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I am currently working on a setting involving our solar system (the Sol system) containing several fictional planets, in addition to all of the real major planets; This question focuses on one of those planets specifically.

This fictional planet, which I have given the temporary name of D-Beta, is somewhat odd, primarily because it has an orbital inclination of 88 degrees, making it only slightly off from a polar orbit of the Sun; This orbit is also highly eccentric, passing between the orbits of Venus and Earth at its closest and between the orbits of Earth and Mars at its furthest.

Assuming those orbital characteristics and a mass between 0.75 and 1.50 times the mass of Earth, what would have been the earliest in history that such a planet could have been reliably observed by astronomers?

The stability of such an orbit over the long-term is not relevant to this question.

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    $\begingroup$ On the contrary, "the stability of such an orbit over the long-term" is crucially related to the period of revolution. By way of being a function of the major axis of the elliptic orbit, which goes to infinity when the eccentricity approaches 1. $\endgroup$ Commented Jun 4, 2025 at 18:43
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    $\begingroup$ This makes periods of tens of kyrs possible, so that the planet could be passing its perihelion for the 1st time since we invented writing; the perihelion may be as near to the Sun as you wish for a given major axis. This relates to the long-term stability because near-parabolic orbits (e ~1) are extremely sensitive to the gravitational influence of the outer planets. $\endgroup$ Commented Jun 4, 2025 at 18:46
  • $\begingroup$ An orbital period of 10kyrs obtains if the major axis of the orbit is 2 million astronomical units (10^6 times that of the Earth), by Kepler's 3rd law. If, in addition, the perihelion is at ~1 AU, the orbit is for all practical purposes a parabola. E. g. the velocity at the perihelion is ~600km/s or ~1/3 AU/day, the escape velocity from the Solar system. $\endgroup$ Commented Jun 4, 2025 at 19:17
  • $\begingroup$ You may want to (re)read Rendezvous with Rama, in which the orbital characteristics of the eponymous asteroid are discussed at length. $\endgroup$ Commented Jun 4, 2025 at 19:23
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    $\begingroup$ Did I miss that the orbit is always nearer to the Sun than Mars, or has it just been added? In which case the planet has been the brightest star in the night sky since Man first stood upright. $\endgroup$ Commented Jun 4, 2025 at 19:36

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Knowledge of the planet would be Older than Dirt

And most certainly before Times Immemorial (1189). You describe a planet that is closer to the sun than Earth, and farther than Venus. It's closer to us than Mars. It'd be somewhere in brightness between Moon and Mars, brighter than Venus and probably even Jupiter in some conditions, and we know Venus, Mars, and even Jupiter were observed since before antiquity and probably back to pre-humans - so they are known to be "older than dirt".

We have seen Venus as the morningstar/eveningstar since we started to use tools, whenever we were in the early or late stage of the day, and the planet Venus was just over the horizon, while the giant plasma fusion reactor we attribute meaning to was not. We'd have seen this planet since the same time, and observed it since then.

Orbital observations

Your eccentric planet would likewise have some times of the year where we would see it cutting through the ecliptic, while Earth just so happens to occlude the sun. While we might confuse it with Venus on some occasions, there will be days on which both are visible, either simultaneously, or at the opposite end of the day from Venus, making sure the earliest astronomers assign a name to it separate from Venus. To keep the naming scheme of planets and because I despite the numerals, I'll call him Bacchus for the moment - Venus is the god of love, Bacchus is the god of parties, and seeing them happens at specific intervals, which is good to plan festivities around.

Venus Orbit is somewhat easy,, and can be mapped to a synodic period, in which its constellation to Earth reoccurs. Using the synodic period, it was in the past very hard to map in terracentric systems. Heliocentricity makes it easy, Venus Orbits once in 225 Earth days. But we are interested in the synodic period of 584 days. Of those, it spends about 263 days as Morningstar (starting at about the green marked line), where Venus is in front of the earth on the orbit, then is invisible for about 50 days as Venus crosses to behind the sun (yellow line), then it spends another 263 days as the evening star (starting at the red line), and finally, Venus passes in front of the sun as it crosses about the magenta line and is invisible to us, when she's closest to us. This is the smallest part of the travel, as the sun is just a few degrees of orbit to us, and so takes only about 8 days.

