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The Closest Extra-solar Planet to Earth:
What's Alpha Centauri Bb Like and How Can We Get There?

by Les Johnson

Alpha Centauri. For science fiction fans of a certain generation, and you know who you are, these two words evoke a sense of wonder and excitement at the prospect of traveling to another star. This is due primarily to one of the more campy television shows we watched in our youth – Lost in Space. Yes, for another generation there was a motion picture reboot of the same name for which I served as a technical consultant (no comments on the science in the movie please – the director simply ignored my advice) but it was not enough of a success at the box office to warrant a sequel.

For the more literary, there have been few extra-solar destinations that have inspired so many excellent science fiction novels as does the Alpha Centauri system. Here are but a few that make this author’s “best of Alpha Centauri” list and adorn the book shelf in my home library:

- Foundation and Earth by Isaac Asimov
- Encounter with Tiber by Buzz Aldrin
- Downbelow Station by C.J. Cherryh
- The Songs of Distant Earth by Arthur C. Clarke
- Voyage from Yesteryear by James P. Hogan
- Footfall by Larry Niven and Jerry Pournelle
- The Killing Star by Charles Pellegrino
- The Sparrow by Mary Doria Russell
- Factoring Humanity by Robert Sawyer
- Revolt on Alpha C by Robert Silverberg
- Starfire by David Weber and Steve White

And these are just the ones I’ve personally read in my years as a fan!

For those who are both fans and followers of science and astronomy discoveries, the news that an Earthlike planet has been found in the Alpha Centauri system probably sent chills down your spines. It certainly did mine. To understand and fully appreciate the significance of this discovery, some technical background is required.

Artist impression of the planet around Alpha Centauri B.

Image courtesy of European Southern Observatory.

First of all, despite the name, Alpha Centauri is not a single star. It’s a binary star with the official name of “Alpha Centauri AB.” A binary star is exactly what its name implies: two stars that are located close together, relatively speaking, and are gravitationally attracted to one another. Astronomers estimate that single stars, like our sun, comprise about two-thirds of all the stars in the galaxy with the remaining one-third being multiple stars, including binaries like Alpha Centauri AB. This is important because scientists once thought that it would be nearly impossible for binary stars to have planets in stable orbits due to the changing gravity they would feel as they orbited – one star would tug more on the planet at times than the other, causing instabilities that would prevent the planet from remaining in a single orbit for any significant length of time. They were wrong.

But wait, the star system gets more complicated. There is a third star, Alpha Centauri C, which appears to be connected gravitationally with Alpha Centauri AB. Alpha Centauri C is a red dwarf (meaning that it is much smaller and cooler than our sun) and although it is gravitationally bound to Alpha Centauri AB, it is some distance away. And Alpha Centauri C just happens to be the closest star to us. Astronomers now talk collectively about the entire three star system as Alpha Centauri AB-C.

Alpha Centauri A and B are particularly interesting for astronomers and science fiction writers because they are both very similar to our sun. If Sol, our sun, is “just right” for producing planets with life, then it would make sense that other similar stars could do the same. And since they are close, at least in astronomical distances – I wouldn’t call being ~25,278 billion miles away “close,” – any planets found there would be exciting and hugely significant. After all, if we ever achieve the ability to travel between the stars, Alpha Centauri AB-C would be our likely first destination and wouldn’t it be great if we found life there?

The relative sizes of the stars that make up the Alpha Centauri system.

Image courtesy of David Benbennick.

Astronomers have been detecting planets outside of our solar system since 1992 and there are now about eight hundred confirmed extrasolar planets and several thousand more found by NASA’s Kepler mission that are awaiting independent confirmation. I wrote about this topic in a recent Baen essay called “Rediscovering the Universe,” where you can learn about the various methods used to find these other worlds. If the discovery is independently confirmed, then we can add the planet Alpha Centauri Bb to that list.

The discovery of a planet circling one of the stars in the Alpha Centauri AB-C system was reported in the Journal Nature on October 17, 2012. The European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS) project found the planet using their 11.8 foot telescope in Chile. Four years of measurements found that Alpha Centauri B is wobbling – wobbling regularly and in such a way that indicates it is being orbited by a planet. But don’t get too excited, Alpha Centauri Bb doesn’t sound like a nice place to visit.

