Does one planet around a star usually indicate others?
Seager: Yes, especially for stars like Alpha Cen B, we have seen that if there's one small planet, there's more. Keep in mind we're still at the very beginning of our journey to try and map out planets around stars, but, in general, I personally expect all stars to have more than one planet. NASA's Kepler Mission has found many, many multiple planet systems around sun-like stars, which is heartening, because not all planets will be detected by the method that Kepler is using, the transit method. Transits occur when a planet goes in front of the star as seen from Earth, but unless the planet and star are lined up just so, Kepler won't see it.
Alpha Centauri is a binary star system, instead of a single star system like ours. I once read that life there might develop two circadian rhythms, corresponding to both the length of day around the primary, and the period of the secondary's orbit. Is that right? Do we know what day and night would look like around Alpha Centauri B?
Seager: Let's focus on the new planet that's been discovered around Alpha Cen B. Think about this planet for a minute. It has a three and a half day period orbit, and it is very close to the star, meaning its sun looms very large in the sky, and likely heats the planet's surface to a temperature of about 1000 K. The other star, Alpha Centauri A, is far away from the planet. It would only be about ten to fifteen times as big as Jupiter looks in our sky, which is not very big.
How can we look for more planets around Alpha Centauri B?
Seager: Right now it's going to be the same way. We're going to want more radial velocity data, and that means looking at the star for a longer period of time, in order to get more signal. The authors of this paper say that finding this planet at 1 Earth mass, at this location, with that same sensitivity, they could reach out to 4 Earth masses, in the habitable zone. So my guess is that it's just a matter of time before more planets are discovered in the same system.
If it turned out that there was a rocky planet in the habitable zone around one of the stars in the Alpha Centauri system, how long would it take a probe to get there with today's propulsion technology?
Seager: Right now we have the Voyager spacecraft at the edge of our solar system, and they're traveling at about 20 km per second. At that rate, it would take them more than 70,000 years to reach the nearest star. That's a really long time.
Now, granted Voyager is pretty slow. There are a lot of people who think we have the capabilities to get to a tenth of the speed of light. People are using that number as a benchmark of what they think is attainable, whether it's with a solar sail or nuclear pulse propulsion. If we could achieve that speed, then we could get to Alpha Centauri in just over 40 years.