Over six years of observations from several telescopes between 2002 and 2007 and a detailed statistical study made by the PLANET (Probing Lensing Anomalies NETwork) collaboration led to the conclusion that the Milky Way contains at least one hundred billion planets.
We’ve grown accustomed to reading news of the discovery of new planets in other solar systems, even planets that orbit two stars or ones of sizes similar to the Earth and Mars. The individual findings, however, tell us nothing about the number of planets across the whole galaxy.
The PLANET collaboration has used in its research the gravitational microlensing technique. When a star passes in front of another star, the gravity of the one in the foreground bends the light of the one in the background, acting as a lens magnifying the light from the star in the background. If the star in the foreground has a planetary system, the planets produce a further light amplification of the star in the background revealing their existence.
Because of the relative motions between the stars, an overlay can last about a month but the amplification caused by a planet may last from a few hours to a couple of days, depending on its motion in its orbit. The gravitational microlensing technique allows to measure the amplification of the light from the star in the background and from it to evaluate the planet’s mass. However, it’s impossible to determine whether the planet is rocky or gaseous.
Various investigation projects such as OGLE (Optical Gravitational Lensing Experiment) and MOA (Microlensing Observations in Astrophysics) examine millions of stars to find star overlapping events that allow the use of the microlensing technique. The follow-up PLANET collaboration keeps under observation selected candidates on an ongoing basis through an international team that uses a network of telescopes around the world.
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This team has been closely observing forty microlensing events and found evidence of planets in three cases. By performing a statistical analysis, the team estimated that one in six stars has a Jupiter-mass planet, half of the stars have a Neptune-mass planet and two-thirds of the stars have Earth-mass planets. In the Milky Way there could be about ten billion planets like the Earth.
The gravitational microlensing technique allows to find planets that have a distance from their star that goes from half of the Earth from the Sun to ten times of the Earth from the Sun. These are more or less the distances of Mercury and Saturn in our solar system, which means that other planets outside of that range may have not been detected.
In essence, according to this research the Milky Way contains at least one hundred billion planets but the actual number could be much higher. At this moment we have no idea how many of these planets might harbor life forms but investigations are becoming more and more sophisticated and their results are more and more accurate so we just have to wait until the next discovery. We’re still talking about the Milky Way, which in astronomical terms is our neighborhood in a universe composed of many billion galaxies.
