Rick Robinson

Feb 3rd 2023

What Happens When a Planet’s Orbit Goes Chaotic?


Can a planet fall out of orbit? Could Earth fall out of orbit and hurtle into the sun, taking us with it? The short and practical answer is no, which should come as good news to anyone worried about this happening after seeing scary headlines in Business Insider or even Smithsonian Magazine.

In short, a planet cannot “fall out of orbit” because it is already falling. A planet’s orbit is a ballistic trajectory, mathematically equivalent in principle to the arc of a thrown ball or of a diver jumping off a high board. The planet doesn’t “fall” in the ordinary sense because its forward motion — much faster than a ball or high diver — carries it so far that, even though its trajectory curves downward, it ends up overshooting and curving all the way around the parent star instead of plunging into it.

You can thank Sir Isaac Newton for the insight that orbiting planets are governed by the same physics as falling apples. But can anything go wrong with a planet’s orbit? If so, what are the consequences, and is Earth in danger? Let’s find out.

Orbital Decay

Orbiting satellites and spacecraft are subject to gradual orbital decay, eventually falling out of orbit to a fiery reentry, either burning up in the atmosphere or crashing to Earth in the process. Thus the International Space Station (ISS) has to be re-boosted every so often to maintain its orbit.

But this is because the ISS and most other satellites are orbiting within the tenuous outer reaches of Earth’s atmosphere, and air friction gradually slows them down. Interplanetary gas is even more tenuous than the outer reaches of the atmosphere, and planets are enormously more massive than spacecraft, so friction is not a problem for them.

That said, a planet’s orbit is not perfectly stable, and there are circumstances in which a planet’s orbit might be so perturbed as to send it in a swan dive into its parent star or, alternatively, hurled into interstellar space to become a “rogue” planet, which describes as a world not orbiting any star. However, these planetary catastrophes take many millions of years to unfold, and there’s no indication that Earth is even slightly at risk.

The Three-Body Problem

If the only astronomical bodies in the universe were one planet and one star (say, Earth and the sun), their gravitational interactions would be entirely straightforward and predictable, and we would not have to worry about orbital chaos even in theory. But in real life, the solar system has eight major planets, along with a dozen or so dwarf planets, many large moons and countless lesser bodies. All of them are influenced by each other’s gravitation, and while the individual effects are mostly tiny, they can add up.

In the 19th century, measured anomalies (or unexpected variations) in the motion of Uranus were enough for mathematicians to successfully predict the existence of Neptune orbiting beyond it. Similar efforts to find a “Planet X” beyond Neptune have never panned out, though by lucky accident, one such attempt led to the discovery of Pluto. What makes the gravitational interactions of three or more bodies (such as a star and two planets) interesting — and potentially alarming — is that, when you add a third body to a gravitational interaction, the result is mathematically chaotic. In orbital mechanics, this challenge is known as the three-body problem.

Not even in theory is there a once-and-done formula for predicting their future motions. They interact with each other too much — each planet’s orbit is being slightly and constantly perturbed by the others. Their future motions can be simulated by a computer model, but the simulation must be performed step by step, with the precision of every step limited by measurement and approximation errors.

An Exoplanet Surprise

While the chaotic potential of the three-body problem was well understood generations ago, for most of the 20th century, it was regarded by astronomers and planetary scientists as a purely theoretical consideration. They developed a theory of the solar system’s evolution in which the planets initially formed in roughly the same orbits where we still find them.

In particular, the sun’s “snow line” — per NASA, this is the distance at which ice grains orbiting the young sun would remain as solid grains rather than boiling away into gas — lined up well with the distinction between the solar system’s small, rocky inner planets (such as Earth) and the much larger outer planets with their enormous atmospheres composed largely of light elements.

Water ice is believed to play a key role in the accumulation of these massive atmospheres, allowing giant planets to form. The fact that the solar system’s giant planets are all beyond the snow line suggested to 20th-century scientists that, while planetary orbits might be chaotic in principle, they were probably much less so in practice. Then, in 1995, a planet was discovered orbiting a nearby sun-like star, 51 Pegasi. According to NASA, it made a complete hash of the accepted ideas about planetary orbits. 51 Pegasi b, as the planet is officially called, is a “hot Jupiter” that’s even larger in size (though with less mass) than Jupiter, yet it orbits its parent star much closer than Mercury orbits the sun.

It seemed impossible for such a large planet made up of mostly volatile “ices” to have formed so close to its star. The theoreticians soon reported that it didn’t need to form there — it could have originated much farther out, at a distance roughly comparable to Jupiter’s. It subsequently “migrated” inward due to gravitation interaction, not with another fully formed planet but with the inner portions of the protoplanetary disk of gas and dust from which the giant planet itself had formed and which had not yet dissipated into space.

The orbit of a planet, it turns out, can change dramatically, far more than anyone had previously imagined.

World in Chaos

Since 1995, the number of known extrasolar planets has grown into the thousands, including hundreds of multi-planet systems cataloged in the Extrasolar Planets Encyclopaedia. Since extrasolar planets are still difficult to detect, we do not have a complete picture of any of these systems. But we can see enough to know that many of them must have undergone chaotic shuffling of planetary orbits.

Thus, a planet’s orbit can undergo dramatic, chaotic change. The past history of many of the planetary systems we observe today proves this is no longer just a theory. But can a planet fall out of orbit? Put in those simple terms, the answer is still no. But a planet’s orbit can drastically change in unpredictable ways by gravitational interaction with other planets or even passing stars. The good news, for us, is that these chaotic orbital changes do not happen abruptly or out of nowhere.

It takes a lot of chaos to make a planet’s orbit undergo dramatic changes, and so long as planets are orbiting far from each other with only subtle mutual interference, the potential for chaos can only build up gradually. The current orbits of solar system planets are not perfectly stable, but they are highly stable, undergoing only tiny changes even in the course of millennia. It would take millions or billions of years for orbital instabilities to build up to the point where drastic changes in orbits could occur.

A Rogue Attack

There is one other possible scenario: a rogue planet from interstellar space passing through the solar system. If the planet is large enough and passes close enough to Earth, it could indeed drastically change Earth’s orbit with catastrophic results.

In this case, mere decades might pass between the initial detection of the approaching rogue world and the final catastrophic pas de deux. The good news is that such close approaches are really, really rare. There are plenty of other things you should probably worry about more.

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