Billions of planets

Billions of Earths in our galaxy

In our solar system, the Sun has captured 99.86% of the total mass of dust and gas of the original nebula. Jupiter, the largest planet in the system, has captured 71% of the remaining mass. The other planets have shared the residue of the gravitational evolution, i.e. 0.038% of the total mass. Even capturing 99.86% of the material of the original nebula of gas and dust, a star leaves enough material for seven planets and billions of terrestrial objects (asteroids, comets, dwarf planets) are developing. There is therefore no wonder that there will be many more planets than stars in galaxies. For there is no planet in a solar system, the star must capture the entire field of the nebula, which is unlikely. Still had to find a way to prove it. In July 2011, more than 1,200 exoplanets had been discovered by conventional methods of oscillations or variation of brightness. A team of scientists at the European Southern Observatory (ESO), has used the technique of gravitational microlensing, to evaluate the average number of planets around a star. After six years of observation, the team concludes that there is at least one planet around a star. The result of this study was published in the journal Nature of January 12, 2012. The search for exoplanets is to identify the oscillations of stars, with identical characteristics to the Sun. This chase is limited mainly to the planets giants that orbit their stars and more stars are new discoveries with cyclical oscillations. Another characteristic of the presence of planets around a star is the variation of brightness of this star. When a planet passes in front of its star, the brightness of the star shows a small variation.

This variation can reveal the size and other characteristics of the planet. Both techniques involve the large planets or planets close to their star, or both. But small planets the size of the Earth are invisible. The method used by the international team of astronomers is new, because it detects that any type of planets. This method is based on gravitational lens (see image opposite). In astrophysics, an illusion, whitch astronomers are familiar with, is the gravitational lensing or gravitational mirage. A massive object, a cluster of galaxies, for example, which is between an observer and a distant light source, prints a strong curvature in Spacetime. This has the effect of deflecting all light rays passing near the object, thereby distorting the images received by the observer. This amplification of light, a distant celestial object by a massive star in front, was predicted by the theory of general relativity in 1917. The massive objects modify the geometry of space and time in their neighborhood. The light on the other hand always takes the shortest path, but in a curved space modified by the presence of a huge mass, the shortest path is not straight. The light path is bent in the vicinity of massive stars.
Arnaud Cassan of the Institut d'Astrophysique de Paris and author of the Nature paper, said: "We looked for evidence of the presence of extrasolar planets by the method of microlenses during six years of observations. The data we obtained show remarkably, that the planets are more common than the stars in our galaxy.
We also found that the less massive planets, like the Neptunes or superterres low-mass, are more common than more massive planets."

Image: This gravitational lens shows a strange blue objects stretched. They are spread in a circle in this picture, but are only multiple views of a single ring galaxy.
The singular form of the galaxy blue background (center of the cluster), allowed to infer that the same galaxy as we see on this image to 4 hours, 10 hours, 11 hours and 12 hours. This amplification of light, a distant celestial object by a massive star in front, was predicted by the theory of general relativity in 1917.

Statistical conclusion

To detect planets, astronomers have used the method of gravitational lens which amplifies the light from a star background. Indeed, the gravitational field of a system (star and its objects), acts as a magnifying glass, magnifying the light from a star behind. This increase in brightness varies when the system foreground has a planet or more.
The observation of these rare events of microlenses, could be implemented through a network of telescopes in the southern hemisphere, Australia, South Africa and Chile.
The monitoring program will detect planets located between 75 million km and 1.5 billion kilometers, weighing between five times the mass of the Earth and 10 times that of Jupiter. By observing thousands of stars, scientists increase their chance of detecting microlensing. Indeed it takes a lot of luck because it requires a particular combination.
We need to find two star systems, with a perfect alignment between the star background and the foreground star, which will serve as a magnifying glass.
Then it is necessary to detect the planet.

This requires that the plane of the orbit of the planet is so aligned, that is seen edge-on (see the picture against).
For all these reasons, scientists have been observed only three exoplanets during six years of intense observation.
Does that mean that astrophysicists have had a substantial chance or that the planets are so numerous that, despite the context, we could not, do not find it?
The statistical conclusion is that a star on six studied, hosts a planet with a mass similar to Jupiter, half of the stars have planets the mass of Neptune and two-thirds super-Earths.
"We used to think that the Earth must be unique in our Galaxy. But now it seems that there are literally billions of planets with a mass similar to Earth in orbit around stars in the Milky Way," concludes Daniel Kubas, principal co-author of this article.

Image: To detect the planet requires that the plane of the orbit of the planet is aligned with the star background.