Could a stellar flyby save Earth from impending doom?

Life on Earth is slowly sliding towards a cliff that leads to cosmic oblivion. Could an astronomical event rescue life on our planet?

If Earth was just 5% closer to the Sun, the Sun would be 10% brighter that it is today. That may not seem like much, but our atmosphere could no longer maintain a stable energy balance. The greenhouse effect would enter a runaway phase, that would no longer be offset by radiative cooling. Earth’s surface would get hotter, causing more water to evaporate into the atmosphere. Water is a strong greenhouse gas, so this would further strengthen the greenhouse effect, which would further heat the planet’s surface. And so on, until the surface was hotter than a pizza oven. 

We’re lucky that Earth isn’t 5% closer to the Sun.  The bad thing is, the Sun is slowly getting brighter. When the Sun reached the main sequence more than 4 billion years ago, it was about 75% as bright as today (this is the so-called faint young Sun paradox).  Looking to the future, the Sun will keep getting brighter.  This is an inevitable consequence of the nuclear reactions in its core that convert hydrogen to helium.

^{Sun luminosity image: this website. Greenhouse image from climate catastrophe post in the How Planets Die series.}

As the Sun brightens, Earth is being slowly pushed to the limit.  I wrote in the end of the Solar System that you can think in terms of the habitable zone — the belt around a star inside which a planet can maintain liquid water, provided it has an appropriate atmosphere. Inside the inner edge of the habitable zone, a planet’s atmosphere will undergo runaway greenhouse heating. Right now, the inner edge of the Sun’s habitable zone is at about 95% of the Earth-Sun distance.

With the brightening Sun, that inner edge of the habitable zone is slowly marching outward. The inner edge of the habitable zone will cross Earth’s orbit in about a billion years (give or take a few hundred million years).  From that point on, there won’t be any more liquid water on Earth. Game over.

But is there anything that might happen in the next billion years that could save the Earth?

Maybe.

Let’s zoom out.  The Sun lives in a ho-hum part of our Galaxy, between two spiral arms.  It’s not the most exciting place, Galactically-speaking. Still, other stars are constantly zooming by.  They almost never come close to the Sun, but sometimes they fly through the Oort cloud (the comet cloud that extends more than a light-year from the Sun).

Every billion years there is a 1% chance that a star will pass within 100 astronomical units of the Sun. Remember, 1 astronomical unit – or au – is the Earth-Sun distance, so a star flying within 100 au has a chance of having an effect on the planets’ orbits. A billion years is the time that Earth has left on its doomsday clock (at least the astronomical one), so this means that there’s a 1% chance that a star will flyby and shake things up before it’s too late.

The question is: what happens when a star flies within 100 astronomical units of the Sun?

I needed help finding an answer, so I recruited three great scientists: Nate Kaib (who knows a ton about comet orbits and stars flying past the Sun), Franck Selsis (a man of many crazy ideas), and Hervé Bouy (of free-floating planet fame).

We decided to to use computer simulations to explore different possible futures of the Solar System. I started with all eight planets on their current orbits, then added a star flying by within 100 astronomical units of the Sun.  I followed the evolution of the system for another 20 million years, using an N-body code, accounting for general relativity and the galactic tide (which allows objects to be captured in the Oort cloud).

I followed through with that plan over the course of 2023. (Personal note: this was one of the funnest projects I’ve ever done — and that’s saying a lot, because I do a lot of fun research projects).

Now, let me show you some possible future trajectories of the Solar System, drawn from the 12,000 simulations that I ran.

I’ll show each example in animated gif form, to make it easy to understand. Even though I’m only showing a simple gif, each simulation was run in three dimensions and the star flew through the Solar System from all angles (entering from above or below the plane of the planets’ orbits). The flyby stars were not yellow cartoons; rather, they came in all masses, from tiny stars of just 5% the mass of the Sun to stars several times more massive than the Sun. Their velocities were what you would expect from stars in the Sun’s local Galactic neighborhood. If you want more details on the setup or the orbital evolution of different systems, you can download our paper here.

I’ll start with examples in which the planets were only weakly perturbed, and ramp it up from there.

First, here is a simulation in which a relatively distant flyby (with a close approach well beyond Neptune’s orbit) did not have a measurable influence on the planets’ orbits.

This simulation, while kind of boring, is pretty characteristic. There is a higher than 90% probability that a star flying within 100 astronomical units of the Sun will have little to no effect on the planets’ orbits. (We will come back to probabilities later).

Let’s get to some more dynamic examples.

Here is a case in which the stellar flyby mainly affected the outer Solar System, by giving strong gravitational kicks to each of the ice giants, stripping them from the Sun and ejecting them into interstellar space (to live out their days as free-floating planets).

Most of the time, the giant planets are the first ones to be perturbed by the passing star. This makes intuitive sense — the star is coming from outside, so the outer planets are closest to the line of fire (although it depends on the exact alignment of the planets when the star passes through, and the geometry of its flyby). Yet the giant planets are way more massive than the rocky planets, and, when their orbits change, they gravitationally transmit this perturbation inward. That process typically takes 100,000 to a million years.

