# I want my aliens to arrive at the edges of the solar system. They would arrive in a week. What do I need to have?

Space is 3d. Which means complications when we introduce FTL travel and in imagining the fleets and their position.

But in my story, FTL is easier beyond the reach of the sun and after the last planet, as the technology beyond FTL travel is effected by planets. So basically the easiest FTL 'jump' is the one between places of no planets or asteroids or suns.

So let us imagine the solar system in a a simplified way like in Mass Effect or in this image or in this.

So the aliens have to appear 'beyond' Pluto.

Now here is what I actually need for my story.

Now I have zero idea about our current space observations capabilities so I don't know if it is possible now or not. If it is not possible I guess the story can be set a couple of decades later.

But then what do I need? Like actual technologies, not handwaving it with: advancements.

For example can the current Kitt Peak National Observatory in Arizona detect them?

Two important points:

Space is big. And we probably, like I said I know little of our astrophysics, are not monitoring every part of it.

True. But such a jump is very 'loud' and when the fleet arrives there is a a release of energy there. So we probably should be warned.

They have FTL travel. They can just do what they want including arrive at earth in an instant.

The aliens speed is not an issue as they are having all sorts of problems regarding their technology and fuel. So they are not using their full tech. Also FTL travel is not doable inside the dead zone of the sun. That's a whole thing in my world but basically means that they are using advanced thrusts to move inside of any solar system. This again is like drawing imaginary lines starting with the sun and ending with the last planet. Anything withing the line of the last planet is a dead zone. After that FTL travel opens.

With current human technology the alien craft would have to be either extremely large (and visible) or very, very noisy for us to detect it anywhere outside the orbit of Mars. So either they're flying around in a polished chrome planetoid or their method of travel produces phenomena that draw our attention.

Our best optical instruments are unlikely to detect them at any reasonable range if they're not planet-sized... or at least small moon size. If the Death Star warped in and sat somewhere on the orbit of Pluto we probably wouldn't notice it for a few years at least. It wasn't until 2005 that we managed to get an image of Nix (Pluto's 3rd moon, although it was the 2nd until they found Styx in 2012) using Hubble, and Nix is about 31 miles long and we were looking for it. As far as I can tell Hubble never managed to get a discernible image of Styx or Kerberos which are both much smaller. So unless your alien ship is extremely large the odds of a visual survey noticing it in the next few decades are vanishingly small.

In order for humans to even start looking we'll need a fairly strong signal source to alert us to the fact that there's something to look for. We'll also need a couple of weeks to get the various observatories re-tasked before they can even really start looking in the right place. There's bound to be a radio observatory like Mauna Kea that can do it quicker, but you probably won't get time on Arecibo inside of a 6 months unless you've already found the object and people are starting to panic.

Honestly, the only way we're even going to notice them coming is if they're making a lot of noise on a fairly clean EM band. They'd have to be sending a few megawatts in our general direction to get noticed in days from that distance. The closer they get the better our odds, so I'd advise that they take their time on in-system drive. If it takes them 2 weeks to get here then they'd better be raising all kinds of hell on the way or there's no way we're even going to notice. We'd maybe figure it out a couple of days before they passed the Moon's orbit. Maybe not.

As for near-future technical advancements, it's unlikely that we'll have anything of interest in the next couple of decades that can really improve the detection odds. In another 50 years or so we might have a functional far-side radio telescope that makes Arecibo look like a portable radio, but it won't do you much good unless it just happens to be pointed in the right direction at the time. A lunar optical observatory would be nice, but again limited by the Moon's orientation at the time.

Give it a hundred years or so and we could find out about the aliens when they destroy an inhabited station or ship out in the outer solar system. A frantic radio call for help would probably get the attention needed to search for the aliens.

