In the multiple star system Kepler-444, there should be no planets at all. A close binary system of two dwarf stars came too close to the orange main star and once scattered the protoplanetary material from which planets could have formed. In fact, however, five companions orbit the 117 Kepler-444 – , light years from Earth, is twice as long as our own solar system has existed. Now, researchers led by Zhoujian Zhang from the University of California in a publication in the»Astronomical Journal», have examined the structure of this system in more detail to find the cause of the unexpected stability.
With its enormous age, the Kepler-444 star system is special even among the thousands of known systems with exoplanets.The system existed billions of years before our sun even formed. The five planets have existed there for just as long, all smaller than our Earth. They need three to ten days each for a round around the main star. The planets are not the star’s only companions: In addition, it is orbited by a close binary system of two red dwarf stars.
Zhoujian Zhang’s team has now remeasured this complicated system, concentrating primarily on determining the radial velocities of the three stars: The measurement provides information about how much the stars pull against each other due to gravity.
One would think that stable planetary orbits would not be possible in such a complex system with several gravitational sources. But it has been known for a while that there may still be exoplanets in it: they can orbit either all the stars of the system or only one star, as is the case with Kepler-444.Nevertheless, exoplanets in multiple star systems are exotic and precarious: While the planets in our solar system were able to develop undisturbed from the protoplanetary disk full of gas and dust, the stars in a multiple system can disturb such a disk of matter and thus limit or even stop planet formation.
And according to current knowledge, this should also have been the case with Kepler-444. Based on previous measurements, researchers assumed that the close binary system of the two dwarfs separated from the main star to 5 AU, five times the distance between the Earth and the Sun. But this should have led to the protoplanetary disk being cut off and not enough material available from which the five rocky planets could have formed.
With the help of the new values collected over a longer observation period, Zhang’s team showed why this did not happen.The orbit of the two dwarf stars around the center of mass with the main star is less elongated than previously thought – and the two dwarfs do not come as close to the main star as thought.
Zhang and his colleagues calculated that the double system only increased to about 23 AU. This means that it has the protoplanetary disk only from a distance of 8 AU truncated. Thus, with an approximate mass of the disk of about 500 Earth masses more than enough material left to form the five tiny rocky planets that orbit the main star to this day, eleven billion years later.
Source — https://www.spektrum.de/news/kepler-444-uraltes-planetensystem-trotzt-stoerenden-sternen/2115705