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Not an astronomer (obviously), but had a curious question. Is it possible to have a planet such that:

  • Its star is right on the edge of a nebula

  • The planet happen to be in the goldilocks zone and has the right atmospheric makeup to support life

  • This planet's orbit takes it in an out of the nebula, creating a "Nebula Season"
  • This season is observed by its inhabitants by the sudden lack of visible celestial bodies in the sky (due to the nebula. Can be gradual but should be a notable difference between June and December, for instance). Perhaps like the effects described in: What would skies look like on Worlds inside Nebulae?

Is this scenario possible? Can such a star system exist in such a location? and Bonus: Can the Nebula be dense enough to produce the mentioned effect?

I'd be happy if you'd correct my assumptions!

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    $\begingroup$ I suspect that a nebula will not have an outer boundary as clearly defined as what you seem to be looking for. I can't prove it with certainty, so I'm not making this an answer, but the scale of a nebula does not lend itself to a convenient and perfectly plain border with immediate contrast between being just inside the edge and being just outside it; you're going to see a vague and fuzzy border as the nebula gradually (over whatever distance, quite possibly in terms of light-years) becomes denser, which means that the exact edge of the nebula is a highly subjective matter. $\endgroup$ – Palarran Mar 29 '18 at 4:12
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    $\begingroup$ No, you have a scale problem. A nebula is often simply a galaxy (!) Orbiting "in and out of" a galaxy is meaningless. $\endgroup$ – Fattie Mar 30 '18 at 16:26
  • $\begingroup$ You might be interested in What are the effects of a planet staying long-term inside of a nebula? Full disclosure: My answer is the accepted one. $\endgroup$ – a CVn Mar 30 '18 at 20:36
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    $\begingroup$ @Fattie, your information is about a century out of date. Galaxies haven't been considered a type of nebula since about the early 1920s. $\endgroup$ – Mark Mar 30 '18 at 20:44
  • $\begingroup$ hmm, i don't really agree Mark, anything you see through a scope that is fuzzy, you say "WTF is that nebula?" (What else would you say until you knew what it was? "that fuzzy thing?") Anyway! indeed even "small" nebula are, what, 10 million? times bigger than the scale the OP is thinking! $\endgroup$ – Fattie Mar 30 '18 at 21:16
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No, for several reasons.

  1. "Edge of a nebula" is a very fuzzy region. A typical nebula is tens to hundreds of light-years across, tapering out over a long distance -- the sharp edges seen in photographs are due to how far we are from them.
  2. Nebulas are incredibly diffuse objects -- if you were to capture a chunk of one, any laboratory on Earth would consider it a high-grade vacuum. They only appear dense because of how thick they are (see point 1).
  3. Light exerts pressure. Not much pressure, but nebula gasses don't weigh much. A star will very quickly clear the nebula from its immediate area, creating a clear bubble a significant fraction of a light-year across.

See Wikipedia's article on nebulas for more details.

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    $\begingroup$ "Nebulas are incredibly diffuse objects" - if one would compare nebula to atmospheres of objects in our solar system, then Moon has much denser atmosphere than your typical nebula. $\endgroup$ – Mołot Mar 29 '18 at 14:19
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    $\begingroup$ It should probably be pointed out that a lot of the Hubble Pictures of Nebula are artificially colored to show properties of the Nebula that are not ordinarily visible to the human eye. They aren't depicting the true coloration of a nebula. $\endgroup$ – hszmv Mar 29 '18 at 14:29
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    $\begingroup$ @Mołot My grandfather used the term "a huge region made up of starfart" to describe nebulas to me, as diffuse objects made up of "gas"-like in space. My grandfather is a weird person. $\endgroup$ – T. Sar Mar 29 '18 at 15:59
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    $\begingroup$ @Pleiades And light isn't the only thing stars produce - there's also the stellar wind, which exerts considerable pressure on its own. The Sun's heliopause (where the outward pressure of the solar wind and the inward pressure of interstellar gas is in an equilibrium) is more than 100 AU; well clear of any planetary orbits. $\endgroup$ – Luaan Mar 30 '18 at 8:47
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    $\begingroup$ You could add that if you were inside a nebula (or close enough to it for it to take up a significant amount of the sky), you probably wouldn't be able to see it at all, as it would be too dim. (Though that does rather depend on what kind of nebula it is, see worldbuilding.stackexchange.com/questions/4875/…) $\endgroup$ – Nathaniel Mar 30 '18 at 8:48
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It doesn't need to orbit in and out. It can face it or face away from it.

When on the side of the sun away from the nebula you see the stars and the nebula would be behind the sun. On the side closest to the nebula you see the nebula and the stars would be behind the sun.

