# What would be the maximum distance from which a space ship could physically see a planet?

A plot point of my novel is that the pilot accidentally finds a planet that is being purposefully hidden (i.e. been deleted from all star maps). It starts with him arriving at a location that is just empty space. Other than distant stars, there's nothing physically around him. There is a minefield of jammers though that prevent him from scanning the area so he starts searching the area and after some time, starts to see a distant planet out of the cockpit.

What would be a realistic distance would the ship be in order for the pilot to physically see a planet (comparable to Earth in size, temperature and age) out of his ship?

• Comments are not for extended discussion; this conversation has been moved to chat. Mar 9, 2020 at 17:48

It's all a conspiracy.

If the planet is actively hidden (removed from maps), it stands to reason that the people in power by and large don't want this world to be found. So the reason nobody finds the planet is that approved instruments or nav software used for long range scanning and navigation are programmed to scrub certain data from the results.

It wouldn't be unthinkable that the companies involved in space travel have to adhere to this, given that even today all maps of china are wrong. Or certain areas containing military bases are not even close to accurate on google maps or similar services.

So your pilot would jump into a system that, according to his sophisticated sensors, is completely empty. But since the planet is only scrubbed from the sensor feeds, he can just see it through his window.[*]

[*]: Space is really big and the chances that you see a planet with your naked eye AND identify it as a planet aren't great. You need to be either very close or invest a lot of time to study its movement. So maybe he just has some after market astronomy hardware installed and uses that. So the answer to your question would be: however far you need it to be for the story.

• Of course, if your ship is configured to erase all traces of a planet from its sensors, chances are it wouldn't have a window for the crew to see the planet out of. Mar 5, 2020 at 17:59
• Someone is paying the Spacing Guild to keep satellites off Arrakis..
– ti7
Mar 5, 2020 at 18:56
• The classic conspiracy problems arise with this - there exists in code on every single ship that says "remove planet X from navigation" - thus, every single ship has code that says "planet X exists, here it is." That means executives in every ship building company and some number of engineers, from all companies, are aware of it. As are GIS/map technicians, librarians, potentially professors, researches, and god knows who many other people. This isn't even considering rogue / enemy nation-states, planets, etc, actively propagandizing the existence of planet X and the efforts to conceal it. Mar 5, 2020 at 21:01
• @CalebJay, yes, it is, but it's still the only remotely plausible explanation for the situation. Because the naked eye works on exactly the same principle, and with much inferior sensitivity and resolution, as one of the sensors. So if the naked eye picks something the sensor does not, the only possible explanation is that the sensor is rigged to ignore it. Mar 6, 2020 at 6:17
• @CalebJay Sure some people would know about some specific changes to the data. But in the end there's just too much there to analyze. Especially if that's not the only change to nav data (which would be plausible, why have one black site if you can have many?). Also, even if information gets out, this would be quite hard to verify independently. So it could just be another conspiracy theory in-universe. Mar 6, 2020 at 9:28

Much of this depends on the technologies used to mask the planet, and the technologies that are being used to scan.

Active jamming is not a viable plan, since it alerts everyone that something is there, and would induce people to send probes or use various electronic counter measures to see through the jamming. A sufficiently powerful active transmitter could simply "brute force" its way through the electronic interference, or the searcher can send millions of nano satellites to take a look. As an aside, active jamming cannot mask the presence of a gravitational field, so simply looking at the trajectories of the nano satellites will be very revealing.

Using metamaterials to actively refract electromagnetic signatures around the object being screened is far more plausible, and done properly, won't even leave a "hole" for sensors to determine that something is there. The problem is metamaterials are generally effective for a limited range of wavelengths, so attempting to screen at all possible wavelengths would require an extremely sophisticated and expensive screen.

Metamaterial cloak in action

The other effect would be that the planet below would be cut off from the outside universe and end up in perpetual darkness. All life on the planet being extinguished is probably not the intention of whoever was hiding the planet...

Gravitational effects are still not hidden by the use of metamaterials, and any infrared or other signatures from the planet may interfere with the metamaterial cloak. As well, if the planet is in communication with the outside world, then holes will need to be created in the cloak to transmit and receive, which are then vulnerable to discovery.