• All of the theory behind the Alcubierre drive suggests that if we ever make it work, one of the effects would be that a warp bubble would sweep up all the debris and stray photons along the path and release them as one massive shockwave at the destination. I suspect the primary shockwave would sterilize us even all the way from Pluto, but if they arrived at just the right angle, the flash would still be bright but not deadly. I suggest leaving the exact angle (also depends on how far the last FTL jump was) that they arrived in unspecified, or even in-universe unknown. – John Dvorak Sep 8 at 9:27
• @JohnDvorak Since Alcubierre's drive requires exotic negative mass components I'm going to pretty much ignore it from now on. I dislike intensely the current fad of taking impossible nonsense and pretending it's only a few years from commercial use. It's dishonest. Maybe when we've rewritten enough of science to be able to even start looking for negative mass particles, maybe then it'll be worth looking into. But not before. – Corey Sep 8 at 9:30
• I think we can give it a bit more leeway here. While for humans Alcubierre is from the story land, @John Dvorak didn't say the humans had it, and for aliens which are KNOWN to be travelling FTL, it might be a practical thing. Yes, we have no clue how to build it, BUT it's one of the very few ways how FTL travel might be theoretically possible without breaking any law of physics we currently assume to hold. – BIOStheZerg Sep 8 at 15:02
• That might be an explanation for why they end their jump in such a large distance from Sol and the inner planets - the extremely-hi-energy debris would be devastating, and they don't want to ruin a system they plan to colonise! – BIOStheZerg Sep 8 at 15:04
• @CharlesBamford 'Oumuamua wasn't discovered until it was about 1 AU away from us. That's a lot closer than the edge of the solar system. – called2voyage Sep 9 at 20:01

As the main point is "Humans need to find out about them. One week in advance", I'd like to offer a new option: We were lucky.

As @Corey mentions in this answer:

(Pluto's 3rd moon, although it was the 2nd until they found Styx in 2012) using Hubble, and Nix is about 31 miles long and we were looking for it.

Why don't you do your people to be looking at that place AND an UFO appears in the screen? You may use all the useful info from the other answers but you don't really need to be so technologically advanced. Something like:

"The researchers were looking at Nix and something unexpected appeared, let's look closer. Gosh! Look at that, what is that object?!"

• This is brilliant. I can't believe I did not think of it DESPITE having a character who is "watching the stars" for a living. I will consider this. Thanks – Seallussus Sep 9 at 5:14
• This is quite elegant. Particularly, considering that many other answers require some sort of noise that is so extreme that humans can pick it up over the distance of the solar system (light minutes). It would be quite an interesting to consider why a space-faring people have no way of hiding their presence from all but the very least sophisticated beings (we have the telescope only since about 400 years) of their presence over large distance. Could be raising another question. Why did they pop up in the plane of the planets? To use the planets gravity to navigate inside the solar system? – P.R. Sep 9 at 19:24
• @P.R., cool questions! Probably Seallussus have an answer to that. Maybe they can warp only in places with low gravity (because of reasons) and then use another movement source. Or it saves energy in exchange to travel slower... Are the aliens aware of us, or are they exploring? If they'd be exploring, it'd make sense going slowly checking the planets. – Chococroc Sep 9 at 21:50

As for detection, the warping out of FTL may create a radio burst that will be picked up by radio telescopes on Earth. It has a very unusual signature, making astronomers direct their telescopes towards the source. They can then spot the exhaust from the alien's engines as they decelerate towards the Earth (the fastest way for the aliens to reach Earth would be to arrive at high speed and then decelerate at maximum thrust to reach Earth with near-zero velocity).

Let us assume that the maximum deceleration that the aliens can stand for extended periods is 30 meters per second squared (three gravities). Then the maximum speed towards Earth they could arrive with at 5 billion km (a bit outside Neptune's orbit) and still brake would be 17.300 km/s, and it would take 160 hours (6 days, 16 hours) to reach Earth, arriving with near-zero velocity (to enter orbit).

This gives you roughly the week you ask for. To extend the time a bit, slightly increase the initial speed and distance or reduce the deceleration.

