Binary stars are not very friendly objects, at least in terms of planet formation. Any object, which orbits a binary star, is subjected to complex and sometimes varying gravitational fields, which act as a destructive force capable of tearing anything it puts a grip on. However, planets may form even in this complex environment, and today there are 17 confirmed circumbinary exoplanets.
But let’s add a bit of additional complexity.
Stars in a binary system may form a common envelope at some moment. Mass transfer from one companion star to another is initiated during this event. The so-called donor star typically expands due to the processes of stellar degeneration, and its convective outer layers may be “stolen” by its counterpart more easily.
This event is highly unstable, as the mass transfer happens relatively fast, causing redistribution of angular momentum budget of the stellar system, and results in a formation of a close binary with very short orbital periods of just hours or days. Needless to say, this cataclysmic transformation profoundly impacts the entire space around a binary star – it may lead to the loss of the planets of even could be fatal to the closest orbiting objects.
NN Serpentis (NN Ser) is one of the most likely candidates of a binary star that has experienced the mutual mass transfer, according to a paper, which has been recently published at arXiv.org. There is also strong evidence that NN Ser contains a planetary system consisting of two massive Jovian planets, 7 and 2 times more massive compared to Jupiter, orbiting their host star at distances of 3 and 5 astronomical units, respectively. The authors of the paper point that only two distinct scenarios are possible: the planets formed together with their host star and survived the common-envelope event, or, alternatively, they formed after the common-envelope event, while possible earlier first-generation planets did not survive.
The scientists performed a computer simulation to analyze the likelihood of the second scenario and to determine the initial conditions, under which the potential first generation planets could have survived the transformation of the host star. The analysis of the resulting orbital parameters based on planetary stability criteria showed that the current planets evolved after the common-envelope event, i.e. were formed prior to the transformation.
The team also claims there is a very high probability that the planets around NN Ser have originated from the fraction of matter of the common stellar envelope that remained bound to the binary star. These planets could be the youngest known so far, and in the future studies they could also provide a lot of valuable insights into formation of the second-generation planets.
By Alius Noreika, Source: www.technology.org