Chuck Bednar for redOrbit.com – @BednarChuck
Most stars in the Milky Way have between one and three planets in the habitable zone, the region where worlds can support liquid surface water, scientists from the Australian National University and the Niels Bohr Institute in Copenhagen report in a new study.
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By analyzing the thousands of exoplanets discovered in our galaxy thus far, the researchers have calculated the probability for the number of stars in the Milky Way that might have planets in the habitable zone. As they reported in the Monthly Notices of the Royal Astronomical Society, the calculations show that billions of those stars could have planets in the habitable zone.
Kepler is on point
Based on information collected by NASA’s Kepler satellite, astronomers have found about 1,000 planets around stars in the Milky Way, as well as approximately 3,000 other potential worlds, the study authors explained. Many of those stars have planetary systems of two to six planets found to date, but they might also have more planets that cannot be detected using Kepler.
Kepler is best suited for finding large planets that orbit relatively close to their stars, and since those that orbit at that distance would be too hot to support life, the researchers used a modified version of the Titius-Bode law: a 250-year-old method technique that uses numbers to show the relationships between planetary orbital periods and their distance from the Sun.
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The law, which correctly calculated the position of Uranus before it was discovered, states that there is a certain ratio between the orbital periods of planets in a solar system. The ratio between the orbital period of the first and second is the same that the ratio between the second and third and so on.
Knowing how long it takes for some planets to orbit around a star can allow scientists to figure out how long it takes for other planets to orbit, and can be used to calculate the position of those worlds in the planetary system It can also be used to detect “missing” planets 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 3 and 6 planets,” Steffen Kjær Jacobsen, a PhD student in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen, explained in a statement.
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“In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets,” added Jacobsen. “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 27 of the 151 planetary systems, observed planets did not initially fit the T-B law, prompting the researchers to place planets into the pattern for where missing worlds would be located. They added those planets between already identified ones, and added one additional planet beyond the outermost known one in each system to predict a total of 228 total planets in the 151 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,” Jacobsen said. “We have encouraged other researchers to look for these. If they are found, it is an indication that the theory stands up.”
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He and his colleagues evaluated the number of planets in the habitable zone based on the extra planets added to each of the 151 systems, and came up with between one and three planets in the habitable zone for each planetary system. They followed that up by taking an additional look at 31 planetary systems where planets had already been found in the habitable zone, or where just one additional 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,” Jacobsen said. “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.”
If the findings are then applied further outward into space, it would mean that there would be billions of stars with planets in the habitable zone in the Milky Way alone, he added. The study authors now hope that other scientists will review the Kepler data for the 40 planetary systems they predict should be well placed for observations conducted using the satellite.
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