Twin star

A second sun

Star in the Dragon constellation is an almost perfect copy of the heavenly object that lights up the Earth

034-036_NovoSOL_196-1The star most like the Sun has just undergone a battery of refined exams. The high resolution spectrometer at the Keck Observatory in Hawaii decomposed the light of the star into its constituent colors and these forms of electromagnetic emission were compared one by one with the Sun. The results confirmed the suspicions of the first diagnosis of the star, carried out five years ago by Peruvian astrophysicist Jorge Meléndez, then at the National University of Australia and today at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP). HIP 56948 is truly the best solar twin known. Its mass, surface temperature, radius, brightness and chemical composition, in fact, the main parameters of the star, are practically identical to those of the Sun. “The difference in the measures between the two stars are within quite acceptable margins of error,” says Meléndez, who is studying the star with the support of a project funded by FAPESP. “Compared with HIP 56948, the other twins are merely distant cousins of the Sun.”

The solar twin lies in the celestial northern hemisphere, in the Dragon constellation, halfway between the stars Alpha Ursae Majoris and the Pole star, the latter famous for having been used since ancient times as a guide for navigators. HIP 56948, sometimes called HD 101364, is located 200 light-years away, something like 12.6 million times further away from the Earth than the Sun is. Before the first comparative study between HIP 56948 and the Sun, which was carried out in 2007, the best clone candidate of our mother-star was 18 Scorpii, on the northern edge of the Scorpius constellation. This star, which is some 45 light-years from Earth, was described as a solar twin in 1997 by astrophysicist Gustavo Porto de Mello, from the Federal University of Rio de Janeiro (UFRJ). “We’re building up a small tradition in Brazil of studying twins,” says Porto de Mello, who did not take part in the work on the star in the Dragon constellation.

In addition to the physical and chemical similarities with the Sun, tests with HIP 56948 have revealed another interesting characteristic in its surroundings. The conditions around the star seem to be compatible with the existence of a group of planets with architecture similar to that of the solar system, where small rocky worlds are located closer to the star and large gassy planets occupy the more peripheral zone. This apparent point in common with the Sun in theory makes HIP 56948 a good candidate for housing Earth-like planets in its vicinity, despite an extra-solar world is yet to be discovered around it. The conclusions form part of a study coordinated by the astrophysicist from USP and accepted for publication in the scientific journal Astronomy & Astrophysics.

034-036_NovoSOL_196-2There is no comprehensive definition about what a solar twin actually is. Up to what point a star needs to be equal or very similar to the Sun to receive this designation is an open question. Some stars are like the Sun when certain parameters are analyzed, but far from similar considering other aspects. As astrophysicists obtain more detailed data about the stars, the similarities and differences become more obvious. For reasons of practicality and due to the limitations of the instrumentation currently available, the search for solar twins is concentrated in an area of the sky no further than 300 light-years away from Earth, where, according to the projections of the astrophysicists, there are likely to be dozens of solar twins. This zone is the equivalent of a minute part of the Universe, but it is necessary to start the search with what is closest to hand.

In the case of HIP 56948 and the Sun, the points in common between the two stars are impressive. The comparison of a series of important parameters leads to a kind of technical ‘draw’ between the stars. For example, the mass of HIP 56948 is just 2% bigger than that of the Sun, within the margin of error of the measurement carried out by Meléndez and his assistants, also  2%. Its radius is 687,000 km, 1.3% smaller than that of the Sun. The average Surface temperature of the stars (on their outermost layer, which gives them their yellow color) is almost the same: it differs by just 0.3%; that of the solar twin is 5,521 °C, 17 °C hotter than that of the Sun. For comparison purposes, the temperature of 18 Scorpii, the second twin most like the Sun, is 54 °C hotter than our mother-star. The difference in the true brightness of HIP and the Sun is almost imperceptible. The twin is 1.4% less luminous.

Despite all of these almost identical features, the two stars have a significant age difference, of approximately 1 billion years, according to the researchers’ most recent calculations. It is as if they were twins, but from different generations. The Sun is 4.57 billion years old; HIP 56948 is 3.52 billion years old. “That’s certainly not bad,” says astrophysicist Ivan Ramirez, from the University of Texas, another author of the article. “In this way we can study how the Sun was evolving one billion years ago.” There is an added problem regarding this parameter. “Determining the age of a star is notoriously difficult,” considers Martin Asplund, from the National University of Australia, another astrophysicist who signed the work in A&A. “Perhaps HIP 56948 is almost the same age as the Sun.” In other words, even older than our mother-star, an idea advocated in other scientific studies, which include an older paper by Meléndez, which was based, however, on data of poorer quality. The margin of error for this parameter is much greater than for other stellar properties.