As for what kind of engines the aliens need, the output would have to be rather extreme to achieve an acceleration of three gravities. The only engines we know of that can do this are chemical ones, and they should have a very visible output. Ion engines have a better fuel-mass-to-thrust ratio but need heavy reactors produce the thrust, resulting in low acceleration (that in turn can be maintained for a very long time). The aliens could feasibly have a matter/antimatter drive that uses the annihilation photons for thrust. Electron/positron produces photons of 511 keV, while proton/antiproton annihilation produces photons of 938 MeV. Both would light up on gamma-ray telescopes like the Fermi Gamma-ray Space Telescope and instantly catch attention. At any rate, it would be difficult to imagine that any engine using non-handwavium technology to provide such an acceleration would not be easioly detectable on Earth.

• The initial burst that raises human attention could as well be a gravitational wave of highly unlikely pattern, detected by all Earth-based gravitational wave detectors. After all, that is the exit of an FTL jump. – lvella Sep 8 at 13:15
• im not sure a pattern is needed for gravity waves as they are so small that any detectable waves that close would be amazing, otoh im not sure how you would create detectable gravity waves without a solar system destroying event, also not sure how well we can detect direction for them anyway – jk. Sep 8 at 14:48
• @jk. Don't forget the distance is a lot less – user253751 Sep 9 at 17:04
• Just out of curiosity, would the "noise" from dropping out of FTL resemble the WOW! Signal? (As best as I recall, there is no good estimate of how far away the WOW! Signal originated. It could have been from Pluto's orbit, or from across the galaxy.) – Codes with Hammer Sep 9 at 19:38

### They aliens' arrival emit gravitational waves when leaving FTL. LIGO detected it.

The alien fleet's FTL technology emits a burst of gravitational waves when they leave supra-luminal speeds. The fleet needs to be big, massive, but not that much.

IRL, gravitational waves are only detected when cosmic events in the scale of black holes fusing or supernova explosions, but these are thousands of light-years away.

Since they decay at 1/r and not 1/r2, the fleet needs to be a sizeable one to create detectable waves.

Compare the required energy/mass in proportion to the square of the distance to "beyond pluto's orbit" (e-4 LY^2) to the square of the furthest black (e+9 LY^2).

The magnitude of the gravity waves produced by the aliens need to be around e-13 times the waves produced by the black hole merger.

A typical stellar-class of black hole has a mass between about 3 and 10 solar masses. ref

So your alien fleet doesn't even have to be massive. A solar mass is about 1.989e+30 kilograms. Multiplying all that, the fleet needs to cause gravitational waves as the collision of a mass in the order of e+17 kilograms, or in the ballpark of a hundred million times the mass of an oil supertanker ships. That's way smaller than the Star Wars' Death Star ref.

An oil tanker's weight is in the 10e+9 kg order of magnitude. ref

Your fictional alien motherships will need to be be bigger than that.

The only thing your FTL technology really needs is to emit gravitational waves. The rest is covered.

LIGO can pinpoint direction, so scientists will know exactly where to look for, and maybe spot your aliens within your timeframe.

• gravitational waves do not follow an inverse square law, they decay as 1/r, being a quadropole wave they do not emit isotropically. depending on your direction, they will either be easy to detect, or impossible. . – camelccc Sep 8 at 13:55
• My (shallow) understanding of graviational waves and LIGO is that LIGO is sensitive only to some frequencies of gravitational waves (e.g. see commons.wikimedia.org/wiki/…), so beyond having enough energy the waves would also need the right frequency. I however have no idea what influences the frequency of gravitational waves... – Martin Modrák Sep 8 at 15:02
• Tankers (all of them on earth collectively) move 2 billion tons of oil per year combined. A single tanker does not move 2 billion tons of oil - the largest move a bit over a quarter million tons in one go and are just over half a million tons fully loaded, so more like E+8kg for a tanker, and your collision needs to be about nine orders of magnitude (a billion times) larger than that - not five. That's about 20 times the size of Eros. – J... Sep 8 at 16:36
• @eps I don't think the idea is that it produces the exact same signature - only that it produces a gravitational signal of some kind that is detectable by LIGO above background. The idea is that it makes us take notice. A different type of signal than a black hole collision, in fact, would probably serve to make the phenomenon even more intriguing because it would be novel. – J... Sep 8 at 16:47
• @DavidZ As far as I understand the procedure, as long as they can pinpoint direction and the signal looks at least not like an obvious error, they will not loose time and alert some optical/radio telescopes, just in case there is something happening in the direction. After all there are Nobel prizes at stake. If they then see a source which starts to blueshift more and more, i.e. is accelerating towards us and which is already close (some observed parallax between distant telescopes), aliens are a believable explanation which will be voiced. Some paranoid generals will then take it from there. – mlk Sep 9 at 7:38

