Earth Likely to Relocate in Galactic Collision

Scott Norris
for National Geographic News
May 16, 2007
The sun and Earth will probably be spun out into a lonely region of space when the Milky Way and Andromeda galaxies finish colliding about five billion years from now, researchers say in a new study.

There's also a small chance that our solar system will be swept from its home in the Milky Way and scooped up by Andromeda during an earlier close encounter, in just three-and-a-half-billion years.

These predictions stem from a new theoretical model that provides the best look yet at how and when the cosmic pile-up will occur.

The model is the first to trace the probable fate of our own solar system as the galaxies merge. (Related: "Solar System Is 'Bullet Shaped' [May 10, 2007].)

Like a pair of circling skaters, the Milky Way and Andromeda will brush past each other and separate again twice before fusing into a single galactic entity—an elliptical galaxy dubbed "Milkomeda" by researchers—on their third and final encounter.

The first close contact between the Milky Way and Andromeda will also come sooner than had previously been thought—just two billion years from now, say model creators T.J. Cox and Avi Loeb. Both scientists work at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

At that time, Loeb noted, the sun will be an aging but still very active star.

"The fact that this merger will take place before the sun dies is very interesting," he said. "There will still be a solar furnace ... potentially allowing life [on Earth] to still exist."

The work is presented in a paper submitted for publication to the Monthly Notices of the Royal Astronomical Society and is available online from the Harvard-Smithsonian Center for Astrophysics.

What's the World Coming To?

Scientists have known for some time the Milky Way and Andromeda are sailing toward one another at about 75 miles (120 kilometers) per second. Less certain is the rate at which Andromeda is slipping sideways, perpendicular to its line of approach.

Based on recent findings, Cox and Loeb's model assumes that Andromeda's sideways velocity is relatively small, making a collision with the Milky Way inevitable.

"The two galaxies are gravitationally bound to each other," Loeb said. "Eventually they will come together."

The model is also based on the probable distribution of mass within the "Local Group"—a region some 10 million light-years across that includes the Milky Way, Andromeda, and a number of smaller galaxies.

Scientists believe the space between the galaxies is occupied by a diffuse medium of gas and invisible "dark matter."

Cox and Loeb say friction from this medium should act like a cosmic braking system, causing the galaxies to lose momentum and ensuring their eventual merger. (Related: "Dark Matter Ring Detected by Hubble" [May 15, 2007].)

"This is a very interesting piece of work," said astronomer Gregory Laughlin of the University of California, Santa Cruz. "They've taken care to incorporate the best available estimate of Andromeda's [sideways] velocity, although that's still the most uncertain ingredient in the simulations."

The University of Toronto's John Dubinski noted that the model can be improved with better estimates once the Space Interferometry Mission (SIM) or the European mission GAIA can measure Andromeda's sideways velocity, in the coming five to ten years.

Welcome to Milkomeda

The new model shows the likelihood of various possible fates for our own solar system as the enormous galaxies bump and merge.

For example, there is about a 10 percent chance that the sun and its planets will be ejected to the trailing edge of the Milky Way by the first close encounter two billion years from now.

There is even a slight chance—about 3 percent—that the solar system will be scooped up by Andromeda on the second passage, leaving the Milky Way completely until the galaxies re-unite.

Such cosmic spectacles would certainly be visible to any future earthbound observers of the night sky.

The aging and warming of the sun may make Earth uninhabitable before the merger is complete.

But Loeb likes to speculate that our descendents might still be around to witness the birth of Milkomeda.

In two billion years, he said, "in addition to the strip of the Milky Way, there will be another dense band of stars. That's the Andromeda galaxy approaching us."

As the galaxies hurtle past and through one another, he added, near-passing stars may unleash spectacular showers of comets.

Ultimately, however, Cox and Loeb's calculations show that the most likely outcome for the solar system is exile to the far periphery of Milkomeda.

"The sun will be kicked to a region with fewer stars," Loeb said. "The chance of having other stars nearby will be far smaller than today."

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