On the other side of the equation, spacecraft will need very sensitive scanners in order to do things like detect distant spacecraft and align antenna for long range communications or weapons. In space, there is no real limit to how large you could build a receiver or mirror (a continent sized mirror of metal foil a molecule thick is theoretically possible, although rather difficult to point). Large synthetic apertures can be made by positioning elements at varying distances from the spacecraft, and having a constellation of sensor drones a light second in diameter (about the distance from the Earth to the Moon) should be no problem for a space ship designed to do scouting or exploration.

The fact there is a planet orbiting the star means that it would be known from the stellar survey. Currently we can detect exosolar planets even hundreds of light years away by looking for the subtle variations that the orbiting planet induces. The star and planets orbit around a "barycentre", so the size, mass and orbit of a planet can be deduced by looking for a "wobble" in the star. Arriving at teh star and not seeing the expected planet would be far more startling and unexpected.

Finding the barycentre allows you to determine mass, orbital parameters etc.

So the idea that an astronaut could simply "look out the window" and see an unexpected planet would be pretty unlikely.

• Thanks for the well researched information. I might have to rephrase or ask a different question then...it's a major plot point that the pilot finds this mysterious planet that no one really knows about. However, you did give me an idea for another plot point later on in the story. So thanks again for your feedback. Mar 5, 2020 at 5:16
• The wobble could be avoided by actively placing another similarly sized planet on the opposite side of the star! (It's unlikely that this configuration is stable in the long run.) Mar 5, 2020 at 16:24
• @HansOlsson if you can haul an entire planet on a heliocentric orbit, keeping a tugboat to do minor corrections every decade or so should be a breeze. Mar 5, 2020 at 17:55
• @Primordial I don't know how important it is to your story that the planet is being actively hidden, but given how stupendously big space is, there are likely to be plenty of areas in the galaxy where starmaps are spotty and incomplete, simply because no one has bothered to look too closely there yet. Hell, the best way to "hide" a planet, might just be to remove/hide all the valuable minerals/metals in the system and count on people just not paying attention to that system once they decide there isn't anything worth having there. Mar 5, 2020 at 19:10

So you're interested in what the pilot can see?

Despite Earth's atmosphere, we can still see Mercury, Venus, Mars, Jupiter, and Saturn. We've been able to do this for millenia.

More recently, Carl Sagan asked NASA to help us take one of the first group-selfies ever. This picture was taken near Saturn's rings. Quite honestly, if they hadn't very helpfully put that arrow and text there, I would have assumed that planet Earth was a buggy pixel in Voyager's camera.

Now, you're asking that a pilot might spot a random planet just by flying around a solar system a little bit. There are a few general ways to spot a planet (using some hand-waving here because fortunately we humans have always known what planet we were looking for when making our robots do our exploring). Here's a few that I can think of:

Orbital aberrations: If this solar system is like ours, then there's a good chance that there are other planets, but this mystery planet (Planet X, if you will) is also there, just unaccounted for/hidden. If you make a supercomputer do some very hard math, you can figure out the orbit periods of each planet and look for any odd features about the ellipse with which each planet goes around its sun. Now, this means that we have to wait a minimum of 1 full orbit of the outermost planet in this system to have reliable information. Recently we have been considering the possibility of a Planet Nine in our solar system (apparently it has a mass of 10 Earths and messes with Neptune's orbit a little bit). But this has taken us this long to even notice - we've been watching the stars as long as we've had eyes, and it took us this long to maybe identify its existence. In fact, we didn't even know that any planet past Saturn existed until the last few centuries.

We might assume that being able to see in space without an atmosphere to distort our vision would make it easier to spot planets by eye, but I'd disagree, because at least on Earth you know to look up. In space, you can look in every direction. All solar systems that we know of are roughly disk-shaped, so even if we know where this 'disk' is, there's a lot of space we have to look through.

If we assume that our ace pilot can spot things as far out as the pale blue dot (assuming that the pilot's vision is equivalent to Voyager cameras), we're suggesting that at the nearest, our pilot can see an Earth-sized object from 6 billion km away, and furthest, we're saying that this pilot can spot it past this solar system's sun at a crazy 12.1 billion km away from the ship. You can make your own judgement on whether this is realistic or not.