So you want us to detect an alien fleet arriving out beyond Pluto's orbit and we have a one week deadline. @Corey has very clearly ruled out our seeing their arrival by visible-light observation. Adding to those arguments, there is also a significant possibility that the sun will be obstructing our direct line of sight to Pluto during many moments of our orbits. If they arrive during any of those moments, our chances of detecting their arrival goes from highly unlikely to near impossible.

So let's see how else we might know they have arrived...

They are utilizing FTL which is something we currently have no scientific explanation for. So if we don't know how it works, we also don't know what side-effects it might have. Perhaps all of our nuclear reactors suddenly flare or dim as the physical laws of the universe ripple as the aliens come out of warp "nearby". Perhaps our cyclotrons release new unexpected particles during the instance of their arrival. Maybe our quantum computers all start suffering an identical rounding errors just because an inactive FTL drive is inside of the dead zone.

We might not know what any of these things mean when they happen, but we would know that something strange is going on, and in response, we might start watching distant space with more effort than we currently do.

• Actually being obscured by Pluto is a highly insignificant probably. Planets are large, but empty space is mind-bogglingly huge; unless the aliens intentionally hide behind a planet or the sun, you can assume there's a direct line of sight. – toolforger Sep 9 at 16:20
• @toolforger, despite the hugeness of all that is, I think the sun can still obscure a significant portion of Pluto's orbital path from our direct observation from Earth. During those moments when the sun is between Earth and Pluto, many large fleets be hidden from us in proximity to Pluto. – Henry Taylor Sep 9 at 19:19
• Googling "sun percentage of sky" (without the quotes) gave me 0.0000241%. Now the actual area unobservable is much larger due to outshine, and Pluto's orbit is just a line, so the real percentage is much better. On the other hand, Pluto's orbit is in a different plane than Earth's, so the Sun never obscures it ;-) (but you need a space-based telescope, because atmospheric telescopes can't see much in daylight, which will be a problem those six months of the year that Pluto isn't on the same side of the Sun as Earth. – toolforger Sep 10 at 10:35
• @toolforger, I stand corrected. Thank you for keeping me honest. – Henry Taylor Sep 10 at 14:23

You can see them when they arrive.

This seems a little goofy to say, but, the simple answer is just that they emit a giant flash of visible light when they drop out of FTL.

Something extremely bright in the visible spectrum is WAAAY more likely to get spotted because we have about 8 billion people, a significant fraction of whom like to look up at the night sky as a recreational activity. Hobby astronomers spot interesting new stuff before professionals all the time, so it doesn't need to be a flash big enough to spot with the naked eye necessarily.

Subsequent confirmation within a decent timeframe (a couple weeks) is well within current tech, we can certainly 'see' something that far away in somewhat decent detail, we just have to know there is something in that section of the sky to look at.

• would it be bright enough though? something that bright would be incredibly hard to produce – Topcode Sep 8 at 19:02
• @Topcode Moving faster than light is also pretty difficult. I'm going to go out on a limb and assume that a light bright enough to be seen on the edge of the solar system is a bit easier. – Iron Gremlin Sep 8 at 19:56
• @Topcode, OPs idea is something I though of. And like in his answer they are FTL capable so doing that is not an issue. The only issue is why? It does not seem like a smart design choice to announce your arrival when you arrive. However for my story there might be an explanation. – Seallussus Sep 9 at 5:20

Neutrinos

If their braking propulsion is showering inner solar system with narrow beam of highly energetic neutrinos from exotic nuclear reactions, they are detectable with existing neutrino detectors and it is possible to determine the direction.

