15 years. They've ruled out the presence of hot Jupiters, and think it also unlikely a large planet lurks at larger distances. However the observations have not been sensitive enough to pick up Earth-sized planets if they exist.
The discovery provides a starting point for more ambitious projects.
Ramirez hopes to use the findings to develop better ways to find many more family members. The Gaia observatory, launched last year, will provide a catalog of 1% of the stars our galaxy, mapping their motions and positions in three dimensions. "The number of stars that we can study will increase by a factor of 10,000," says Ramirez. Having this data and processing it effectively are two different things however, and Ramirez hopes his work will find a way through the maze. "You can concentrate on certain key chemical elements that are going to be very useful," he says. Barium and yttrium stand out as the elements whose concentration is both easy to measure and most useful in establishing kinship.
With a large enough family tree, Ramirez believes he will be able to identify the place in the galaxy where this scattered sample of stars began, as well as something about the cloud from which we came.
A far more speculative undertaking is to study HD 162826 for planets planets in the habitable zone. The two stars must have been quite close together in the period when large asteroids and planet sized objects were crashing into each other, blasting bits off in the process. It's not impossible a few bits of Earth got knocked from the solar system and ended up in neighboring systems. It's more of a stretch that these bits could have transferred life either from the Earth to HD 162826, or in the other direction, seeding life here, but Ramirez still considers “solar siblings key candidates in the search for extraterrestrial life.”
Ramirez was previously involved in the identification of the best “solar twin” then found. The term solar twin refers not to stars born close to the sun, but ones that most closely resemble our star in mass and chemical composition.