Getting the image of an exoplanet next to its host star would be like successfully photographing a seabird flying around a lighthouse several thousand kilometers away, according to our partner The Conversation.
However, we have succeeded in detecting exoplanets with properties similar to those of the Earth (size, mass, distance from its star) and new, more precise instruments are being developed.
The analysis of this phenomenon was carried out by Guillaume Hébrard, CNRS research director at the Institut d'Astrophysique de Paris.
Our star, the Sun, is accompanied by a procession of planets.
We are on one of them, the Earth, and from its surface we can observe the other planets of the Solar System in the sky.
The night sky is also filled with thousands of stars, whose fixed positions relative to each other draw constellations.
Our observations of the sky allowed us to understand that the Earth and the other planets were cold spheres orbiting the Sun, much larger and hot.
We also understood that the Sun was a star similar to the others;
it is simply its greater proximity that makes it appear very bright to us, compared to other stars which are considerably more distant and therefore much less bright.
Let's get out of the solar system
The Sun being a star similar to the others, a question naturally arises: are the other stars also accompanied by planets orbiting around them?
And if they exist, do these “exoplanets” or “extrasolar planets” have similar characteristics to those of the planets in the Solar System, or are they very different?
Are these possible extrasolar planets very abundant, or are they rare, with only a few stars hosting them?
And finally, if some exoplanets are similar to Earth, have they experienced the appearance and then the development of life forms on their surface?
As we can see, these questions are far-reaching, from a scientific, philosophical or societal point of view.
Asking them and trying to answer them is also the bearer of many emotions.
The problem of the uniqueness of our universe or the plurality of worlds has been regularly addressed, since Antiquity and throughout history.
From the Copernican revolution of heliocentrism in the 16th century, the planets of the Solar System are understood to be stars similar to the Earth, and therefore capable of constituting new worlds.
Giordano Bruno even goes so far as to state the existence of exoplanets around other stars, which are more inhabited.
Overview of the Solar System (the Sun, planets, dwarf planets and natural satellites are scaled for their relative sizes, not distances) © Beinahegut (vf: Charlestpt) / Wikimedia CC BY-SA 4.0
In the 17th century, notably with Johannes Kepler and Isaac Newton, knowledge was ripe to really tackle the question of exoplanets with a scientific approach: if they actually exist, we now know roughly what their movement should be around their host stars.
In fact, few astronomers have doubted their existence since that time;
the Sun being accompanied by a planetary system, it seems reasonable to think that many other stars are also, if not all.
Presumably, the exoplanets would eventually be detected one day.
Observe planets around other stars
But such an observation is arduous and represents a challenge, beyond the reach of telescopes for several centuries.
Indeed, as the possible exoplanets are smaller in size and much less massive than their host stars, their effects on the latter are tenuous.
And getting the image of an exoplanet directly next to its host star would be like successfully photographing a seabird flying around a lighthouse several thousand kilometers away.
With the improvement of instrumentation in astronomy, it was not until the end of the twentieth century that programs were undertaken which were likely to detect it, but these remained for a long time unsuccessful as the task was so difficult.
The first exoplanet was detected in the mid-1990s by Swiss astronomers Michel Mayor and Didier Queloz at the Observatoire de Haute-Provence, France.
The Observatory of Haute-Provence, in the South of France, from where Michel Mayor and Didier Queloz discovered the first exoplanet in 1995 © OHP-OSU Institut Pythéas / AMU / CNRS Photo library (via The Conversation)
Using and perfecting the so-called “radial velocity” method, they obtain this result by measuring very precisely the movement of a star and by interpreting its slight periodic variations as being caused by the presence of a planet. After having verified their analyzes several times, it is with great emotion that they present, during a conference in Florence on October 6, 1995, the discovery of the planet "51 Pegasi b". Their results were published the following month in the journal
Nature
.
This announcement generates upheaval and an emotion shared by the entire scientific community: if the existence of extrasolar planets was little contested, their discovery makes it concrete, finally gives the answer to this centuries-old question, and opens the way. to many other detections.
Observation programs such as theoretical studies devoted to exoplanets will multiply.
Exoplanetology will develop considerably from 1995, and hundreds of astronomers around the world are now devoting their research to it.
The Nobel Prize in physics that Michel Mayor and Didier Queloz received in 2019 for this discovery underlines the revolution it has constituted, for astrophysics and more generally for human knowledge.
Dider Queloz, Michel Mayor and their 2019 Nobel Prize in Physics co-winner, James E. Peebles © Jonas Ekstromer / AFP (via The Conversation)
The omnipresence and great diversity of planetary systems
Since then, exoplanets have been detected, characterized and studied by the thousands.
By comparing their properties with the predictions of theoretical models, we better understand how planetary systems form and evolve.
These detections also show the omnipresence of planets in our galaxy - it seems that most stars host them - and reveal a very large diversity of planets.
Indeed, if some are similar to those of the Solar System, many others show very different and sometimes surprising properties.
We can cite for example the "hot Jupiter", that is to say planets as large and massive as our Jupiter, but located so close to their star that they do not go around it in a few days.
51 Pegasi b belongs to this category.
Heated by their star, the atmospheres of these planets have temperatures above 1000 ° C.
There are also "super-Earths" or "mini-Neptunes", about two to three times the size of our planet.
Depending on their internal structure, they can resemble large terrestrial planets or small gas planets.
Completely absent around the Sun, they are nevertheless very abundant around other stars.
Artist's impression of 51 Pegasi B © Kirk39 / Wikimedia CC BY-SA
We can also study the atmospheres of certain exoplanets, for example measuring their thicknesses or their temperatures, identifying chemical species, detecting movements.
In addition, if some exoplanets seem for the moment to be alone in orbit around their star, many others are located in systems comprising several planets revolving around the same star, sometimes in orbits very close to each other. .
Some orbits are particularly eccentric or inclined, which is not the case for the planets of the Solar System.
These different configurations are probably the signature of several types of dynamic evolutions.
Towards other lands?
Finally, we begin to detect exoplanets with properties similar to those of the Earth, for example its size, its mass or the distance from its star.
New instruments are being developed to try to detect others and to study them more precisely by further improving the techniques currently used.
Methods allowing the detection of possible traces of life are beginning to be developed, in particular by the study of their atmospheres;
they should be implemented in the decades to come.
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Thus, we are currently living through a privileged, unprecedented and particularly moving period.
After centuries of waiting, we begin to have very concrete elements allowing us to compare the Earth and the Solar System with other planetary systems, and to know how they are unique… or very ordinary.
These questions are directly related to the place we and the Earth occupy in the universe.
Their answers have great implications from a scientific point of view, but also and beyond.
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This analysis was written by Guillaume Hébrard, CNRS research director at the Institut d'Astrophysique de Paris.
The original article was published on The Conversation website.
Declaration of interests
Guillaume Hébrard has received funding from public research organizations.
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