Theoretical models of such propulsion already exist. It is possible to check the source with existing x-ray and gamma-ray telescopes/spectrometers whether the spectrum conforms to these models. Compared to visible spectrum and radio, the x-ray and gamma-ray space background is quieter and aliens would stand out much more. This way it can be determined that it is artificial process and measured their speed by doppler shift.

• This definitely seems like one of the most plausible (or least implausible?) options to me. A neutrino telescope like IceCube covers a wide range of angles, so it's one of the few things that could detect a signal from space without having to be pointed in the right direction ahead of time, and neutrinos are not all that well understood, so it'd be easy to pass off a strong neutrino signal as a side-effect of FTL travel. And we already have the infrastructure in place to quickly point X-ray/gamma-ray/optical telescopes at a neutrino source. – David Z Sep 8 at 21:52

# We detect their STL rocket burns.

One week from Earth to Pluto would require one heck of a rocket burn. Given a time of 3.5 days (half the time used for acceleration and half for deceleration) and a distance of 33 AU, we can calculate their acceleration as being approximately 96 m/s, or approximately 10g, and a maximum speed of .097c (the relativistic factor of which is still approximately 1, so we can neglect relativistic effects here).

That is a very powerful level of acceleration sustained for a very long time - it would require either some form of superscience reactionless drive, or an extremely powerful nuclear rocket. The amount of energy such a rocket would produce would likely make it one of the brightest objects in the sky - though how bright would depend on how much mass these ships have.

Space is big and your alien ship is small, so the odds that someone notices it are very low. It's incredibly difficult for such a (relatively) tiny object to draw our attention at such extreme range. A much easier approach is for the ship to go somewhere we're already looking.

Have ship approach along the vector between Earth and Polaris. This is a very prominent star that is perhaps the most important star in the Northern hemisphere for navigation purposes. Every amateur astronomer knows how to find it, and it can be seen with the naked eye. People are always looking for it for reference purposes.

If your ship came in along a trajectory that blocked the view of Polaris, people on Earth would notice it pretty quickly. Stars get eclipsed all the time by objects moving through space, but your ship would block the star suddenly and unpredictably, and the star would remain out of sight for an extended period of time. That's definitely not normal, and would draw attention fairly quickly.

I think the most likely way to detect them is if the fleet (their engines or some other of their devices) were to produce radio noise.
Considering it would be a by product of a device and not an intended signal its emission may be erratic. But still something unusual enough to be picked up and drive attention on. Pinpointing the source from different radio telescope stations astronomers would be able to find the location of the emission and in that position it would have no astonomical explanation. Furthermore acceleration by the fleet would not be consistent with the gravitational field and so raise even more attention to the unexplained phenomenon (see all the fuss that Oumuamua made).

If you have to be detailed about it check the frequencies that radiotelescopes use.

Channel 37: 608 to 614 MHz.
The "Hydrogen line", also known as the "21 centimeter line": 1420.40575177 MHz, used by many radio telescopes including The Big Ear in its discovery of the Wow! signal 1406 MHz and 430 MHz [7].
The Waterhole: 1,420 to 1,666 MHz.
The Arecibo Observatory has several receivers that together cover the whole 1–10 GHz range.

I would suggest the hydrogen line as it also may have plausable explanation of emission by alien technology.

The signal may be faint considering it is not focused to Earth and spreads out in a sphere from the fleet but (for story reasons) has to be picked up. So at the source the emission should be very powerful. Engines first come to mind.
I would suggest to avoid having the FTL arrival to produce the signal because it would be an instantaneous event, much more likely to be missed. And it would mean further observation and confirmation would be impossible. Astronomers would not be able to go from "a strange radio signal from that direction" to "an alien fleet is coming towards us!".

The aliens may be aware of emitting radio noise and not care. Or just underestimate human technology. Usually a good engineer would shield that. Maybe they are having technical problems as you mention?

The signal would take from 7 to 4 hours to reach Earth from Pluto (depending if Pluto is at its farthest or closest to Earth).

One week only seems a tight costrain though. Consider that astronomers would verify their observations before coming out to the public or to their government. Observations would need to be performed by different radio stations on Earth in order to do the triangulation necessary to locate the source. More stations are also needed due to Earth rotation. As the signal will set for one station another will pick it up.

