Alpha Centauri Bb: An Interstellar Target?

Could the discovery of an Earth-sized world in the Alpha Centauri system invigorate demand for an interstellar probe to investigate up close?

A little exoplanet living in a neighboring star system has caused a very big stir this week.

It may be too hot for life to survive on its surface, but the discovery of an Earth-sized planet orbiting Alpha Centauri B has ignited hope that the star - one of two that orbit one another as a binary pair - could play host to a whole system of rocky worlds.

NEWS: Earth-Sized World Found Next Door

The very fact that scientists using the European High Accuracy Radial velocity Planet Searcher (HARPS) telescope could detect the tiny gravitational tug of the orbiting world on its parent star is amazing enough, but the ramifications for the future of mankind have the potential to be nothing less than historic.

"The discovery of an Earth-sized world that is so close to us, when measured in interstellar distance scales, means that when we select target solar systems for exploration, we may not have to choose systems that are prohibitively far from our own solar system," said Richard Obousy, co-founder and president of Icarus Interstellar Inc.

Icarus Interstellar is a project to realize the possibility of sending an unmanned probe to another star system within the next century. Interstellar distances are, currently, prohibitively vast, so it's desirable to look for new worlds to explore that are located in our cosmic backyard. Now, astronomers have discovered evidence of an Earth-sized world located on our proverbial cosmic doorstep.

To bridge the gap between the stars, novel propulsion techniques will be required to accelerate a starship to an appreciable fraction of the speed of light, a challenge the Icarus team are currently investigating. For more on the projects Icarus Interstellar are developing and the technology behind them, browse our special Wide Angle: "Reaching for Interstellar Space."

A Ripe Target

Alpha Centauri is located around 4.4 light-years from our solar system and can be found in the southern constellation of Centaurus. Apart from Proxima Centauri, a small star located 4.2 light-years away and thought to be gravitationally bound to the Alpha Centauri binary, Alpha Centauri is the closest star system and a ripe target for a future interstellar probe.

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There may be a small planet orbiting a nearby star, there could even be a whole as-yet-to-be-discovered system of planets, but what would be the incentive to drive us to send probes and, possibly, send humans across the interstellar expanse?

"One compelling reason is the idea of colonization of other solar systems, which would not only be a natural extension of mankind's compulsion to explore and settle new lands, but also serve as an excellent hedge for the very survival of our species should Earth experience some catastrophic event leaving it uninhabitable," Obousy said in an email to Discovery News.

"This is a tremendously exciting discovery," said Ian Crawford, professor at the Department of Earth and Planetary Sciences at Birkbeck College London and lead designer for Icarus science and target selection. "The alpha Cen system was already the front runner as an Icarus target, because of the three different types of star it contains. So the discovery of a planetary system just reinforces the system's priority as a target."

Last year, Crawford wrote a Discovery News guest article "Which Exoplanet to Visit?" detailing the priorities of target star selection for an Icarus probe. Keeping in mind that one of the design requirements for an interstellar vehicle is that it must arrive at its destination within 100 years from launch; suddenly it becomes a question of how far, realistically, could a starship go within that tight timeframe?

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"It follows that the actual target will probably have to be significantly closer than 15 light-years (from Earth)," Crawford wrote in his February 2011 article. "Within 15 light-years of the sun there are approximately 56 stars, in 38 separate stellar systems."

Although many of those 56 stars hold promise, it would be wonderfully fortuitous if the star right next door played host to a bounty of Earth-sized exoplanets.

An Interstellar Demand?

"I have often imagined the day when scientists directly image an Earth-like extra-solar planet," Icarus Deputy Project Leader Robert Freeland told Discovery News. "We would be able to determine the planet's atmosphere and surface temperature from its spectrum, and we would thus know whether it might be able to sustain life as we know it. I suspect that once such a discovery hits the news, people worldwide are going to demand that we send a probe to determine whether the planet has life (of any type) and/or could be suitable for human habitation. If the latter proves true, then a manned mission would eventually follow."

Freeland added that in the case of this most recent discovery, we're not looking at a viable exploration target. This world's orbit is ten-times closer to its star as Mercury is to the sun - it would be a hellish, (likely) rocky, molten world. Alpha Centauri Bb orbits well inside the nearest edge of the star's ‘habitable zone' - the region where liquid water can exist on the surface - so this exoplanet is the very antithesis of ‘habitable.' "The discovery of (Alpha) Centauri Bb isn't quite as momentous as this owing to its proximity to the star, but this discovery is a warning shot that the news is coming a lot sooner than we think," he continued.

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By Freeland's reckoning, when we do discover a bona fide Earth-like world encircling one of the stars in the Alpha Centauri system (i.e. a world of the approximate mass of Earth orbiting inside the habitable zone), the world's space agencies (particularly NASA) would need to be prepared, lest be caught "flat-footed" when a wave of public pressure to mount a mission to that planet demands why a plan isn't in place.

"Icarus Interstellar and its partner organizations - the British Interplanetary Society and the Tau Zero Foundation - have been working on interstellar mission designs for years, and we're eager to help NASA/ESA jump-start a serious, fully-funded interstellar program.

"It is my strong opinion that the quest for life on other planets should be the core mission of our major space agencies for the 21st century and beyond, and that such a mission would galvanize public support behind them," he concluded.

No Wobble?

Although there's reason for some guarded excitement about the discovery of Alpha Centauri Bb, Crawford cautions that detecting small exoplanets at larger orbital distances from the star cannot be done using the "radial velocity" detection technique. Earth-mass worlds orbiting further away will have less of a gravitational impact on the host star, thereby causing it to wobble less.

"Even these very sensitive radial velocity measurements are incapable of detecting Earth-mass planets in the alpha Cen B habitable zone - with the lowest mass detectable at (habitable zone) orbital distances being 4 Earth-mass super-Earths," he said. "Therefore, despite the very real cause for excitement about this detection, it may still be a long wait before we know whether or not this star also has Earth-mass planets in its habitable zone."

For now, we'll likely need to be patient before we discover a low-mass alien world orbiting within a neighboring star's habitable zone, but as we develop exoplanetary observation techniques, there's hope that we may not be waiting very long.

"It's my hope that this discovery not only increases the exposure of the wonderful work that astronomers are undertaking, but that it also invigorates the field of interstellar flight, by giving us a, comparatively, ‘easy' destination," concludes Obousy. "I also hope that further work is performed on the Alpha Centauri B system, as the discovery of a planet in the habitable zone would be even more exciting than this, already profound, discovery."

Image: The second stage of the British Interstellar Society's Project Daedalus interstellar spacecraft arrives at its target star system. Could that target be Alpha Centauri? Image courtesy Adrian Mann