Is Our Planet Unique in the Universe? Two Experts Offer Diverging Insights

The response of Jean-Pierre Bibring, astrophysicist at the Institute of Spatial Astrophysics at Paris-Saclay University

As long as no observation could guide the answer, this question was a vision of dogmatic essence, without possible validation. Many theologies have made the earth a unique object, to result from a singular creation. Several thinkers, on the contrary, proposed a “plurality of the worlds”. Epicurus, in his letter to Herodotus dated 301 BCE, proposed that “it is not only the number of atoms, it is that of the worlds which is infinite in the universe. There is an infinite number of worlds similar to ours, and an infinite number of different worlds ”. Giordano Bruno did the same, in 1583, on the same basis of an infinite universe. Basically heretical, this thesis earned him a tragic end.

[Article issu de The Conversation, écrit par Jean-Pierre Bibring Professeur émerite de physique et astrophysicien à l’Institut d’astrophysique spatiale, Université Paris-Saclay et Michel Mayor Professeur émerite d’astrophysique, Université de Genève ]

Since then, physics has invited itself in the debate, by highlighting that of the same “laws”, universaloperate in the same way at all scales of the universe: the processes that have built the earth could perfectly – should even have operated elsewhere. In this sense, the earth cannot be unique. This was the situation at the dawn of the space era. The spatial exploration of the solar system, hoped to validate this principle, would lead to its deep questioning.

Spatial exploration rebates cards

One after the other, planets, comets and asteroids of the solar system were finely characterized on site. This resulted in an upheaval of our vision of the planetary worlds, on the contrary, highlighting their deep diversity as a new paradigm.

Simultaneously, initiated by the seminal discovery of Michel Mayor and Didier Queloz in 1995, the general existence “of exoplanets”, these planets which revolve around other stars than the sun, is accompanied by a major observation: the 'immense diversity “exoplanetary” systems themselves. Just as one cannot extrapolate the properties of the earth from the other planets of the solar system, one cannot generalize its properties to the other planetary systems of the galaxy.

Universal physical laws but extraordinarily varied circumstances

By a fertile coupling between observations and modeling, the processes responsible for the diversity of planets and exoplanets are beginning to be identified: planetary migrations, gravitational instabilities, giant collisions … These processes have a common point: while these are processes generics, in that they operate in a very wide variety of contexts, it is their form specificlinked to the context in which they arise, which plays the major role in the evolution of the objects concerned.

For example, the collisions were very numerous among the “protoplanetary” objects. The collision that affected the earth and gave birth to the Moon shaped a large number of terrestrial properties: particular tectonics, surface water, climate stability linked to that of the obliquity of the diurnal rotation axis compared to the plan of Annual revolution around the sun, etc. Barely different parameters (shock speed and geometry, impactor size, etc.) would have led to a completely different soil.

By such events, planetary developments, throughout their history, are made up of countless bifurcations whose exit paths are oriented, selected by the context. From this point of view, each situation is unique: there are never two strictly identical configurations. Departments, even tiny, will result in different developments. This is how the contingency Acts in the determinism of laws, to forge an immense variety of evolutionary paths; They banish any arrow, any meaning in evolution.

Are the earth-and life-are they unique in the universe?

The answer therefore depends on what the notion of “earth” covers. If we limit the characteristics of the earth to its size, its distance to the sun and to a few other global properties, there are certainly “exo-terros” quantity.

If on the other hand the earth is defined by an extended field of its properties, which result from very specific events, often unprodible (“hazards”), the probability that there are other lands in the universe, which is Finished both in space and time, falling vertiginously: each planet is unique in its properties, because it results from a fundamentally singular evolutionary sequence.

It would be one of the specific properties of the earth for having built, by countless reaction stages, a very singular cosmic chemistry: the alive. Singular, because from extraterrestrial grains-some properties of which have been synthesized in the particular environment of the proto-solar disc (which will form the sun and the planets)-immersed in primordial terrestrial oceans, rich in ingredients and special catalysts , then continuously adapted to the particular evolution of their environment. The living would be terrestrial in essence!

