Man's Next Migration

Alpha Centauri: Binary Possibilities

The Alpha Centauri system is perhaps the first destination of a human space mission if present technology will accelerate to the point of developing ships capable of going close to the speed of light. In futuristic space science fiction, Alpha Centauri almost always becomes the first "port of call" or the human's "gateway" to the stars. Understandably, the star system is after all the closest to Earth at about 1.34 parsecs (4.37 light years). The Goldilocks Mission may choose Alpha Centauri as its first destination given that scientists in the last decades have come up with convincing calculations that a terrestrial planetary system may exist within the binary star's habitable zone.

That's right, Alpha Centauri is a binary star system consisting of two stars, α Cen A and α Cen B orbiting a common center of gravity. The larger yellowish-white star, α Cen A, is a brighter solar analog with a mass that is 10% greater than the Sun's. Its companion star, the yellow-orange α Cen B, is a smaller solar analog that has a mass 90% that of the sun. The two stars are 23.7 AUs apart (average) but with their elliptical paths, the minimum distance is at 11.4 AU and the farthest at 36 AU, an orbit that is completed every 80 years. Because of this, the brightness of the stars differ with their positions in the sky when viewed from an possible exoplanet within the system. Both the elliptical orbit and the resulting brightness observation does not have significant effects on the habitability of the planet. In fact, when the stars are at their nearest, the distance between them is at least 2 AUs greater than the mean distance of Saturn from the sun. At their farthest, the separation is at 6 AUs greater than the mean orbital distance of Neptune.

Scientists agree that the major factor in determing the stability of a planet within orbit of any of the two stars is not based on the distance between the stars but it is rather dependent on the planet's distance from its parent star. Calculations suggest that an exoplanet with a stable orbit may exist in any of the two stars as long as the distance to its parent star is no greater than one-fifth of the closest approach of the other star. Ideally, Alpha Centauri A or B could host as much as four terrestial exoplanets, at a distance less than 3 AU for planets bound to either one of the stars in a parallel orbit, and 70 AU for planets that orbit both of the stars!

Just like any other solar analog, both stars possess high metallicity, characteristics that favor the formation of terrestrial exoplanets. A study published in 2008 showed the majority of  simulations ran in period of 200 million years worth of the binary star system evolution always resulted to the formation of earth-like planets in α Cen B's habitable zone.

In the same year, two groups of astronomers began their detailed hunt for earth-like planets within the binary star using observatories in Chile. The radial velocity method is being employed to detect "wobbles", spectral changes that result from the gravitational effect of planets orbiting their star. The team at the Cerro Tololo Inter-American Observatory ran computer simulations that show successful evolutionary planet formation. Greg Laughlin, one of the astronomers who was from the University of California in Santa Cruz has said that they will focus on α Cen B which is calmer than its twin in terms of changes in the star atmosphere which will make spectral observation easier. The other team uses the High Accuracy Radial velocity Planet Searcher which the group claims to be sensitive enough to detect planets as small as Mars. Both teams need several years worth of data to come up with viable observations. In 2009, a third team based at the Mt John Observatory in New Zealand have also started focusing on Alpha Centauri using a Hercules spectograph and a technique called gravitational lensing, considered to be more sensitive than the previous methods. The astronomers, led by John Hearnshaw of the University of Canterbury, claim that three years worth of 30,000 spectral images could yield detection of a goldilocks planet at 1.2 AU from α Cen A or 0.75 AU from α Cen B. Most major studies of exoplanets require years and decades of data. Three years is indeed something!

NASA's Space Interferometry Mission (SIM), scheduled for a tentative launch in 2015, will be using eight groundbreaking technologies developed in less than a decade that will be able to accurately detect small habitable exoplanets that have a mass as small as three times that of the earth. Alpha Centauri is on its top list of target stars to search for exoplanets.

Although Alpha Centauri is just 136,379 times as far from Earth as Mars is, it is not a far-fetched possibility for a space mission that will further confirm the presence of exoplanets. Gamma Cephei is also a binary star system similar to Alpha Centauri which situated 45 light years away. In 2002, after almost a decade of contrasting views within the astronomical community, it was finally confirmed to host a Jupiter-like exoplanet. This in itself further cements the possibility of detecting terrestrial, and perhaps habitable planets in the Alpha Centauri binary star system.

References:

Sol Company. "Alpha Centauri 3".

John Hearnshaw. Cosmic Diary. "The race to find Earth-like planets in their habitable zones, and the prospects for alpha Centauri". 22 Oct. 2009

ScienceDaily. "Alpha Centauri Should Harbor Detectable, Earth-like Planets". 07 Mar 2008.

Stephen Battersby. New Scientist. "Nearest star's wobbles could reveal Earth's twin". 29 Feb 2008.

Wikipedia. "Alpha Centauri".

NASA Solar System Exploration. "Space Interferometry Mission PlanetQuest".