Stars are gravitationally attached to their galaxies and move in harmony with their environment. But sometimes something breaks the bond. For example, if a star gets too close to a supermassive black hole, the black hole can push it into space like a rogue star.
What would happen to Earth if one of these stellar invaders got too close?
It’s not a very likely event, but the probability is not zero.
After several billion years, our solar system has evolved towards sedentary predictability. The planets move as they move, and the sun is firmly at the center of it all.
But if another star came too close, the invisible gravitational bonds that keep everything as it is would be stretched or broken.
Earth is a small planet, containing only about three millionths of the mass of the Sun. Our planet exists under the whims of the sun and its powerful gravity, and if another star forces its way into our orderly arrangement, Earth will be completely at the mercy of the new gravitational paradigm.
A new paper explores what would happen if a rogue planet came within 100 AU of the Sun. The title of the article is “Future trajectories of the solar system: dynamical simulations of stellar encounters within 100 au.” It will be published in Monthly notices of the Royal Astronomical Society. The lead author is Sean Raymond, astronomer at the Laboratoire d’Astrophysique de Bordeaux, CNRS (National Center for Scientific Research) and the Université de Bordeaux.
We know that stable predictability in our solar system will not last long. The sun will continue to evolve and become brighter over the next billion years. Earth is terribly close to the inner edge of the habitable zone. Just a little closer to the sun will disrupt the delicate balance that allows liquid water to persist on the surface.
Within that same billion-year range, there is about a 1 percent chance of an encounter with a rogue state. What will happen to the Earth if that happens? Will Earth be pushed out of the habitable zone?
“Earth has habitable surface conditions remaining for about a billion years,” the authors write. That is in a closed system, which is for the most part our solar system.
“Although the orbital evolution of the planets is largely determined by secular and resonant perturbations,” the authors explain, “passing stars can have a consequential influence on the planets’ orbits.”
If a passing star comes too close, our solar system is no longer a closed system.
Most rogue stars, also called intergalactic stars or hyper-velocity stars because their orbits will take them out of the Milky Way, don’t go anywhere near Earth. For example, Kappa Cassiopeiae is 4,000 light years away and will never come closer. Others, like the 675 rogue stars that Vanderbilt University astronomers discovered in 2012, were ejected after tangling with the Milky Way’s supermassive black hole, and their trajectories took them nowhere near Earth.
Even in the Milky Way, space is largely empty, and most stellar flights will never approach another solar system.
“Statistically speaking, flybys closer than 100 au, which would have a strong influence on the planets’ orbits, only occur about once every 100 Gyr in the current Galactic neighborhood,” the researchers explain.
Although the chances are slim, it is a possibility. Looking at the galaxy as a whole, it’s almost certain that a stellar flyby will come within 100 AU of another star somewhere in the galaxy.
If that star is our sun, what will happen to Earth?
The team conducted N-body simulations to determine the possible outcomes for Earth. They started with the eight planets of the solar system and added a single rogue star. They linked the masses of the simulated rogue stars to the masses of stars in our stellar neighborhood. They also matched the speeds of the rogue stars to the neighborhood. They simulated different velocities and trajectories for the star to see what the outcomes for Earth look like. The researchers performed a total of 12,000 simulations.
“If a star passes within 100 au of the Sun, there is still a very high probability that all eight planets in the solar system will survive,” the authors write. There is more than a 95 percent chance that no planets will be lost.
The angular momentum deficit (AMD) resulting from the flyby largely determines what happens next. AMD is a measure of a planetary system’s orbital excitation and long-term stability. It is the difference between an “idealized system with the same planets of the real system orbiting the star on the same semimajor axes in circular and planar orbits and the norm of angular momentum of the real planetary system,” according to this definition.
But what does it look like if one of our solar system’s planets is lost?
The simulation produced varying results. Mercury is most vulnerable and sometimes lost when it collides with the Sun. Other results include Earth’s collision with Venus, the ejection of the ice giants Uranus and Neptune, the survival of only Earth and Jupiter, or the survival of only Jupiter. In one apocalyptic outcome, all eight planets are ejected.
Other results are less dramatic. All eight planets are unperturbed, all eight are slightly disturbed, or all eight are very disturbed.
Although in most simulations all eight planets survive, survival can mean different things. Even though they remain in the solar system and remain gravitationally bound to the sun, their orbits can be greatly disrupted. Some can even be pushed far into the Oort Cloud.
The researchers also mapped out the ten most likely outcomes involving planet destruction.
“We have determined the most common routes by which planets can be lost, keeping in mind that there is a greater than or equal to 95 percent chance that no planet will be lost if a star passes within 100 au,” they write.
- Mercury collides with the Sun (2.54% chance).
- Mars collides with the sun (1.21%).
- Venus has an impact on another planet (1.17%).
- Uranus is ejected (1.06%).
- Neptune is ejected (0.81%).
- Mercury has an impact on another planet (0.80%).
- Earth influences another planet (0.48%).
- Saturn is ejected (0.32%).
- Mars has an impact on another planet (0.27%).
- The Earth collides with the Sun (0.24%).
When it comes to ejected planets, Uranus and Neptune face the worst odds. That’s not surprising, since they are farthest from the sun and most weakly bound to it by gravity. It’s also not surprising that Mercury has the highest chance of colliding with the Sun. As the least massive planet, it is at greater risk of disruption due to stellar flight.
When it comes to Earth, there are a wide variety of possible outcomes. In the list above, Earth has a 0.48 percent chance of colliding with another planet. But there is another potential fate awaiting Earth, and it’s not pleasant to think about: exile to the Oort Cloud.
“Earth’s long-term survival in the Oort Cloud is not guaranteed,” the authors said.
Another exotic result of the simulations is worth considering: the capture of Earth by a passing star. That simulation had a star that was slightly less massive than the Sun and that approached our solar system at a relatively low speed.
The result was a devastating destruction of the solar system as we know it. Earth left the sun and took off with the star, while six of the other planets crashed into the sun. The only remaining planet was Jupiter. No surprise, as it is the most massive planet.
The article presents a wide range of outcomes, including the moon impacting Earth, both Earth and moon being occupied by the passing star, and even all the planets and their moons being destroyed. But the chance of this happening is extremely small.
But how likely is it that Earth will remain habitable in such an encounter? If Earth’s orbit is changed, it will make the planet warmer or cooler.
There are other potential fates. Earth could survive as a rogue planet for about a million years, until its surface froze over. Or perhaps, if captured by the rogue state, it would somehow be habitable in a new setup.
Ultimately, the chance of a star flight of 100 AU is infinitesimally small. And the simulations show that if it were to happen, the most likely outcome is that all eight planets survive, albeit on orbits slightly different from the ones they follow now.
“Despite the diversity of potential evolutionary pathways, there is a good chance that the current situation of our Solar System will not change,” the authors conclude.
This article was originally published by Universe Today. Read the original article.