Since March 2009, the Kepler space probe traces the sky in search of other habitable planets. For four years, you will observe more than 150,000 stars. Beyond our Solar System, billions of planets orbit around other stars. These planets are called extrasolars or exoplanets. Of them, only a small part could harbor life.
Since NASA launched the Kepler Project, new planets are discovered every month. The planets are formed from the cloud of gas and dust that surrounds the stars when they are born. Since the 90s, we know that they are very common in the universe. But now the search focuses on planets with conditions similar to Earth. Planets that can develop life, or welcome us when the Earth dies.
Only those that orbit at an optimum distance from their star are considered potentially habitable planets. The habitable zone depends on the mass and luminosity of each star. The more mass a star has, the wider its habitable zone. But if it is too massive, it will deplete its fuel quickly and life will not have time to evolve. If it has less mass, its habitable zone is smaller but stable. Stars of very low mass have no strength to generate habitable areas. Ideally, stars of similar size to our Sun.
Rocky planets are sought, with moderate temperatures, capable of harboring liquid water and retaining an atmosphere capable of regenerating. Even meeting these conditions, it would be necessary to analyze its chemical composition and that of its atmosphere to know whether or not it is habitable. But, for now, our technology does not allow us to go that far. We have to settle for what we see.
The Kepler probe does not make direct observations. Unlike other telescopes such as Hubble, the Kepler does not take images. It captures the light emitted by the stars and elaborates its Doppler spectrogram. The stars emit wave frequencies, of light, that form their spectrogram. Small variations recorded in the spectrogram of a star can indicate the existence of a planet.
When a planet passes in front of the solar disk, it produces a small decrease in the luminosity of the star, which is reflected in its spectrogram. It also detects the small oscillations caused by gravity. Every body with mass emits gravity. A star, a planet, or even ourselves. The gravity of the star on the planet keeps it in orbit. The planet's gravity on its star is much less, but it produces a very slight oscillation of its mass, which is also reflected in its spectrogram.
Kepler's observations are directed to a specific region of the sky. Between the constellations of Lira and Swan. Most planets discovered so far are gas giants, such as Jupiter and much older. So far, the closest thing to a habitable planet is Gliese 581d. It has a mass seven times greater than Earth and orbits a red dwarf smaller than the Sun.
NASA has high expectations for the coming years, and believes that finding a twin planet to Earth is only a matter of time.
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