The response of Michel Mayor, astrophysicist at the University of Geneva, pioneer of the discovery of exoplanets, Nobel Prize in Physics 2019 for this discovery

The plurality of the worlds and the plurality of the inhabited worlds are questions already present in the time of Greek philosophers; Questions always current for the science of our time; Questions that have seen major paradigms changes, if only for a century.

Other planetary systems in the galaxy

It is interesting to note that during the first half of the twentieth century, astronomers published many studies claiming all that the solar system was unique among the hundreds of billions of stars in the Milky Way. They thought that only infinitely improbable circumstances made it possible to create a protoplanetary disk, that is to say a disc of dust and rotation gas which ended up becoming more likely.

In 1952, the American astronomer, emigrant of the Russian Empire, Otto Strive notes on the contrary that the stars comparable to our sun turned too quickly compared to what is expected, the stars forming from turbulent clouds gas which contracts by turning at a speed which should be transferred to the rotation of the star, but which seems missing. He concludes that the excess of “angular moment” is transferred to a protoplanetary disk. Dust and gas aggregate to form planets. In other words, Strive shows that planets are compulsory by-products in the training of the stars themselves. Planetary systems must therefore be innumerable in the galaxy.

At the end of the XX^e A century, the first planet detections around other stars than our sun (or “exoplanets”), then the first observations of planetary systems largely confirmed this postulate of Strive. Since then, systematic measures, both from the ground (thanks to Doppler spectroscopy) and from space (with the measurements of “planetary transits”, that is to say the fall of light when the planets pass in front of their stars ), indicate that almost all stars are surrounded by planetary systems.

Our land, is a rocky planet a rare, unique object?

These measures also show that rocky planets of masses comparable to our land are by far the most frequent – in accordance with the models of astrophysicists. No, our land is not unique as a rocky planet.

On the other hand, the diversity of planetary systems is astonishing, and this had not been anticipated before the discovery of the first exoplanets. It is also interesting to recall that it was admitted that the giant planets could only have orbital periods greater than 10 years, therefore sufficient orbital axes half-axis for ice grains can exist. The discoveries of 51 PEGASI B and the countless other planetary systems highlighted the phenomenon of orbital migration, responsible for many of the observed diversity of planetary systems. Orbital migration results from the interaction of the planet in training with the protoplanetary disc – a phenomenon which was not taken into account until then in the planetary training scenarios. It is a major change in our understanding of planetary training.

Habitable worlds: are there life elsewhere, on other rocky planets in the universe?

We know that life as it exists on earth needs water, so a first axis to answer this question is to look for water on other rocky planets. What is the order of magnitude of the number of rocky planets at a distance from its star that the surface temperature is compatible with the presence of liquid water on its surface? Today, estimates of the frequency of rocky planets in planetary systems are still very imprecise (a few percent of all planetary systems). If, pessimistic estimate, only 1 % of the systems shelter a rocky planet in the temperature zone where the complex chemistry caused life could take place, it is deduced from the order of the billion temperate rocky planets are present In the Milky Way – not to mention the whole universe, which contains hundreds of billions of galaxies.

Let us look at the diversity of environments present on earth, their variety of temperatures, pH, chemical composition, underwater volcanic sources, interfaces with different solids. Can we imagine the chemistry that takes place on these various environments, for hundreds of millions of years, out of the 510 million square kilometers of the earth's surface and the hundreds of millions of planets comparable to our land … and to affirm ” The living only exists on our earth ”? Far from having an answer to this question, I think that today, the look that we get to leave of the solar system alone Certainly not be able to resolve the question: “Is there life elsewhere in the universe?” »»

To date, faced with the enormous complexity of this problem, there is only a possible scientific response for me:

“Look for, look for if life exists outside our land, for example in the ocean of Europa depths, or in the atmospheres of rocky exoplanets called” lukewarm “. »»

Does life exist on other “worlds”? This is a superb question that thousands of researchers work today in multidisciplinary exobiology institutes. How many years will be necessary before science provides an answer to this question asked here are more than two millennia?