P.S: Unrelated question: why would the aliens need to arrive near Pluto? How about arriving above or under the plane of the solar system?

• "The signal may be faint considering it is not focused to Earth and spreads out in a sphere from the fleet but.." but does it? if its pointed at earth, perhaps more radiation would head that way, rather than another – Topcode Sep 8 at 19:00
• @Topcode Fair point. As this radio noise is a byproduct of some device its emission would be dependent on the device itself. We don't know any details. A spherical emission is just a rough approximation (given, it may be inaccurate). We have no special reason to think the emission would be specifically targeted towards Earth. The chances for it to randomly happen are low. If the fleet route were to determine the direction of emission then the signal would be towards the position Earth will orbit to in 7 (as OP requirement) days. We may miss it completely, depends on how much it spreads out. – Duncan Drake Sep 8 at 19:40

Anyone looking up

I'll assume their engines are reaction engines with 50% efficiency.

Their ship is the mass of an aircraft carrier -- about 100 million kg.

Going from twice pluto's orbit (100 au) to the sun is $$2 * 10^{10} km$$.

$$2 * 10^{10} km = \frac{1}{2} a(1 week)^2$$ gives us $$82 \frac{m}{s^2}$$, or just shy of 10 Gs of acceleration.

At 50% efficiency, this thrust is $$4*10^{17}$$ Watts.

Solar irradiance is $$0.873 \frac{W}{m^2}$$ at Pluto's distance from the sun.

So the ship is as bright as a perfectly circular reflective mirror of radius 1 million km, or 1000x larger (in each dimension) than Pluto.

While it is hard to see Pluto at that range, something literally a million times brighter...

Pluto is magnitude 14 brightness. Each step is 2.512, so 1 million times brighter is magnitude -1. Pluto isn't a perfect reflector, so the ship would be a bit brighter; call it 1 more magnitude.

A magnitude -2 object is brighter than the brightest star in the sky. On a clear night, anyone looking up on the right side of the planet would see it.

Even if their engines are less bright than this, it would have to be much much much much much less bright not be spotted by curious amateur astronomers.

Basically, trust is bright. It takes a lot of energy to move fast, and energy use means emitting light; entropy doesn't really give you other options.

I won't type a huge answer with a bunch of math because other people have already done that, and the math checks out. What I will do is give you two points that will help get the gears turning in a more creative way, plus one means of detection that I don't think anyone has mentioned yet!

1. Regarding the one week deadline from detection to arrival: the distance between Pluto and Earth is 4.6 light-hours, which simply means that travelling exactly at the speed of light it would take them 4.6 hours to make it to Earth. Since they are having tech problems, and they're starting slightly beyond Pluto, you can basically adjust these numbers however you want to make it work out for the story. Example: if 1.00c = 4.6hr travel time, then multiplying the travel time by roughly 36x would give you a travel time of 168 hours, or almost a week. That would be (1/36)c, or 0.027777c, or 2.7% the speed of light, which is completely believable for a species capable of FTL travel but currently experiencing some technical difficulties! Feel free to customize this based on your needs, or even do something like require the ship to slowly accelerate and build up speed, which changes the equation. I didn't consider this until now, but if the typical FTL drive is broken on the ship, they will need to resort to conventional means of acceleration, which will result in G-Forces being exerted on the crew. They may not be evolved for that, and it won't be an insignificant amount of G-forces either; it would probably kill humans to accelerate like that. I don't know exactly what the Gforces would be, but there are some physics calculators online that are based around F=ma, and will tell you the acceleration required to reach a certain speed in a certain amount of time, and then you can look a that acceleration and compare it to the acceleration of Earth's own gravity (9.8 m/s^2) which is one "g" of acceleration, to see how strong it would really be. They'll need to use some kind of tech that we don't have yet to cancel these g-forces, like "inertial dampeners" which have been a staple in science fiction for decades but don't have much basis in reality, or maybe a shield that provides a different "frame of reference" inside the bubble, so from the ship's perspective and the perspective of the crew inside the ship, it doesn't feel like you're moving at all because the shield bubble is like a bubble of "still" space, but the bubble can accelerate through our space as fast as it wants to, or as fast as their malfunctions allow them to. This is similar to how the Alcubierre Drive would work, except it would only be a bubble of local space around the ship and wouldn't have any type of physical shielding capabilities.

