You've made it to another star! Now, how do you find its planets?
From Earth, we can detect extrasolar planets by a number of methods; primary are detecting the wobble in a star's motion caused by a large orbiting planet, and the dimming of the star's light as the planet passes between it and Earth. Neither of these seems very amenable to quickly finding planets, nor to finding planets while at the star.
So: You're looking for habitable planets and have used your hyperspeed device to reach a distant G-class star. Now what can you do to find its solar system? I'm trying to see how to do this relatively quickly (on the order of weeks), with reasonable extrapolations of current technology (except that we can move really really fast). Scaling up the methods we currently use to find e.g. Kuiper belt objects don't really seem amenable to this time frame; could we even do a complete astronomical survey in a few weeks? Would it give us enough information to find the planets?
We're looking for habitable planets, so I'd be OK with just looking in the Goldilocks zone, but finding all of the planets would also be interesting.
EDIT: It looks like the simplest method would be to scan the sky from a bunch of points in (or near) the system, then compare the pictures to 3D-locate nearby points. I'm still not certain of the optical constraints here, though. Could someone who knows something about astrophotography comment on how big / sensitive the camera would have to be to pull this off?
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2 answers
You can do it most effectively by looking before you get there.
Point your big telescope at the sky in the direction of your destination star. Record the image it captures.
Repeat step 1 every month for a year or so.
Combine the resulting images into one. Mark anything that appears round the star that is not another star, and using the varying positions, draw on a rough orbit.
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Use the orbits and sizes of the objects around the star to determine what they are likely to be:
- Comets have trails and usually very eccentrically elliptical orbits. They are small and their trails are long.
- Asteroids are small and usually have fairly circular orbits.
- Planets' orbits can vary, but they are larger than comets and asteroids and appear brighter.
Use the size of the star to work out its Goldilocks zone. Note any planets in this zone. These are your target planets.
Compute, as a function of time to get to the star and orbital period, the position of the target planets when you get there.
Go there. Investigate the planets and decide on one or more to colonize.
N.B.:
- In Step 2, I say a year or so because planets' orbital periods vary but a year should give you enough data to be able to extrapolate their orbits.
- Step 4 may not be completely accurate, so you'll want to check your conclusions as much as possible on the way, and of course when you get there.
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You could quite easily detect the presence of any planets by taking an image of the sky in a specific direction, move some small distance, and take another image of the sky in the same direction. There is no need to move to the opposite end of the system for this (as suggested by Darth Wedgius); moving a fraction of the distance of the orbit around the star at the distance you are should be plenty enough for our purposes.
Stars are so far away that they won't have moved, or if you shifted the perspective enough they will all appear to have moved by the same amount in the same direction, which is trivial to detect by simply overlaying the images. However, planets, especially in the habitable zone, are going to have moved enough that the difference is relatively easy to spot. This is the parallax effect.
It is also very similar to how Pluto was discovered after its approximate position was known.
In the situation you're describing, you wouldn't know the position of any orbiting objects (the planets) but you would know that anything that has moved when comparing two pictures taken some distance apart but near each other in time must be moving relative to the background stars, and thus is a very good candidate for being a planet that you can aim your high-resolution equipment toward and take a closer look.
Once you know in what direction to look, move some distance orthogonally to your original direction of movement and look again. The two angles will form a triangle with a focal point at the planet, which with some trivial trigonometry will give you the distance from your current location to the planet. Since we can likely assume that you know your distance and direction to the central star, this tells you both how to get to the planet and how far from the star it currently is.
Given a high enough speed of movement between taking the images that the planet won't have moved significantly in its orbit in the interim, with a maximum of (I think) four such maneuvers, you will have located the planet in 3D space.
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