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Imagine a generation ship heading for a distant star. Humanity had means to observe that star up to a point, that it is 99.9% sure the star has a colonizable Earth-like planet (with water and atmosphere). So sure enough to send the ship in the first place.

On the way to this star the ship needs to refuel its fusion engine and passes some other stars that have Jupiter like planets. It is going to collect the fuel there to continue its journey. When it approaches this intermidiate star system, it discovers an Earth-like planet in the colonizable area.

Now the question. Is there a plausible explanation as to why this Earth-like planet in the middle-station star system wasn't discovered before (from Earth, when planning the expedition)? Which [future] technology could allow on one hand to predict the conditions on a far away planet and simultaneously completely overlook another planet?

I was thinking that e.g. the Earth-planet is kind of always behind the Jupiter-planet when observed from Earth, so always covered by it. Are such trajectories of two planets even possible?

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    $\begingroup$ This doesn't address your question but since you're asking about plausibility I thought you might like to know that the premise of refueling at an intermediate star system isn't very plausible. Unlike travel on Earth, space travel doesn't require additional fuel to go further; what matters is getting up to, and down from, speed (delta-V.) Stopping at an intermediate star means that you're going to do that twice, doubling your fuel requirement and nullifying the advantage of refueling. It's just going to cost you a ton of time, which is a risky proposition for a generational ship. $\endgroup$ – Gene Feb 24 at 19:31
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    $\begingroup$ It's a small detail, but I wanted to point out: One planet cannot hide behind another. That is not a possible orbit. The big planet can make it harder to detect the smaller one, e.g. If the small planet is a moon of the big one, Earth based satellites might not be able to tell them apart because they always show up as one blurry blob, but the "always covered by the other" idea does not work. $\endgroup$ – Luke Feb 24 at 22:33
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    $\begingroup$ @Gene But as a more plausible alternative, the spaceship could be getting a gravitational slingshot in an intermediate star system with a super-Jovian planet, to speed it up with no fuel cost. $\endgroup$ – Mike Scott Feb 25 at 6:33
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    $\begingroup$ @MikeScott That adds a real high stakes situation to the decision, similar to the Rich Purnell maneuver in The Martian, they have two choices, either they decelerate and stop (using most of their remaining fuel and stranding them here if the planet turns out not to be habitable after all) or they continue the original plan, speed up and continue to their original destination as planned in a few more generations. I imagine a lot of the crew would be enamoured with the idea of stopping. $\endgroup$ – Ruadhan Feb 25 at 8:44
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    $\begingroup$ @Gene that depends on how your fuel is being used, a generational ship capable of carrying a decent population pool could easily be using far more fuel up keeping the lights on and oxygen available than it uses to accelerate and decelerate. $\endgroup$ – J.Doe Feb 25 at 14:27
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A simple answer is that the planet is on an orbit with a high inclination relative to our line of sight. The other planets in the system may appear, from our perspective, to be in line with us and the star; we could then discover them through transits. This new planet, on the other hand, would never produce a transit as seen from our vantage point on Earth.

If the planet is habitable, it's likely low-mass and thus unlikely to produce strong radial velocity shifts in the motion of the star. This, combined with its orbit outside the plane of the system means that it is unlikely to gravitationally affect the other planets, making it hard to detect it indirectly (e.g. by transit timing variations).

Plus, as Luke suggested, the lack of transit observations would have made it difficult or impossible to perform spectroscopic observations of the planet's atmosphere. Non-transit methods wouldn't have revealed much about its habitability from an atmospheric perspective.

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    $\begingroup$ Also, our best methods of determining atmospheric composition involve observing starlight refracting through the planetary atmosphere during a transit. Even if it were discovered by other methods, it might be discarded as a destination because no one knew it had water. $\endgroup$ – Luke Feb 24 at 16:35
  • $\begingroup$ @Luke Excellent point. $\endgroup$ – HDE 226868 Feb 24 at 18:11
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It depends on how the planets were discovered.

One way of finding planets orbiting distant stars is to observe the oscillation of the star caused by the bodies orbiting around them.

A large planet like a Jovian one can plausibly hide the oscillations caused by an Earth-like planet, which would instead be noticeable in a system with no gas giant.

