The exoplanet census is about to exceed 4000 celestial bodies, a little less than 24 years after the discovery of a first planet orbiting a star other than the Sun. But the quest for humanity to find a world that can accommodate conditions conducive to life remains intact. Point.
On October 6, 1995, Swiss scientists Michel Mayor and Didier Queloz of the Geneva Observatory announced the discovery of 51 Pegasi b, the first exoplanet detected to orbit a solar-type star.
Since then, astrophysicists have identified as many as 3926 exoplanets (source: NASA, March 24), and another 3402 are being confirmed.
The confirmed exosolar planets are in orbit around 2927 different stars.
Worlds to discover
To date, the few studied regions of the sky in search of exoplanets are in our vicinity of the Milky Way, the galaxy where the Sun is located.
On the galactic scale, this represents a small area, like a grain of sand on a beach.
Since it is estimated that at least one planet orbits around each star in the galaxy, that means there could be more than one trillion planets in our Milky Way, many of which could be the size of the Earth.
Research is therefore only in its infancy.
Detect an exoplanet
Several terrestrial telescopes (from ESO, NASA, ESA, etc.) and space observatories (Kepler, Hubble, CoRot, Spitzer) scan the skies for other worlds.
There are four main ways to detect planets around stars:
- The transit technique is by far the one that has confirmed its presence (77.4% of them) to date. It consists of detecting a drop in the brightness of a star when an object passes in front of it;
- The radial velocity method revealed 18.6%, the first of which was in 1995. It uses the Doppler-Fizeau effect to achieve this. The movement of a planet around a star induces a slight movement of the latter, which is detectable by this effect;
- The gravitational microlens technique accounts for 1.9% of discoveries. This effect occurs when the gravitational field of a star distorts the space-time, which deflects the light from a distant star behind, in the manner of a lens;
- Direct observation, extremely difficult, allowed 1.1% of observations. As they are small and dark, the planets are easily lost in the glare of the brilliant giant stars they orbit. However, thanks to current telescopes, there are particular circumstances in which a planet can be observed directly. Three Quebecers have also participated in the creation of the first direct image of exoplanets. They were named Radio-Canada’s 2008 Scientist of the Year for their achievement.
Gaseous planets
These celestial bodies are the easiest to observe because of their size. Their masses vary from 10 times those of the Earth to more than 50 times.
- Joviennes are mostly composed of hydrogen and helium, like the planet Jupiter of our system. 1214 of these gaseous giants have been detected to date. There are also “super Jupiter”, whose size is 2 to 3 times that of Jupiter.
- Neptunians, composed mainly of methane and water, are icy giants. They are classified according to their size, similar to that of the planet Neptune. No less than 1666 exoplanets discovered to date are in this category.
Rocky planets
These telluric planets, that is to say rocky, are the most likely to shelter life as we know it. There are two types :
- Earth-type, which equates to a dimension equivalent to that of the Earth. We found 156;
- “Super Earth” type, rocky planets of dimension equivalent to more than twice that of the Earth. The census is currently 878.
A new era is coming
The chances of life developing on the surface of a rocky exoplanet are closely related to the type and strength of light emitted by its host star, and the presence of water on the surface.
The combined presence of these two factors could lead, on the planets identified, to the discovery of elements constituting life.
The detection of this type of planet is about to receive a big boost with the arrival around 2021 of the successor of the Hubble Space Telescope.
The even more sophisticated instruments of the James Webb telescope will make it possible to identify systems whose planets will be even smaller, but also to better understand their composition.
Discoveries worthy of mention
Kepler 452b, detected in 2015 by NASA , is often presented as a twin of the Earth. Its diameter is 60% larger than that of the Earth.
Seven planets around Trappist-1 . The size of the Earth and moderate temperature, they revolve around a star located within 39 light-years. They were observed in 2017 . At least three of them even have conditions compatible with the presence of water on their surface.
Wolf 503b . Twice as large as Earth, this planet is located in the constellation of the Virgin. It is thus very close to its star, at a distance 10 times smaller than that between Mercury and the Sun. She was discovered by a master’s student at the University of Montreal’s Exoplanets Research Institute (iREx)and foreign colleagues. This exoplanet is located at a distance of about 145 light-years from Earth.
LHS 1140b. This super-Earth is located in the constellation of the Whale (The Sea Monster). His star is a red dwarf much smaller and much cooler than the sun . But the distance between the planet and its star is ten times smaller than that of the Earth and the Sun. It receives only half of the sunshine and occupies the center of the habitable zone. According to the scientists, it has probably retained a large part of its atmosphere.
Proxima B. It is the closest exoplanet to our system discovered to date. It is four light years from the solar system , 40 000 billion kilometers. A probe using current technology would still take thousands of years to achieve. It occupies a habitable zone around its star, and the estimated temperature on its surface allows to consider the presence of water.