Here is an example in which the giant planets’ orbits were shaken up by the passing star, causing the rocky planets’ orbits to also be shaken up (but not stirred). In this case, Mercury ended up colliding with the Sun, and Venus and Mars bashed into each other.

If a flyby star shakes up the giant planets’ orbits strongly enough, it will trigger a dynamical instability. This is when the planets’ orbits cross paths with their neighbors’. This leads to gravitational encounters between the planets (sometimes called planet-planet scattering), and usually with the ejection of one or more planets.

Here is an example in which a dynamical instability was triggered, and Earth was captured on an orbit in the Oort cloud, on the edge of interstellar space. After being scattered by Neptune, Earth was on such a wide orbit that it felt the force of the Galaxy’s gravitational field (specifically, the difference in gravity arising from the fact that the Galaxy’s mass distribution is not perfectly smooth). Earth was trapped on such a wide orbit that it took more than a million years to complete a single loop around the Sun.

In my simulations, Uranus or Neptune were the planets most likely to be scattered into the Oort cloud. (See also my post on Oort cloud planets).

Some stellar encounters trigger system-wide dynamical instabilities. When that happens, all of the planets’ orbits are affected and the outcome can be disastrous for the Solar System. The rocky planets end up as innocent bystanders, often driven into the Sun or ejected by the giant planets’ (especially Jupiter’s) wild orbital swings (see here for more on the destructive power of Jupiters).

In the next example, the flyby star set off a system-wide instability that led to the ejection of most of the planets (apart from Mercury, which collided with the Sun). When the dust settled, Jupiter was the only surviving planet. Yet Jupiter’s orbit was stretched-out (or eccentric), a scar from this violent event.

Among our 12,000 simulations, more than 300 finished with a single planet orbiting the Sun. There were single-planet Solar Systems in which each planet was the lone survivor. (Most often it was Jupiter).

The orbits of all of these systems are shown in the following image — the x axis is the average orbital distance and the y axis is the eccentricity, a measure of how stretched-out each surviving planet’s orbit was. (The habitable zone is shifted outward at high eccentricity because the total amount of energy from the Sun also increases).

In most dramatic examples, perturbations from the flyby star are so strong that all of the planets are ejected into interstellar space. This is a very low probability event (less than 1 chance in 100,000), and these cases leave the Sun as a naked star.

These happened only for the strongest encounters, usually when a massive star passed passed slowly closer than Mars’ orbit.

Planetary capture is my personal favorite outcome of a stellar flyby.

When the stars align (pun intended), the flyby star can gravitationally steal a planet from the Sun. Sometimes a single planet is captured, although up to five planets were captured by a single stellar flyby in our simulations.

Here is an example in which Earth was captured during a flyby. Note also that such a close flyby would certainly have triggered instability among the surviving Solar System planets, but I didn’t include that in the gif.

The final orbits of captured planets tend to be very stretched-out. There is also no reason for those orbits to be aligned with the capturing star’s equator (or with the orbits of other planets that may be in orbit around the other star — those were not included in our simulations).

Sometimes a planet can be captured onto an orbit that is favorable for life.

The next image shows the orbital distances of the planets captured by flyby stars in all of our simulations. The capturing stars ranged from puny red dwarf stars with masses less than 10% of Earth’s mass to Sun-like stars, up to stars several times more massive than the Sun. The habitable zone is much closer-in for red dwarf stars and farther out for massive ones (as discussed in many posts on this blog, such as here and here). The horizontal error bars represent how stretched-out each planet’s orbit is.

We did the math to estimate the probability of all sorts of different outcomes. Of course, these are only relevant if a star comes within 100 astronomical units of the Sun, which has a 1% chance of happening sometime in the next billion years (or in any billion-year period, but after a billion years from now the Earth’s water will be gone so it won’t matter).

Here are the top ten most likely outcomes if a star passes within 100 astronomical units of the Sun (which, remember, has a 1% chance of happening in the next billion years).

By far, the most likely outcome is that all of the Solar System’s planets will survive, with very small perturbations to their orbits.

The most probable destructive outcome is that Mercury will fall onto the Sun (2.5% probability). This happens not as a direct result of a stellar flyby, but rather as a byproduct of the giant planets’ orbits being perturbed by the flyby, and that perturbation being transmitted to the rocky planets. Mercury is already the most vulnerable planet in the Solar System, even with no flyby, with about a 1% chance of falling onto the Sun over the next 5 billion years (for more, see the end of the Solar System).

The next most likely destructive outcomes involve various planets falling onto the Sun (Mars in particular), collisions between the rocky planets (especially those involving Venus, and the ejection of the ice giants. Pretty standard.

Exotic outcomes are rare. The probability that Earth will be ejected into interstellar space is only 0.06% (about one in 1700) in the 1% per billion year event of a star passing within 100 astronomical units. The probability that Earth will be captured by another star is about one in 100,000, and the odds of Earth ending up in the Oort cloud is one in about 500,000. I’ve included a table with all the numbers at the end.

What will happen to Earth?