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The planet may be habitable but has been ruled out for other reasons. Perhaps it is poor on metals or its sun is prone to massive solar flares. Either of those factors would limit the planet's usefulness to a high-tech society. So the planet was noticed during the mission planning but ruled out as a final destination.

From there, it just takes a clerical oversight to neglect to tell the generation ship crew about the planet and why it was ruled out. That leaves the crew believing that they have discovered something new and wonderful. And since they are multiple light years away from Earth by then, any queries that they radio back concerning the "overlooked" planet won't be answered for many years.

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  • $\begingroup$ I find it rather hard to believe that the ship's computer systems will not contain all the knowledge of the human race at the time of departure, and will not also received updated data transmitted from earth periodically. So the file on the refueling system should contain all the date available about the seemingly Earth like planet, unless there is some glitch in the data system. If too much data has been lost due to system failure, the colonists may be doomed to die out no matter which planet they settle on. $\endgroup$ – M. A. Golding Feb 24 at 17:02
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    $\begingroup$ @M.A.Golding, no glitch necessary. The OP said this is a generation ship. The earth-trained professional data officer, who started this journey, would have no difficulty querying the fully functional and up-to-date database about the refueling location; but her grandson, whose mother replaced that fine officer 65 years earlier, lacks such skills. He can read the refueling section of the mission plan, but it is a summary, written by the long-dead bureaucrat who overlooked the habitable planet. A perfectly functional information system and a well informed crew are not the same thing. $\endgroup$ – Henry Taylor Feb 24 at 19:06
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The earth like planet orbits the star at the L4 or L5 points of the gas giant. That would hide any effects of the terrestrial planet's gravity on the system, and if it does not transit the star, you would not notice it. From far away any light that you might see from the planet would get swamped out by the star and the gas giant.

My hasty checking suggests that the Earth like planet would be safe from radiation from the gas giant in this scenario. The L4 and L5 points near a Jupiter like planet at 1 AU from its star would be about about 149 million km away. Our Jupiter is about 5.2 AU from the Sun. The bow shock of its magnetosphere is about 9 million km away in the direction of the Sun. Moving the Jupiter closer to Sun would shrink the size of its magnetosphere and move the bow show closer to the surface, but even with its current size it is unlikely to reach the L4 and L5 points in the hypothetical solar system. Outside the gas giant's magnetosphere the star's radiation is dominant. An Earth like planet would be able to handle the radiation at 1 AU from a Sun like star.

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  • $\begingroup$ I would add however, that at the L4 or L5 LaGrange points the small planet would probably not be habitable because of being bathed in intense radiation from a planet the size of Jupiter. $\endgroup$ – levininja Feb 25 at 3:47
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    $\begingroup$ @levininja You are probably thinking of the L1 and L2 points. Those would be close to the planet. $\endgroup$ – David Elm Feb 25 at 6:32
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    $\begingroup$ "An Earth like planet would be able to handle the radiation at 1 AU from a Sun like star." [citation needed] $\endgroup$ – Mr47 Feb 25 at 10:16
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    $\begingroup$ @Mr47 Luckily we discovered one of these planets through direct imaging. en.wikipedia.org/wiki/The_Blue_Marble $\endgroup$ – Futoque Feb 25 at 16:09
  • $\begingroup$ Balderdash. L4+L5 of a planet at unit distance from its star are at that same distance both from the planet and the star. $\endgroup$ – Karl Feb 25 at 22:47
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This is a rogue planet that was only very recently captured by this star system. This would also mean that it is in the process of thawing out after its recent interstellar wandering, but it could be in the habitable zone.

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  • $\begingroup$ But it would be hardly colonizable. $\endgroup$ – user28434 Feb 26 at 12:04
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    $\begingroup$ An interesting point as to what constitutes earth like and colonizable area. You are probably right in the sense that they won't be able to start farming out doors, but it might well act as a suitable base with Earth gravity and all required resources. In fact it probably might make a good question in its own right... $\endgroup$ – Slarty Feb 27 at 17:41
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The planet became Earth-like only recently.

This planet is being rapidly terraformed. The humans have arrived shortly after the oxygenation of the atmosphere, and newly seeded plants are spreading over the surface.