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    $\begingroup$ This is a good idea, but it took me a while to understand what you meant. When the planet is opposite the nebula in its orbit, then the nebula would be on the day side of the planet, and so would not be visible. At night you would see the stars in the other direction. Half-a-year later, the nebula would dominate the night sky, while the non-nebula-covered sky would be hidden in daylight. $\endgroup$ – Paul Sinclair Mar 30 '18 at 17:11
  • $\begingroup$ A quarter of the year half the sky would have stars and half would be empty $\endgroup$ – Thorne Mar 31 '18 at 11:19
  • $\begingroup$ @Thorne except that the nebula would probably not be dense enough to block out all the stars. It would be a rare nebula that was dense enough to block out all the light of the stars behind it. $\endgroup$ – M. A. Golding Mar 31 '18 at 23:13
  • $\begingroup$ It's a story. I think he can have said rare nebula..... $\endgroup$ – Thorne Apr 3 '18 at 1:23
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Mark's answer is right on spot.

I'll just add for a planet to be habitable like Earth, it will probably take billions of years. During that time, planets within a star system will have cleared out any debris from the star's formation that is close to their orbits. Any mass of dust, rocks or ice that does not make it into a planet, and does not get captured by one, should become part of a belt or a cloud outside any planet's orbit.

If your planet is young enough that there are still nebulae remains within its system, and crossing its orbit, it will probably be going through a hadean phase. That would not be inhabitable.

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  • $\begingroup$ The sun is about 4.6 billion years, years old. First life on earth was about 3.8-4.3 billion years ago - so less than a billion years for earth to become habitable. If you insist on oxygen in the atmosphere, that's still only a couple of billion years from the formation of the sun. $\endgroup$ – Martin Bonner Apr 1 '18 at 13:29
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Going by the title of your question alone:

Can a habitable world exist that would orbit in and out of a nebula?

The answer would be yes. As the planet orbits its star that orbits the center of the galaxy, the planet and its star could enter a nebula and pass through that nebula for thousands or millions of years and then emerge on the other side of the nebula. And then, after making a full orbit around the galaxy lasting for maybe two hundred million years, the star and its planet might reenter the nebula again if the nebula was still there.

Of course entering the nebula every 200,000,000 years is not exactly like having a regular "nebula season" every year, but at least it is possible.

Exoplanet GU Piscum b orbits GU Piscum at a distance of about 2,000 astronomical units and thus the opposite sides of its orbit are separated by a distance of about 4,000 astronomical units. An astronomical unit is the distance between Earth and the Sun. Thus a planet with such a wide orbit as GU Piscum b could travel about 2,000 times farther into and out of a nebula's borders than a planet with Earth's orbit could. I don't know if a nebula would have border sharp enough for that to make a difference.

Of course a planet with such a wide orbit would probably be many, many times as far from GU Piscum as the outer limits of GU Piscum's habitable zone, so unless there is a very exotic and alien type of life on that very cold planet there would nobody to notice the nebula seasons.

Of course there could be something like a brown dwarf star orbiting a star at a distance of 2,000 Astronomical units and there could be a habitable planet orbiting the brown dwarf very closely, close enough to be heated to a habitable temperature.

Of course the year of GU Piscum b is calculated to last about 163,000 Earth years, so the natives would probably not live long enough to notice nebula seasons.

Among the smallest known nebula types are planetary nebulae, emitted by stars during a certain brief phase of their lives. At any one moment there will be only one planetary nebula for millions of stars. Planetary nebulae are about a light year in diameter, and thus about 63,241.077 Astronomical units wide. So even if a planetary nebula has a relatively sharp border, a planet of a star right at the border of the nebula, with an orbit only a couple of thousand astronomical units wide, probably isn't going to take travel into very much denser or thinner regions of the nebula.

Furthermore, most nebulae are much thinner than the thinnest vacuums which can be produced on Earth. Astronomical photographs of nebulae are taken with long exposures many thousands of times longer than the time it takes for a human eye to see successive images. Thus nebulae look bright and opaque in photographs but look pale and translucent when seen through telescopes.

So the sky probably wouldn't look much different when a planet was deep inside a nebula than when the planet was outside the nebula, and people on even the widest orbiting planet wouldn't live thousands of years to notice what little difference there was.

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  • $\begingroup$ The nebula must be orbiting the galaxy too (otherwise it would get sucked to the galactic centre by gravity). $\endgroup$ – Martin Bonner Apr 1 '18 at 13:30
  • $\begingroup$ @Martin Bonner - Yes, and there is no reason why the two orbits would be similar enough that they would ever intersect again. As objects orbit the galactic center they pass close to many other objects and their orbits are constantly changing. $\endgroup$ – M. A. Golding Apr 4 '18 at 3:36
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Not as you describe for a whole planet (Mark's answer), but you could achieve a similar effect for 1/2 the planet (actually far less due to the visible sky at various longitudes and latitudes).

  1. If it were tidal locked one side of the world would always be facing the star, while the other side would alternate between facing the nebula (Nebula season) and facing the visible universe (Star season).

  2. If the day and year were equal in length one side of the planet would always face the nebula (Nebula season), while the other side would always face the visible universe (Star season).

For both scenarios playing with the axis of the rotation could add some variety by mixing meteorological seasons with celestial ones.

Whether such planets would be inhabitable becomes a new question however, as being tidal locked or having other convenient orbital mechanics pushes the planet well outside earth-norms.

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Yes. We look up and things astonish us. If your world is fictional, by all means - yes - make it so. A nebula spire pulled toward a blackhole for instance... is obviously not impossible - in the vastness of the cosmos.

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