In planetary systems formed by stars similar to the Sun, but which have severe dynamic processes that cause reconfigurations in their architecture, some planets may have been “devoured” by the host star.
An international team of astronomers — led by Lorenzo Spina, from the Istituto Nazionale di Astrofisica (INAF), Padua, Italy, and including Jorge Meléndez, from the Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo (IAG-USP) — studied the chemical composition of solar-type stars in more than one hundred binary systems in order to identify signatures of planets eventually “swallowed”. An article about it was published today (30) in the journal Nature Astronomy.
“In a binary system, the two stars are formed from the same material and therefore should be chemically identical. However, when a planet falls into a star, it is dissolved in the outermost region of the stellar interior, called the convective zone, and can change the composition of this region, increasing the content of chemical elements, called ‘refractory’, which are abundant on rocky planets. In the stars whose signatures indicate the engulfment of planets, larger amounts of lithium and iron are observed in relation to their twin companion star in the binary system”, says Meléndez to the FAPESP Agency.
According to the researcher, lithium is destroyed inside stars, but preserved in the material that makes up the planets. Therefore, an abnormally high abundance of this chemical element in a star may indicate that planetary material has been swallowed by it.
The study was based on observations of 31 binary pairs, therefore, of 62 stars, obtained with the HARPS spectrograph in the 3.6 meter telescope at the La Silla Observatory, operated by the European Southern Observatory (ESO). The data collected on site were complemented with previous results, already recorded in the specialized literature.
The La Silla Observatory is located in the Atacama Desert, in the Chilean Andes, in an extremely dry, lonely region, far from light pollution, which has one of the darkest night skies on Earth.
“This was the largest sample of similar stars in binary systems ever studied, and the results showed that at least a quarter of solar-type stars ‘devour’ their own planets. The finding suggests that a significant fraction of planetary systems had a very dynamic past — unlike our Solar System, which has preserved an orderly architecture,” says Meléndez.
According to the study coordinator, Lorenzo Spina, “the search for Earth-like planets is like looking for a ‘needle in a haystack’. However, this result opens up the possibility of using abundances of certain chemical elements to identify stars with a composition similar to that of the Sun”. Stars deficient in so-called refractory elements are more likely to host structures analogous to our Solar System.
A well-studied binary system is 16 Cygni, located at a distance of approximately 69 light-years from Earth. The system is made up of two yellow Sun-like dwarf stars, 16 Cygni A and 16 Cygni B. It may also include a red dwarf star. It is estimated that 16 Cygni A and 16 Cygni B are separated by a distance of 860 AU [sendo UA, a unidade astronômica, definida pela distância entre a Terra e o Sol]. For comparison purposes, the distance between the Sun and the so-called Heliopause, which constitutes the farthest boundary of the Solar System, is estimated to be between 110 and 160 AU. Despite the enormous distance separating the twin stars, the strongly eccentric orbit of a planet larger than Jupiter that orbits the star 16 Cygni B may be due to the gravitational perturbation produced by the star 16 Cygni A.
“It is interesting to note that component 16 Cygni A, which has no detected planets, is overabundant in refractory elements, which suggests that perhaps this star has already swallowed planets,” comments Meléndez.
The research was supported by FAPESP, through the thematic project “High-precision spectroscopy: from the first stars to the planets”, coordinated by Meléndez.