Exoplanets: could Alpha Centauri have any?

My fiction recently gave me cause to examine interstellar travel. Many writers tend to shy away from the reality that we’re a long way from anywhere. It’s too hard, too intimidating, too depressing. I, too, have done the wormhole thing, you know, where your characters can zip between worlds, but fun as it is, and I won’t stop writing space opera, it always feels like cheating to me. In my next project, I wanted a more realistic approach. How realistic? I haven’t answered that yet, but I’ve started out by looking at the facts.

Our nearest stellar neighbour is Alpha Centauri (it’s actually a multiple star, but a bit more about that later), the brightest star in the constellation Centaur, the forth-brightest star in the sky (less bright than Sirius, which is more than twice the distance), and mostly visible in the southern hemisphere. At a distance of a mere 4.22 light years, and a theoretically achievable travel speed at 10% of the speed of light, allowing for speeding up and slowing down, it would take roughly 50 years to get there. Wow. I am waiting on the arrival in my real-life mail box of some material about how to get there, and will write more about that later, but let’s assume, for the sake of the argument, we could send a ship there within a human lifetime.

Question is: why would you? What is there? If Alpha Centauri had planets the size of Earth, or Mars in the habitable zone, wouldn’t they already have been detected?

There is a long and a short answer to these questions. Let’s have the long answer first.

What methods do we have to detect planets?

Direct Imaging:
There is no denying that emotionally the best way to determine if something is there is to see it. Humans tend to be visual creatures and have a great ‘I’ll believe it when I see it’ instinct. Surprisingly, telescopes have seen some planets. There is some debate as to whether those objects are planets or brown dwarfs, but something is definitely orbiting those stars. To see a planet, the size of the planet and brightness of the star are going to matter. The reality is that in most cases our telescopes are nowhere near detailed enough, and the above examples are exceptions.

Radial Velocity method (also called Doppler Spectometry):
When a planet moves around a star, the star wobbles a tiny bit. Light waves behave in a manner similar to sound waves with an approaching and passing ambulance. If the star moves away from us, the waves become longer, the light more red; if the star moves towards us, the waves become shorter, more blue. This is called redshift or blueshift and can be picked up with very sensitive instruments. This method tends to detect planets whose weight ratio compares favourably with their stars – relatively large planets orbiting relatively small stars. It also doesn’t take into account any part of the star’s movement that is not towards or away from us. This is where astrometry helps. These two methods combined are the most common in planet-hunting.

Transit Photometry method:
If you’re lucky enough, the plane of the orbit of a planet around a star is exactly the same as our field of vision. In other words, we view the solar system edge-on. In that case, there will be times that the light from the star dims, because the planet passes in front. Because you need more than one pass of the planet, this method obviously favours large planets with a short orbital period.

When a star moves in front of another, the closest star distorts the light of the more distant one, making it appear 1000 times brighter than normal. This effect usually lasts a few weeks, until both stars move on. When, during this time, a planet happens to pass in between, it adds to the effect. You obviously have to be pretty lucky for this to occur at a time you happen to be watching.

This method relies on extremely accurate measurements on how a star moves in the sky. The Radial Velocity method discussed above works best when a solar system is viewed edge-on; astrometry works best when the solar system is viewed face-on. This method has enormous potential, and astronomers predict that we will be able to detect Earth-sized planets. But not yet. At the moment, distortions from the Earth’s atmosphere hamper measurements, but projects like the European Gaia mission, scheduled for 2011 will change that.

Now about Alpha Centauri:
The trouble with Alpha Centauri is that it’s not one star, but three, denoted by the capital letters A, B and C (the small letters are used for any planets discovered). Alpha Centauri A is also called Rigil Kentaurus, the largest and brightest of the group, and is a G star similar to the Sun. Alpha Centauri B is an orange K type star (see here for a cheat sheet on star types). A and B form a close binary, with a distance between them of 23AU (1 AU is the distance from Earth to the Sun), about as far apart as the Sun and Uranus. This is considered to be a piddle of a distance. The star Alpha Centauri C is also called Proxima Centauri and is actually the closest star to us. It’s a red dwarf, and 14,000AU from A. You can imagine that it is hard for planets to form in a system where they are influenced by the gravity from more than one star. However, it is not impossible. Various references that are listed at the bottom of this Wikipedia page suggest that A could possibly have planets in a zone not further than 2.5AU from the Sun-like star. In our solar system, the habitable zone is considered to be roughly between 0.9 to 1.7AU (this includes Mars, but not Venus). Additionally, planets have been detected in close binaries.

The current count of extra-solar planets numbers 418, listed here (be sure to visit the rest of the site, because it is excellent). It’s an entertaining list, which includes a small visual of what the system looks like.

When you go through the list in detail, you will notice a few things. In the first place, most planets are massive. Masses are given in Jupiter masses, so a planetary mass of 1 is equal to Jupiter, and a solar mass of 1 is equal to the Sun. Many, but not all, orbit close to their parent star, and lastly, their parent star is relatively small. Strangely enough, distance from us doesn’t seem to be a factor. The furthest planet detected I could see in the list was more than 20,000ly away. Distance between the star and the planet matters only in that it increases the time necessary to be looking at the planet to reliably detect it. The planet Gliese 581g, the source of the kerfuffle earlier this year, was the result of eleven years’ study of the parent star. Astronomy is not a short-time career. We come back to the planet mass (compared to Jupiter)/star mass (compared to the Sun) ratio. For Earth, the ratio is 0.003. For Mars, 0.0003. For Uranus, it’s 0.04. The smallest ratio I could find in the table was 0.016 for a planet called CoRot 7b. Gliese 581g, which is not in the table, has a ratio of 0.029. It has three times the mass of Earth, and its star is 31% of the Sun’s mass. I expect that new methods will bring this ratio down.

That was the long answer.

The short answer is that there might be planets, but because we haven’t seen them, they are unlikely to be Jupiter-sized, and we don’t yet seem to have the technology to detect planets much smaller than Uranus ratio, not even for our closest stars.

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