Finding New Planets: Tracking a Star’s Wobble

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Trust, but verify: Astronomers detected telltale motion in a star four light years away. S. Brunier / European Southern Observatory

I don’t know which I find more remarkable, the fact that there appears to be a small rocky, Earth-mass planet orbiting the closest star to our sun, or the fact that astronomers were able to discover it.

The latter fact may seem surprising. After all, if the Alpha Centauri system of three stars is our closest neighbor, shouldn’t it be the easiest place to find extra solar planets?

If it was a matter of merely seeing them, like we see stars, that might be the case, but planets don’t, in general, shine (although they can reflect the light from their host stars, which is why we can see the planets in our own solar system). Astronomers at the University of Geneva and their colleagues used a much subtler technique they helped develop almost two decades ago to discover the first extra-solar planets.

As a planet orbits its host star, it tugs at it in different directions, causing the star to wobble ever so slightly. The more massive the planet, the greater the wobble. The closer the planet, the faster it orbits its star, and the faster the period of the wobble.

But what is most astounding about the purported new planet, discovered around Alpha Centauri B, is that it is so light that it barely tugs its star at all. As it orbits every 3.2 days around its star, it causes the star to move back and forth at a speed of just 50 centimeters a second! That is the speed at which you might take a leisurely walk in the park.

That astronomers can deduce this kind of motion in a star four light years from us is so amazing, in fact, that we should be cautious about accepting the validity of the new claim, which was published in Nature last week, until it is confirmed using another independent set of observations. The observed signal is so small that it must be extracted from far bigger random jitter in the star’s light. Many of my colleagues have expressed skepticism on this point, but if we weren’t skeptical we wouldn’t be scientists.

If the new planet is indeed real, the result is groundbreaking. One could actually imagine sending probes to this star system in real time, taking maybe a century or two to complete the trip. We wouldn’t want to send astronauts to visit of course, because it is so close to its host star that the temperature on the planet approaches 1,000 degrees. But where there is one planet, there may be more.

The authors of the current work claim that with further observations over a longer period they could detect an earth- like planet in the “habitable zone,” where liquid water could exist, so perhaps such a discovery is around the corner.

Maybe we will discover life in a nearby hospitable solar system. Maybe we will find a place for humanity to retreat if things go badly here. Or maybe the discovery will simply demonstrate that solar systems are so common that there are perhaps 100 billion of them around the 100 billion stars in our galaxy.

If that is the case, the likelihood that we are alone in the universe will continue to decrease. Whether you think that is a good thing or a bad thing depends upon your background and prejudices, but a galaxy full of potential earths means things will never be the same.

Lawrence Krauss is director of the Origin Project at Arizona State University.

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