A few weeks ago, I discussed the possibility of finding habitable planets in binary star systems. Just a couple of days ago, astronomers announced the discovery of an Earthlike exoplanet—called Proxima Centauri b—orbiting Proxima Centauri, one of the closest stars to Earth. Proxima Centauri also appears to belong to a multiple star system that includes the binary stars Alpha Centauri A and B. Let’s look at what this discovery means.
Details of the Host Star
Proxima Centauri sits 15,000 astronomical units (AU) away from the Alpha Centauri binary pair. If it orbits this pair of stars, it does so at least every 500,000 years. When viewed from Proxima Centauri b, the Alpha Centauri binary system would appear as two stars, both much brighter than Venus—the brightest object in the sky after the sun and the moon. While Alpha Centauri A and B are similar in size to the sun, Proxima Centauri has only 12 percent the mass of the sun. Consequently, it gives off about one-tenth of a percent of the sun’s radiation. The small mass and luminosity mean that the habitable zone (as usually defined) has an inner boundary of 0.042 AU and an outer boundary of 0.082 AU. (See the article in Nature for a full description of Proxima Centauri and Proxima Centauri b.)
One other aspect of Proxima Centauri warrants mention. Astronomers have monitored the star for a number of years and have noticed frequent bursts of high-energy radiation from the star. Proxima Centauri is classified as moderately active, but it gives off bursts of radiation that would measure 400 times larger on Proxima Centauri b than anything Earth experiences from the sun.
Details of the Exoplanet
While most of the recent exoplanet discoveries arise from the transit method and the successful Kepler mission, Proxima Centauri b was found using the radial velocity (or Doppler) method. Given the limitations of this method, astronomers can only determine a minimum mass for the exoplanet of 1.3 times Earth’s mass. Proxima Centauri b orbits at 0.05 astronomical units (one AU is the distance from Earth to the sun), ten times closer to its star than Mercury is from the sun. Given this distance and the size of the star Proxima Centauri, it takes 11.2 days for the planet to complete an orbit. However, this orbit places Proxima Centauri b well within the habitable zone of Proxima Centauri!
Proxima Centauri b also has an Earthlike mass and orbits in its star’s liquid water habitable zone. However, the small distance between Proxima Centauri and Proxima Centauri b means that the planet is tidally locked, i.e., the same side of the planet always faces the star. While research shows that some tidally locked exoplanets could still have liquid water, the lack of a magnetic field (since there is no rotation) and the intense radiation bursts emitted by Proxima Centauri put tremendous stress on the atmosphere and likely remove any water from the planet.
The Big Deal
Why am I excited about this discovery? I am excited because the proximity of the exoplanet to Earth enhances the chance that astronomers could actually observe Proxima Centauri b (within a decade or two) in enough detail to determine if the planet has liquid water, if it has continents or clouds, and if it has any signs of life. Only data of this nature will truly determine what characteristics a planet must have to be truly habitable.