Can a rogue star kick Earth out of the solar system?

A NASA camera aboard the Deep Space Climate Observatory satellite caught this view of Africa and the entire sunlit side of Earth on July 9, 2015.
A NASA camera aboard the Deep Space Climate Observatory satellite caught this view of Africa and the entire sunlit side of Earth on July 9, 2015. (Image credit: NASA)

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights

It's a dangerous universe out there, and life on any planet faces numerous existential threats — including the possibility of the planet getting kicked out of its solar system altogether. One team of researchers ran the numbers and found that the Earth is safe from such a catastrophe — almost.

Related: Earth quiz: Do you really know your planet?

One swift kick

Gravity can be a tricky thing. One moment, it's holding a planet in orbit around a star, safely chained to that life-giving warmth for billions of years. The next, that very same gravity is sending the planet hurtling into the depths of interstellar space, doomed to wander the frozen wastelands of our galaxy, with any life on that planet quickly and permanently erased.

Gravity is so tricky because it's actually rather complex. When there are just two objects — say, a massive star and a single little planet — the math is easy to work out. Once a planet finds itself in a stable orbit, it will remain in place for billions upon billions of years without deviation, the regular rhythms of its motion repeating like the gears of a great clockwork mechanism.

But throw a third object into the mix? Anything goes — chaos. As in, literal chaos. The problem of trying to predict the motion of three objects interacting through gravity has been notorious for centuries, with intellectual heavyweights through the ages trying to come up with a solution and failing. The problem is that with three objects, any little deviation or shift can lead to massive changes in a surprisingly short amount of time. 

In other words, what looks calm, cool and collected one moment — say, the fact that the Earth has orbited the sun placidly for billions of years — can become dangerously unstable the next.

But so far, so good, right? The rest of the planets in the solar system are relatively small, and while they do tweak and affect the Earth's orbit in subtle ways, they don't cause outright havoc and general destabilization. Things will be just fine for billions of years to come, as long as nothing massive — say, a passing star — comes close.

Related: Our solar system: A photo tour of the planets

The danger zone

But sometimes neighboring stars do come close to our solar system, and when they get too close, it's nasty.

As a star nears the solar system, it can start to change the orbit of the Earth. When the Earth and the interloper are near, our planet can get a little bit of energy, a gentle gravitational tug-of-the-leash from the foreign visitor. This may not amount to much, but if this happens year after year, the Earth can steadily gain energy, moving its position farther away from the sun, out of the "habitable zone," the band of orbits around the sun that aren't too close for surface water to vaporize or too far for it to turn to ice.

And with enough tugs, the Earth could potentially gain so much energy that it achieves escape velocity from the sun, leaving the solar system in the blink of a cosmic eye.

Thankfully, this scenario hasn't happened in the past four billion years (obviously, because we're still here), but that doesn't mean it couldn't happen in our future. Recently, a pair of astronomers set about simulating the effects of a passing star on the orbit of the Earth, reporting their results in a paper appearing on the preprint journal arXiv.

They looked at all potential scenarios: different masses of stars incoming at all sorts of angles with all sorts of speeds, to see what circumstances could potentially move us out of the habitable zone — and end life on our planet as we know it.

Generally, the faster the star blows through our solar system, the better it is for us; for a star in a hurry, there just isn't enough time to really mess things up gravitationally. If the star is moving at least as quickly as the Earth orbits the sun (a little over 66,000 mph, or 106,000 km/h), then it has to pass within the orbit of Jupiter to have a decent chance of knocking us from our orbit; otherwise, its gravitational influence is just too inconsequential. But slow-moving stars can cause much more trouble: they only have to skirt the edges of the solar system to cause carnage.

Naturally, the mass of the passing star is critical, as well as the angle of impact. If the star is passing along the plane of our solar system, it has a lot of chances to get near enough to the Earth to destabilize us, and if the star comes within twice the distance of Earth's orbit around the sun, regardless of other factors, we're simply toast.

I'm a survivor

The researchers then took these results and applied them to our galactic neighborhood, using our knowledge of the positions, masses, and velocities of the stars nearest to us. They found that, on average, our planet only had a 1/15,000 chance of orbital destabilization over the course of our 4-billion-year history. In other words, for every 10,000 Earth-like planets in our neighborhood, only one has suffered a disaster.

But things get much worse in the busy, compact central bulge of the Milky Way. With all those stars, the chance of gravitational catastrophe is 160 times higher.

Generally, that's pretty safe. But it does inform searches for life on distant exoplanets. The center of a galaxy is a generally unfriendly place, a hotbed of high-energy radiation and cosmic rays. And now we know it's also full of potentially deadly close calls between stars, close enough to knock life-bearing worlds out of their orbits. If life has managed to arise on any of the worlds there, it's in for a tough time.

Read more: "Dangerous stellar encounters with an Earth-like planet in the Milky Way galaxy"

You can listen to the Ask A Spaceman podcast on iTunes, and on the Web at http://www.askaspaceman.com. Ask your own question on Twitter using #AskASpaceman, or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom or Facebook

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Paul Sutter
Space.com Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

  • rod
    Admin said:
    Life on any planet faces numerous existential threats — including the possibility of the planet getting kicked out of its solar system altogether. One team of researchers ran the numbers and found that the Earth is safe from such a catastrophe — almost.