Venus and earth orbits, starts of the described periods marked

Bacchus, is a little more complicated. Placing it circularly and directly between Earth and Venus, would result in an orbit taking some 286 days if my back of the napkin math of picking a number between Earth and Venus is right - a little shorter than an Earth year. But Bacchus also moves 88° to the ecliptic and Earth, so it has a node to Earth: the point when the orbits are closest and their planes cross.

Wherever we place this node, we will never see Bacchus unless Bacchus is in a position far enough below or above the sun, because the Sun and Bacchus would be from our observation point very close in that alignment. Often, they might be too close to have only one covered by the Earth's horizon, unless Bacchus rose high enough above (or dipped enough below) the sun. So, within the quarters in which Earth is close to the node, we might only see Bacchus if it is within the top or bottom area of its orbit, which is maybe a 15° angle on either side of the sun before dipping into the glare of the sun - by conservative estimation based on a napkin sketch.1 Putting Earth on the quarters perpendicular to the nodes, we would have the best chance to see Bacchus, either as a morning or evening object, akin to Venus, provided that it is within about... 50° off the ecliptic in that time by the same estimation2 - so there is at least a 200° window of Bacchus orbit on which we can see it in the two quarters of Earth orbit in which we are far from the nodes. The orbit we'd map to Bacchus under terracentric models would be more strange than the countercircles we attributed to Venus, but again, heliocentricity makes it easier. If it is relevant, it would take modern humans only a little bit of processing power with a program akin to Universe Sandbox to get the actual visibility periods and even brightness, and when you can see Bacchus in each year.

All in all, it's a solvable problem, and astronomers would probably have taken Bacchus appearance as morning or evening star to calculate their year from - in fact, observing Bacchus disappearance or position above or below the sun would be the best indicator of a specific point in time of the year, even better than the equinox or solstices, whole the appearance as morning or evening star would laud other times of the year.

Naming

I chose Bacchus, because I thought I could make its appearance perfect for the wine harvest, but then I figured, depending on the node alignment, you could map the whole harvest cycle to the position of the planet. Depending on the exact alignment of the node to the Earth year, the harvest god Proserpina (Persephone) might also be a good name, especially if the node lines up with the equinox or solstice, making it easier observable.

Simulating D-Beta

Plugging in a 0.85 to 1.42 AU orbit at 88° in Universe Sandbox at 186° ascending node granted a somewhat circular orbit (eccentricity 0.253), an orbital period of 1.21 Earth years. The average temperature of the planet is between -7°C to -15°C. The situation is stable in the mid term: no orbit had catastrophic failure after 5500 years of simulation, despite D-Beta getting as close as 0.15 AU to Earth and 0.18 AU to Mars, if we assume Earth mass and density. The ascending node had shifted to 215°, but nothing else. View from the eclipticView from top

Getting more extreme settings: I placed D-Beta with 0.8 Pericentric distance, put the half axis to 1.23 (resulting in 1.37 Earth years per D-Beta year). Eccentricity ended at 0.351, Inclination 88°, Pericentric argument 299° with the ascending node at 0°

Start Conditions - extreme orbit - from eclipticFrom Above

D-Beta has seasons... kinda... average temperature is still -15°C to -20°C, and distance to the star is a super interesting graphically: getting as close as 0.8 AU, it strays out to Mars (orange) and back!

Average Temperature over a few years

That's a direct result of getting a very variable influx of energy: 80 and 90 Petawatt of solar energy, where Earth gains 120 PW constantly, and Mars gains 16 PW.

Energy from Sun on the Planets

As I estimated, D-Beta would be more luminous than Mars and Venus, but Jupiter would still the most luminous reflector in the solar system - though the apparent power of D-Beta might be higher in some situations:

enter image description here

Some further observation after watching the simulation for some hours: With an even more eccentric orbit that passes under the sun closely, there might be a point where the conditions allow to always observe the planet, but for its closest approach to the sun. The orbital period of such a planet could be some 6 years for an eccentricity of about 0.8, but the surface temperature would be super frosty. It'd spend only about three months in the "dip" area, before rising far enough above the sun so Earth could occlude the latter but not the former in the worst condition of Earth being exactly opposite the ascending node...

