The search for one’s family tree and genealogy is an important journey for every human being, often leading to a better sense of self-identity. Like humans, stars have their own distinct origins and unique lifespans in the Universe. Tracing their evolutionary path could, among other things, provide us with important information regarding the very beginnings of our own Solar System. For this reason, scientists have been hunting for solar “siblings” for decades—stars that share the same origins and birthplace with our Sun. Now, a team of astronomers has reported that such a star might have been found, possibly providing us with unique insights into the evolution of our own Solar System.
According to the leading theories of stellar formation, most if not all stars are born in groups inside giant interstellar clouds of gas and dust, resulting in the formation of what are known as open star clusters, which usually contain thousands of stars that loosely bound together by their mutual gravity. One such example is the famous Pleiades, or M45 cluster, easily visible with the naked eye, located just 440 light-years away in the direction of the Taurus constellation. Even after the remaining leftovers of the original interstellar cloud are stripped away by the cluster’s intense radiation pressure, its constituent stars will still retain a similar trajectory in space for as much as half a billion years, before eventually breaking free from their mutual gravitational embrace and left to follow their own independent paths around the galactic center.
Like most stars, the Sun is believed to have been part of an open cluster, for at least the first hundred million years following its formation, approximately 4.5 billion years ago. In the time since, the solar cluster had ample time to dissipate; its members gravitationally unbounding themselves from each other during their long journeys around the center of the Milky Way galaxy. Yet identifying the exact location where our Sun was born has long been a goal for astronomers in their efforts to understand the evolutionary process of our Solar System and the eventual appearance of life on Earth. Yet the absence of the needed astrometric measurements of the stars’ positions and movements in the galaxy and detailed spectroscopic analyses of their chemical compositions made this prospect seem an unlikely proposition. Despite these observational limitations, a team of astronomers led by Ivan Ramirez, a Research Fellow at the University of Texas at Austin, announced the identification of such a possible sibling to the Sun in a new study that is scheduled to be published in the June 1 issue of The Astrophysical Journal.
The team conducted a detailed chemical analysis of a total of 30 stars that had already been tagged by previous studies as solar sibling candidates. To achieve the sensitivity needed for their measurements, the astronomers used high-resolution spectroscopy during two periods of observations in December 2012 with the 2.7-meter Harlan J. Smith Telescope at the McDonald Observatory in Jeff Davis County, Texas, and in March and April 2013 with the 6.5-meter Magellan Telescopes at the Las Campanas Observatory in Chile.
In order to check the stars’ possible relationship to the Sun, the team studied their metallicity—the proportion of atoms heavier than hydrogen and helium that were present in their atmospheres. Since different stars can have similar chemical compositions, thus masquerading as false solar siblings, the team focused on looking for the lines of specific trace elements in the stars’ spectra, like aluminium, barium, sodium, vanadium, and yttrium, and compared them with the Sun’s spectrum. The presence of these elements in the stellar spectra is dependent on the exact location of the star’s formation in the galaxy, thus a star exhibiting these chemical signatures would qualify as a real solar sibling candidate, indicating that it shared the same birth place with the Sun. “A star with the same composition as the Sun must have all values [of the above chemical elements] around zero within the [study’s measurement] errors,” writes the team in their study. “Two stars [out of the 30 stars studied], stand out in this context: HD 154747 and HD 162826.”
HD 154747 is a G-type yellow subgiant star, located 316 light-years away in the southern constellation Apus, while HD 162826 is a 6th magnitude F-type yellow-white dwarf, located in the constellation of Hercules, 110 light-years away. While both stars were shown to have an age and metallicity essentially identical to that of the Sun, Ramirez’s team checked their orbital dynamics as well using computer models of galactic gravitational potential to see whether the orbits of these stars had intersected that of the Sun at any times in the past, something that would point toward a common place of origin. This led astronomers to ultimately narrow down the solar sibling candidates to just one. “This leaves us with only one true solar sibling candidate: HD 162826,” concludes Ramirez’s team in their study. “Its chemical composition is Solar, within the [measurement] errors and its past orbit includes a number of close encounters with the Sun (within a relative distance of approximately 32 light-years), which happened with relative velocities of about 10 km/s−1 or less. The encounter parameters are particularly favorable around t= 4 billion years ago, at an epoch when the solar cluster may have not fully dissipated yet … Not surprisingly, these findings demonstrate that elemental abundance analysis alone is not sufficient, and neither is the dynamical argument by itself. Both are required to make a proper solar sibling identification … Only the star HD 162826 satisfies both our dynamical and chemical criteria for being a true sibling of the Sun.”
HD 162826 has also been studied extensively for many years by the McDonald Observatory’s Planet Search team in an effort to discover any orbiting exoplanets. Yet the star seemed to lack any major exoplanetary companions. “High-precision radial velocity observations carried out over a period of time longer than 15 years, rule out the presence of hot-Jupiter planets,” write the astronomers in their study. “These data also suggest a 2/3 chance that a Jupiter analog is not present either. Smaller terrestrial planets cannot be ruled out at this moment.”
The method used by Ramirez and his team could prove to be a valuable tool for the identification of other possible solar siblings in the future. This search is expected to receive a huge boost by ESA’s Gaia space observatory, which was launched successfully in December 2013 and is currently in the middle of its commissioning and instrument check-out phase, at the Sun-Earth L2 Lagrangian point, 1.5 million km away. During its planned five-year mission, Gaia will make astrometric measurements of unprecedented detail of the proper motions and velocities of approximately a billion stars within the Milky Way galaxy. These observations will lead to the creation of precise three-dimensional maps of star motions, yielding detailed physical properties, characterizing luminosities, pegging effective temperatures, and compiling databases of gravitational and elemental compositions. “The combination of astrometric data from the ongoing Gaia mission and spectroscopic data from surveys of comparable large size such as the Gaia survey, APOGEE, and/or GALAH, will allow us to discover many more solar siblings in a very near future,” writes Ramirez’s team. “We expect that the analysis presented in this paper will guide future endeavors in this field and allow us to perform these searches more efficiently.”
The search for the Sun’s other family members in the galaxy and the subsequent results from Gaia are bound to be monumental, promising to forever alter our view of our place in the Cosmos. “We want to know where we were born,” says Ramirez. “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.”
With the exception of the discovery of life elsewhere in the Universe, space exploration, while once again showcasing its incalculable value to society, could hardly answer a more fundamental question for any human being.