Alpha Centauri Bb is just a little more massive than the Earth but it orbits way too close for life as we know it to exist. The Earth orbits the Sun at an average distance of 93 million miles. Alpha Centauri Bb is only 3.5 million miles from its parent star so the discovering science team estimates that its surface temperature is about 2240 degrees Fahrenheit. Another interesting bit of data is the length of its year, or the amount of time it takes to go around its parent star one time. For the Earth, one year is 365 days; for Alpha Centauri Bb one year is only about 3 days. Unless you are a Horta, you won’t find Alpha Centauri Bb a nice place to live – but it would still be a way cool place to visit.

There is a planet orbiting Alpha Centauri. What do we do about it?

We go there, of course!Unfortunately, Alpha Centauri AB-C is about 4.3 light years distant. A light year is the distance light travels in one year; light travels at ~186,000 miles per second. The Voyager spacecraft, launched in 1977, is one of our fastest spaceships and it is on its way out of the solar system and into interstellar space. If it were pointed in the correct direction, which it is not, then it wouldn’t arrive in the Alpha Centauri AB-C system for another 74,000 years. Chemical rockets, the primary method we currently use to explore our solar system, are incapable of reducing the trip time to anything reasonable. We clearly need a new type of space propulsion. Fortunately, nature has provided us with some possibilities and none will require us to rewrite the laws of physics: matter/antimatter annihilation, nuclear fusion, solar and laser sails, and nuclear pulse propulsion systems. All are physically possible but they will be very, very hard to engineer.

A fusion driven starship enroute to Alpha Centauri Bb.

Image courtesy of Adiran Mann.

Matter/Antimatter Annihilation: Antimatter is real and is produced regularly by nature and by humans in our nuclear research facilities like the European’s CERN (Conseil Européen pour la Recherche Nucléaire). If large quantities of antimatter can be created, stored and safely controlled, then it could be used to propel a spacecraft to the stars. The annihilation of matter and antimatter is the most energetic reaction known and converts all of the reaction products’ mass into energy according to Einstein’s famous E=mc2 equation. Unfortunately, it is beyond our current engineering capability to do any of these three so the total global production of antimatter remains under one nanogram or ~0.000000000002 pounds (yes, that is a lot of zeros!), it isn’t stored at all, and we have no idea if we can actually control it in any safe way. And, oh yes, to propel a starship to Alpha Centauri AB-C would require thousands of pounds of antimatter.

Nuclear Fusion: The Sun is powered by fusion, we can cause atoms to fuse in the laboratory, and fusion is what powers a hydrogen bomb. Several countries have been working for years to develop fusion energy for use in power plants and progress is being made. Unfortunately, as of this writing, we’re currently not getting more energy out of these fusion reactions than we put in to cause the reaction in the first place nor are the research facilities designed to lead to the compact fusion reactors that might actually be launchable into space someday. Nuclear fusion as an interstellar space propulsion system is closer to being real than antimatter based devices, but I wouldn’t hold my breath.

Solar and Laser Sails: A solar sail reflects light and gains momentum, hence velocity, from the interaction. They are typically made of large, lightweight reflective materials and many countries are working to develop the technology for robotic interplanetary travel. IKAROS, the world’s first interplanetary solar sail, was launched by Japan in 2010. If a very large solar sail (the size of Texas) were to be deployed very close to the Sun (inside the orbit of Mercury) in order to capture lots of light and get a big push, then it might make the trip to Alpha Centauri in a thousand years. If we were to put huge lasers into space (think the Death Star), then we could continue to push the sail as it departs our solar system, accelerating it to the point where a trip to Alpha Centauri AB-C might be possible within a hundred years of launch. Unfortunately, we don’t have the materials that are light enough, strong enough or temperature resistant enough to fly one of these sails; we don’t know how to make one the size of Texas and launch it; and we certainly don’t have any Death Star lasers ready to put into space. Alas.

