Rare stellar alignments aids hunt for planets around closest stellar neighbour

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In the search for planets orbiting around other stars, the Kepler Space Observatory usually gets all the attention, but it's not the only one on the hunt. The Hubble Space Telescope is also one of the pack, and twice in the next three years it will have a rare opportunity to find planets orbiting around our closest stellar neighbour.

The closest star system to ours is called Alpha Centauri (or Rigel Kent if you prefer), and as far as astronomers can figure, it's a triple, or 'trinary', star system. There are two stars (yellow A and orange B) that are fairly close together, orbiting each other every 80 years or so, and there's a third star, a red dwarf called Proxima Centauri, orbiting much further out from the other two, which is currently the closest star to our solar system.

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Astronomers have searched for planets around these stars, and there's even evidence that there's an Earth-sized one orbiting close to Alpha Centauri B — with a subsequent naming-contest, the controversy surrounding that, and now word that it may not even exist), but finding smaller planets around dim, red Proxima is no easy task.

However, Hubble planet hunters are going to catch a lucky break, first in October 2014 and then again in February 2016, when Proxima Centauri will line up with Alpha Centauri — first with one of the stars, and then the other.

The reason this is a lucky break is because when Hubble searches for planets, it uses a method called 'gravitational microlensing'.

This technique takes advantage of one of the basic properties of the universe — anything with mass bends the fabric of spacetime. Think of the fabric of spacetime like a big sheet of memory foam, and a star in space as a bowling ball placed on that sheet of foam. Just as the bowling ball's weight presses the foam down into a well, a star's mass forms a well in spacetime. We can't see this effect, of course, but we experience it as gravity, and call it a 'gravity well'. Planets have their own gravity wells, and they form smaller depressions inside the larger well created by the star they orbit.

Looking at a far-away star, the well in spacetime surrounding it will warp our view of anything that's on the other side of it from us, as if the star was encased in a giant glass lens. This 'gravitational lens' will make the object on the other side of the star appear brighter and shifted slightly from its true position. The addition of a planet around the star, along with its smaller gravity well, will make the background object appear even brighter, and its position will be warped slightly to one side too.

If we're fortunate enough that the planet has a fairly short orbit, we can even see the brightness and position of the background object change as the position of the planet's gravity well changes.

So, as Proxima Centauri passes in front of Alpha Centauri A and B, Hubble will be able to watch as the position of the background star shifts. If the star simply seems to shift straight away from Proxima Centauri, it's likely that there's no planets around it, but if the background star shifts in a slightly skewed way, we may have a 'hit'.

This particular case — with the closest star to us passing in front of the next closest star, and doing so twice — is an incredible opportunity for the Hubble astronomers.

With Proxima Centauri so close, we can see its gravity well much easier than the gravity well of a star further away, so we'll be able to notice smaller changes in the brightness and position of any stars it passes in front of. Also, with Alpha Centauri A and B so close to both Proxima Centauri and us, the effect of the gravity well on them will be much more noticeable than on more distant stars. This all combines together to give astronomers the chance to see very small changes, which means they should be able to detect the effects of relatively small gravity wells, such as one generated by a planet like Earth.

The fact that astronomers will have not one, but two chances to see this is just 'icing on the cake', so to speak.

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With the setback of possibly losing Kepler in the hunt for planets, astronomers may have to rely on the other methods of detection for awhile, until TESS (Transiting Exoplanet Survey Satellite) goes into operation in 2017. The list of planets and planet candidates that are being found around other stars seems to grow daily, but the most exciting finds are those that are similar to Earth, and that have the possibility of harbouring life. The odds of finding such a planet orbiting around the star closest to us are probably a long-shot, but having this rare astronomical alignment to help us look is an awesome opportunity to at least be able to roll the dice.

(Images courtesy: ESA, NASA/Hubble, Space Telescope Science Institute)

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