The atmosphere of terrestrial planets is sometimes not terrestrial. This already shows a comparison between Mars and Venus. While the Red Planet has only a very thin and cool atmosphere, the Venus is thick with hot temperatures. Little is known about the gas envelopes of the more than 4000 confirmed extrasolar planets. Only a few Jupiter-like planets have so far been able to detect an atmosphere. Now, for the first time, astronomers have found an atmosphere with an exoplanet that is significantly smaller than Jupiter. The find confirms the assumption that living conditions prevail in the distant world, for there is vaporous water in the atmosphere there.
It’s about the exoplanet K2-18b. As researchers around Angelos Tsiaras of University College in London write in the journal “Nature Astronomy” , he was already discovered in 2015 by the Kepler Space Telescope of NASA in about 110 light years away. In the two following years also observed the Hubble telescope two Vorübergänge the planet in front of his home star. During these transits, on the one hand the light of the star was partially blocked by the planet, on the other hand the remaining starlight was filtered through its gas envelope. The archived data was re-analyzed by the research team. They found the water vapor, and the measurements point to hydrogen and helium in the atmosphere.
No second earth
K2-18b is not a second earth, but about eight times as heavy. It falls into the weight category of the so-called super-earth, for which there is no equivalent among the eight planets of the sun. With a density of 3.3 grams per cubic centimeter (Earth: 5.5), it could have a rocky crust, much like a terrestrial planet. But it could also be a world of water dominated by a global ocean. Which scenario is more appropriate is unknown. Water can exist on the surface probably because the planet receives just enough heat from its native red dwarf star that the temperatures are potentially earth-like – at least mathematically K2-18b circles in the habitable zone of its star.
The co-author Giovanna Tinetti sees in K2-18 b therefore one of the most interesting planets for future studies. However, the researchers must confuse many ambiguities in the current data situation: For example, it is still unknown which other molecules exist in the air of the distant planet. Are nitrogen and methane also present, as the authors suspect? Besides, you do not know how much the sky is clouded by clouds on K2-18b. The percentage of atmospheric water can therefore be estimated only very roughly, depending on the assumptions used between 0.01 and 50 percent.
An exciting find
Kevin Heng from the University of Bern, who himself was not involved in the study, nevertheless welcomes it as a milestone. Similar investigations have already been carried out many times, but here for the first time a really clear signal was found for a super-earth. That was clearly an exciting find. For such analyzes are not exclusively about the chemistry of atmospheres. The long-term goal is to deduce a possible biological activity from certain substances that are classified as biomarkers. The expert, however, brakes the expectations: “We are not that far yet.”
In general, the characterization of extrasolar gas hoses is a very difficult business. In the case of Trappist 1, populated by seven exoplanets, the search for atmospheres has been unsuccessful. This is partly due to the fact that the central star of this planetary system glows only with 0.5 per thousand of the luminosity of the sun. The signal is therefore too weak for the previous telescopes.
That’s why the researchers opted for the next generation of space telescopes. These include the international James Webb telescope and the European Ariel mission; the expected start dates are 2021 and 2028. Already since April 2018, the NASA satellite Tess in space. He is to discover hundreds of other super-earths like K2-18b in the coming years. According to co-author Ingo Waldmann, it is likely that many potentially habitable planets will soon be found. Because unlike in our solar system, super-earths are the most common planets in the Milky Way.