Alpha Centauri Harbors Earth-size Planet!

Artist impression of the planet Alpha Centauri Bb orbiting the star Alpha Centauri B, a member of the triple star system including Alpha Centauri A (left) and Proxima Centauri (out of the frame). The sun shines at upper right. Credit: ESO/L.Calcada

Big news today! A team of European astronomers announced the discovery of a planet orbiting Alpha Centauri. Located only 4.4 light years away, Alpha Centauri is a triple star and the closest star to the sun. It’s one of the few places where the sun holds its own as a first magnitude star in the constellation Cassiopeia. Not only is the new planet the closest extrasolar planet ever found, it’s also about the same size as the Earth, making it the lightest planet yet discovered orbiting a sun-like star.

Exoplanet surveys typically turn up massive planets because the big (and close-in) ones tug on their parent stars hardest. Astronomers use sensitive instruments called spectrographs to measure the slight changes in a star’s velocity induced by the circling planet. These tiny wobbles of the star toward and away from Earth can be measured and the planet’s mass determined.

Alpha Centauri is a bright triple star in the southern constellation Centaurus visible in the spring evening sky. It’s visible from southern Florida, Hawaii and points south. Credit: ESO/Digitized Sky Survey 2

In Alpha Centauri’s case, the team employed the super-sensitive HARPS spectrograph on the European Southern Observatory’s 142-inch (3.6 meter) telescope in La Silla, Chile for more than four years to reveal telltale velocity changes caused by an Earth-size planet orbiting Alpha Centauri B every 3.2 days.

The wobble is minute. The planet causes the star to move back and forth by no more than 1.1 mile per hour, about the speed of a baby crawling. Earth’s effect on our sun would be similar. Sadly, the planet is nowhere near a clement zone around the star; it orbits just 3.7 million miles from Alpha Centauri B, almost 10 times closer than Mercury to the sun. You could broil a steak there in a minute, but I doubt life could set up house on this heat-heavy world.

Alpha Centauri is a triple system with two bright components – Alpha Centauri A and B – and one more distant, fainter red dwarf called Proxima Centauri. A and B are separated by 13,000 times the distance between sun and Earth. From the new planet’s perspective, B would be far and away the dominate sun, while A would be brilliant pinprick of light off in the distance.

I’ve always wanted a planet to be found around Alpha Centauri if only because it feels so close to Earth. While Alpha Centauri Bb’s still over 26 trillion miles away, the news is most exciting. Of the 842 confirmed extrasolar planets found to date, it’s already my favorite. More about the discovery can be found HERE.

11 Responses

  1. caralex

    Bob, is our solar system somewhat unique, in that its planets are spaced at a ‘reasonable’ distance from the sun, not whizzing round it in just a few days, as many of the exo-planets discovered to date are?

    Why are these planets so close to their suns?

    1. astrobob

      It’s undoubtedly a selection effect because our instruments aren’t sensitive enough yet to detect the tiny changes in radial velocity (Doppler shifts) caused by Earth-like and smaller planets orbiting farther from their host stars. The numbers are biased toward hot (close-in) Jupiter-like planets and others near enough to tug in a measurable way.

      1. caralex

        Thanks, Bob. Do you think, though, that Jupiter-sized planets close to their suns might be the norm, with rocky planets further out, or are we fairly average here with rocky planets close in, and gas giants much farther out? I suppose it’s hard to tell, but do you have any ideas yourself?

        1. astrobob

          Excellent question Carol and I don’t think there’s an answer yet. One thing’s for sure, we’re learning there’s more than one way to build one. We also could be seeing lots of these new solar systems at different points in their evolution. In our own, Jupiter migrated closer to the sun before backing out to its current location.

          1. caralex

            Really? I did’t know Jupiter was a vagrant! How do we know that planets move closer and further from the sun?

          2. astrobob

            I’d heard about it before but really got a lot more insight this summer when I went to a conference and heard the whole spiel by a planetary scientist at the U. of Colorado. It’s a bit complicated but Jupiter and Saturn both migrated inward and then out. The migrations were caused by interactions with the solar system’s dust/gas disk and explain things like the current locations and populations of the asteroid belts.

    1. astrobob

      He would be – a passionate guy with grey beard and ponytail and a good sense of humor. I do plan a blog on the topic sometime since I took notes.

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