



Alpha Centauri, the Sun's nearest neighboring star system, seen by the Cassini orbiter above the limb of Saturn. / NASA/JPL/Space Science Institute

Sometimes I worry that the popularity of cosmology—the study of the universe on the very largest and smallest of scales—has created a significant misconception about astronomy among the public. Specifically, in cosmology, the perceived worthiness of a question seems roughly proportional to the increasing number of light-years that separate us from its answer. In this view, most events considered interesting will occur a trillion years from now, or already took place many billions of years ago on the other side of the universe. This is exactly opposite from much of the rest of observational astronomy, where many of the most treasured objects are those that, because they are relatively nearby, offer plentiful photons for astronomers to gather and study.

This is particularly true in the search for habitable exoplanets. Planet-hunters need lots of photons from a star to confidently determine whether it harbors any planets, and they need even more photons from a planet to have any notion of whether it harbors anything alive. The relatively low number of available photons from the distant stars in the Kepler field is what will keep us from closely examining more than a handful of the many transiting, potentially habitable worlds we will glimpse there. And it's the pursuit of more photons that drives the costs and specifications of ambitious space telescopes that, once built, should be able to study the atmospheres of small, rocky worlds like ours.

What this means, and what I worry many people don't realize, is that we probably don't need to gaze across the universe or even the galaxy to find another Earth-like planet. In fact, we may only need to look right next door. It could be that, to have a good chance of gathering enough photons to find life beyond our solar system, we need to only build a very large and expensive space telescope for perhaps $5-10 billion, rather than a ridiculously large space telescope that would be an order of magnitude more expensive.

I recently wrote a feature story for Nature exploring how focusing on nearby stars could help reduce the astronomical projected costs of finding and verifying living worlds beyond our solar system, and I'm planning to talk more about the field's near- and long-term prospects later here on BoingBoing, but for now, here's a quick listing of five relatively nearby stars that could soon yield a high-value, world-changing discovery, and kickstart serious efforts to make the next giant leap in observational astronomy.

Pay attention—before you know it, one of these stars may be making front-page news.

1. Alpha Centauri

I'm cheating a bit here, because Alpha Centauri is actually two Sun-like stars, Alpha Centauri A and Alpha Centauri B, orbiting each other with an average separation of a bit more than the distance between our Sun and Uranus. There's also a red dwarf, Proxima Centauri, drifting in the outskirts of the system. Both A and B have stable regions in their habitable zones where small planets could exist, though there is some debate over how easily planets can form around such binary pairs. Most importantly, at only 4.3 or so light-years away, the stars of Alpha Centauri are the closest in the sky other than our own Sun. They're very bright, and very easy to study, though no planets have yet been found. At least three planet-hunting teams are currently performing radial-velocity surveys of Alpha Centauri, with a particular focus on B, which is a more quiescent star, and thus easier to monitor. For additional context about these stars and two of the competing teams, check out my 2009 feature story in Seed magazine, The Long Shot.

2. Gliese 581

This red dwarf star is relatively close by, just over 20 light-years away, and is one of the few stars of this very promising type to have been closely scrutinized for planets. Searches have discovered 4 planets and 2 candidates that await confirmation. Two of these objects, Gliese 581 d and the to-be-confirmed Gliese 581 g, are small enough and close enough to the star that they could conceivably be habitable. I wrote about Gliese 581 g, the so-called "Goldilocks" world, shortly after its discovery announcement last year, describing what its environment might be like. Since then, doubts about g's existence have received lots of media coverage, but one aspect of this story has gone largely overlooked: Even if many of the details of Gliese 581 g as reported in its discovery paper are proved incorrect, that particular orbital slot could well be occupied by another, smaller, potentially more habitable planet that has yet to be indisputably detected. Given the public uproar and resulting increased observational cadence for this system, I'm guessing we'll know soon whether Gliese 581 has a Goldilocks.

3. GJ 1214

At 40 light-years away, this red dwarf star is close to the cusp of my comfort-zone in labeling it "nearby." And I'm also being a bit generous in its potential to deliver a game-changing discovery for habitable planets. The only known planet this star does have, GJ 1214 b, is a warm "super-Earth" of about 6 Earth-masses, and isn't really very Earth-like at all. But GJ 1214 b is very special, because it transits—which allows astronomers to examine its atmosphere and learn more about the planet's composition and history.



Deeper investigation of worlds like GJ 1214 b is very important, because while Kepler and other surveys are finding them seemingly everywhere they look, no planets like this exist in our own solar system. Consequently, at present it's quite difficult for astronomers to say whether planets of several Earth-masses could realistically be habitable. These worlds may almost always turn out to be more like "mini-Neptunes" than super-Earths. GJ 1214 b is the most easily-studied planet of this sort known to us, but hopefully more examples will be found soon transiting nearby stars.

4. HD 40307

HD 40307 is a K star slightly smaller and cooler than our Sun. It's even further away than GJ 1214, lying 42 light-years distant in the constellation Pictor, but it's of great interest to planet-hunters because of its confirmed planets, three worlds discovered in 2008 in scorching, nearly circular orbits, all less than ten times Earth's mass.



These worlds were discovered via the radial-velocity technique, meaning that astronomers can only estimate their mass and don't have reliable information on their density or composition. Just as with most other super-Earths, it's not entirely clear whether they're large rocky planets with thin atmospheres or bloated balls of gas with smaller rocky cores. Given the growing number of known planets of this kind, this system doesn't at first glance appear very notable. But there are tantalizing residual motions in its radial-velocity signals that hint at an additional orbiting companion further out in the system, perhaps even in the star's habitable zone. The European Southern Observatory's HARPS spectrometer, the world's premier radial-velocity instrument, is closely monitoring HD 40307 in hopes of finding additional, more habitable planets there.

5. 61 Virginis

61 Virginis is a star very similar to our Sun, though probably a billion or so years older, just less than 28 light-years away. Like HD 40307, it's known to harbor at least three planets in scorching, tightly packed orbits, a super-Earth and two Neptune-sized worlds that were discovered in 2009. The system also appears to have a debris disk of cold dust in its outer reaches, likely the product of cometary impacts in an analog of our own solar system's Kuiper belt. According to calculations by members of the discovery team at the University of California's Lick Observatory, stable orbits exist in 61 Virginis' habitable zone. 61 Virginis is one of the top targets for Lick Observatory's new Automated Planet Finder Telescope, a facility dedicated exclusively to radial-velocity planet-hunting that is beginning operations this year. By having the ability to observe this and other target stars each and every night instead of in infrequent chunks, the APF could rapidly discover many small, rocky planets.

Got questions about why I chose these stars instead of others? Do you have your own preferred list of most promising places to look for potentially habitable planets? Let me know in the comments!