Billions of stars in our galaxy have between one and three planets which could potentially support life . Astronomers came to the conclusion after calculating the chances of planets orbiting in the 'Goldilocks zone ' . This is the belt around a star where temperatures are ideal for liquid water to pool on a planet 's surface , and for alien life to exist . The illustration shows the habitable zone for different types of stars . The green area is the habitable zone ( HZ ) , where liquid water can exist on a planet ’ s surface . The red area is too hot for liquid water on the planetary surface and the blue area is too cold for liquid water on the planetary surface . 'According to the statistics and the 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 , ' said Steffen Kjaer Jacobsen , from the University of Copenhagen . The findings are based on data from the American space agency Nasa 's Kepler space telescope , which has confirmed the existence of around 1,000 planets orbiting stars in the Milky Way . It has also identified an estimated 3,000 other candidate worlds . Many of the stars have solar systems of two to six planets , but scientists believe there could be more that remain undiscovered by Kepler , which is best suited to finding large planets in relatively close orbits . To estimate the positions of the 'hidden ' planets , the researchers updated a 250-year-old mathematical concept called the Titius-Bode law . Pictured are exoplanetary systems where the previously known planets are marked with blue dots , while the red dots show the planets predicted by the Titius-Bode law on the composition of planetary systems . 124 planetary systems in the survey – based on data from the Kepler satellite , fit with this formula . The law , which inferred the existence of Uranus before it was seen , states that there is a certain ratio between the orbital periods of planets in a solar system . 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 . This means if you know how long it takes for some of the planets to orbit around a star , you can calculate how long it takes for the other planets to orbit and can calculate their position in the planetary system . You can also calculate if a planet is 'missing ' in the sequence . Because of this , if you know how long it takes some planets to circle a star , you can work out the time taken for other planets to orbit and fix their positions in the planetary system . You can also calculate if a planet is missing in the sequence . 'We decided to use this method to calculate the potential planetary positions in 151 planetary systems , where the Kepler satellite had found between three and six planets , ' said Mr Jacobsen . 'In 124 of the planetary systems , the Titius-Bode law fit with the position of the planets . 'Using T-B 's law we tried to predict where there could be more planets further out in the planetary systems . 'But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite . ' In this way , the scientists predicted a total of 228 planets in the 151 systems and concluded that each system should have an average of one to three planets in the habitable zone . Extrapolating further , this suggested that there could be billions of stars in the Milky Way with habitable zone planets where surface water and life could exist . The scientists , whose findings appear in the journal Monthly Notices of the Royal Astronomical Society , have made a shortlist of 77 'hidden ' planets . They say these planets have a good chance of being spotted as they pass in front of their parent stars . Light curves of the some of the planets orbiting the star Kepler-62 . The dip in the light curve occurs when the planet moves in front of the host star , thereby dimming the light of the star . The dip in the light curve is proportional to the size of the planet . The Kepler satellite observes exoplanets by measuring the light curve of a star . When a planet moves in front of the star there is a small dip in brightness . If this little dip in brightness occurs regularly , there might be a planet orbiting the star and obscuring its light .
Astronomers used an improved version of a 250-year old equation . It helped predict a total of 228 planets in the 151 planetary systems . They believe there are one to three planets in the habitable zone of each . Extending equation to the entire the Milky Way means there are more habitable worlds in existence than previously believed .