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Hard and Lifeless ASTRONOMY y Hard and lifeless Even though they resemble Earth, the crusts of some rocky exoplanets may be too rigid to be habitable Igor Zolnerkevic PUBLISHED IN AprIL 2017 he illustration on these pages was the first rocky exoplanet discovered international team of researchers, the as- shows how one artist imag- in the habitable zone of its star. Inspired tronomer Kátia Cunha from the National ined the exoplanet Kepler-186f by this discovery, the artwork shows the Observatory in Rio de Janeiro and her Tin April 2014. At that time, as- surface on Kepler-186f resembling that doctoral student Diogo Souto conducted tronomers confirmed that the mass and of Earth, with continents and oceans, the first detailed analysis of the chemi- size of this planet 500 light years from creating a landscape conducive to the cal composition of the star Kepler-186. the solar system was similar to that of development of life forms similar to Their study was published in February Earth. Observations made by the Kepler those of our planet. of this year in the Astrophysical Journal space telescope also suggested that the However, new observations of its star and presents the chemical analysis of distance between this planet and its star, suggest that the surface of Kepler-186f another red dwarf, Kepler-138, which is the red dwarf Kepler-186, would allow may be very different to and much less orbited by the smallest rocky exoplanet liquid water to exist. For this reason, favorable for the existence of life, at least, discovered so far, with a body the size of researchers announced that Kepler-186f to life as we know it. Working with an Mars. This was the first time that astron- 52 z FEBRUARY 2018 Artist’s conception of the exoplanet Kepler-186f, the first to be discovered of a size similar to Earth omers were able to measure the chemi- ecules. When these stars are observed contains more silicon than the Sun. This cal abundances of red dwarf stars with in the visible light range, titanium oxide excess silicon would cause the planets accuracies resembling those possible masks the presence of several chemical around the red dwarf to be made of rocks when observing stars similar to the Sun. elements. Even so, Souto showed that, so hard that they would prevent the for- The analysis of the light emitted by a by using infrared, it is possible to iden- mation of tectonic plates in their crusts. star, which is known as the star's spec- tify and measure the abundances of 13 Without tectonic plates, there would trum, provides a general understanding chemical elements in red dwarf stars. be no processes to recycle gases, liquids E L of the abundances of the chemical ele- Souto and Cunha used data obtained and rocks, which, on Earth, over billions Y ments that comprise it. However, Souto from APOGEE, a high-precision spec- of years, have determined the chemi- H /T. P H /T. explains that the temperatures in the trograph installed on a telescope in New cal compositions of the atmosphere, tec L A -C atmospheres of red dwarf stars are low Mexico, to estimate the concentrations the continents and the oceans. With- L enough to allow the formation of water, of different chemical elements in the out oceans or continents constantly al- NASA /JP titanium oxide and vanadium oxide mol- two stars and concluded that Kepler-186f tered by the movement of tectonic plates, PESQUISA FAPESP z 53 Kepler-186f would have a relatively un- changing and possibly desert surface. The other red dwarf, Kepler-138, showed silicon levels similar to those of the Sun, so its small rocky exoplanet would have a composition that could favor the formation of tectonic plates. Even so, it is too close to the star to have liquid water on its surface. “Studies like this are of great impor- tance for the astronomy of exoplanets,” says Souto. “A future NASA mission, TESS, will look especially at low-mass stars, which are the most abundant in the galaxy, and allow detailed study of their chemical composition, which is important for understanding the prop- erties of their exoplanets.” IRON, OXYGEN, AND OCeaNS The conclusions made by Souto, Cunha and their collaborators about the surface of Kepler-186f result from the applica- tion of a mathematical model developed in 2016 by the American geophysicists Cayman Unterborn of Arizona State University and Wendy Panero of Ohio State University. The model uses astro- nomical observations of the chemical composition of a star to estimate the mineral compositions of rocky planets formed around it. “The composition of the star serves as a reference for the pos- sible compositions of its planets,” says Unterborn. Artistic Unterborn