Astronomers conducted a kind of census of water on exoplanets and came to the conclusion that it is found in their atmosphere quite often, but at the same time it is extremely small. Only in one planet out of 19 investigated - hot Saturn WASP-39b from the constellation Virgo - there was a lot of water. The planetary scientists published their research in the Astrophysical Journal Letters.
"Given that the presence of water on Earth is considered one of the main reasons for life on it, we wanted to understand how much moisture is present on worlds outside our planetary system. We were very surprised to find out how little water is in the atmospheres of various types of exoplanets.", - commented on the study one of its authors, astrophysicist from the University of Cambridge Nikku Madusudan.
Over the past few years, astronomers have discovered more than a thousand exoplanets and several thousand possible candidates for this role. Most of them are among the so-called hot Jupiters, but scientists are increasingly finding smaller planets that are comparable in size to our world. The growing number of potential Earth twins makes one wonder how many of them have the conditions for the existence of life.
To answer this question, you need to know whether there is water in the atmosphere or on the surface of such worlds, as well as how much of it is on this or that planet. Modern telescopes cannot get detailed images of even the closest exoplanets, and therefore scientists estimate the amount of water on other planets in an indirect way, observing how light from stars travels through their atmospheres.
Water deserts of space
Madusudan and his colleagues combined all such measurements and tried to compare different types of exoplanets with each other, using data from the Hubble and Spitzer space observatories, as well as dozens of ground-based telescopes that observed a total of 19 exoplanets. These included both relatively small super-Earths, such as the planet K2-18b in the constellation Leo, and large gas giants that are twice the mass of Jupiter, WASP-33b in the constellation Andromeda.
An analysis of the spectra of the atmosphere of all these exomers unexpectedly showed that even if there was water in their atmospheres, its amount in all cases was extremely low, regardless of the type of planet, its size and position in orbit. The only exception was the hot Saturn WASP-39b in the constellation Virgo, in whose atmosphere planetary scientists have found record large amounts of water, ammonia and other volatile substances.
The lack of water on other gas giants surprised scientists, since theoretical calculations and observations of Jupiter show that there should be a lot of it in their atmosphere. These discrepancies, accordingly, may indicate that the planets of the solar system could have formed in unique conditions, not similar to those in which all worlds outside of it were born.
On the other hand, astronomers do not exclude the possibility that the discrepancies may be due to the fact that they studied only a small number of planets, many of which may be anomalies in themselves. Madusudan and his team hope that follow-up observations of exoplanets, as well as measurements from Jupiter's probe Juneau, will help test whether this is actually the case.