Observations of already discovered exoplanets helped scientists to find out that the probability of the formation of large and small planets in different stars is not accidental and depends on how many elements are heavier than hydrogen and helium, the star contains, says an article published in the journal Astronomy & Astrophysics.
“After examining already published data on exoplanets, we found that giant planets like Jupiter and its“big brothers”, whose mass is thousands of times greater than that of the Earth, form in completely different conditions than small Earth twins and small gas giants. There are two distinct populations of planets,”says Vardan Adibekyan from the University of Porto (Portugal).
Over the past ten years, astronomers have discovered thousands of planets outside the solar system, some of which are similar in size to Earth, while others are smaller or larger copies of Jupiter. Now planetary scientists are actively working on studying their atmosphere to assess the likelihood of life on them and to understand the history of the formation of the planet.
Adibekyan and his colleagues uncovered an unusual pattern in the formation of both celestial bodies by analyzing the properties of more than 3.5 thousand exoplanets discovered in recent years by the Kepler orbiting telescope, the HARPS ground-based instrument and a number of other observatories.
Studying the properties of gas giants, not inferior in size to Jupiter and much larger than it, they noticed that the probability of discovering large and small gas giants in the vicinity of this or that star was not accidental.
In particular, large "cousins" of the largest planet in the solar system, whose mass exceeded Jupiter's by about four times, were most often found in stars, in whose depths astronomical "metals", elements heavier than hydrogen and helium, were almost absent. As a rule, the “cleaner” a star was, the greater the chance of discovering really large gas giants, and the larger their average sizes were.
Likewise, small planets like Jupiter and smaller gas giants have sprung up in the vicinity of stars whose bowels contain as much or more "metals" as the Sun.
Similar, but less significant connections existed between the orbits of the planets and the chemical composition of stars - on average, the less "metals" a star contained, the farther the planets were removed from it. As scientists admit, this trend was rather weak and has yet to be confirmed by observations of a larger set of planets and stars.
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Such a clear distinction in the masses of large planets, according to scientists, suggests that they are formed according to two different scenarios. Small planets, similar in mass and size to Jupiter, arose as a result of the accumulation of gas on the surface of a rocky "embryo" similar in size and density to the Earth, and large gas giants - as a result of the direct gravitational collapse of a part of the gas in the protoplanetary disk due to the appearance it contains areas with abnormally high density.
All of this, as Adibekyan and colleagues note, may indicate why Earth's "cousins" and other small planets are more likely to form in the vicinity of red dwarfs and other small stars, whose bowels usually contain more "metals" than large stars. If astronomers can confirm this, then they will have a new way to limit the number of stars in which to expect the discovery of full-fledged Earth twins.