It seems that the answer to this question is rather elusive. About a week ago we said that after the fall of Pluto from the planetary pantheon, the answer was 8. Then we saw some more can be found planets orbiting strange and inhospitable objects, such as neutron stars, but we only found a dozen by this method .
But as I said, knowing the number of stars in the universe (a number) accounts simply do not exist. Indeed, we are already seeing more and more planets, which convinces us that are not so strange.
And how can we discover these extra solar planets? Because it’s one of the most active branches in current astrophysics, detection methods have multiplied in recent years. The following image, published by M. Perryman in 2000, shows one of the most commonly used methods:
The method of radial velocities is based on the oscillatory motion. This wobble of the star will be more noticeable the larger the mass of the planet in relation to the mass of the star, so this method is more efficient to detect large planets, several Jupiter masses. It is called radial velocity because thanks to the Doppler effect, one can measure when the star moves away or approaches us, measuring the frequency shift of a spectral line. That is, when the star approaches us (because the planet is pulling it toward us in its movement) the spectral line will run a little to the blue, while if the star is moving away, the line will run until the Red. By measuring the frequency shift and having an estimate of the mass of the star, one can limit the size of the planet and some orbital parameters. This all sounds we known, because the method is similar to how stars were discovered pulsars .
The other method mentioned is the method of transits, which measure the changes in brightness in a star as the planet orbits it passes in front of it (relative to us). These small “eclipses” produce variations of brightness tiny but measurable. As in the previous case, the high – mass planets tend to obscure more than smaller planets star, so the first will be easier to detect.
Today they have reached more than 750 planets detected by making use of all these methods, and the census can go at every day in the catalog of the Extrasolar Planets Encyclopaedia .
There is currently a great competition in terms observatories and instruments capable of detecting the highest number of extrasolar planets. Examples are space missions COROT (the European Space Agency ) and Kepler (from NASA ) using the transit method. On Earth, we have as an example the project SuperWASP (transits) in which participates actively IAC , and the instrument HARPS (radial velocities) of the European Southern Observatory (ESO), which runs on the 3.6m telescope at La Silla Observatory in Chile.
Finally, I cite the recent press release from the ESO, very relevant as to the question at hand:
“Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in their habitable zone, a zone that allows the existence of liquid water on the surface of the planet.” says Xavier Bonfils (IPAG, Observatoire des Sciences of the Universe of Grenoble, France), who leads the team. “Because red dwarfs are so common – there are about 160 billion in the Milky Way – this leads us to the conclusion that there are tens of billions of such planets in our galaxy alone.“
There is still much to learn and to say about this fascinating topic so much interest. But what is certain is that in the coming years will go knowing more about planets, the composition and conditions of their atmospheres, and if they meet the requirements to support life.