"Death comes to all, but great achievements build a monument which shall endure until the sun grows cold." -Ralph Waldo Emerson

In the great cosmic ocean, there's only one planet that we know -- for certain -- has the right conditions and history to result in intelligent life: our own.

Image credit: NASA, from the Space Shuttle, for Sun-Earth Day 2008.

Life -- or even intelligent life -- may be possible in environments vastly different than our own: around different classes of stars, at different temperatures, and even with different molecules and/or chemical elements.

But we know for certain that it's definitely possible with the conditions we have here on Earth, given our Sun with its properties and its distance from us.

Image credit: F. Espenak of http://astropixels.com/, from this past August, 2012.

Our Sun is the only star in its system, has the temperature and luminosity properties of a G-type star, and the Earth is located at just the right distance that liquid water -- existing for long periods of time -- is prevalent on our world's surface.

You might be curious, looking up at the night sky, what the nearest star to us is that is also a G-type star, and the only star in its Solar System. To find it, just look in the constellation of Cetus, and find this clearly visible naked-eye star.

Image credit: Wikipedia user Till Credner's own work: via AlltheSky.com.

Tau Ceti is the 20th closest star system to us, only 12 light years distant. (That's less than a third the length of Han Solo's famous Kessel Run!) It's the same class as our Sun -- nearly the same temperature -- with 78% of our Sun's mass, and a corresponding smaller radius. It's also less active and less variable than our own Sun, and slightly "yellower" due to the slightly lower temperature.

Viewed from the same distance, the Sun and Tau Ceti would appear slightly different from one another, with the Sun on the left and Tau Ceti on the right.

Image credit: PaintShopPro illustration by contributor R.J. Hall.

For a long time, we didn't know whether Tau Ceti had any possibly habitable planets around it, or -- for that matter -- any planets at all! It's true, of course, that we've found thousands of planets around thousands of stars, and we now believe that the vast majority of stars do have planets around them.

Image credit: NASA / Kepler Mission / Kepler Science Team.

But that doesn't mean that we can look at any star we choose and know whether there are planets around it or not. The way we've found most of these planets is by the transit method, where we look at the variability in the amount of light coming from a distant star. If there's a planet (or multiple planets) that pass in between that star and our line-of-sight, it will block a small amount of that light during that planetary transit.

And over time (or, I should say, over many orbits), we can observe multiple transits by the same extra-solar planet, allowing us to determine how many planets there are in that star system that transit their parent star, as well as to measure the radius and semimajor axes of those worlds.

Image credit: NASA / Kepler Mission.

Even though this is likely to soon become the most prolific method of detecting extra-solar planets, it also has a fatal flaw which makes it unsuitable for detecting most of the planets around most of the stars out there.

Think about it for a second. The Sun is a pretty big object: it's slightly over one million kilometers (1.38 million, to be precise) in diameter, or about 109 times the diameter of the Earth. But it's really, really far away from the Earth, at an average distance of 150 million kilometers. We don't usually draw the Earth, Sun and Moon to scale when we talk about them, and there's a good reason for it: the distances between them are tremendous compared to the actual sizes of these objects. If we drew it to scale, it would look like this.

Image credit: The old Rapt in Awe blog / Driftway Observatory, http://www.giveyoujoy.net/awe/.

If you can't see the Earth, that's because it'd be just about a twentieth of one pixel in the image above! If you were at a random location in the sky, you'd have under a 1% chance of being able to detect Earth via this transit method; only very fortuitously located worlds would have a shot.

But there is another, older method that could have a shot even if you weren't favorably aligned: Doppler Spectroscopy.

Image credit: Wikipedia User Zhatt.

The key to this method is that, while it's a very good approximation in our Solar System that the Sun remains fixed at the center while the planets orbit it in ellipses, a more accurate picture is that each of the planets also exert a gravitational pull on the Sun while they orbit it. This means -- as it moves towards and away from us -- the light from it will be blue-and-redshifted (respectively) in a periodic fashion.

Image credit: Wikimedia Commons user Reyk; original work.

This works best if the planet does transit the star, but it also allows, in principle, for the detection of any planet in a system that's not exactly "face-on" to our line-of-sight. Because we know how gravitation works, when we observe a star "wobbling" (or moving forwards and backwards, periodically) with respect to our line-of-sight, we can infer the mass of every planet that causes this radial motion.

As long, that is, as we can find a signal that exceeds the noise of our measurements.

Image credit: The Exoplanet Orbit Database and the Exoplanet Data Explorer at exoplanets.org, maintained by Dr. Jason Wright, Dr. Geoff Marcy, and the California Planet Survey consortium.

For small-mass planets, the effect is tiny, and so Earth-sized planets have been very difficult to detect using this method unless they're extremely close to their parent star.

But a newly discovered technique may have just doubled the sensitivity of this method, severely reducing the noise of this radial velocity technique. And our nearest Sun-like star system, Tau Ceti, is the first beneficiary!

Image credit: J. Pinfield / RoPACS network / University of Hertfordshire.

Five candidate planets have been discovered around this lone G-star just 12 light years away, using this new technique. It took 14 years of observational data, and many detailed spectroscopic measurements of this star, to build up enough orbits of the inner planets to find these worlds. Although the announcement is still tentative and the discovery unconfirmed, this is very exciting. According to the Australian Broadcasting Company:

Tau Ceti was selected to calibrate the new technique because it's a very stable star, which after 14 years of study, showed no signs of a planetary system. "Because it's so close, bright and similar to the Sun, it's a particularly valuable target for study," says [Dr. Jonti] Horner. Once all the noise had been accounted for using the new modelling techniques, astronomers detected a signal indicating the presence of a planetary system.

Oh, and did I mention, one of those planets is about the same distance from Tau Ceti as Venus is from the Sun, putting it squarely in Tau Ceti's habitable zone!

Image credit: Wikipedia user Chewie, derived from Ignacio javier igjav's work.

No one expected there to be planets around this star, much less a rocky world in the habitable zone; it was pure serendipity that this star, selected for its stability and proximity to develop a noise-reducing technique for doppler spectroscopy, happened to have an interesting rocky, inner solar system.

And if these planets do get confirmed, and turn out to be real? The proximity and stability of Tau Ceti means that it will likely not be long before we measure the atmospheres of these worlds!

Image credit: Exoplanetary spectrum around HR 8799, via ESO / M. Janson.

There's still plenty of work to be done and plenty of caution to be exercised, but you have every reason to get excited about the future of planet hunting in general and the worlds around Tau Ceti in particular! If confirmed, this would be the closest-ever potentially habitable world to our own! Follow all the news on Exoplanets here; you won't regret it!

So get to work on those starships, Mr. Musk and Mr. Branson; there's too much to explore and discover to remain confined to Earth!