Jacques Laskar (born 28 April 1955 in

Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...

) is a French

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

. He is a research director at the

French National Centre for Scientific Research The French National Centre for Scientific Research (french: link=no, Centre national de la recherche scientifique, CNRS) is the French state research organisation and is the largest fundamental science agency in Europe. In 2016, it employed 31,637 ...

(CNRS), and a member of ''Astronomy and dynamical systems'' of the Institute of Celestial Mechanics and Computation of Ephemerides (French: IMCCE) of the

Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...

. He received the

CNRS Silver Medal The CNRS Silver Medal is a scientific award given every year to about fifteen researchers by the French National Centre for Scientific Research The French National Centre for Scientific Research (french: link=no, Centre national de la recherch ...

in 1994 and the

Milutin Milankovic Medal The Milutin Milankovic Medal is an annual award in Earth science presented by the European Geosciences Union (EGU). The award was introduced in 1993 by the European Geophysical Society (EGS). After a merger with the European Union of Geosciences ...

in 2019. Since 2003, he is a member of the

French Academy of Sciences The French Academy of Sciences (French: ''Académie des sciences'') is a learned society, founded in 1666 by Louis XIV of France, Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French Scientific me ...

Education and early teaching career

After attending the

École Normale Supérieure de Cachan École may refer to: * an elementary school in the French educational stages normally followed by secondary education Secondary education or post-primary education covers two phases on the International Standard Classification of Education scal ...

, Jacques Laskar taught secondary school from 1977 to 1980 and passed the aggregation in mathematics in 1981. He then studied astronomy and celestial mechanics, finishing his thesis in 1984. He became a CNRS researcher at the Bureau des Longitudes in 1985.

Research work

Stability of the Solar System

In 1989, Laskar provided evidence that the Solar System is

chaotic Chaotic was originally a Danish trading card game. It expanded to an online game in America which then became a television program based on the game. The program was able to be seen on 4Kids TV (Fox affiliates, nationwide), Jetix, The CW4Kid ...

instead of

quasi-periodic Quasiperiodicity is the property of a system that displays irregular periodicity. Periodic behavior is defined as recurring at regular intervals, such as "every 24 hours". Quasiperiodic behavior is a pattern of recurrence with a component of unpr ...

as originally determined by Laplace and Lagrange. More specifically, his estimate of the maximum Lyapunov exponent measuring the exponential divergence of two nearby orbits is

1/5 \; \text^

, meaning that it is possible to predict the trajectories of the Solar System over 10 Myr but fundamentally impossible over more than 100 Myr. This chaoticity comes mainly from the inner planets

Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

, the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, and

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

. In 2009, he and his colleague Mickaël Gastineau generated numerical simulations of orbital instability over the next five billion years. Their model, unlike those used by previous researchers, took into account Albert Einstein's theory of general relativity. This made little difference over a short time span, but resulted in dramatically different orbital paths over long times. The researchers looked at 2501 possible scenarios, 25 of which ended with a severely disrupted solar system.

Chaotic obliquity of the planets

Laskar also contributed to the study of the evolution of the skew planets of the solar system. One can for example include his work on

retrograde rotation Retrograde motion in astronomy is, in general, orbital or rotational motion of an object in the direction opposite the rotation of its primary, that is, the central object (right figure). It may also describe other motions such as precession o ...

of Venus. With his colleague Alexandre Correia, at Astronomie et Systemes Dynamiques of Paris, he found out that the atmosphere may simply have slowed the planet down and then started it spinning the other way. This insidious process would have been the unique result of the thick atmosphere always lagging behind as the planet rotates.

Paleoclimates

He has contributed to the astronomical theory of paleoclimates, studying the orbits of the planets and the obliquity of the solar system and relating it to the study of climate on geological time scales Astronomical study of paleoclimate Earth and Mars
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References

External links

Jacques Laskar's home page
on the site of the IMCCE {{DEFAULTSORT:Laskar, Jacques 20th-century French astronomers Members of the French Academy of Sciences 1955 births Living people Scientists from Paris 21st-century French astronomers Research directors of the French National Centre for Scientific Research