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Is Proxima Centauri Flying Solo?

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Proxima Centauri.
X-Ray image of Proxima Centauri. Credit: Chandra.

If you want to send an interstellar probe, you're going to chose the closest star. And that would be Proxima Centauri, located only 4.2 light years away. Since they first calculated its distance, Astronomers have always assumed that Proxima Centauri was part of the Alpha Centauri triple star system. But recent calculations threw that assumption into doubt. Was its location purely a coincidence? Is Proxima Centauri flying solo through the Milky Way?

The Alpha Centauri system was first calculated as the closest star system in 1839 by the Scottish Astronomer Thomas Henderson, using the parallax method - measuring the shift of a star against the background from opposite sides of the Earth's orbit. He calculated that Alpha Centauri was 2.7 light years away. Modern Astronomers now know it's 4.4 light years away. Still, not bad.

Proxima Centauri took the title of closest star when its parallax was measured accurately in 1917, and it was found to be only 4.2 light years away. Since Proxima Centauri is so close to Alpha Centauri A and B, and moving in the same direction across the sky, Astronomers have always assumed they were a triple star system.

This assumption came under fire about 12 years ago when a group of researchers used the best data on hand to calculate that the Alpha Centauri system doesn't have enough gravity to hold onto Proxima Centauri. One possibility is that Astronomers just miscalculated their masses. Or maybe the stars are all moving together, so it just looks like Proxima is moving too quickly.

But maybe Proxima Centauri isn't a member of the Alpha Centauri system at all. It began life from a different cloud of gas and dust, and its proximity to the other stars is just random chance. The likelihood of this is extremely remote, but then, it's a big Universe, and almost anything is possible.

In 1997, ESA's Hipparcos delivered exquisite data about all three stars in the Centauri system, and this has enabled new calculations about their mass, speed, and motions. Further observations over the last few years have delivered even more data for Astronomers to crunch.

Jeremy G. Wertheimer and Gregory Laughlin from the University of California, Santa Cruz, have used all this new data to do the calculations again. They calculated that Proxima Centauri could actually be orbiting the gravitational centre of the Alpha Centauri system. Assuming, however, that its orbit is extremely eccentric, and Proxima Centauri is currently in the most distant point of this orbit.

So go ahead and launch that interstellar probe towards our nearest neighbour. Proxima Centauri should be right where you're expecting it.

Written by Fraser Cain.

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