• Don't need a "trained" eye to see the classical planets. They are quite bright. Venus is the third brightest object in the sky (after the Sun and the Moon) and Jupiter is the fourth. Mar 5, 2020 at 4:51
• Yes, but they need to be spotted in the right weather conditions and are only that bright at the right time of the day (and kind of know where you're looking). Not much of an astronomer here sadly, I've mostly only lived in places with enough clouds and air pollution to make me question the sky itself lol Mar 5, 2020 at 4:52
• And Jupiter and Venus are both pretty easy to spot even in heavy light pollution -- they're sometimes the only things you can see besides the moon. Mar 5, 2020 at 7:48
• The thing is, if you simply look up at the sky, you are going to see Venus. But will the average person pause and think, "huh - that's a planet?" Probably not. I bet many people would just assume it's a bright star. In other words, I think there's a difference between seeing a planet and knowing that something you see is a planet. Mar 5, 2020 at 16:07

If the planet is far from any star, you could very well rub your face on it and still not see. You already can do that in a room with lights off. It would be practically pitch black. There is simply nothing more than starlight to be reflected by its surface. And if it is black in color it will get way worse to see it. If it is very cold (near 0K), then even on IR, it would be pitch black.

If you are near enough, it could be perceived as a disk oculting some stars. But you would need to be very near it, like no more than 10 times the distance from the Earth to the Moon (and probably much less). And without knowing where to search for it in the vastness of interstellar space, it would be like searching for a needle in a haystack.

However, if it is as hot as Earth, it would glow in IR light. It would still be pitch black for human eyes, but some telescopes similar to WISE, but much more powerful, could perhaps see it as a faint dot from a few light years away. However, seeing and perceiving are different things. Seeing a small dot in the middle of billions of other similar and uninteresting other dots spread out randomly means that in would still be very unlikely to be noted (so, although visible, it would still be very well camoufled). It would only be noted when you are already very near to it (say, at least 20 LY, possibly much less) or if you are intensively and actively scanning each one of those dots looking for a needle in the haystack.

• Thanks Victor. Appreciate the feedback. Mar 5, 2020 at 4:47

To reframe the question a bit...

On average, the human eye has an angular resolution of ~1 arcminute (1/60th of a degree). The diameter of the Earth is ~12750 km.

$$12750 / x = \tan(1') = 0.00029‬$$ $$12750 / 0.00029 = x= 43,965,517$$

So at ~44,000,000 km, an astronaut looking out the window of their spacecraft would be able to resolve the Earth as a disk and not merely as a point source of light.

• Earth's diameter is 12756.2 km at the equator. Mar 8, 2020 at 0:36
• ...aaaand the number I was using was for miles. sigh Lemme fix Mar 8, 2020 at 3:50

Jamming sensors to hide a planet will not work, as others have explained. The better solution is to hide the planet in plain sight.

There are 100 billion stars in our galaxy alone, and probably at least that many planets. That is so many that simply reading a list of them is impossible; you would die of old age before making a dent! So, folks search databases to find planets they want to visit. Simply change the database entry for your planet such that it isn't interesting, and nobody (except your protagonist) will ever come by and notice what you've done.

I assume your story has FTL travel. What is its nature? For instance, if you have something like wormholes or hyperspace lanes, your planet is simply off the beaten path and there is no record of any resources (or a habitable atmosphere) that justifies the time and expense of getting there when so many other planets are easier to get to and have a reason to go there. If the wormholes or whatever are man-made, nobody has put one near this planet because nobody thinks it's interesting.

Move the planet. Plot device used in James H. Schmitz "The Witches of Karres" The witches periodically move the planet when the authorities get too annoyed with them.

Invisible tech: So you have a planet. It has forests and lakes, and plains, and no cities. Sparse population. Posit contra-gravity (as in H. Beam Piper's works) and there are no trails/roads.

Radar? What about it. Right now commercial planes have transponders so they show up decently on radar (and controllers know who's who.) A piper cub with 400 pounds of engine at the front, and the rest mostly wood and fabric doesn't give a decent radar reflection.

Use paint with an impedance of 377 ohms per square, and radar energy is absorbed.

Radar generally is tuned for the specific application. It's trained to disregard things that aren't interesting. Flocks of birds, planes that move at car speeds, things below 300 feet.

(If you wanted to create some excitement hang a 4 foot diameter corner reflector from a drone or kite, and fly it near an airport.)

So, advance the tech. Flying vehicles made with little metal and absorbing plastics. Low populations that can live on agroforestry.