Earth could fling out of orbit or into the sun

 The Earth could flung out of orbit or into the sun because of a passing star

Computer models of the solar system’s future reveal a new risk facing us all. The gravitational tug of a passing star could cause another planet to smack into Earth. Or it might fling our planet into the sun. Or it could send Earth far away from the sun, where any inhabitants would freeze. The chance of this happening is very small, but not impossible. The gravity of passing stars can jostle our solar system. That might someday cause another planet to smash into Earth or eject our planet from the solar system altogether. Either way, the sun is likely slated to consume Earth in billions of years. “None of these things are probable,” says Nathan Kaib with a laugh. This astronomer works at the Planetary Science Institute, based in Iowa. He teamed up for this research with astronomer Sean Raymond, at the University of Bordeaux in France. 

It’s difficult to know if such an outcome is likely. Recently, researchers have found the Milky Way likely won’t crash into its neighbouring galaxy any time soon. Our blue marble is already slated to be eaten by our sun in several billion years, after it turns into a red giant and expands. But researchers said in a recent study that thousands of computer simulations indicate there’s a chance a passing field sta, a star that appears in the same region of the sky as another object being studied, could cause more havoc than previously believed. “Our simulations indicate that isolated models of the solar system can underestimate the degree of our giant planets' future secular orbital changes by over an order of magnitude. In addition, our planets and Pluto are significantly less stable than previously thought,” Nathan Kaib and Sean Raymond, a pair of astronomers, said. The study say passing stars are the most probable trigger for instability during the course of the next four billion years.  It’s difficult to know, but researchers say simulations show a chance.

Over the next 5 billion years, they calculate, Earth’s chance of an apocalypse caused by a passing star is only 0.2 %. The figure is based on the number of stars passing near our solar system. That’s pretty slim odds of a drive-by star destroying or ejecting Earth within the lifespan of our sun. Still, it’s a much greater risk than past studies had found. Those estimates hadn’t accounted for the long-term influence of passing stars. The gravitational tug could cause instability to completely stable objects, including Pluto: formerly the ninth planet of our solar system. Over the course of five billion years, stars could transform Pluto from a completely stable object to one with a chaotic set of gravitational interactions that sets it off its orbit. While the odds of those changes occurring in that time frame from Pluto are approximately 5%, they are exponentially greater for Mercury.

If another star gets too close to us, Mercury will be key to the risk of Earth’s demise. The innermost planet’s orbit around the sun is fairly oval-shaped. And astronomers have long known that Jupiter’s gravity can stretch its orbit out even more. The new models show that passing stars heighten this danger. The risk of instability for the solar system’s fifth planet would increase by between around 50 and 80%. “We also find an approximately 0.3% chance that Mars will be lost through collision or ejection and an approximately 0.2% probability that Earth will be involved in a planetary collision or ejected,” as per their findings. Kaib previously published work suggested Earth's orbit was altered by a passing star three million years ago. “We looked at the typical, run-of-the-mill flybys,” Raymond said. “These are the stars that really do pass by the sun all the time, cosmically speaking.” Pluto, once our solar system’s ninth planet, could be impacted by one of these stars. So could Mercury and Mars. Still, these simulations aside, Kaib said that “none of these things are probable.”

In the models, Mercury’s orbit becomes so elongated that the planet typically collides with the sun or Venus. The resulting chaos sometimes causes Venus or Mars to crash into Earth. Other times, Earth crashes into the sun. Or Venus and Mars can fling our world toward Jupiter. The giant planet’s gravity then ejects Earth from the solar system. “It’s a little scary how vulnerable we may be to planetary chaos,” says Renu Malhotra. A planetary scientist, she works at the University of Arizona in Tucson. Poor Pluto faces a 4 % risk of getting booted from the solar system or smashing into a giant planet during the next 5 billion years, thanks to passing stars. Malhotra thinks past encounters with other stars have already influenced the solar system. For instance, one would expect the giant planets to have nearly circular paths around the sun. That’s because the planets formed from a disk of gas and dust with a nearly circular orbit. But today, three of the giant planets, Jupiter, Saturn and Uranus, have somewhat oval-shaped orbits. The gravity of passing stars may have tugged them onto these paths.

The most dangerous stars, Kaib says, are those that come closest, less than 100 times as far from the sun as Earth is. Stars that move slowly are also risky. Especially ones that move less than 10 km's (6 miles) per second relative to the sun. That cosmic snail’s pace extends how long their gravity will be able to tug on the planets. Without passing stars, Pluto was thought to have an even more stable orbit than Earth. “But once you allow stars to alter the solar system and push things around,” Kaib says, Pluto is in trouble. It can skirt by the giant planets. Then their gravity can kick it out of the solar system. Or Pluto can smash into one of them. Over the next 5 billion years, the chance of such a fate befalling Pluto is about 4 %. That’s 20 times greater than the risk facing Earth. There is one upside to Pluto getting booted from the solar system, though. It might just end the long-standing debate over whether Pluto is really a planet in our universe.