The aliens responsible will probably not be too pleased that humans traipsing around their planet have introduced a bunch of earth germs and hookworm eggs. Or possibly the terraforming entities are automatic robots fulfilling some ancient directive, and so will not be pleased or displeased.

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If the habitability is determined using spectroscopy, that gives only approximate abundancies of chemical compounds in the atmosphere (or surface if it is visible). If the planet had some very unusual atmosphere chemistry, it could mask oxygen and water - and the planet would appear uninhabitable or not having right temperature. Perhaps the planet could be enriched by helium, which would make puffy envelope around it and obscure the breathable atmosphere below it.

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This is a fun question. The reasons why an Earth-like planet could go unnoticed for so long generates some interesting ideas.

In our own solar system, the main reason we discovered the non-classical planets (the classical planets being Mercury, Venus, Mars, Jupiter, and Saturn), is because we were actively looking for things in the night sky. (This is especially true for Neptune and Pluto.)

It's important to realize that things do slip through the cracks, in that some observation was misidentified, and it took a while to realize the mistake. This happened with Uranus, in that Uranus was seen by the ancients, but not recognized as a planet until until 1781. Pluto also has a history of having misconceptions: In 1931, Pluto was thought to be as massive as Earth; finally, in 1978 it was shown to be only 0.2% as massive of Earth. And from the mid-1800s to the mid-1900s, we assumed that our sister planet, Venus, could likely harbor life with an Earth-like environment.

We assumed Uranus was a star, when it was actually a planet. We assumed Pluto was Earth's rival, mass-wise. And we guessed that Venus could easily support life.

We didn't make these assumptions because we were stupid; we were just going with what we knew, based on the other things we knew at the time. And our assumptions occasionally cause us to incorrectly think about certain things for a long time, until we finally discover more of the truth.

We often speak badly about assuming things, but when you have so much information to process, and so little knowledge to go off of, sometimes you don't have much choice but to make certain assumptions. (And, believe it or not, often assumptions can be pretty accurate.)

That's why I say that it's pretty reasonable that certain things can "slip through the cracks." For instance, a solar system could have many, many planets that orbit in a exceptionally flat and circular plane. Therefore, it would be easy to miss a planet that circles its sun nearly perpendicularly. If you're only looking at the ecliptic for planets, why would you look elsewhere? That might not make sense to you or anyone else -- but that doesn't mean that there can't be a planet outside of the ecliptic plane.

It's not easy to find planets from one snapshot of the night sky, so a common technique is to take several photographs over a period of several days/weeks/months/years, and try to find points of light that move predictably, independently of the stars in the sky. But there may be literally billions of objects that move. How can you identify whether one is a sizeable planet, or just a relative speck of dust?

Often, these points of light are identified as asteroids, comets, or a Trans-Neptunian object. And since there are so many, you can't spend a lot of time investigating every single one, making it easy to assume that a cluster of moving points is just a cluster of rocks in an asteroid field.

Conceivably, your planet could have been previously seen off the ecliptic of planets, but since it was surrounded (or in the path of) asteroids, it was just dismissed to be tracked as "just another asteroid." When, in the future, it is noticed again by a younger astronomer, he/she would look it up in the archives, learn that it was classified as an asteroid, and then move on and forget about it.

It would take several of these sightings, and a realization that this asteroid is being noticed more frequently, before a theory develops that something is different about this asteroid, making it deserving of a closer look.

Just like Neptune, Pluto, and Venus, something was misidentified, and it takes a while to discover the mistake after we dismiss this important planet as an asteroid. There could be many reasons for this dismissal/assumption, such as:

  • In a solar system of so many gas giants, it is unlikely an Earth-like planet could exist.
  • We've scoured the ecliptic plane, and we're 99.99% sure there are no planets we haven't already found.
  • If there were any other planets out there, we'd surely have found them by now.

Regardless of how invalid these reasons are, they're still reasons, and they all work against discovering any new planets.

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I'll answer the:

I was thinking that e.g. the Earth-planet is kind of always behind the Jupiter-planet when observed from Earth, so always covered by it. Are such trajectories of two planets even possible?

No, but kind of. It is impossible for two planets but a habitable moon of the gas giant that has the same orbital period as the gas giant does could hide behind the gas giant long enough and have been hard enough to detect before that. The orbital planes also have match with each other and line up with direction to Earth.

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