An international team led by astronomers in Brazil has used ESO’s Very Large Telescope to identify and study the oldest solar twin known to date. Located 250 light-years from Earth, the star HIP 102152 is more like the Sun than any other solar twin — except that it is nearly four billion years older.
This older, but almost identical, twin gives us an unprecedented chance to see how the Sun will look when it ages. The new observations also provide an important first clear link between a star’s age and its lithium content, and in addition suggest that HIP 102152 may be host to rocky terrestrial planets.
Astronomers have only been observing the Sun with telescopes for 400 years — a tiny fraction of the Sun’s age of 4.6 billion years. It is very hard to study the history and future evolution of our star, but we can do this by hunting for rare stars that are almost exactly like our own, but at different stages of their lives. Now astronomers have identified a star that is essentially an identical twin to our Sun, but 4 billion years older — almost like seeing a real version of the twin paradox in action .
Jorge Melendez (Universidade de São Paulo, Brazil), the leader of the team and co-author of the new paper explains: “For decades, astronomers have been searching for solar twins in order to know our own life-giving Sun better. But very few have been found since the first one was discovered in 1997. 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.”
The team studied two solar twins  — one that was thought to be younger than the Sun (18 Scorpii) and one that was expected to be older (HIP 102152). They used the UVES spectrograph on the Very Large Telescope (VLT) at ESO’s Paranal Observatory to split up the light into its component colours so that the chemical composition and other properties of these stars could be studied in great detail.
They found that HIP 102152 in the constellation of Capricornus (The Sea Goat) is the oldest solar twin known to date. It is estimated to be 8.2 billion years old, compared to 4.6 billion years for our own Sun. On the other hand 18 Scorpii was confirmed to be younger than the Sun — about 2.9 billion years old.
Studying the ancient solar twin HIP 102152 allows scientists to predict what may happen to our own Sun when it reaches that age, and they have already made one significant discovery. “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?”
Lithium, the third element in the periodic table, was created in the Big Bang along with hydrogen and helium. Astronomers have pondered for years over why some stars appear to have less lithium than others. With the new observations of HIP 102152, astronomers have taken a big step towards solving this mystery by pinning down a strong correlation between a Sun-like star’s age and its lithium content.
Our own Sun now has just 1% of the lithium content that was present in the material from which it formed. Examinations of younger solar twins have hinted that these younger siblings contain significantly larger amounts of lithium, but up to now scientists could not prove a clear correlation between age and lithium content .
TalaWanda Monroe (Universidade de São Paulo), the lead author on the new paper, concludes: “We have found that HIP 102152 has very low levels of lithium. 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.” 
A final twist in the story is that HIP 102152 has an unusual chemical composition pattern that is subtly different to most other solar twins, but similar to the Sun. They both show a deficiency of the elements that are abundant in meteorites and on Earth. This is a strong hint that HIP 102152 may host terrestrial rocky planets .
 Many people have heard of the twin paradox
: one identical twin takes a space journey and comes back to Earth younger than their sibling. Although there is no time travelling involved here, we see two distinctly different ages for these two very similar stars — snapshots of the Sun’s life at different stages.
 Solar twins, solar analogues and solar-type stars are categories of stars according to their similarity to our own Sun. Solar twins are the most similar to our Sun, as they have very similar masses, temperatures, and chemical abundances. Solar twins are rare but the other classes, where the similarity is less precise, are much more common.
 Previous studies have indicated that a star’s lithium content could also be affected if it hosts giant planets (eso0942
), although these results have been debated (ann1046
 It is still unclear exactly how lithium is destroyed within the stars, although several processes have been proposed to transport lithium from the surface of a star into its deeper layers, where it is then destroyed.
 If a star contains less of the elements that we commonly find in rocky bodies, this indicates that it is likely to host rocky terrestrial planets because such planets lock up these elements as they form from a large disc surrounding the star. The suggestion that HIP 102152 may host such planets is further reinforced by the radial velocity monitoring of this star with ESO’s HARPS spectrograph, which indicates that inside the star’s habitable zone there are no giant planets. This would allow the existence of potential Earth-like planets around HIP 102152; in systems with giant planets existing close in to their star, the chances of finding terrestrial planets are much less as these small rocky bodies are disturbed and disrupted.
Stanford solar scientists have solved one of the few remaining fundamental mysteries of how the sun works.
The mechanism, known as meridional flow, works something like a conveyor belt. Magnetic plasma migrates north to south on the sun’s surface, from the equator to the poles, and then cycles into the sun’s interior on its way back to the equator.
The rate and depth beneath the surface of the sun at which this process occurs is critical for predicting the sun’s magnetic and flare activity, but has remained largely unknown until now.
The solar scientists used the Stanford-operated Helioseismic and Magnetic Imager
(HMI) – an instrument onboard NASA’s Solar Dynamic Observatory satellite – to track solar waves in much the way seismologists would study seismic movements beneath the surface of the Earth. Every 45 seconds for the past two years, the HMI’s Doppler radar snapped images of plasma waves moving across the sun’s surface.
By identifying patterns of sets of waves, the scientists could recognize how the solar materials move from the sun’s equator toward the poles, and how they return to the equator through the sun’s interior.
“Once we understood how long it takes the wave to pass across the exterior, we determined how fast it moves inside, and thus how deep it goes,” said Junwei Zhao, a senior research scientist at the Hansen Experimental Physics Laboratory at Stanford, and lead author on the paper.
Although solar physicists have long hypothesized such a mechanism, at least in general terms, the new observations redefine solar currents in a few ways. First, the returning currents occur 100,000 kilometers below the surface of the sun, roughly half as deep as suspected. As such, solar materials pass through the interior and return to the equator more quickly than hypothesized.
More startling, Zhao said, is that the equator-ward flow is actually sandwiched between two “layers” of pole-ward currents, a more complicated mechanism than previously thought, and one that could help refine predictions of the sun’s activity.
“Considered together, this means that our previously held beliefs about the solar cycle are not totally accurate, and that we may need to make accommodations,” Zhao said.
For example, some computer models projected that the current solar cycle would be strong, but observations have since showed it is actually much weaker than the previous cycle. This inconsistency could be due to the previously unknown inaccuracies of the meridional circulation mechanism used in the simulations.
Improving the accuracy of simulations, Zhao said, will produce a better picture of fluctuations of the sun’s magnetic field, which can interfere with satellites and communications technology on Earth. The sun’s magnetic field resets every 11 years – the next reset will occur sometime in the next few months – and there is evidence that changes in the meridional flow can influence how the magnetic field evolves during a particular cycle.
“We want to continue monitoring variations of the meridional flow,” he said, “so that we can better predict the next solar cycle, when it will come and how active it will be.”