After analysing satellite data, an international team of researchers have calculated the probability for the number of stars in the Milky Way that might have planets in the habitable zone.
Their calculations, published in the journal Monthly Notices of the Royal Astronomical Society, show that billions of stars in the Milky Way will have one to three planets in the habitable zone, where there is a potential for liquid water and where life could exist.
The team from the Australian National University and the Niels Bohr Institute at the University of Copenhagen made calculations based on a new version of a 250-year-old method called the Titius-Bode law.
The Titius-Bode law was formulated around 1770 and correctly calculated the position of Uranus before it was even discovered.
The law states that there is a certain ratio between the orbital periods of planets in a solar system.
So the ratio between the orbital period of the first and second planet is the same as the ratio between the second and the third planet and so on.
If you then take the calculations further out into space, it would mean that just in our galaxy, the Milky Way, there could be billions of stars with planets in the habitable zone, where there could be liquid water and where life could exist, the team wrote.
"We decided to use this method to calculate the potential planetary positions in 151 planetary systems, where NASA's Kepler satellite had found between three and six planets.
"In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets," explained Steffen Kjaer Jacobsen, PhD student at the Niels Bohr Institute.
In this way, the team predicted a total of 228 planets in the 151 planetary systems.
"We then made a priority list with 77 planets in 40 planetary systems to focus on because they have a high probability of making a transit, so you can see them with Kepler. We have encouraged other researchers to look for these. If they are found, it is an indication that the theory stands up," Jacobsen noted.
Out of the 151 planetary systems, they made an additional check on 31 planetary systems where they had already found planets in the habitable zone or where only a single extra planet was needed to meet the requirements.
"In these 31 planetary systems that were close to the habitable zone, our calculations showed that there was an average of two planets in the habitable zone. According to the statistics and indications we have, a good share of the planets in the habitable zone will be solid planets where there might be liquid water and where life could exist," explains Steffen Kjaer Jacobsen.
Using NASA's Kepler satellite, astronomers have found about 1,000 planets around stars in the Milky Way and they have also found about 3,000 other potential planets.
Many of the stars have planetary systems with two-to-six planets, but the stars could very well have more planets than those observable with the Kepler satellite, which is best suited for finding large planets that orbit relatively close to their stars.