Planet Found in the Alpha Centauri System – Could Pandora Be Discovered Soon?

Artist's illustration of the Alpha Centauri System. Credit: ESO/L. Calçada/Nick Risinger (

Reminiscent of the movie AVATAR, a planet has been discovered in the nearest star system to our Sun, Alpha Centauri. This is a trinary system consisting of three stars: Alpha Centauri A, B, and C. Alpha Centauri A is the same type of star as our Sun but slightly larger while its companion, Alpha Centauri B is slightly smaller and cooler. Alpha Centauri C is a red dwarf star also known as Proxima Centauri and is the closest star to our solar system at a distance of 4.22 lightyears. Alpha Centauri A and B orbit each other at a distance of about 23 AU (Astronomical Unit: 93 million miles/150 million kilometers) or about the distance between the Sun and Uranus.

This newly discovered planet is no Polyphemus, the gas giant in the movie that the moon Pandora orbited. The planetary system was in orbit around the star Alpha Centauri A. This planet (designated Alpha Centauri B b) is in orbit about Alpha Centauri B and has an orbital period or year of 3.236 days. It’s mass (minimum mass) is 1.13 times that of Earth and it orbits its star at a distance of about six million kilometers, 3.6 million miles.

The simple facts about this planet belies the huge effort that was put forth to push the envelope of the technology and analysis techniques to find the planet.  This information was gleaned out of data collected from over of four years of observations using the HARPS spectrograph at the ESO LaSilla Observatory (See Finding Exoplanets – Part 2: It’s All About the Mass for more information on the HARPS instrument.) The team of astronomers, lead by Xavier Dumusque (Geneva Observatory, Switzerland and Centro de Astrofisica da Universidade do Porto, Portugal), lead author of the paper were able to improve on the sensitivity of the HARPS instrument by taking into account:

  • The radial motion of the Alpha Centauri star system relative to Earth
  • The stellar oscillation modes for Alpha Centauri B, akin to seismic vibrations
  • The granulation of the star’s surface (the convective zones of rising hot plasma and sinking cooler plasma on the surface, which contribute noise to the measured radial-velocity of the star)

    Image of the granulation of the Sun's surface. Image courtesy of ESA

  • The rotational contribution of the star (as the star rotates, the side moving toward us will be blue shifted while the side rotating away from us will be red shifted)
  • Spots on the surface that are brighter or darker than the mean
  • Magnetic cycle activity
  • Light from Alpha Centauri A contaminating the spectrum of the B star
  • Instrument noise.

After extensive data reduction and analysis, the team determined that the star was wobbling at a velocity of 51 cm/sec (20 inches/sec) due to the planet’s motion. This is about 1.8 km/hr or 1.1 mile per hour!

Although the planet discovered is too close to its parent star to be habitable, at least with life as we know it, the analysis techniques developed to pull the presence of the planet out of the noise can be used to identify planets with a minimum mass of 4 times Earth’s mass in the habitable zone of a star. This opens up a new category of planets that can be searched for.  Note that this is the first planet found in the Alpha Centauri, it may not be the last. It may only be a matter of time before a planet (or moon) like Pandora from AVATAR is found in a star system in the Milky Way.

Till next time,

RC Davison

Planet Found in Nearest Star System to Earth:

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