    Can a rogue star kick Earth out of the solar system? : Read more

    Interesting report closing remarks concerning the habitable Earth.

    "The researchers then took these results and applied them to our galactic neighborhood, using our knowledge of the positions, masses, and velocities of the stars nearest to us. They found that, on average, our planet only had a 1/15,000 chance of orbital destabilization over the course of our 4-billion-year history. In other words, for every 10,000 Earth-like planets in our neighborhood, only one has suffered a disaster. But things get much worse in the busy, compact central bulge of the Milky Way. With all those stars, the chance of gravitational catastrophe is 160 times higher. Generally, that's pretty safe. But it does inform searches for life on distant exoplanets. The center of a galaxy is a generally unfriendly place, a hotbed of high-energy radiation and cosmic rays. And now we know it's also full of potentially deadly close calls between stars, close enough to knock life-bearing worlds out of their orbits. If life has managed to arise on any of the worlds there, it's in for a tough time."

    At the moment, we have 4186 exoplanets confirmed, The Extrasolar Planets Encyclopaedia
    In exoplanet research, there is not *10,000 Earth-like planets in our neighborhood* confirmed at this time. The simulation is interesting though in pointing the stat out here.
    Reply
  • Craig Willms
    The answer is yes. It's likely that it's already happened to Earth according some theories. Earth quite possibly was a planet around a brown dwarf star we now know as Saturn. Saturn was captured by the Sun and it's planets Mars, Earth and Venus were ripped away and took up orbit around the Sun. One clue is the axial tilt of Saturn and the three smaller planets is nearly the same.
    Reply
  • Omikron777
    rod said:
    At the moment, we have 4186 exoplanets confirmed, The Extrasolar Planets Encyclopaedia
    In exoplanet research, there is not *10,000 Earth-like planets in our neighborhood* confirmed at this time. The simulation is interesting though in pointing the stat out here.

    You are correct, but I think they meant to emphasize just the 10,000 nearest stars; then, in the purely hypothetical sense, they just treated these stars' relative habitable zones as though they were habitable & did indeed have Earth-like worlds:

    "The researchers then took these results and applied them to our galactic neighborhood, using our knowledge of the positions, masses, and velocities of the stars nearest to us."

    The wording makes it confusing after that, but I assume that's how they ran their statistics. I don't think they even looked at a documented potential Earth-like exoplanet. Their 1 planet from 10,000 in 4 Billion years is a statistical artifact and not real....or could be, but to the most absolute miniscule of odds... The bad part is, this actually overestimated the odds of an Earth-like planet getting ejected out of a star's orbit...
    Reply
  • Earth to Rick
    Craig Willms said:
    The answer is yes. It's likely that it's already happened to Earth according some theories. Earth quite possibly was a planet around a brown dwarf star we now know as Saturn. Saturn was captured by the Sun and it's planets Mars, Earth and Venus were ripped away and took up orbit around the Sun. One clue is the axial tilt of Saturn and the three smaller planets is nearly the same.
    Well, Saturn does not have nearly enough mass to be a brown dwarf. I think the threshold is a few multiples of Jupiter`s mass.
    Reply
  • Catastrophe
    Just for the record:
    Search Results
    Web results

    Brown dwarf - Wikipediaen.wikipedia.org \203a wiki \203a Brown_dwarf
    Comparison: most brown dwarfs are only slightly larger than Jupiter (10–15%) but up to 80 times more massive due to greater density. Image is approximately to ...
    ‎Sub-brown dwarf · ‎List of brown dwarfs · ‎Luhman 16 · ‎HR 2562 b

    Cat :)
    Reply
  • Catastrophe
    On the subject of Jupiter's possible development, here is an interesting snippet from Astronomy of January 2019:

    "Jupiter's growth into a giant planet was delayed for about 2 million years while it absorbed kilometre-sized rocks, which temporarily stifled the planet's ability to accrete more gas."

    Cat
    Reply
  • Catastrophe
    Can a rogue star kick Earth out of the solar system?

    I think we probably agree that the answer is yes, and probably that not only stars but also large planets can effect the same result.
    Reply
  • COLGeek
    Catastrophe said:
    Can a rogue star kick Earth out of the solar system?

    I think we probably agree that the answer is yes, and probably that not only stars but also large planets can effect the same result.
    And the impact would be catastrophic (pun intended).
    Reply
  • Catastrophe
    COLGeek

    "And the impact would be catastrophic (pun intended)."

    Where do you think I got my pretty little picture from?
    I am a great Asteroid fan. If you keep looking for the 'one that got away' you may find it too late!

    Cat ;)
    Reply
  • COLGeek
    It would seem that anything of substantial mass, enough to disrupt the orbital equilibrium of our planetary system, could produce very unfavorable conditions for us. A direct strike of sufficient mass and we could be the next dinosaurs.
    Reply