Our Sun is approximately 4.6 billion years old, far older than humankind, and still older than our ability to observe it with telescopes. Thus, our best source for information about how the Sun has evolved and will evolve is other stars similar to our own. Those similar stars fall into one of three categories, based upon how like our Sun they are: solar-type stars, solar analogues, and solar twins, which are most similar to the Sun. Solar twins are very much like the Sun in terms of mass, temperature, and relative abundance of different chemicals, and are also the rarest of the three classes.
Their rarity makes searching for solar twins an arduous task, but worthwhile; solar twins that are different ages than the Sun can provide a tremendous quantity of data on the life stages of our own star. Now, new observations by the European Southern Observatory’s Very Large Telescope (VLT) in Chile have revealed the oldest known solar twin: HIP 102152. The new study was led by Jorge Melendez of the Universidade de São Paulo in Brazil.
Melendez and team actually studied two solar twins to more precisely determine their ages and chemical compositions. One of the stars, 18 Scorpii, was found to be quite a bit younger than the Sun, at only 2.9 billion years old. In contrast, HIP 102152 turned out to be nearly twice as old as the Sun, 8.2 billion years old, providing a unique opportunity to learn what our Sun will be like when it reaches that great age.
In particular, HIP 102152 has shed light on an enduring mystery surrounding the chemical abundances within our Sun. Melendez and colleagues used the UVES spectrograph on the VLT, which splits light into its component colors to reveal the chemical composition and other properties of the object that emitted the light – in this case, the stars 18 Scorpii and HIP 102152. The spectrographic data indicated that HIP 102152 contains less lithium than the Sun.
Scientists have long puzzled over why the Sun contains only 1% of the lithium that was present in the matter from which it originated. Previous studies showed that younger solar twins, such as 18 Scorpii, contain significantly higher abundances of lithium. Now, the new findings about HIP 102152 suggest that lithium content does decrease as a star ages, though the mechanism behind this remains unknown.
HIP 102152 is also remarkable in that its chemical composition is subtly different from other solar twins, but very similar to that of the Sun, showing a dearth of elements that are common in rocky planets, such as Earth and Mars. Also, information from the HARPS spectrograph indicates that there are no giant gas planets in the star’s habitable zone that could disrupt the formation of rocky planets.
The new research will be published in the Astrophysical Journal Letters.