Your spacecraft is speeding toward Alpha Centauri at nearly 5% the speed of light. At that speed, your 95 year-long journey from Earth would end in a flyby lasting only a day or so. You’d like to stay a while, but to do that you have to slow down. How do you get the job done? 

Traveling the great distances of interstellar space will always have a tension between time and speed. The faster you go, the less time it takes to get there, but greater speed demands more energy, and all that speed has to be gotten rid of if you want to visit a while.

To date, the fastest spacecraft we’ve ever launched was New Horizons, which had a launch speed of about 58,000 km/hr. Even after getting a gravitational boost during it’s flyby of Jupiter, New Horizons approached Pluto with a speed of only 48,000 km/hr.

Voyager 2’s heliocentric velocity vs it’s distance. Credit: Wikipedia user Cmglee (CC BY-SA 3.0)

The fastest spacecraft to leave our solar system is Voyager 2, which used gravitational flybys of the four outer planets to leave our solar system with a speed of about 60,000 km/hr. At that speed a trip to Alpha Centauri would take about 78,000 years. Even then, a spacecraft such as Voyager would need to make another series of gravitational flybys to slow down once it arrived. While we know at least one planet orbits the smallest member of the Alpha Centauri system, Proxima Centauri, we don’t know of any planets large enough to slow a spacecraft down.  Without a gravitational assist, the alternative would be to bring fuel on the long journey. Unfortunately the amount of fuel you would need to slow down at the end of your journey is about the same as the amount of fuel it took to speed up in the first place. Your original launch would then have to accelerate both your spacecraft and the fuel it carries, making the whole thing unfeasible.

Part of the problem with sending a Voyager-type spacecraft to another star is its mass. Voyager 2 had a mass of over 700 kg, for example. But an alternative approach favored by the Breakthrough Starshot project is to send extremely light spacecraft, with a mass on the order of grams. Such spacecraft would be accelerated to a few percent of light speed by an array of lasers. We can’t currently build such small starcraft, but if we could, we might be able to slow down the spacecraft by the light of Alpha Centauri itself.

A new paper published in the Astrophysical Journal outlines the idea. Since light exerts a tiny amount of force on anything it lands upon, a small craft with a large enough sail could use light to slow down. The idea has been around for a long time, but this new work demonstrates that a craft the size and mass proposed by Breakthrough Starshot could orient itself in such a way to use the light of Alpha Centauri A and B to both slow down and change its trajectory toward Proxima Centauri. Instead of taking 95 years to make a quick flyby of the Proxima system, it would reach Alpha Centauri A in 95 years, swing past Alpha Centauri B, and make a slow 46 year journey to Proxima Centauri. Sure, the idea would increase the journey time by about 50%, but it would allow the spacecraft to stay for a while, gathering data until its systems failed.

Ideas like this are still in the earliest stages, but they do demonstrate that interstellar missions aren’t simply science fiction. Just as we once made our first tentative explorations of the solar system, in a century or two we may have tiny spacecraft orbiting a planet around another star.

Paper: Heller, R., & Hippke, M. Deceleration of high-velocity interstellar sails into bound orbits at Alpha Centauri. The Astrophysical Journal Letters, Volume 835, L32, DOI: 10.3847/2041-8213/835/2/L32 (2017)

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One of the biggest challenges in the search for exoplanets is distinguishing real evidence of a planet from random fluctuations that give a false positive. In most cases we can’t observe an exoplanet directly, so we have to look at things such as a dip in a star’s brightness, or a star’s wobble from a planet’s gravitational tug. Just because some data looks like the effect of a planet, we can’t announce its discovery without being sure. This is why we distinguish between candidate planets and confirmed planets. But even when we’re careful there are times when a planet can be confirmed in error, such as the recent case of Alpha Centauri Bb.

The discovery of a planet orbiting Alpha Centauri B was announced about 2 years ago. It was found by observing the wobble motion of the star (what’s known as its radial velocity). The amount of radial motion was small, about the speed at which a person could walk. That’s a small signal to pull out of the noise, but the evidence seemed pretty reasonable, which is why the results were published. But further study placed doubts on the existence of a planet, and now a new paper demonstrates that the planet could be a false positive. The authors looked at simulated data of stellar fluctuations with no planet data, to see if results might look like a planet. What they found was that the rate at which data was taken could produce a false result.

The original observations were taken at La Silla observatory, which is a ground-based telescope. That means data could only be gathered when skies were clear, and the observing time had to be scheduled between other observations. It turns out that the observation sessions also had a periodicity to them, and this affected the analysis of the original data. It’s a subtle effect, and it’s not surprising that the original authors didn’t see it. This new work demonstrates that patterns in your sampling can affect a result as well as random fluctuations in your data. Particularly when the signal is weak.

So it’s very likely that Alpha Centauri Bb isn’t a planet after all. Of course this raises an interesting question. Technically, the name A Cen Bb should go back into play, so if a different (real) planet is discovered around the star it should be given that name. But since the name is already in the literature that would be confusing. It might be better to name any new planet A Cen Bc. If that’s the case, then Alpha Centauri Bb would become a ghost planet, being the name of a planet that doesn’t exist.

Paper: Vinesh Rajpaul, et al. Ghost in the time series: no planet for Alpha Cen B. arXiv:1510.05598 [astro-ph.EP]

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Alpha Centauri is the nearest star to us. (Third nearest if you count the Sun and Proxima Centauri.)  It is also a binary system.  Those two facts make it instantly appealing to a child with an interest in astronomy.  What’s more, the primary star of the system is much like our own sun, so it is easy to imagine an earth-like world orbiting it. Surely it would be a world much like ours, but with two suns in the sky.  You almost wish it to be true.  I remember rainy summer days when I was about 12, working with a copy of Asimov’s Alpha Centauri to imagine whole solar systems around the two stars.

So when we started looking for extrasolar planets, Alpha Centauri is high on the list of any astronomer’s inner child.  But despite its closeness, finding planets there would be difficult.  The orbit of Alpha Centauri B is at an angle relative to us, so there isn’t likely to be any transiting planets that we can detect.  That means planets would have to be found by looking at the wobble of the star as the planet orbits it, which is much harder to measure.

In the early days of speculation regarding extrasolar planets, it was thought that double systems weren’t likely to have planets because the two suns would disturb planetary orbits too much.  As we developed more sophisticated computer models, we found that wasn’t the case.  It would even be possible for planets to lie within a habitable zone around either star.  But even if they existed, they would still be hard to detect.

Recently, however, we have found one around Alpha Centauri B.  (You can see the original article here)  This planet was found by observing the Doppler shift of the star as it wobbles.  Of course, B wobbles due to A as well as any planets, so there is a lot of noise in the observations, but if you average the results you get the figure below.  The red line represents the best fit curve.  If you look at the vertical scale, you find they had to measure Doppler motion at around half a meter per second.  That would be like measuring the Doppler shift of someone walking down the street, which is amazing in itself.

The big thing is that we’ve found an earth-sized planet around our nearest neighbor.  This planet is only about 10% larger than Earth, but it is very close to the star and its “year” is only 3.2 days long.  So don’t plan a vacation trip just yet. But with one planet, there are likely others, and that means Alpha Centauri likely has a solar system.

Which of course makes my inner child squee with joy!

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