Notes on the napkin estimations

After simulating and trying my best to get some angling done, I need to revise my napkin math:

  1. We might see Bacchus on the circular orbit for at least the top quarter and bottom quarter, if we are aligned with the nodes. This shifts rapidly if we make Bacchus more eccentric to one side, and can result in the total occlusion in one direction.
  2. Again, I should have taken a cut from Venus here and realized we'd see not 200° but anything but a thin sliver from the points orthogonal to the nodes. On that point, we can see all but a 50° cone around the sun, and Bacchus will never be in that cone if we are at that spot. My Brain failed in not doing the 90° shift. Of course, how much this cone occludes of Bacchus Orbit shifts between us being at the nodes and orthogonal to them.

Concluding, we should have a good chance to see Bacchus on at least 300° of its orbit.

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    $\begingroup$ @MichaelRichardson I mentioned explicitly that I did the napkin math based on a circular orbit - that simplified a lot of it, but you are correct for the actual schedule. $\endgroup$ Commented Jun 4, 2025 at 16:32
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    $\begingroup$ This is a really high quality answer. +1. $\endgroup$ Commented Jun 5, 2025 at 1:37
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    $\begingroup$ The additional details and graphics are incredible! $\endgroup$ Commented Jun 5, 2025 at 13:57
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    $\begingroup$ When you forget you're on Worldbuilding, not Speculative Astronomy. +1 $\endgroup$ Commented Jun 6, 2025 at 8:52
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    $\begingroup$ @controlgroup Universe Sandbox $\endgroup$ Commented Jun 10, 2025 at 19:59
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Prehistoric. Orbital inclination doesn't affect brightness and it'll be brighter than Mars, which is already one of the brighter objects in the sky. Practically everybody will know about the crosswise sky-traveler long before anyone writes anything down.

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  • $\begingroup$ Orbital inclination makes the planet less bright because it spends more time close to earth. It doesn't impact the maximum brightness very much $\endgroup$ Commented Jun 11, 2025 at 14:56
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It would have been observable since antiquity. A terrestrial planet with an orbit that is closer than that of Mars would not require anything more than the naked eye to observe. At that distance and with the maximum size of 1.5 Earth mass it would probably be one of the brightest objects in the night sky. So most ancient cultures would at least be aware of it while astronomically inclined civilizations like the Maya would have highly detailed records of its observation and movement.

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It depends upon its size (though the OP has set that at around that of Earth) and albedo, which is the amount of incident light that is reflected back into space. Earth has an albedo of around 0.3. Venus has an albedo of around 0.7. Mars has an albedo of around 0.2. However, the (exo)planet with the lowest known albedo is TrES-2b, with an albedo of 0.0004, which is blacker than charcoal.

With a high albedo like Venus or Earth, this planet would have been discovered in antiquity. However, at particularly low albedo scores (<0.01), this world might no longer reflect enough light to be seen with the naked eye, and would only be readily visible with a telescope, putting its discovery later than the 1600s AD, potentially considerably later if its albedo is much lower. With an albedo lower than 0.001, this world might not be seen unless it transits the sun while someone is looking... which might be as early as the 1600s AD, or it might be considerably later.

Failing the optical observation of a particularly low albedo world, it might be discovered by examining the perturbations its presence would have upon other planets. This would necessarily follow Newton's 1684 theory of gravitation.

However, low-albedo planets like TrES-2b appear to be rare, so the greatest likelihood is that this eccentrically-orbiting world would have been known from antiquity.

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  • $\begingroup$ Halving the albedo of Mars would make it dimmer by about 0.8 magnitudes. It would still be one of the brightest objects in the sky. (And remember than Mars is small. This planet is big, so that it would reflect the same amount of light as Mars even if its albedo was only 1/4 of that of Mars.) $\endgroup$ Commented Jun 4, 2025 at 19:24
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    $\begingroup$ @AlexP Plus it is close... this question prompts me to get universe sandbox... $\endgroup$ Commented Jun 4, 2025 at 19:36
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    $\begingroup$ Since it's supposed to be 1900K, TrES-2B is "black" in the way that the filament in a lit incandescent light bulb is "black". Taking luminosity $L = L_\odot (R/R_\odot)^2 (T/T_\odot)^4$, at a distance of 1AU would be significantly brighter than the full moon. Artists' depictions and wikipedia descriptions are silly sometimes. $\endgroup$ Commented Jun 5, 2025 at 2:05

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