# Three Planets I See Large and Small...

1. Apr 5, 2017

### Kumustaka

I have by no means any education on the question I am about to ask. I have looked on the internet and find myself here, signing up for an account to see if anyone can point me to any research on the subject. I thank you in advance for any help that is in layman terms.

Scenario: 3 planets. 1,2,3. Planet 2 can see planets 1 and 2 but planets 1 and 3 are only visible to each other briefly, like only a minute to each other daily. Is this possible and where can I research this scenario?

If I need to make this a theory that is not possible but fun to entertain, a "what if" if you will I can. This is for a possible book I am thinking on writing and want to be at least close to a theory or fact. I appreciate your time in reading my post and possibly responding.

2. Apr 5, 2017

### Noisy Rhysling

You might work that out by having long orbits, say around a large star. At any given time three bodies would be visible, partially visible, or not visible to each other. It's finding that period in their orbits when they are in the positions you desire that you want, I think.

3. Apr 5, 2017

### Grinkle

Seems possible. Imagine 3 earths in the same orbit around the sun. Earth 2 is at 9 o'clock, Earth 1 at 11:59, and earth 3 at 6:01. You might posit that 1 and 3 can only see each other briefly at sunset and sunrise because , while earth 2 can see both all of the time, if one happens to be on the proper side of earth 2 to get a viewing. I'm not good at all with picturing geometries, so maybe I'm all wrong.

4. Apr 5, 2017

### Drakkith

Staff Emeritus
Such an arrangement is unstable unless some external force actively prevents the planets from being perturbed out of their orbits over time. So you need some kind of crazy sci-fi phenomenon or technology.

5. Apr 5, 2017

### Algr

Creating a one-way viewability situation sounds like a problem to me. If 2 can see 1, what prevents 1 from seeing 2? I suppose 2 could be extremely close to the sun, thus lost in the glare, but as soon as they learn to look for transits, (17th century technology.) the planet would become obvious.

6. Apr 6, 2017

### Grinkle

That is what I was picturing.

7. Apr 6, 2017

### Kumustaka

Sorry! Planet two in the middle can see both planets 1 and two...

8. Apr 6, 2017

### Drakkith

Staff Emeritus
Do you mean that planet two can see planets one and three?

9. Apr 6, 2017

### Algr

Well yes I was just looking at part of the problem, but if you include planet 3 you get the same issue twice over. The glare trick only works for planets that are nowhere near capable of space travel, and also renders planet 2 utterly uninhabitable.

A marginally better solution might be if planet 2 is by far the outermost planet and has an extremely low albedo. Perhaps its surface is all coal dust and no atmosphere. I think one of the moons of Neptune (?) is like that, but we still saw it from earth.

The technology available to the viewers is very important. If you've got people traveling between planets 1 and 3, you can forget hiding anything remotely habitable in the system without resorting to pure technobabble.

10. Apr 7, 2017

### Staff: Mentor

You cannot really hide planets with natural methods.

You can put one planet extremely close to the star, but that makes the planet a boring hot place, and you can still study it with telescopes, especially with space telescopes. It also does not work in reverse, the inner planet can observe the outer planet easily.

You can put planets extremely far away, but then they are hard to see all the time.

You cannot hide the planets behind the star for a longer time, because every orbit keeping them there is unstable.

You can make objects vary in apparent brightness a lot with artificial influence. The Iridium flares on Earth are a prominent example: The satellites have large reflective antennas, if they are oriented in the right direction you see a very bright point in the sky (so bright it can cast a visible shadow).
If you have a civilization installing a lot of mirror-like objects on the surface (solar panels? They reflect some light. Mirrors to counter global warming?), all pointing in the same direction for some reason, then the reflection can hit other planets once per day (day of the planet with mirrors). The duration would depend on the planets and the star system, but it would be of the order of minutes. That effect is one-sided, but you can have those mirror-like systems on both planets. The third planet can be outside the reflection cones most of the time.

11. Apr 7, 2017

### Staff: Mentor

There's a book by Cixin Liu called the Three Body Problem that has similar although different premise using suns.

12. Apr 8, 2017

### guywithdoubts

Make 1 and 3 have really thick atmospheres like Venus, so that they're very bright and very visible from 2, but doing astronomy from them is either too hard or impossible. You'd have to make 2 invisible to them too, so that you can only see 1 and 3 <from each other> if and only if they're very close and the weather isn't so bad.

13. Apr 8, 2017

### Drakkith

Staff Emeritus
Reminds me of an idea I had for a short story about what astronomy would be like for an alien culture developing on a world perpetually under cloud cover.

14. Apr 9, 2017

### Staff: Mentor

I guess you can do radio astronomy. The first airplane or rocket going above the cloud cover must be an amazing moment.