If it’s destroyed, Earth will most likely collide with another planet (0.5% probability; most likely planet to collide with is Venus). It could also fall into the Sun (0.24% probability). Far less likely is for Earth to be ejected into interstellar space (0.06% probability), captured by another star (0.001%) or kicked into the Oort cloud (0.0002%).

What are the odds that Earth survives on a cooler orbit than the current one? That’s really the best-case scenario, because it would rescue our planet from the brightening Sun.

It turns out that there is only a 0.28% probability (about 1 chance in 350) of Earth surviving on an orbit that is at least 10% cooler than the present-day one. The chance of Earth surviving on an orbit that is at least 10% hotter is more than twice as high, at 0.8% probability.

Although the odds are small, Earth could end up on an extremely cold orbit after a stellar flyby. The absolute coldest possibility is if Earth was ejected from the Solar System entirely, to live out its days as a free-floating (or “rogue”) planet. A close second is if Earth is trapped in the Oort cloud, on such a wide orbit that the energy it received from the Sun would be basically zero.

Despite the extreme cold, a free-floating or Oort cloud-bound Earth might not be the worst situation for life. It is much harder to un-roast a fried planet than to heat up a frozen world. It would take about a million years for Earth to freeze over completely, giving any lifeforms a decent amount of time to scramble to adapt (biologically or, just maybe, using technology).

If they possess modestly-thick hydrogen atmospheres, free-floating planets can trap enough heat to maintain liquid water on the surface. Another way for this to happen is by trapping heat under a thick layer of ice. There are organisms called chemoautotrophs that rely solely on chemical energy rather than Solar energy, and, as long as there was enough residual heat and a thick enough thermal blanket (in the form of a thicker atmosphere or a layer of ice), they could remain quite happy on a rogue Earth. See here for an essay I wrote a while back about life on free-floating worlds.

(Side note: Cixin Liu’s the Wandering Earth tells the story of Earth becoming a free-floating planet. It is not completely accurate, but it is nonetheless an awesome story and I highly recommend it.)

Another way to frame the statistics is that, for every 100 Suns, a star will indeed pass within 100 astronomical units in the next billion years. If it doesn’t happen here, it will happen out there, among exoplanet systems. In fact, a star must have flown close by at least a few of the thousands of exoplanet systems that we know of. That’s something to explore in the future…

What about the Moon?

In the 12,000 simulations that you’ve seen to this point, the Earth and Moon were merged together as a single particle. This is standard in this type of analysis. It’s done because resolving the Moon’s orbit around the Earth slows down the computation by a lot.

In the interests of exploring the importance of the Moon, I ran about 800 additional simulations including the Moon as a separate particle orbiting the Earth.

It turns out that the Moon doesn’t do all that much. It never has a measurable effect on other planets’ orbits. However, when things go haywire and the rocky planets’ orbits start to cross, the Earth-Moon system is often disrupted. The Moon sometimes escapes to be its own planet orbiting the Sun — although this is generally a very short-lived adventure, because it usually ends up hitting a planet or the Sun, or being ejected.

More often, when the Moon’s orbit around the Earth is disrupted, it collides with Earth. This humongous impact would melt our planet’s surface and completely wipe out all life. Not good for business.

(It’s not clear from these simulations just how often the Moon really hurts the Earth’s chances for life, but I’m curious and am working on figuring that out now.)

Now let’s loop back to the beginning and answer the central question of this post: could a passing star save life on Earth?

The answer is yes, it could. But the odds are awfully small. There’s about 1 chance in 100 that a star will pass within 100 astronomical units of the Sun in next billion years. And if it does, there’s only a 1 in 350 chance that Earth will survive around the Sun on an orbit that is cold enough to extend the plausible lifetime of our planetary habitat.

Put together, that’s a 1 in 35,000 chance that the long-term prospects for life on Earth will be rescued by a passing star. That’s about the same odds as randomly pulling the ace of spades from two separate decks of cards while also rolling a combined 10 with two dice. Not the best odds.

A better option would be for humanity to take its survival into its own hands.

This could take the form of using technology to alter Earth’s orbit (perhaps by using an asteroid to exchange angular momentum with Jupiter), or devising some sort of shield to reduce the amount of Solar energy reaching the Earth. On a personal note, I feel like humanity should really be taking its survival into its own hands right now, by more strongly addressing the climate crisis — this is in some sense a trial run before a much bigger challenge (albeit one that is far far away in time).

To wrap things up, here are the main takeaway points of this project, in animated gif form.

Questions? Comments? Words of Wisdom?

---

Additional Resources

• Our paper, Future trajectories of the Solar System: dynamical simulations of stellar encounters within 100 au, is now published in Monthly Notices of the Royal Astronomical Society (downloadable here).
• A historical note. This project started with a discussion between Franck Selsis and the science fiction writer luvan (check out her website here) about the possibility of Earth becoming a free-floating planet. After Franck asked me about it, I recruited Nate Kaib and Herve Bouy to help out.
• Finally, here is the table listing the detailed outcomes from our 12,000 simulations for each planet, including the probabilities.