Actually, there have been radar studies of both Mars and Venus, as well as other planets. I’ve got a book on it. But Venus is clearly the case where we had to do it that way.
Planets don’t lie exactly on that plane. They mostly lie a few degrees of that plane, but that’s still a big enough range that your method won’t work. We orbit the Sun every year but it’s extremely rare to see Mercury or Venus cross the face of the sun; usually it passes “above” or “below” the sun.
Except it’s a lot easier to see a light spot against black space than to see a black spot in front of the Sun. Yes, your method could work…but you could find those inner planets in a few hours with just an eyeball and a telescope by looking for bright specks. And there’s no guarantee that the inner planets would transit the star…Venus transits the Sun only very rarely when seen from the Earth.
Detecting the emissions of an actve radar, and detecting the reflection of that same signal from 5O AU or so is an entirely different set of parameters.
They would see us comming, for sure. but us see them? Not so much.
Tris
Since you have a spectrograph anyhow, just use it to look at the brightest object. The spectrograms of planets are way different from those of stars, right, so you’ll have your list real quickly, without even having to move. Though a blink comparator would work also.
Come on, people, haven’t you ever seen a diagram? Just look for the big white ellipses. The planets will be lined up nicely on them, with the star at one end.
With no equally-advanced sensors or scanners, I have to assume our navigation capabilites would be severely limited. We would most certainly run into a celestial body before arriving at our destionaion - or run smack into our destination itself. Flying at light speed without advanced sensors or scanneers would be copmparable to dflying down the highway at 100 mph with no steering or brakes.
But why?
We know nothing about how this FTL works. Do we transit space? Do we “warp”? Do we go through a “wormhole”? Why don’t we have “steering”? Why don’t we have “brakes”?
Look, space is really really big. Traveling down the highway at 100 miles an hour is not a good analogy. Traveling across an open desert at 100 miles an hour is more like it. Maybe we’ll hit a rock or a bump and crash…but we’re not going to hit a planet or a star because if such an object existed we’d be able to SEE it, there is no edge out in space equivalent to the shoulder of a highway.
I admit that since we don’t know how FTL works, it is reasonable to assume that “looking out the window” isn’t going to work because we’ll be traveling faster than the light we see by. But why assume anything beyond that?
Besides the “space is empty” explanation already given, there are two other possible answers to that:
(1) The original post assumes your alien technology FTL starship is usable, despite having no advanced scanners. That’s simply part of the scenario. IOW the stated task is how to identify the planets at the destination system, not speculate whether the FTL drive would be usable without equally advanced navigation. That’s an interesting question, just not what was originally asked.
(2) It’s unclear you’d even need navigation during FTL flight. Given nearly unlimited propulsion capability, you don’t really need precise navigation at all – you just roughly aim and jump (or boost). You get fairly close, then repeat. The iterative process avoids the need for extreme precision of a single-jump transit.
And as already stated, space (esp. interstellar space) is unbelievably empty. It’s not like science fiction, where you’re constantly ducking swarms of meteors, comets, asteroids, etc. Imagine if you and one other person where randomly placed in the Pacific ocean, each in your own rowboat. What would be the odds of colliding? Whether you rowed blindfolded or kept looking over your shoulder would make no difference. The emptiness of space is like that, only more so.
Yep. That is to say, not at all difficult. It’s one of the brightest objects in the sky, noticeably a disk even with low power binoculars, and usually has two to four readily visible moons.
I’ve got it! We land on the sun’s equator and look outwards for all the really bright objects.
What?
Apparently I need a better telescope or better training. It took quite a while for me to find Jupiter.
Jim
I came in to mention this exact thing. I read that book as a kid, and had the same sort of epiphany about the popular notion of planets around a sun, and the probable reality.
I pulled out my old copy of Orphans of the Sky and looked up the quote:
I love that book!
Heinlein was a great writer, but he was a mechanical engineer not an astronomer or electronics man. Commercial digital computers didn’t even exist until the year Orphans was published (Univac, 1951). Therefore his technical scenarios typically were of a mechanical nature – a rocket controlled by a cam, a drafting machine similar to a typewriter, etc.
The situation quoted from Orphans is typical of that – it assumes no computers no electronics, no information automation. That’s understandable – it mostly didn’t exist in 1951. Also he was trying to weave some education into a fictional account intended for juvenile audiences, by emphasizing stars and planets often look like point sources. In reality even in that fictional situation you wouldn’t use a dime-store telescope to commit the trajectory of a starship.
However the technology situation is very different today, and it affects the scenario of finding planets using 2006 technology on an advanced starship that traveled to another system.
Today a single hobbyist astronomer can buy a computer-controlled telescope and write his own software to scan the skies and catalog objects. A large national or multinational effort like building a starship from alien plans would design appropriate scanning capability using 2006 technology to quickly locate planets at the destination system, and emplace that on the ship.
Point of note: Heinlein was a noted amateur astronomer in his youth (he lectured on the topic extensively) and was well versed in celestial and orbital mechanics (witness his exposition in “Misfit”, Methuselah’s Children, The Cat Who Walks Through Walls, et cetera). He was also reasonably well versed in the electronics of the day and in logical extensions thereof (see The Moon Is A Harsh Mistress, The Door Into Summer).
It remains that finding new large, planetary-class bodies in a system is an elementary exercise, even given the limitations of mid-20th Century technology. Getting there, on the other hand, is the great trick only managed by science fiction writers. But someday…
Stranger
http://www.newscientist.com/article.ns?id=mg18524911.600
In the above article, they talk about the Kuiper Cliff. There is a region of space that is clear of space debris, so they theorize that there is some celestial body that is orbiting out there.
I assume a version of this can be used to give you place to start looking for planets.
The same thought has crossed my mind more than once. Thanks, What Exit! 
… and thanks, everybody, for the many interesting and informative answers to my OP! When the Thermians show up, you’ll all be hearing from me.
It’s also a straightforward task for computers to move one’s point of view to another star and show you the night sky as seen from there. You can download shareware from the internet that’ll do it.
The computed view from Alpha Centauri or another close star will certainly be accurate, but if you go farther afield, there may be some locally bright stars that aren’t in the database. But probably not more than one or two, unless you’re going really far away, like 500 light years or more.