Researchers from the Australian National University and the Niels Bohr Institute in Copenhagen have calculated the percentage of stars in the Milky Way that could host planets in the habitable zone. The results show that billions of stars may have from one to three planets in the habitable zone Thanks to the data collected by the satellite Kepler, astronomers have discovered thousands of extrasolar planetary systems in our galaxy, many of whom have more planets orbiting a single host star. Analyzing these planetary systems, researchers at the Australian National University and the Niels Bohr Institute in Copenhagen have calculated the percentage of stars in the Milky Way that might host planets in the habitable zone. The results show that billions of stars may have from one to three planets in the habitable zone, or where you can find liquid water. This work was published in the journal Monthly Notices of the Royal Astronomical Society.
Credit: NASA/Kepler Mission/Dana Berry
The planets that orbit close to their stars have temperatures too high to support life, so find out if within those systems we can find planets in the habitable zone, a group of researchers at the Australian National University and the Niels Bohr Institute of University of Copenhagen has made calculations based on a revised version of a 250 years ago method: the law of Titius-Bode. The Titius-Bode law dates back to around 1770 and allowed to calculate the position of Uranus before it was discovered. The law states that there is a precise relationship between the orbital periods of the planets in a planetary system. Or the ratio between the orbital period of the first and second planet is equal to the ratio between the second and the third planet and so on. Therefore, knowing how long it takes to some planets to orbit its star, it is possible to calculate the orbits of the other planets in the planetary system. We can also determine whether a planet is ‘missing’ in the sequence. “We decided to use this method to calculate the potential positions of the planets in 151 systems in which the satellite Kepler had already found 3 to 6 planets. In 124 of these planetary systems, the law of Titius-Bode provides correct results for the position of the planets, and so we tried to use it to predict where it might be other planets not yet observed. For this study we selected only those systems where there is a good chance of being able to observe the planets that are missing with the Kepler satellite,” says Steffen Kjær Jacobsen, PhD student in the research group of Astrophysics and Planetary Science at the Niels Bohr Institute of Copenhagen University and co-author of the study. In 27 of the 151 planetary systems studied, the planets that were observed did not fit the Titius-Bode. The researchers then tried to place the planets in position predicted by the model, and then added the planets that seemed to be missing between the planets already known. In this manner, we have predicted a total of 228 planets in 151 systems. “We then selected 77 planets in 40 planetary systems, as were those with the highest probability to transit in front of its host star, and so that can be seen by Kepler. We have also encouraged other researchers to look for these planets. If we find them, it would be an indication that the theory works, “says Steffen Kjær Jacobsen. The planets that orbit very close to a star are too hot. In contrast, the planets are too far away from the host star would be too cold. The area of intermediate space, where there is the possibility of the presence of liquid water, is not located at a fixed distance. The habitable zone of a planetary system varies depending on the size and brightness of the host star. The researchers estimated the number of planets in the habitable zone on the basis of the planets added using the Titius-Bode law, obtaining a value of 1-3 for each of the 151 planets systems studied. From these 151, we are now proceeding to a further control of the planetary systems 31 within which the planets were already present in the area or space where it is needed the addition of one more planet. If we extend these calculations to larger regions, this result indicates that only in our galaxy there could be billions of stars with planets in the habitable zone. Steffen Kjær Jacobsen explains that the next step will encourage other researchers to analyze the Kepler data again for 40 planetary systems that seem well positioned for observations with the satellite Kepler.