Astronomers from the United States, France and Scotland, thanks to observations with the Hubble, confirmed the presence of water in the atmosphere of exoplanet HAT-P-26b. In addition, the researchers estimated the proportion of elements heavier than hydrogen in the planet's gas envelope - it turned out that it is small, which is outside the previously discovered pattern. According to the authors, most likely the atmosphere of HAT-P-26b has remained practically unchanged since the planet's origin. The research is published in the journal Science.
The vast majority of known exoplanets were discovered in one of two ways: by the transit method (when the planet slightly dims the star by passing between it and the terrestrial observer) or by Doppler analysis (when the planet's gravity causes the star to wobble slightly back and forth). With their help, you can determine some parameters of the planet's orbit, make restrictions on its size or mass. However, it is impossible to find out what this or that exoplanet consists of using Doppler methods.
Transits of the planet allow you to explore its gas shell, if it is large enough. At the moment when the planet begins to move in front of the disk of the star, part of the light of the latter passes through its gas envelope. Depending on what gases the atmosphere is made of, parts of the star's spectrum begin to be absorbed. For example, water and carbon dioxide have characteristic absorption bands - they lie in the infrared region of the spectrum. By comparing the spectrum of a star during and before transit, astronomers can determine exactly in which spectral ranges the exoplanet's atmosphere is absorbing and make predictions about its composition.
For the first time, water traces in HAT-P-26b were recorded in 2015 - using the combined data of the Spitzer telescope and ground-based observations. This exoplanet is located about 430 light years from Earth and is a "hot Neptune", the equilibrium temperature on the surface of which is about 1000 Kelvin (730 degrees Celsius). Due to the small acceleration of gravity, a celestial body can have a dense and high atmosphere. The planet orbits the star of the system - an orange dwarf - in about 4.2 days.
Hannah R. Wakeford et al. / Science, 2017
In the new work, the authors expanded the spectrum of observations of the exoplanet and used the Hubble Space Telescope to observe transits in the visible and near infrared ranges. This made it possible to see additional absorption bands, which reliably indicated the presence of water in the planet's atmosphere. It should be noted that HAT-P-26b cannot be called a water world due to the planet's too high temperature.
In addition to observing water in the atmosphere, astronomers were able to estimate the metallicity of the gas giant's shell. This is the relative proportion of elements heavier than helium in the composition of the object. Based on observations in the solar system and in a number of exoplanets (WASP-43b and HAT-P-11b), astronomers have noticed a pattern - with an increase in the size of the planet, metallicity decreases. In other words, the proportion of heavy elements in Jupiter is much less than in Uranus or Neptune. This observation became the basis for some hypotheses about the evolution of the planets. HAT-P-26b is out of this pattern: with sizes comparable to Neptune, its metallicity is about the same as that of Jupiter.
According to astronomers, the differences between HAT-P-26b and other planets with known metallicity and mass may mean that the process of its evolution, for some reason, differed from the generally accepted one. The authors point out that, most likely, the gas envelope of HAT-P-26b is the same as in the first periods of the exoplanet's existence. Moreover, the exoplanet probably did not collide with other planetesimals and most of its heavy elements are concentrated in the core.
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The authors note that this is a unique situation when an exoplanet has been studied in such detail. In itself, such a result is already an important achievement.
Vladimir Korolev