The planetary orbits of our solar system are ultimately chaotic, according to a French astronomer


While the term itself immediately conjures up the kind of alchemy at the heart of sci-fi’s best fantasy, celestial mechanics – the digital nuts and bolts of how planetary bodies move, orbit, and rotate over time. cosmic – is as old as astronomy itself.

In fact, celestial mechanics is the study of celestial motion; planets and minor bodies; their orbits, their rotation, their inclination and their evolution over time, Jacques Laskar, astronomer at the Paris Observatory, told me recently. And it all started with the end of Isaac Newton’s 17th birthdaye laws of motion of the century, he said.

Laskar uses both historical and current data to predict the movements of planetary bodies within our solar system. However, with current computer technology, celestial mechanics can be both innovative and revealing.

So, on a recent weeknight in Paris, I found Laskar in his fifth-floor office well after 5 p.m. conversing remotely with a colleague via computer in an office crammed with reference books, journals and even a collection of original multi-volume works by the famous French astronomer and celestial mechanic Urbain le Verrier.

“When I first started in Celestial Mechanics, it was really considered a very dusty subject,” Laskar said. “But I was really fascinated by this subject.” However, at the time, he says, the mainstream astronomical community seemed to have underestimated how much celestial mechanics can influence the physics of the solar system.

“But now they understand it’s not,” Laskar said.

We often think of our own inner solar system as a golden loop-shaped construct of pure serendipity, with largely circular orbits, and a natural stabilizing satellite for our own planet. And while this is largely true, Laskar’s four decades of calculations reveal that our own internal solar system is not immune to chaotic intangibles on time scales ranging from tens of thousands of years to hundreds of millions of years.

Even though the motions of our planets haven’t changed much in 3.5 billion years, by definition the inner solar system is still chaotic. That means you can’t predict the movements of planets in the past or the future beyond 60 million years, Laskar explains.

This is because in celestial mechanics, the orbital motions of the planets – and even minor bodies like the asteroids Ceres and Vesta – as they move around the sun have a dynamic effect on other planets in the solar system.

“It’s a chaotic system the same way time is chaotic,” Laskar said.

In my book “Distant Wanderers”, I note that if it had not been for the fortuitous formation of our moon due to the collision of an earth impactor the size of Mars about 4 billion years ago, the obliquity (or axial tilt) of our planet would be insanely chaotic. It would vary by up to 60% over a period of 2 million years.

In fact, Laskar and his colleagues have already found that on timescales of millions of years, the obliquity of Mars has deviated between zero and 60%. This is because Mars only has two tiny moons, Phobos and Deimos, and does not have the stabilizing influence of an abnormally large natural satellite like our Moon.

As Laskar told me in my book, “The Moon’s fortuitous influence on the Earth’s obliquity means that the likelihood of finding extra-solar planets with stable obliquities in habitable areas is probably less likely than to win the lottery ”.

Without the Moon, the tilt of the Earth would have been chaotic, Laskar says. This means that we are not in a generic situation, he says.

As for the ultimate stability of planetary orbits within our own inner solar system?

In a 2009 article in the journal Nature, Laskar and co-author Mickael Gastineau have run numerical models of some 2,500 orbits using current knowledge of the initial orbital parameters of our solar system. They found that in about one percent of their models, over a period of five billion years, Mercury’s orbit would become so unstable that the tiny planet would either collide with Venus or be engulfed by the Sun.

Is there any evidence of an additional planet in the inner solar system from 100 million years ago, asks Laskar? The one that could have been ejected from our solar system or the one that could have folded inward and been cannibalized by the Sun?

“I’m trying to prove that there was no extra planet 100 to 500 million years ago,” Laskar said. “It’s difficult because you need observations.”

And, of course, you can’t ask dinosaurs for their observations of celestial mechanics, Laskar said with a smile. So I am relying on geological data beyond 60 million years in the past to try to constrain the past evolution of the solar system, he says.

Laskar is using geological sediments here on Earth to search for signatures that reveal that the orbital motions of the planets we see now are roughly the same as they are today.

“I guess I won’t find an additional planet in the past, what I will instead prove is that the solar system was pretty much what it is now,” Laskar said.

By the end of the interview, it was well after dusk and Laskar led me into the main building of the original observatory and into a long, high-ceilinged room now known as the Meridian Room. This is where a longitudinal meridian line has been drawn across the floor of the observatory and the place remains a point of interest for tour groups.

So, in just a few meters, we were back from the present to a time when Newton and the celestial mechanics Verrier and Joseph-Louis Lagrange were desperately trying to model the motions of our own solar system. And in a way, not much has changed. We still struggle to understand the vagaries of how our internal solar system has changed over time.


Arline J. Mercier