NOTE: The information being received by scientists/military on the ground will be 4.6 hours out of date due to the distance and light delay. This means that when the scientists first spot the ship and make the prediction that the ship will reach Earth in roughly 168 hours, the ship actually appeared 4.6 hours ago and began moving then, giving it a 4.6 hour head start. The scientists should be aware of this, as physicists have known about light delay for hundreds of years and have been dealing with it personally since the dawn of radio communication. Also, as the alien ship gets closer and closer, the light delay will shrink and shrink until it gets to 0 (light delay from Earth to the Moon is about 1.3 seconds for reference) so it isn't like the alien ship will just appear on Earth when it looks like it is still 4.6 hours away.

1. Regarding the detection: we currently have some pretty new devices called gravitational wave detectors that we are using to gather information about fast rotating black holes and pulsars. You're right there would be a "BANG!" when the ships jumped back to local relativity, but it wouldn't be a sound wave since space is a vacuum. While there could certainly be a blast of radiation released when the ship drops out of FTL speed (as is theorized would be released by an Alcubierre Drive, which is one of the current ideas for a true FTL drive) there will also be "gravitational ripples" released by the sudden appearance of large ships. Usually, there are NO such ripples ever caused by matter appearing or disappearing suddenly, other than at the quantum scale. Even when a star explodes, nearly 99.9% of all the matter is still there, it's just expanding now. Similarly, when matter is sucked into a black hole, all the matter is still there contained inside the black hole: the mass of the singularity just rises based on the mass it is consuming. However, for matter to just appear seemingly out of nowhere? Even at the edge of our solar system, this could be detectable to us as a gravitational anomaly just like the ones that the LIGO and VIRGO groups began researching in 2015 when looking at pulsars and binary star systems far away from us. It would really probably be more like a "gravitational shockwave" since there will have probably never been readings like this before, especially if the ships are large, there are extremely many of them, or there are a lot of normal sized ships but one massive mothership that also appears. The radiation burst and, later, images from orbiting telescopes, could merely server as a confirmation that you're dealing with aliens, but you could make the initial detection via gravitational wave equipment to take advantage of some brand new technology in a way that would make all the scientists scratch their heads initially.

Good luck on this, it sounds awesome!

As you point out, space is 3D. So, don't forget that our solar system is (mostly) a flat plane with Pluto being a little off of that plane. Depending on your alien FTL technology and the interference of planets, the fleet could arrive directly above or below our plane - perhaps allowing them to be closer to Earth for detection.

As for detection methods, radio waves, gravity waves, supernova light bursts, etc. could all trip a sky survey mission here on Earth and raise awareness.

One alternative the the "FTL jump" or warp drive approach is to have their interstellar technology use some sort of wormhole. (Optionally require a device like the Mass Effect mass relay at the origin point to generate the wormhole.) Lots of scifi has the ship open a wormhole instantly, accelerate in, and then pop out instantly, but you can certainly stretch it out to have the wormhole take as much time as you'd like to fully materialize/open wide enough for the fleet to come through.

Maybe it takes a week, maybe it takes a month -- plenty of time for people to get curious about some weird point-source radiation or gravitational waves or accidental radio transmissions through the baby wormhole and have someone pointing a telescope in the right direction when the fleet starts coming through.

# Detection

The detection question is easy: we can't. It's not like in the movies. We don't have telescopes sweeping the sky like radar. We can't zoom and enhance.

To get an idea of Earth's detection capabilities, just look at our best picture of Pluto taken from Earth (actually Low Earth Orbit).

This is a composite of hundreds of images taken over the course of a year by the Hubble Space Telescope, and it is 1000 km wide. If your spacecraft is 1km wide, huge by spacecraft standards, it is 1000 times smaller than Pluto and would be a single pixel, if that.

