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100 billion planets in the Milky Way

Dr Amanda Doyle, Feature writer
Jan 14, 2012, 8:00 UTC

Sen—Using a phenomenon predicted by Albert Einstein, astronomers have come to the conclusion that there are more planets than stars in the Milky Way.

It took many years of dedication and patience before the first exoplanet around an ordinary star was discovered in 1995. Today, we have discovered over 700 planets and that number is rising rapidly. But it seems that this is only the tip of the iceberg as a new statistical analysis shows that there are in fact at least 100 billion planets in our Galaxy.

Most of the planets discovered to date were found by using the Doppler and transit methods. The Doppler method reveals giant planets close to a star by the way they make the star wobble, and the transit method uses a dip in the light received from a star to detect a planet passing in front of the star. Both of these methods are limited in that they mostly detect giant planets that orbit close to their parent star, although NASA’s Kepler mission is currently pushing these limits by coming ever closer to finding an Earth-type planet. But the planets used for this analysis were discovered using a method called microlensing, which has no such limits. Microlensing is sensitive to planets of various masses at various distances from a star.

Gravitational lensing was first predicted by Albert Einstein and it is based on the fact that light will bend around massive objects. The bending of light by the “lens” can create multiple or distorted images of the background object. Gravitational lensing is only evident when the observer, the lens and the source are perfectly lined up. If the lens is particularly massive, such as a galaxy cluster, then the background galaxies can appear in many places or be stretched into impressive arcs. However if the lens and the source objects are just stars, then no images can be seen and this is known as microlensing. The only thing we actually see from microlensing is an increase in light as a source star passes behind a lens star. The lens effectively magnifies light from the source star, and if there happens to be any planets orbiting the lens star then this will show up as a bump in the light curve. Microlensing events typically last for a few months, with the planet popping up on the light curve for only a few hours.

Gravitational microlensing infographic

Click on the above graphic or here for a larger version of the infographic

In order to detect microlensing events, collaborations such as the Optical Gravitational Lensing Experiment (OGLE) observe millions of stars every night. If a microlensing event is detected, then an alert is sent out which allows follow up networks such as the Probing Lensing Anomalies NETwork (PLANET) to observe the event in more detail. Microlensing events are incredibly rare, and finding a planet via this method is even rarer. As such, the data can be used to estimate the number of planets in the Galaxy.

“There is not only information in the detections of planets, but also in the non-detections,” explains Daniel Kubas, co-author of the Nature paper explaining the results. “If you assume a certain distribution or frequency of planets and know your detection sensitivity, you can predict how many planets you would detect during the experiment. While we did not know which planet frequency to assume in the first place, we tried in a statistical analysis many different possible planet frequencies and compared each possible planet frequency to our known detection sensitivity and the detections and non detections. This way we could constrain the unknown planet frequency to be able to conclude that on average each star has at least a planet.”

Their results show that an average of 1.6 planets, which are 5 times the mass of the Earth or greater, will exist in the region equivalent to the area between Venus and Saturn in our Solar System. And within an area spanning only 50 light years from Earth, there should be at least 1500 planets. This ultimately means that there are more planets than stars in the Galaxy, and giant terrestrial planets are much more common than hot Jupiters.

However Kubas is cautious and explains that a statistical analysis such as this can have uncertainties. It is possible that the discovery of planets with microlensing was just extremely fortunate. For example the odds of finding the planet OGLE-2005-BLG-390Lb were about 1 in 100 million. However, since thirteen planets have been discovered with microlensing it seems unlikely that these are all simply lucky coincidences, and a more likely explanation is that planets are common. “Using Ockham's razor given the choice between something very unlikely and a more likely reasonable alternative explanation, we go with the more likely explanation,” Kubas tells Sen.

So if there are billions of planets in the Galaxy, then what are the chances of any of them hosting life? “The large number of planets suggests that statistically we may expect a fraction of them to lie within the habitable zone,” says Arnaud Cassan, lead author of the paper. The habitable zone is the area surrounding a star where the temperatures are just right for liquid water to exist on the surface. However, just because a planet is in the habitable zone doesn’t mean that it is actually habitable or that it is home to life.

“The probability for having other civilisations is the product of the number of planets times the probability for a planet to host life, so to be sure life is abundant throughout the Galaxy, we need not only a great number of planets but also a high probability that planets can host life,” says Cassan. “We know that the latter probability is likely to be small...”