and Panero based their representation model on what astronomers and geo- of the red dwarf star Kepler-138 physicists know about the composition and its rocky of the Sun and the formation of the so- exoplanet, lar system. The planets formed from a which is smaller disk of gases and dust made of the same than Mars primordial material that produced the Sun. A series of collisions that occurred over hundreds of millions of years be- thick mantle; this, in turn, is covered by tween the materials in this disk caused If part of the a thin crust that forms the planet's sur- the specks of dust to stick together, form- face. “In the mantle, oxygen reacts with ing increasingly larger bodies until they crust is denser the iron and creates oxides that are too formed the rocky planets. It is this pro- than the material light to sink to the center of the planet,” cess of planetary formation that the explains Unterborn. “Instead, these ox- American geophysicists' model simulates in the mantle, ides remain in the mantle and affect the in a simplified manner to calculate the compositions of minerals.” The quantity mineral compositions of exoplanets from the planet of oxygen also controls the presence of the chemical makeups of their stars. water in the mantle and the chance that One of the model’s main conclusions can develop a the planet will have oceans. is that the abundance of the element dynamic of While the core is made almost entire- oxygen in this protoplanetary disk can ly of iron, the mantle and the crust are restrict the sizes of the cores of these tectonic plates composed of minerals containing a va- planets. A rocky planet like Earth has riety of elements but are mostly silicon. an iron core, which is surrounded by a “In estimating the chemical composi- 54 z FEBRUARY 2018 tion of the silicon-rich portion of the its planet Kepler-407b has dimensions planet outside the core, we can model similar to Earth and is expected to have a the process that heats up the rocks in The chemistry crust rich in diopside and a mantle with the mantle and forms the crust,” adds an abundance of garnets, minerals that Unterborn. “If part of the crust is com- of the planets are tougher and denser than olivines, posed of a material that is denser than which are the most abundant on Earth. the mantle beneath it, the planet can The concentration of the This combination, according to Unter- develop a dynamic of tectonic plates.” element silicon changes born, would prevent the formation of To test this model against astronomi- the stiffness of the crust tectonic plates. cal observations, Unterborn and Pane- Because of these denser minerals, ro have been working since the middle The images below show the Kepler-407b may have a greater mass of last year with Johanna Teske of the possible mineral compositions of than Earth, although its radius would Carnegie Institute and other astrono- the planets formed around the be similar. Future observations of the mers using the APOGEE spectrograph stars Kepler-102 and Kepler-407. planet made with a new generation of mounted on the Sloan Foundation tele- The silicon concentration of more powerful telescopes could confirm scope at the Apache Point Observatory Kepler-102 is similar to that of the or disprove this prediction. “For now, the in New Mexico. APOGEE has already sun but that of Kepler-407 is uncertainties in the measurements are analyzed the spectral lines in the infra- higher. Kepler-102‘s planets are still significant,” says Unterborn. red frequency range of approximately rich in olivine and could have plate As the work by Souto, Cunha, and 200,000 stars in the Milky Way. The tectonics like those of Earth; their colleagues has shown, the model by main objective is to use the chemical Kepler‘s-407‘s planets have more Unterborn and Panero can also be used compositions of these stars to under- garnets and diopsides and to estimate the compositions of exoplan- stand the history of the galaxy’s forma- probably have a rigid crust ets orbiting other types of stars, not just tion (see Pesquisa FAPESP, issue No. 232). yellow dwarfs like the Sun. Most im- portant of all are the red dwarfs, which Some of the stars APOGEE has ob- KEPLER-102 served were also the target of the Kepler represent 70% of the stars in the Milky Olivine space telescope, which was designed to Way; yellow dwarfs make up 7% to 8% look for signs of exoplanets in variations Wadsleyite of the stars in the galaxy. Because they of the brightnesses of their stars, and are Garnet and are small, less than half the size of our now being investigated by Teske and her ringwoodite Sun, red dwarfs facilitate the detection colleagues.
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