Put on your thinking caps and let's get planning!
Artist's rendering of an exoplanet (NASA)
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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.
How was this planet detected? Is this a candidate or has it been confirmed?
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.
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.
"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.
Whenever I give a talk to a public audience I explain the hazards of living on a spacecraft for 40 years, the fact that life could be extremely tedious, and could possibly even include some kind of induced hibernation. But then I always ask if anyone in the audience would volunteer for a 40+ year journey, and every single time I get a show of hands. And then I say "oh I forgot to mention, it's a one way trip," and even then I get the same show of hands. This tells me that our drive to explore is so great that if and when engineers succeed at traveling at least 10 percent of the speed of light, there will be people willing to make the journey. It's just a mater of time.
Do you think the first mission to Alpha Centauri will be manned or would it be a robotic probe?
Seager: The first mission would be a probe. I actually worry about the Star Trek scenario where you send out a probe that's so slow that technology in the not-too-distant future supersedes it. And then the next probe reaches Alpha Centauri before the original one you sent out. It's a bit of a catch-22, because you can always wait until technology gets better, but unless you actually try something technology never will get better.
Is this something that could be industry-supported? You and I have talked in the past about space science hitching its wagon to industries that would benefit from the technologies it needs. Do you see that as a possibility for interstellar travel? Is there an industry you can think of that would be benefitted by the kind of breakthroughs you'd need to go to the stars?
Seager: Right now, I can't see any connection between capabilities for interstellar travel and industry -- at this point, it would be purely for exploration. But we can still benefit if industry decides a low-Earth-orbit platform for assembly and launch would be useful. That's because much of the fuel used for space missions is used to combat Earth's gravity. If you have a way to assemble these probes in space, you actually have a chance to be more efficient and go faster. It's conceivable that the private sector would be interested in something like that.
This was a group of European researchers that discovered this planet orbiting Alpha Centauri B. Should we be surprised that this discovery wasn't made in the United States?
Seager: At the present time, we shouldn't be surprised. It's pretty clear that many of the people working on radial velocity planet detection have figured out how to get more and more precise measurements, and so really it's just a matter of time: telescope time. You need to be able to collect photons from these stars, more and more of them. The Swiss radial velocity planet-hunting group has a fantastic, nearly dedicated telescope in Chile, with an awesome spectrograph, the instrument that measures the radial velocity of stars. They have enough telescope time to just bang away at all of the interesting stars, way more time than the United States has. So it's not a surprise that they have managed to lead the way here. We just aren't leading the world anymore when it comes to detecting these worlds.
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