Alpha Centauri May Have Habitable Planets, if Only We Can Get There
Monday, June 1, 2009
The nearest Earth out there in space? It might be right next door, galactically speaking.
Two teams of astronomers, one from the United States and one from Europe, are in a race to find a planet orbiting our near neighbors Alpha Centauri A and Alpha Centauri B, twin stars that appear from Earth as a single point of light.
"I'm betting that there are planets like Earth or Mars or Venus around either or both of those stars, and the only question is whether we'll be able to detect them," said Debra Fischer, an astronomer at San Francisco State University. Backed with U.S. government funding, she is using a telescope in Chile to assemble 100,000 observations of the Centauri system.
If the astronomers succeed in detecting a planet there, it will be a scientific bombshell -- and it will raise the question of how we might someday send a probe to get a closer look. Alpha Centauri A and B may be our nearest sunlike neighbors (a third, smaller star in the Centauri system, the red dwarf Proxima Centauri, is a hair closer to the Earth), but it's still a long haul from here.
The problem with interstellar spaceflight is the "interstellar" part. We happen to live in a universe that is strikingly vacuous. The Centauri system is nearly 26 trillion miles away.
That's roughly 280,000 times the distance of the Earth from the sun. It's so distant that a beam of light traveling at 186,000 miles per second needs more than four years to cross the interstellar void. If you looked at Alpha Centauri tonight (it's overhead in the Southern Hemisphere), you'd be seeing light emitted right around the time of the second inauguration of George W. Bush.
To frame it another way: The venerable robotic probe Voyager I, which has traveled farther from Earth than any man-made spacecraft, is racing away at nearly 11 miles per second and has already traveled 10 billion miles. It would need on the order of 80,000 years to reach Alpha Centauri were it traveling in that direction.
Conventional chemical rockets are useless for interstellar flight because the ratio of the weight of the fuel to the size of the payload rises exponentially (at some point, you'd need a fuel tank the size of the entire universe). Cutting-edge, nuclear-electric propulsion systems can improve velocity by a factor of 10, but that still doesn't make an interstellar trip plausible. And NASA has stopped funding its most speculative propulsion programs.
Marc Millis, a NASA engineer and propulsion physicist who ran one of the defunct projects (and says he is speaking for himself and not for the space agency), has started a nonprofit organization, the Tau Zero Foundation, in his spare time, hoping to attract attention to research on interstellar flight.
"You need physics breakthroughs," Millis said. "Or undiscovered physics."
In the 1970s, the British Interplanetary Society drew up an elaborate blueprint for what it called Project Daedalus, centered around a 55,000-ton unmanned starship that would make a 50-year journey to Barnard's star, which is 5.9 light-years distant and at the time was thought by some scientists to have at least one planet. It would use a fusion engine (not yet invented) that would propel the spacecraft to 12 percent the speed of light.
Another popular notion is for a laser-guided light sail. A laser in space, powered by sunlight, would beam energy via a lens to a spacecraft fitted with a thin, light-catching sail. But Fischer said research shows that a light sail wouldn't work for an Alpha Centauri mission.
Advances in miniaturization have made smaller spacecraft more plausible. The Starship Enterprise of "Star Trek" is roughly the size of an ocean liner. In real life, proponents of interstellar spaceflight are dreaming of something more like the Cellphone Enterprise: a small, speedy spacecraft, crammed with nanotechnology and capable of beaming snapshots back home.
Perhaps the first probe would be the size of a sewing needle, says Paul Gilster, author of "Centauri Dreams" and of a Web site devoted to interstellar travel. The needle probe might be capable of carrying out elaborate auto-assembly instructions, building a research station using natural resources on that distant world. The spacecraft would be like the seed of a machine, rather than the machine itself.
"I'm certain we're going at some point," Gilster said. "We're not only going to get probes out there, we're going to get people out there someday. It might take centuries for the people."
Robert Cassanova, former director of the now-defunct NASA Institute for Advanced Concepts -- a forward-thinking outfit that, until NASA pulled the plug in 2007 during a budget crunch, funded research on futuristic spaceflight -- doubts that we'll see an interstellar mission anytime soon.
"I would bet that most of the money will be going to exploring planets in our own solar system rather than Alpha Centauri," Cassanova said.
What's certain is that "extrasolar" planets are out there, possibly in great numbers. Hundreds have been found so far, most of them gas giants, their presence inferred in most cases from the subtle wobble in a star's motion caused by the gravitational influence of the unseen planet.
No one has yet found an Earth-size planet, much less one in the likely habitable zone -- at a distance from the star where water might be liquid at the surface. But with planet-hunting at a feverish pitch, such an announcement is considered likely within a few years at most.
Fischer's search was motivated by Gregory Laughlin, a theorist at the University of California at Santa Cruz. Laughlin developed models of planet formation that suggested that double stars, long viewed as unlikely to possess planets, might yet be good targets for a search. He is particularly intrigued by Alpha Centauri B, which is somewhat less massive than our sun and quite a bit less radiant. The habitable zone around Centauri B would be closer in, and the tug of such a planet would be more easily discerned.
A European team has also been looking at Alpha Centauri, and it has instruments that give it an edge in sensitivity, the Americans acknowledge. But the Europeans are casting a wider net. Fischer is devoting more telescope time to the double star.
Laughlin, the theorist, makes a final point about star trekking: This is not a realistic alternative to taking care of our home planet.
"We have what is to all appearances by far the best planet in the galaxy," Laughlin said. "And we have no workable backup plan."