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Starts With A Bang

Yes, two planets really can share the same orbit

Can two planets stably share the same orbit? Conventional wisdom says no, but a look at Saturn’s moons might tell a different story.
double planet illustration
The idea of two planets sharing the same orbit has been considered an unstable situation by many. However, our Solar System has an example in two of Saturn's moons that may prove this stable situation possible, after all.
Credit: ESA/Venus Express
Key Takeaways
  • Gravitational dynamics are a tricky situation, as over long timescales, the effects of other, nearby masses on an orbiting body’s stability might cause an ejection or collision.
  • We’ve seen the evidence for gravitational instabilities all across the Universe, from our Solar System to rogue exoplanets to hypervelocity stars and more.
  • But in our own Solar System, Saturn’s twin moons Janus and Epimetheus not only share the same orbit, but swap positions periodically. Stability might be truly possible.

Despite the dangers posed to planet Earth by a comet or asteroid strike, our Solar System is actually an incredibly stable place. All eight of our planets are expected to remain in their orbits, stably, for as long as the Sun remains a normal, main sequence star. Simulations, in fact, indicate only about a ~1% chance of any of our eight planets being ejected over the remaining 5-7 billion years of our Sun’s life. But this isn’t necessarily the case for all stellar systems, as instabilities can often lead to planetary ejections.

If two planets pass closely by one another in orbit, one can perturb the other, resulting in a massive orbital change. These two planets could collide, one of them could get ejected, or one could even get hurled into their central star. But there’s another possibility: these two planets could successfully share a single orbit together, remaining in orbit around their parent star indefinitely. It might seem counterintuitive, but our Solar System offers a clue to how this could occur.

solar system planets
Of the eight planets in our Solar System, the four gas giant worlds are the least dense, with less than half the density of the least dense rocky planet (Mars), and with Saturn being even less dense than water.
Credit: NASA/Lunar and Planetary Institute

According to the International Astronomical Union (IAU), there are three things an orbiting body needs to do in order to be a planet:

  1. It needs to be in hydrostatic equilibrium or have enough gravity to pull it into a spheroidal shape. (In other words, a perfect sphere, plus whatever rotational and other effects distort it.)
  2. It needs to orbit the Sun and not any other body (e.g., it cannot orbit another planet).
  3. And it needs to clear its orbit of any planetesimals, proto-planets, or planetary competitors.

This last definition, strictly speaking, rules out two planets sharing the same orbit, since the orbit wouldn’t be considered “cleared” if there were two of them.

two planets exoplanet system illustration
In principle, even two gas giant planets that were in orbit around the same star would not be considered planets if they shared an orbit. The IAU definition is inadequate in many ways, even for planetary and exoplanetary astronomers.
Credit: ESA/Hubble & NASA

Fortunately, we’re not bound by the IAU’s questionable definition in considering co-orbiting planets. We can instead choose to worry about whether it would be possible to have two Earth-like planets that share the same orbit around their star. The big worry, of course, is gravitation.

Gravitation is capable of ruining a dual orbit in one of the two ways we imagined earlier:

  1. a gravitational interaction can “kick” one of the planets very hard, either sending it into the sun or out of the solar system,
  2. or the mutual gravitational attraction of the two planets can cause them to merge, resulting in a spectacular collision.

In simulations that we run to model the formations of solar systems from proto-planetary disks, both of these effects are seen extremely frequently.

earth moon synestia formation giant impact
A synestia will consist of a mixture of vaporized material from both the larger mass planet/protoplanet and the smaller impactor, which will form one or more large moons inside of it from the coalescence of moonlets. This is a general scenario capable of creating one single, large moon with the physical and chemical properties we observe Earth’s moon to have, multiple moons like those found around Mars or Pluto, or more complex systems around higher-massed worlds.
Credit: S.J. Lock et al., J. Geophys. Research, 2018

This latter case is, in fact, something that may have happened to Earth when the Solar System was only a few tens of millions of years old! There was definitely a collision, some 4.5 billion years ago, that resulted in the formation of our modern Earth-Moon system. In addition, it very likely caused a major resurfacing event on our planet; even the oldest rocks we find on Earth are not as old as the oldest meteorites — likely arising from the primitive asteroid belt — that we’ve discovered.

Two planets don’t do a great job of occupying the same exact orbit, however, because there’s no such thing as true stability in these cases. The best you can do is hope for a quasi-stable orbit. In this context, quasi-stable means that technically, on infinitely long timescales, everything is unstable, and these planets will play a game of Thunderdome: where at most one will remain.

lagrange points
A contour plot of the effective potential of the Earth-Sun system. Objects can be in a stable, lunar-like orbit around the Earth or a quasi-stable orbit leading-or-trailing (or alternating between both) the Earth. The L1, L2, and L3 points are points of unstable equilibrium, but an object in orbit around the L4 or L5 point can remain stable for indefinitely long periods of time.
Credit: NASA

However, you can obtain configurations that will sustain themselves billions of years before one of those two “bad” events occurs. To understand how, you need to take a look at the above diagram, and in particular at the five labeled (in green) points: Lagrange points.

If you only considered two masses — the Sun and a single planet — there are five specific points where the gravitational effects of the Sun and the planet cancel out, and all three bodies move in a stable orbit forever. Unfortunately, only two of these Lagrange points, L4 and L5, are stable; anything that starts out at the other three (L1, L2, or L3) will unstably move away, winding up either colliding with the main planet or getting ejected.

