Speaking of Hubble...

The Lure of the Exoplanet Next Door

October 31, 2012 by Ray Villard
Artist’s impression of the the planet orbiting the star Alpha Centauri B. CREDIT: ESO/L. Calçada/N. Risinger

Artist’s impression of the the planet orbiting the star Alpha Centauri B. CREDIT: ESO/L. Calçada/N. Risinger

Despite the hundreds of extrasolar planets found so far, the discovery of a planet orbiting the nearest star to our Sun has extraordinary consequences for astronomers and the public alike.

The Sun’s nearest neighbor is a multiple star system that lies merely 4.3 light years away – a galactic stone’s throw.

Alpha Centauri is only clearly visible from southern skies as a brilliant white star (it is actually two stars that are too close together to be seen separately by the naked eye). The faint third member of the system, a red dwarf star Proxima Centauri, yields no evidence for planets and is so far from the binary pair as to be inconsequential.

The European Southern Observatory reported the planet discovery on October 17. An instrument called the High Accuracy Radial velocity Planet Searcher, or HARPS, precisely measured small wobbles in the slightly smaller companion star, Alpha Centauri B. This yielded telltale evidence for the presence of an Earth-sized world whirling around the star, completing an orbit every 3.2 days.

The bad news is that the planet is too close to the star to support life as we know it. The surface roasts at over 2,000 degrees Fahrenheit – hot enough for hellish oceans of molten magma.

Alpha Centauri A & B are separated by as much as two billion miles. According to models, they are each capable of forming terrestrial planets despite the perturbing influence of a binary companion. In fact, a companion star can be a gravitationally stabilizing influence, like massive Jupiter is in our solar system.

On the heels of this discovery, the Alpha Centauri system is ripe for far-future exploration. That’s because NASA’s planet-hunting Kepler observatory is showing us that entire planetary systems are common. Kepler has identified several thousand other exoplanets, though most remain to be confirmed by follow-up observations.

What’s more, Kepler is finding that rocky Earth-sized planets are increasingly common. (Kepler is monitoring over 150,000 stars in the constellation Cygnus for telltale planet transits, and Alpha Centauri is not in its field of view). Therefore, it’s probably only a matter of time before other observations turn up additional planets at Alpha Centauri.

It’s also possible that one or more planets could be in the stars’ habitable zones, where temperature are mild enough for stable oceans to exist. The discovery of such a world would eventually be followed by a large enough optical-infrared space observatory that could spectroscopically sample the planet’s atmosphere. Attempts would also be made with NASA’s upcoming 6.5 meter mirror James Webb Space Telescope.

If spectroscopic observations made perhaps with an 8- to 16-meter mirror space telescope confirmed that a planet’s atmosphere has biotracers – such as oxygen, ozone, nitrous oxide, methane, and chlorine – there would be motivation to build a dedicated interferometric array of optical space telescopes. Observations could reveal the waxing and waning of continents and oceans as the planet rotates, and the changing tapestry of weather patterns. SETI observations would monitor the planet for evidence of alien telecommunications.

Today’s fastest space probes would take 40,000 years to get to Alpha Centauri. But by the next century, there could be attempts to send a small probe to the system at extraordinarily faster speeds. Such a mission would have a cruise phase of only 40 years if the technology were to develop for extraordinarily powerful propulsion systems that could accelerate a probe for 10 percent the speed of light. That no small task, but it doesn’t require some imaginary warp drive, simply Newtonian physics. The builders of the spacecraft could live long enough to see data returned from Alpha Centauri.

An ambitious mission would enter orbit around any potentially habitable planet. Robotic landers would be dispatched to observe life up-close and personal. We could behold the effects of Darwinian evolution on an extraterrestrial Serengeti of unimaginably exotic creatures.  By the time we are ready for such a mission we will have matured the required artificial intelligence and nanomachine technology. The autonomous probe would direct its own exploration mission.

Because Alpha Centauri A and B are Sun-like in terms of age and temperature, there has been plenty of time for life to develop on any planets we might find in the habitable zones around either star.

The view from a terrestrial planet in an approximately Earth-sized orbit around Alpha Centauri would certainly be exotic to our experience.  At one point during the planet’s year, the two stars would be in conjunction – in other words, side-by-side in the sky. The closer star would have a glowing disk like the Sun; the other would be more star-like and far more brilliant than Venus.

A planet would be influenced by the radiation from both stars. Every 70 years Alpha Centauri A & B come closest to each other. Warming on an Earth-like world would be brief but intense, raising planet-wide temperature by a few degrees.

Given the awesome power of biological evolution, life would evolve to cope with living with a second star. Life on such a planet might develop two circadian rhythms corresponding to both the length of day on the planet, and orbital period of the binary stars.  There may be planet-wide migrations in anticipation of the approaching “super-summer.” And, there would be a variety of other novel coping mechanisms.

If there were intelligent life present out there, Alpha Centauri astronomers would routinely turn their attention on a bright first-magnitude star in the W-shaped constellation Cassiopeia. It would be our Sun. And the astronomers might muse on what life would be like around a single yellow dwarf star.