Are we alone in the Universe? Though we are no closer to finding the answer, a recently conducted study could be an important step in that direction. On January 12, Nature magazine published findings of a group of researchers, led by Arnaud Cassan of the Paris Institute of Astrophysics, which said that our galaxy, the Milky Way, is teeming with hundreds of billions of planets. In an exclusive interview at the Institute of Astrophysics, Professor Cassan tells Dhananjay Khadilkar the significance and implications of these startling results

Could you tell us about your findings?
Our statistical derivation is based on six years of micro-lensing observations conducted between 2002 and 2007. We found that for the total number of stars in the Milky Way, the average number of planets per star is 1.6. In terms of numbers, our galaxy has 200 billion stars. According to our statistical derivations, there should be 320 billion planets. Even if you consider the margin of error, there are still around 160 billion planets. One important thing to be noted is that the planets that we studied were all present in the distances between 0.5 AU to 10 AU from the star. (1 AU or Astronautical Unit is the distance between the Sun and the Earth). The Kepler observatory is used to study planets that are well within the Mercury orbit. Our method is complementary to the findings of Kepler observatory and to another microlensing result that took into account planetary bodies that exist beyond 10 Astronomical Units, some of which were not even bound to stars.
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What is the microlensing technique?
The micro-lensing effect is based on the deflection of light by general relativity. There are many stars in the centre of the Milky Way. From time to time, some stars pass in front of them. When there is an alignment between the background star, foreground star and the observer, you have a microlensing effect which is the brightening of the flux of the background star, observed as a bell-shaped curve. When the passing star has a planetary system, then you observe an asymmetric side curve along with the smooth bell-shaped curve. With this one can determine the presence of a planetary system. We used a network of telescopes in the southern hemisphere from Chile, South Africa and Australia. We observed the centre of Milky Way 24 hours a day. (The centre of Milky Way, which has the largest density of stars, can be seen only from the southern hemisphere)


Arnaud Cassan of the Paris Institute of Astrophysics
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Were you surprised by these findings?
We didn't expect this result. It was a pleasant surprise. Our goal was to gather as much information about planets as possible. The larger the perimeter of space you observe, the more information you get. What was known before was that smaller mass planets are numerous. The problem is that it is very hard to detect them. What we have done in the study is to take a representative sample of stars in the Milky Way and statistically derive the proportion of small, medium and large planets in our galaxy. What we found was that 2/3rd of the stars in our galaxy have super Earth planets (5 to 10 times the size of Earth). Half the stars have Neptune like planets and 1/6th of the stars have big planets the size of Saturn or Jupiter. Some of these planets are 10 times the size of Jupiter.