Earth and 3753 Cruithne in orbit
The orbits of Cruithne and Earth over the course of a year. Cruithne’s location is indicated by the red box as it is too small to be seen at this distance. Earth is the white dot moving along the blue circle. The yellow circle in the center is our Sun. Although 3753 Cruithne isn’t exactly stable, it has remained in an apparent orbit around one of Earth’s Lagrange points (from our perspective) for hundreds of years, and will remain for hundreds more.
Credit: Jecowa/Wikimedia Commons

But L4 and L5 are the points around which asteroids collect. The gas giant worlds all have thousands, but even Earth has one: the asteroid 3753 Cruithne, which is presently in a quasi-stable orbit with our world!

Although this asteroid in particular isn’t stable on billion-year timescales, it is definitely possible for two planets to share an orbit just like this. It’s also possible to have a binary planet, which would be a lot like the Earth/Moon system (or the Pluto/Charon system), except with no clear “winner” as to who’s the planet and who’s the moon. If you had a system where two planets were comparable in mass/size, and only separated by a short distance, you could have what’s known as either a binary or double planet system. Recent studies indicate that this is legitimately possible.

ALMA PDS 70 system
This annotated image of ALMA’s view of the system PDS 70 showcases the central star, the two known planets, the outer protoplanetary disk, and also a possible co-orbiting companion of the innermore planet, PDS 70b.
Credit: ALMA (ESO/NAOJ/NRAO) /Balsalobre-Ruza et al.; Annotations: E. Siegel

But there’s one more way to do it, and this is something you might not have thought was stable: you can have two planets of comparable mass in two separate orbits, one interior to the other, where the orbits swap periodically as the inner world overtakes the outer world. You might think this is crazy, but our Solar System has an example where this happens: two of Saturn’s Moons, Epimetheus and Janus.

Every four years, whichever moon is interior (closer to Saturn) comes to overtake the exterior one, and their mutual gravitational pull causes the inner moon to move outward, while the outer moon moves inward, and they swap.

janus epimetheus orbit swap saturn
The physics of how Janus and Epimetheus swap orbits can be explained by simple gravitational dynamics of two low-mass objects in orbit around a much higher-mass object. Mutual gravitational interactions can exist in a quasi-stable fashion like this, creating orbits that are stable for billions of years or longer.
Credit: Emily Lakdawalla, 2006

Over the past three decades, we’ve observed these two moons dance quite a bit, with the configurations repeating themselves without perceptible changes over an eight year period. As far as we can tell, this configuration is not just stable on human timescales, but ought to be stable over the lifetime of our Solar System.

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Resonances appear in many different ways in planetary dynamics, including the way that Neptune affects the distribution of Kuiper belt objects, the way that Jupiter’s moons Io, Europa, and Ganymede obey a simple 1:2:4 orbital pattern, and in how Mercury’s rotational speed and orbital motion obey a 3:2 resonance.

a graph showing the frequency of a sound wave.
Janus and Epimetheus are two moons of Saturn that share the same orbit via orbit swapping. Because of the differences in mass between them, Janus’ orbit varies by approximately three times as much in its semimajor axis as Epimetheus’ orbit. These two moons swap positions every four years, but appear to have never collided.
Credit: NASA/JPL/David Seal

It’s no surprise that planetary orbits might also obey an orbit-swapping resonance, with Janus and Epimethius providing a spectacular example. You might object that these are moons around a planet, not planets around a star, but gravity is gravity, mass is mass, and orbits are orbits. The exact magnitude is the only difference, while the dynamics can be extremely similar.

Considering that we now know of exoplanetary systems that exist in great abundance around M-class, red dwarf stars, and that they appear analogous to either the Jovian or Saturnian systems, in other words, it’s totally conceivable that we’d have a planetary system somewhere in our galaxy with two planets (rather than moons) that do exactly this!

jupiter solar system trappist-1 exoplanets
TRAPPIST-1 system compared to the inner planets of the solar system and the moons of Jupiter. Although it might seem arbitrary how these objects are classified, there are definitive links between the formation and evolutionary history of all of these bodies and the physical properties that they have today. The solar systems around red dwarf stars appear to be just scaled-up analogues of either Jupiter or Saturn.
Credit: NASA/JPL-Caltech/R. Hurt, T. Pyle (IPAC)

The unfortunate news, at least for now, is that out of the thousands of discovered planets around other stars, we don’t have any binary planet candidates yet. There was one candidate that was announced in the early days of the Kepler mission, but it was retracted, as one of the co-orbiting planetary candidates was discovered to actually have twice the period of the main planet. But absence of evidence is not evidence of absence. These co-orbiting planets may be rare, but with more and better data, we fully expect to find them.

Give us a better planet-finding telescope, a million stars with planets around them, and about 10 years of observing time. With facilities like those, we’d likely find examples of all three possible examples of planet-sharing orbits. The laws of gravity and our simulations tell us they ought to be there. Perhaps a young version of this has been found around the star PDS 70, but the mature examples remain elusive. The only step left is to find them.

Ethan Siegel is on vacation this week. Please enjoy this article from the Starts With A Bang archives!


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