Paris (AFP)

The discovery of the first exoplanet in 1995, which earned Tuesday the Nobel physics Michel Mayor and Didier Queloz, has decompartmentalized the solar system and opened the quest for life in the universe.

What is an exoplanet?

It is a planet that revolves around a star other than the sun, which is outside the solar system.

Before 1995, "other exoplanets had been discovered but they were around pulsars, which are dead stars", explains to AFP Vincent Coude Foresto, Observatory of Paris. But these discoveries had not had the same repercussions.

"Now, it is estimated that there are at least as many planets in the galaxy as there are stars, about 100 billion each!", Adds the astronomer.

51 Pegasi b, a first

Detected in 1995, 51 Pegasi b is a giant planet, the size of Jupiter, gas and boiling (there is about 1,200 degrees Celcius). It orbits around a solar-type star, about 51 light-years from Earth.

"She was totally weird, and not at all placed as one might think," recalls Didier Quelloz, the first to see it.

51 Pegasi b is close to her star - even closer than Mercury is to the sun - she goes around in a little over four days. A positioning never observed in our solar system.

"It was thought until then that for a giant planet to be created, it had to be cold, and therefore it is far from its star," says François Forget, a planet scientist at CNRS.

A surprise discovery

It is difficult to observe a planet near a star because of its high brightness.

Michel Mayor has developed a technique that allows not to see the planet, but to detect its presence via the disturbance that its gravity inflicts on the star.

This ultra-precise instrument, called Elodie, was able to detect the planet from the Haute-Provence Observatory of the CNRS in France. "The data collected told us a story that could only be that of a planet," said Didier Queloz, then a young researcher.

This discovery was a surprise, as the two men did not expect to do it so quickly. They saw themselves instead to observe for years "the slow movement of a massive planet away from its star," says François Forget.

The discovery was announced on October 6, 1995 in Florence, 24 years ago to the day.

A new field of research

This discovery "changed the way we saw our place in the universe," said David Clements of Imperial College London. And "ushered in a new era for cosmology," Judge Stephen Toope of Cambridge University.

51 Pegasi ba open the ball: Since then, nearly 4,000 exoplanets have been flushed out. "A sufficient number to make statistics, + + demography + on planetary systems," explains Vincent Coude of Foresto.

Before 51 Pegasi b, "we only had the example of our solar system, suddenly we enriched our zoo + of planets, as in medicine, it is good to observe other animals for better understand the human being, "analyzes François Forget.

Among these planets, about fifty are in the habitable zone of their star, that is to say just where the temperature allows the water to exist in the liquid state and where the life, such as knows it, could develop.

So it's "a step closer to the fascinating issue of detecting evidence of life," notes Martin Rees of Cambridge University.

© 2019 AFP