How will life on Earth end? The answer, of course, is unknown, but two new studies suggest a collision with Mercury or Mars could doom life long before the Sun swells into a red giant and bakes the planet to a crisp in about 5 billion years.

The studies suggest that the solar system's planets will continue to orbit the Sun stably for at least 40 million years. But after that, they show there is a small but not insignificant chance that things could go terribly awry.

On human timescales, the solar system seems to move as regularly as clockwork. But Isaac Newton realised three centuries ago that the gravitational tugs the planets exert on each other can potentially nudge them out of their orbits over time.

Predicting what will happen is extremely challenging because so many bodies are involved. Even small errors in the observed positions of the planets today can translate into huge uncertainties in projections of the future. Because of this, astronomers can only say for sure that the solar system will remain stable for the next 40 million years.

Although no one can say for sure what will happen beyond that, new calculations are now providing a rough guide to the more distant future. These suggest that there is a 1 to 2% chance that Mercury's orbit will get seriously out of whack within the next 5 billion years.

This would tend to destabilise the whole inner solar system and could lead to a catastrophic collision between Earth and either Mercury or Mars, wiping out any life still present at that time.

In the case of a smash-up with Mars, for example, "all life gets extinguished immediately, and Earth glows at the temperature of a red giant star for about 1000 years", says Gregory Laughlin, a co-author of one of the studies at the University of California in Santa Cruz, US.

Highly eccentric

Jacques Laskar of the Observatoire de Paris in France authored the other study. He ran 1001 computer simulations of the solar system over time, each with slightly different starting conditions for the planets based on the range of uncertainties in the observations.

In 1 to 2% of the cases, Mercury's orbit became very elongated over time due to gravitational tugs by Jupiter. In these cases, its orbit reached an "eccentricity" of 0.6 or more (an eccentricity of 0 means the orbit is a perfect circle, while 1 is the maximum possible elongation).

Putting Mercury into such an elongated orbit increases the interactions between Mercury, Venus, Mars and Earth. Previous simulations by Laskar have suggested this can throw the whole solar system into disarray, a scenario confirmed in simulations by Laughlin and Konstantin Batygin, also of UCSC.

'All bets are off'

"Once Mercury's eccentricity gets up above about 0.6, then it's getting close to crossing Venus's orbit," Laughlin told New Scientist. "Once you get orbit crossings, you sort of transition from the orderly yet chaotic configuration that the solar system is in currently to a much more violently chaotic situation. Then all bets are off – a lot of bad things can happen."

Mercury and Mars tend to get thrown around the most when the solar system destabilises, because at 6 and 11% of Earth's mass, respectively, they are relatively easy to move. It is harder to budge Venus, on the other hand, because it has 82% of Earth's mass.

In one of Batygin and Laughlin's simulations, Mercury was thrown into the Sun 1.3 billion years from now. In another, Mars was flung out of the solar system after 820 million years, then 40 million years later Mercury and Venus collided.

Lava ocean

These were the disasters that happened to occur in the limited number of simulations that Batygin and Laughlin carried out. But Laughlin says there are many other ways for the solar system to unravel.

"You open yourself up to a huge number of possible disasters that can occur," he told New Scientist. "In each case, the gory details are completely different."

Direst for Earth is the possibility of a collision with a wayward Mercury or Mars.

A fair bit is known about what Mars could do to Earth. Many scientists think a Mars-sized object bashed into Earth in the early solar system, throwing out debris that eventually formed the Moon.

Earth was heated to thousands of degrees by the impact, with an ocean of lava covering its surface. A future replay of that event would be disastrous, Laughlin says.

But there is a 98 to 99% chance that the solar system will still be running like clockwork 5 billion years from now. Says Laughlin: "The glass is 98% full or 2% empty."

Journal references: Laskar, Icarus (in press); Batygin & Laughlin, Astrophysical Journal (in press)

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