Nuclear Pulse: Now this is a propulsion system we could probably build and use it to travel to Alpha Centauri Bb within only a few hundred years of launch. Originally known as Project Orion, the idea is simple: detonate small nuclear bombs underneath a very large ship equipped with radiation shields and a pusher plate (beneath which we would detonate the bombs) and off you go. The idea was actually studied extensively in the 1950s and ‘60s before it was canceled. To launch the starship, we’d likely end up trashing and radioactively polluting a large part of Earth. One could build it in space, but getting all that mass off the Earth in the first place would be a huge challenge. Nuclear pulse is an idea whose time has come – and gone.

So, without a breakthrough, we’re centuries away from having the technology to actually go there and check it out for ourselves. What else can we do?

A picture is the next best thing to being there, right? So why don’t we build a telescope to observe Alpha Centauri Bb and any other extrasolar planets of interest? Can we just point the Hubble Space Telescope toward it and take a look?

Despite being one of the brightest stars in the sky, without a telescope there is no way to see that the Alpha Centauri AB-C system is actually three stars. And, even if it were possible and if you are a reader in the northern hemisphere of the Earth, then you’d be out of luck anyway. Alpha Centauri AB-C is only visible from the southern hemisphere.

The Hubble is great telescope but it simply isn’t designed to see a relatively tiny planet orbiting another star, let alone image it with the kind of clarity we’d like to have. We’d like to use a telescope to not just glimpse the planet, but also to image it like we do the other planets on our solar system to answer some of these questions: Does it have an atmosphere? Are there oceans? What gases are in its atmosphere-- if it has one? And are there any signs of life?

Aside from having optics large enough to gather the light reflecting from the planet and traveling the immense interstellar distance to Earth, the telescope would have to see the planet’s reflected light without the glare of the star itself blinding it. Remember, from Earth, Alpha Centauri B and its planet are very close to each other and resolving them in the bright light of the star would be like trying to see a firefly flash while staring at a spotlight. To do this would require a telescope at least four times larger than the Hubble with a mirror several orders of magnitude more precise. Telescopes with this capability were assessed by NASA in their Terrestrial Planet Finder mission studies. Various ideas for blotting out the starlight have been proposed, including flying a large occulter that would block the light and cast a shadow on the telescope that is performing the imaging. These telescopes would help us find and study extrasolar planets, but they would not give us the view we’re really like to have.

My personal favorite approach is known at the Focal mission. The idea is to use an interesting aspect of Einstein’s General Relativity Theory, known as gravitational lensing. Light is constrained to move in space-time and space-time can be bent by the presence of mass. The larger the mass, the more the bending. Bending light is exactly what lenses do, using a different physical process, but the end result is the same – bending light can produce focused light. The mass of our Sun can bend light with a focus about 550 Astronomical Units (AU) from the Sun – or at about 51 billion miles. If we were to place a powerful telescope near this focus, then we could use the gravity bending of light by our sun to image the surface of Alpha Centauri Bb with unprecedented clarity. This mission would be challenging with today’s technology, but it is certainly far from impossible.

Artist concept of the Focal mission that would place a telescope beyond 550 Astronomical Units from the sun in order to image distant objects taking advantage of the Sun's ability to bend light.

Image courtesy of Adrian Mann.

Scientists and science fiction fans should be excited about the news that a planet orbits a star so close to home. Will we be able to go there in our lifetime to check it out? Unfortunately, barring a breakthrough, the answer is no. Will we be able to look at the planet? Perhaps, but not unless it becomes a higher priority on the agenda of space scientists. Will there continue to be exciting and thoughtful new books and movies that have setting on or near this nearby alien world? I certainly hope so!

Suggested Reading:

Deep Space Propulsion: A Roadmap to Interstellar Flight, by Kelvin Long

The Starflight Handbook by Eugene Mallove and Gregory Matloff

Going Interstellar edited by Les Johnson and Jack McDevitt

“The New Age of Exploration: Crazy Far,” National Geographic Magazine, January 2013

Copyright © 2013 by Les Johnson

Baen author Les Johnson is Deputy Manager for the Advanced Concepts Office at the NASA George C. Marshall Space Flight Center in Huntsville, Alabama. Johnson is the coeditor of science and science fiction anthology Going Interstellar, and the coauthor with Travis S. Taylor of science fiction novel Back to the Moon, and of the forthcoming Rescue Mode, with Ben Bova.

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