320 billion planets larger than Earth discovered

Jan 15, 2012, 11:43 IST | Dhananjay Khadilkar

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.
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
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.

Will it be fair to say that Earth may not be unique after all and that there could be numerous habitable planets in our neighbourhood?
In order to determine if life is unique to Earth, one needs to know two things: the number of planets and the probability of a given planet to develop life, whether the planet exists in the exact habitable zone. In case of Earth, you need very special circumstances. For example, if you move the Earth a bit further, the water and atmosphere would be gone. There is a probability for each planet to have these conditions. This probability is extremely small even if you have large number of planets.

What is the significance of this discovery?
We know that stars are everywhere. We can see them with our eyes. However, we cannot see the planets with our eyes. Earlier, people thought we were alone in our galaxy. The new picture now however is that there are as many planets as there are stars or even more.  So next time, when you look at any star in the night sky, be rest assured that it will have a super Earth planet around it!

Your study determines the presence of numerous super Earth planets. Were these planets orbiting the Sun like stars?
Every kind of star can pass in front of another star. The stars we study are the ones representative of the Milky Way. This means that the most abundant stars in our galaxy will be the most frequent to make the microlensing effect.  The most common stars in our galaxy are what we call Red Dwarfs, whose mass is 30 to 40 per cent that of the Sun. The Sun is a special star as it is less abundant.

Did you find any Earth-like planets?
No. The smallest planet detected using micro-lensing technique is three times the size of Earth. Of course, in the coming years, there is a possibility that we have microlensing observations from the Euclid satellite (European Space Agency mission which is expected to be launched in 2019). Maybe then, we will be able to look at smaller planets (size of Mars) in a statistical way. We will be able to refine our statistical derivations once we conduct the study from the space. The probablity of finding Earth like planets from space would still be small but that would be one of the goals.

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