September 4, 2013
Our Sun, Scorpii 18 and HIP 102152 on a timeline.
The Sun seems to be an ordinary star, but so far we have found very few stars that are similar to it, the so-called solar twins. One such star was recently discovered by an international team led by Brazilian astronomers. The star is 8.2 billion years old and it could tell us a lot about the future of our own star. The observation also hints at possible rocky planet around it and suggests that Sun's low lithium levels could be normal for this star type.
The star, designated HIP 102152, appears to have more in common with the Sun than any other star we know of. It is also the oldest known solar twin. It is about 250 light-years away in the constellation Capricornus and was observed with the Very Large Telescope (VLT) in Chile.
"For decades, astronomers have been searching for solar twins in order to know our own life-giving Sun better," said Jorge Melendez from Universidade de São Paulo, Brazil. He is the leader of the team and co-author of the new paper that will be published in the Astrophysical Journal Letters. "But very few have been found since the first one was discovered in 1997," Melendez said. "We have now obtained superb-quality spectra from the VLT and can scrutinise solar twins with extreme precision, to answer the question of whether the Sun is special."
Another solar twin was observed, named 18 Scorpii – a star that is only 2.9 billion years old – but it was nowhere near as interesting as HIP 102152. The reason is the amount of lithium that the team found in HIP 102152. Using the UVES spectrograph of the VLT, they found out that it has significantly less lithium than our Sun.
"One issue we wanted to address is whether or not the Sun is typical in composition," says Melendez. "Most importantly, why does it have such a strangely low lithium content?"
For years, astronomers have asked themselves why does the Sun have less than 1% lithium, although it should be pretty common since it's the third element in the periodic table. It appears that the lithium gets destroyed inside the star with time, so older stars have less lithium than younger stars as is the case with the Sun and HIP 102152. Earlier observations of younger solar twins, like Scorpii 18, have shown that they have more lithium than the Sun, so it is important to observe the star that is older than our nearest star and confirm that it is normal for it to lose lithium with age.
"We have found that HIP 102152 has very low levels of lithium," said TalaWanda Monroe, Universidade de São Paulo, Brazil. "This demonstrates clearly for the first time that older solar twins do indeed have less lithium than our own Sun or younger solar twins. We can now be certain that stars somehow destroy their lithium as they age, and that the Sun's lithium content appears to be normal for its age."
Further chemical composition analysis shows that HIP 102152, just like the Sun, has a deficiency of some elements that are common in meteorites and on Earth. Incidentally, compared to other solar twins, both HIP 102152 and the Sun have a slightly different chemical composition. Astronomers hope this is a strong indication that HIP 102152 too has rocky planets in its orbit.
This is because during the formation of the system, rocky planets "collect" certain materials as they revolve around their star. These materials end up as building blocks of these planets instead of being a part of the star.