Put on your thinking caps and let's get planning!
During the last three years, astronomers have identified thousands of planet candidates in the Milky Way galaxy. And yet, until yesterday, so far as we knew, they had yet to discover any orbiting the nearest stars to Earth. But all that changed on Tuesday afternoon, when news began to leak that a European planet-hunting team had discovered a planet orbiting Alpha Centauri B -- a star in the closest stellar system to our own. The planet sits extremely close to its star, too close to host life, but its mass is strikingly similar to that of our own planet. And more tantalizing still, it may have neighbors; this planet may be the Mercury that helps us find another Earth.
To get a handle on the import of this discovery, I reached out to planet-hunter extraordinaire Sara Seager, Professor of Physics and Planetary Science at MIT. Seager told me she expects this discovery to galvanize exoplanet researchers, but also the small community of interstellar engineers who hope to one day send a probe to the stars. "To imagine that there's a planet right here on our doorstep is absolutely phenomenal," she said. "It raises my hopes that every star has a planet, and that every star has rocky planets, which increases our chances of finding a true Earth twin around one of our nearest sun-like stars."
What makes the discovery of a planet orbiting Alpha Centauri B so extraordinary?
Seager: Alpha Centauri is the nearest star system to Earth. Alpha Centauri has captured our imagination for decades, even though it's four light years away, which is really far, but close for a star. People have written about it in science fiction for a long, long time; people who want to travel to other stars use Alpha Centauri as their target. This discovery is a huge motivator for those who want to send probes to another star system.
But it's more than that; in the field of exoplanets we need planets that we can study in detail, and we need those planets to be around the very nearest stars. So this is a huge boost in morale. To imagine that there's a planet right here on our doorstep is absolutely phenomenal. It raises my hopes that every star has a planet, and that every star has rocky planets, which would increases our chances of finding a true Earth twin around one of our nearest sun-like stars.
The reason we like planets that are close to Earth so much is because our currency is photons. The more photons we have the better measurements we can make, and the more we can follow up. In the coming months, people are going to spend a lot of time figuring out whether there's anyway to follow up with this planet. Anyone that works on planet characterization is going to be thinking really, really hard about how to make follow-up observations of this object, because even though the star is close and we're getting a lot of photons, the planet is small. If we're extraordinarily fortunate and the planet, or another one in the system, transits the host star as seen from Earth, that would open a wealth of observations for us.
Seager: This planet was detected by the so-called radial velocity method; people also like to call it the "wobble" method. When a planet and star orbit their common center of mass, we can see the motion of the star along our line of sight, even though it's tiny. In this particular case, the planet only caused movement of about a half-meter per second. That's walking speed. We're essentially measuring the tiny, walking-speed movements of a star that is four light years away. To answer your second question, we consider this particular finding a detection. It's not a candidate; it doesn't need to be confirmed. It's a detection.
When you say the planet is Earth-mass, does that mean it's roughly the size of Earth, or something else?
Seager: When we say something is Earth-mass we actually don't know exactly what size it is, it really depends on what the planet is made of. Think of the planet that is closest to the sun: Mercury. Mercury is actually three quarters iron, and iron is much more dense than the rocky material that dominates Earth. If this planet is made of iron it will be smaller than Earth, and if it's made of material like Earth than yes, it will be Earth's size. But there's another subtlety here: the detection technique that was used doesn't tell us the exact mass of the planet. It only tells us the minimum mass, the lowest mass it could be.
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.