034-036_NovoSOL_196-3HIP 56948 has a chemical signature very similar to the peculiar composition of the Sun, which is less rich in certain metals when compared with other types of star. The solar twin is also deficient in certain elements, such as nickel and iron, although to a degree that is between 2% and 3% less pronounced than that of our star. A group of astrophysicists, among them the researcher from USP, believes that the scarcity of some metals in the composition of the Sun may be linked to the formation process of the planets around it. For supporters of this interpretation, some of the material in the primordial gas cloud that gave rise to the Sun condensed in the form of dust, from which the bigger structures that subsequently formed the planets, above all the rocky ones (Mercury, Venus, Earth and Mars), originated. Therefore, according to this line of reasoning, our star “lost” a fraction of its raw material in order to give rise to the planets that surround it. As a result, it ended up with a smaller amount of some metals relative to the usual pattern of these elements in stars.

If this hypothesis is correct, the best solar twin known may be home to a planetary system similar to ours. “We speculate that perhaps HIP 56948 has a planetary system that is a twin of that of the Sun,” says Meléndez. For the time being, the researchers have not found any giant gassy world, like Jupiter, in the closest orbits or in the so-called habitable zone around the star, the region in which, due to the local temperature conditions, life forms along the lines of those that exist on Earth might flourish, at least in theory. Data from American observatories (Keck in Hawaii and McDonald in Texas) were used in the search for new worlds around the star.

The news seems bad but is good. If there were an enormous gassy planet close to HIP 56948, the chance of a small rocky world such as the Earth existing there would be almost nil. Because of the gravitational interactions, large planets, when they are close to their star, tend to cause the destruction of smaller worlds, which are pushed out of the system or into the scalding interior of the bright celestial body. Therefore, not having found a hot Jupiter, as gassy worlds located in hot zones close to stars are called, was a relief for the researchers.

The method used to look for planets in the vicinity of the solar twin was radial velocity, the most traditional technique used for this since the mid-1990s, when the first extra-solar world was discovered. Since then, the majority of the almost 700 exoplanets known were identified using this technique. Radial velocity measures the gravitational effect exercised periodically by a planet when it passes very close to its star. Roughly speaking, the presence of the planet makes the star suffer oscillations or disturbances on its orbit. The bigger the world around, it the bigger the impact felt by the star. “With current instrumentation we would only be able to detect a planet 10 times bigger than the Earth,” comments Meléndez, who recently obtained the right to use the facilities of the European Southern Observatory (ESO) in Chile for 88 nights to observe solar twins.

The new measurements ratified the status of HIP 56948 as the known star that is most similar to the Sun. In one detail, however, 18 Scorpii, the star ‘dethroned’ by HIP 56948 five years ago from its condition of best solar twin, proves to be more similar to the Sun. “There’s no star that is a perfect clone of the Sun. Depending on the parameters used, one star or the other may be more like the Sun,” says astrophysicist José Dias do Nascimento Jr, from the Federal University of Rio Grande do Norte (UFRN), another specialist in solar twins. “If, for example, we basically take into account the characteristics of the magnetic field, 18 Scorpii is more like the Sun than HIP 56948.”

Our star has a more or less regular magnetic cycle. Every 11 years, the Sun enters a period of maximum activity, marked by the appearance of a larger number of spots on its surface, spectacular ejections of matter from its corona (the equivalent of its “atmosphere”) and a variety of explosions. The peaks of solar activity are so strong that they interfere with life on Earth. The weather may change and satellite communication and electricity grids may suffer interruptions. The magnetic cycle of 18 Scorpii, whose estimated age is 4.2 billion years – fairly close to that of the Sun – is around 7 years. “It’s still a notable solar twin,” says Gustavo Porto de Mello, from UFRJ.

We still do not know the energy activity pattern of HIP 56948, the studies of which started later. It is possible that its magnetic cycle has a periodicity of 5 to 10 years. “If magnetic activity on HIP 56948 is extremely strong, the chance of there being other planets with good conditions for life around the star is smaller,” says Nascimento Jr. Nevertheless, according to Meléndez, preliminary data suggest that this parameter of HIP 56948 is similar to that of the Sun. “Deep down, we’re trying to discover if stars that are very like the Sun tend to produce planetary systems like ours. If this relationship exists, finding solar twins may be a way of discovering planets similar to Earth,” says Porto de Mello.

The Project
Influence of the formation of planets on the chemical composition of solar-type stars (nº 2010/17510-3); Modality Regular Research Project Funding; Coordinator Jorge Meléndez – IAG-USP; Investment R$ 184,263.76 (FAPESP).

Scientific article
MELÉNDEZ, J. et al. The remarkable solar twin HIP 56948: a prime target in the quest for other Earths. Astronomy & Astrophysics. In press, 2012.