Alpha Centauri has a Posse

In Exoplanets by Brian Koberlein6 Comments

Alpha Centauri is the nearest star to us. (Third nearest if you count the Sun and Proxima Centauri.)  It is also a binary system.  Those two facts make it instantly appealing to a child with an interest in astronomy.  What’s more, the primary star of the system is much like our own sun, so it is easy to imagine an earth-like world orbiting it. Surely it would be a world much like ours, but with two suns in the sky.  You almost wish it to be true.  I remember rainy summer days when I was about 12, working with a copy of Asimov’s Alpha Centauri to imagine whole solar systems around the two stars.

So when we started looking for extrasolar planets, Alpha Centauri is high on the list of any astronomer’s inner child.  But despite its closeness, finding planets there would be difficult.  The orbit of Alpha Centauri B is at an angle relative to us, so there isn’t likely to be any transiting planets that we can detect.  That means planets would have to be found by looking at the wobble of the star as the planet orbits it, which is much harder to measure.

In the early days of speculation regarding extrasolar planets, it was thought that double systems weren’t likely to have planets because the two suns would disturb planetary orbits too much.  As we developed more sophisticated computer models, we found that wasn’t the case.  It would even be possible for planets to lie within a habitable zone around either star.  But even if they existed, they would still be hard to detect.

Recently, however, we have found one around Alpha Centauri B.  (You can see the original article here)  This planet was found by observing the Doppler shift of the star as it wobbles.  Of course, B wobbles due to A as well as any planets, so there is a lot of noise in the observations, but if you average the results you get the figure below.  The red line represents the best fit curve.  If you look at the vertical scale, you find they had to measure Doppler motion at around half a meter per second.  That would be like measuring the Doppler shift of someone walking down the street, which is amazing in itself.


The big thing is that we’ve found an earth-sized planet around our nearest neighbor.  This planet is only about 10% larger than Earth, but it is very close to the star and its “year” is only 3.2 days long.  So don’t plan a vacation trip just yet. But with one planet, there are likely others, and that means Alpha Centauri likely has a solar system.

Which of course makes my inner child squee with joy!


  1. “The big thing is that we’ve found an earth-sized planet around our nearest neighbor. This planet is only about 10% larger than Earth, but it is very close to the star and its “year” is only 3.2 days long.”

    It seems that many of the exoplanets (all of them, if I’m not mistaken) that have been “found” by Doppler shift have very short years. All of which are much shorter that Earth’s year.

    From hearing the news of the first exoplanet discovered, leading up to the present, with over 1,500 in our database, I have had a theory. The theory, Mr. Koberlein, you may be able to quickly debunk, but it is a theory I would like to submit because my theory just seems too easy for astronomers to have overlooked for this long.

    My theory: Measuring the Doppler shift and/or the planetary transit of planets is an inaccurate method of determining exoplanet sizes and orbital periods. I have come up with the conclusion that by using Doppler shift when hunting for exoplanets the Doppler motion of a star within a solar system containing multiple planetary bodies will give observers the false sense that less planetary bodies exist within the solar system. Also, the Doppler motion of the star will make it appear that the orbital period for such exoplanets are shorter than the observer’s calculations based on the fact that other exoplanets belonging to the same solar system are causing reverse effects on the star’s Doppler motion. In other words, the observer can calculate 1 exoplanet orbit a star every 3.2 days, but this data alone is inconclusive because this star may have 8 planets orbiting it, making this solar system appear to have only 1 planet because of the other 7 planet’s positions.

    1. Author

      Actually, we can distinguish between one planet and several by looking at oscillations in the Doppler shift. Where there is bias is that it is easier to find large close planets than small distant ones. This is why we tend to find planets with short orbits.

  2. So, how can we advance exoplanet research? I’d rather spend money on almost anything other than research that’s going to discover exoplanets that orbit so close to their star, because once we find 1,500 exoplanets, the search needs to narrow to closely study a few of these systems so that we can form algorithms in which these stars move through space and discover all the exoplanets around particular systems.

  3. If we are looking at a doppler shift, wouldn’t that imply that the orbits are lined up at a 0 degree angle with our POV? What about planets that orbit at say, a 89 degree angle?

  4. In the future we will get much better data, you are off to a very good start….

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