# An Extrasolar Maneuvering Object!

We do now have some wide-field instruments and sky surveys constantly sweeping the sky looking for minor planets and asteroids, but at the distance of Pluto a ship-sized object would not even register.

Your ship would eventually be spotted as it moved closer, much closer, and would be registered just as some new asteroid. Scientists would take note of the new object, more telescopes would start observing to determine its orbit, and they'd eventually notice a few things.

First, it's headed for Earth. This would get it more attention as a possible threat to Earth. This would not cause a panic, we detect these all the time, and as we take more observations and get a more accurate orbit we're more and more confident it will miss. But observations will reveal the next issue...

It's extrasolar! Meaning its orbit indicates that it must have come from outside our solar system. The discovery of 'Oumuamua recently caused a stir, but it was still a scientific curiosity. This would cause more observations. Eventually they'd realize...

It's maneuvering! A hunk of space-rock should follow a predictable orbit around the Sun. Any corrections and maneuvers the ship makes would cause it to not follow predictions. However, unless it's something really dramatic, there Earth scientists would still not label it a spacecraft and would be extremely hesitant to do so. There are plenty of reasons for "non-gravitational acceleration" and scientists will search for a natural explanation.

As the ship gets closer to Earth it will have to maneuver more dramatically to achieve Earth orbit and the evidence will mount that there can be no natural explanation, or it will get close enough that it can be directly imaged. By that point, the ship is practically on top of us.

# Extremely fast, but still slow.

Because our detection capabilities are so bad, and because it would take us quite some time to determine that it's not a natural object, your spaceship has to move quite slowly... but still far, far faster than any man-made object. For example, the journey from Earth to Mars takes months, in part because we're coasting most of the way. New Horizons, one of the fastest spacecraft ever, took almost 10 years to reach Pluto.

So you can choose however fast you like for your spacecraft to go to get your week, it will still be fantastically fast compared to Earth spacecraft.

So basically the easiest FTL 'jump' is the one between places of no planets or asteroids or suns.

If so, that ship will have to come out of FTL waaay before Pluto.

# Which is the last "planet"?

What is and is not a "planet" is decided less by physics and more by human definition. For example, Pluto is no longer considered a "planet" by humans, it is a "dwarf planet", but that should not affect an alien's FTL drive.

There's plenty of other dwarf planets past Pluto, including some, like Eris, which are more massive than Pluto.

In addition, the solar system doesn't just stop at Pluto. There is the Kuiper Belt which is a second asteroid belt far larger and more massive (but less dense) than the belt between Mars and Jupiter. Neither of these are very dense at all, certainly not like depicted in the movies, and we send spacecraft through hem all the time.

# Neptune... or Saturn.

As you say, FTL drives are affected by gravity, but Pluto and other Trans-Neptunian Objects are far, far too small and spread out to have an influence.

Instead I'd suggest they need to drop out of FTL because of the first really massive object which can influence its entire orbit: Neptune. It is nearly as far out as Pluto, and it is massive enough to be a plausible influence.

Or you can decide Neptune and Uranus were on the other side of the solar system and it jumps in near Saturn. It really doesn't matter, we still wouldn't see it.

# Just Don't Mention It.

In the finest tradition of good sci-fi, if you don't have to explain it don't explain it. Especially if you're not an expert in that field.

Since our detection capabilities are so bad, you don't have to explain the details at all. As above, the first Earth will know of it is vague observations of some new asteroid. If it comes up later in the story, all you need the aliens say is their FTL drive is influenced by gravity and they had to come out of FTL because they got too close to the Sun. Done.

Behind Pluto are alot more small Objekts. Ultima Thule for examle is 30 km long and was detected 2014. The New Horizon Misson went first to pluo and than to this rock. So its possible. Now a lot of Teleskopes are looking for Planet 9 much furher outside. They are looking for tiny peaces of light. That increases the chance findign it. You culd say it was just luck or you use the Radiation from your FTL witch could be very bright. It dosen't need to be bright in the visible spctrum. a lot of telescopes are using infared or ultraviolet.